Download EC3445 Trafalgar Centre Seismic Evaluation

SEISMIC EVALUATION REPORT
S T R U C T U R A L
A N D
N E L S O N
C I V I L
C I T Y
T R A F A L GA R
P R E P A R E D
N E L S O N
C O U N C I L
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C O U N C I L
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N E L S O N
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2 0 1 4
TRALAFGAR CENTRE, NELSON – DETAILED SEISMIC ASSESSMENT REPORT
Prepared For:
NELSON CITY COUNCIL
Date:
Project No:
Revision No:
5th February 2014
108899.00
5
Prepared By:
Reviewed By:
Mark Browne
Mark Whiteside
PROJECT ENGINEER
SENIOR PROJECT ENGINEER
Matthew Franklin
DESIGN ENGINEER
Holmes Consulting Group LP
Christchurch Office
108899 Trafalgar Centre Seismic Evaluation Report Northern extension.doc
REPORT ISSUE REGISTER
DATE
REV. NO.
REASON FOR ISSUE
October
2012
1
(DRAFT)
Draft issue for comment
November
2012
2
(DRAFT)
Draft issue for information
December
2012
3
First Issue to Nelson City Council
May
2013
4
Draft issue updated to include northern buildings
5th
February
2014
(DRAFT)
5
Draft watermark removed - no change to content.
(FINAL)
108899 Trafalgar Centre Seismic Evaluation Report Northern extension.doc
CONTENTS
Page
EXECUTIVE SUMMARY
1.
2.
ES-1
INTRODUCTION
1-1
1.1
The Building
1-1
1.2
Information Used for the Evaluation
1-2
1.3
Limitations
1-2
BUILDING DESCRIPTIONS
2-3
2.1
Northern Building
2-3
2.1.1
Lateral Load Resisting System
2-3
2.1.1.1
Transverse direction
2-3
2.1.1.2
Longitudinal direction
2-3
2.2
2.1.2
Gravity Load Resisting System
2-4
2.1.3
Foundations
2-5
Main Hall
2.2.1
2.3
2-6
Lateral Load Resisting System
2-6
2.2.1.1
Transverse direction
2-6
2.2.1.2
Longitudinal direction
2-7
2.2.2
Gravity Load Resisting System
2-8
2.2.3
Foundations
2-9
Southern Addition
2-9
2.3.1
2-9
Lateral Load Resisting System
2.3.1.1
Transverse Direction
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2-10
i
2.3.1.2
2.4
3.
4.
Longitudinal Direction
2-10
2.3.2
Gravity Load Resisting System
2-10
2.3.3
Foundations
2-11
Civil Defence Office
2-11
2.4.1
Lateral Load Resisting System
2-11
2.4.1.1
Transverse direction
2-12
2.4.1.2
Longitudinal direction
2-12
2.4.2
Gravity Load Resisting System
2-12
2.4.3
Foundations
2-12
STATUTORY REQUIREMENTS
3-1
3.1
Building Act
3-1
3.2
Building Code
3-2
3.3
Scope of This Report
3-2
BUILDING EVALUATION
4-1
4.1
Northern Building
4-1
4.1.1
Building Seismic Parameters
4-1
4.1.1.1
Importance Levels
4-2
4.1.1.2
Structure Ductility
4-3
4.1.2
Material Properties
4-4
4.1.3
Modelling Assumptions
4-4
4.1.4
Estimation of Building Strength
4-5
4.1.4.1
Transverse Direction Analysis
4-5
4.1.4.2
Longitudinal Direction Analysis
4-6
4.1.5
Foundation Analysis
4-8
4.1.6
Geotechnical Considerations
4-8
4.1.7
Summary
4-8
4.1.8
Strengthening Required
4-9
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ii
4.2
Main Hall
4.2.1
4.3
4.4
4-10
Building Seismic Parameters
4-10
4.2.1.1
Importance Levels
4-11
4.2.1.2
Structure Ductility
4-12
4.2.2
Material Properties
4-12
4.2.3
Modelling Assumptions
4-13
4.2.4
Estimation of Building Strength
4-14
4.2.4.1
Transverse Direction Analysis
4-14
4.2.4.2
Longitudinal Direction Analysis
4-15
4.2.5
Geotechnical Considerations
4-16
4.2.6
Summary
4-16
4.2.7
Strengthening Required
4-16
SOUTHERN ADDITION
4-18
4.3.1
Building Seismic Parameters
4-18
4.3.1.1
Importance Levels
4-19
4.3.1.2
Structure Ductility
4-20
4.3.2
Material Strengths
4-21
4.3.3
Modelling Assumptions
4-21
4.3.4
Estimation of Building Strength
4-22
4.3.4.1
Transverse Direction analysis
4-22
4.3.4.2
Longitudinal Direction Analysis
4-24
4.3.5
Geotechnical Considerations
4-24
4.3.6
Summary
4-25
4.3.7
Strengthening Required
4-25
Civil Defence Office
4-26
4.4.1
Building Seismic Parameters
4-26
4.4.1.1
4-27
Importance Levels
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iii
4.4.1.2
5.
6.
Structure Ductility
4-28
4.4.2
Material Properties
4-29
4.4.3
Mo delling Assumptions
4-29
4.4.4
Estimation of Building Strength
4-29
4.4.4.1
4-29
Lateral Resistance Analysis
4.4.5
Geotechnical Considerations
4-31
4.4.6
Summary
4-31
4.4.7
Strengthening Required
4-31
RECOMMENDATIONS - CONCLUSIONS
5-1
5.1
5-1
Recommendations
REFERENCES
TABLES
6-2
Page
Table 2-1: Priorities and Timeframes – Earthquake Prone Building Policy
3-2
Table 3-1: Seismic Parameters
4-1
Table 3-2: Importance Levels
4-3
Table 3-3: Seismic Parameters
4-10
Table 3-4: Importance Levels
4-11
Table 3-5: Seismic Parameters
4-18
Table 3-6: Importance Levels
4-20
Table 3-7: Seismic Parameters
4-26
Table 3-8: Importance Levels
4-28
FIGURES
Page
Figure ES-1-1: Nelson Trafalgar Centre
2
Figure 1-1: Trafalgar Centre
1-1
Figure 1-2: Plan of Current Layout of Buildings
1-2
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iv
Figure 1-3: Plan of Northern Building showing lateral load resisting systems
2-4
Figure 1-4: G ra v it y L oa d Res is t i n g S ys t e m ins id e t h e N o rt h e r n B u i ld in g
2-5
Figure 1-5: Typical wall and foundation in the Northern Building
2-6
Figure 1-6: Plan of Roof Bracing
2-7
Figure 1-7: Elevation of Glulam Timber Arch Beam
2-8
Figure 1-8: Plan of Gallery Seating
2-8
Figure 1-9: Timber Joist to Bearer to Concrete Pile System
2-9
Figure 1-10: Detailed plan of the Civil Defence Office
2-11
Figure 3-1: Building Acceleration Spectra Comparison NZSS 1900 versus NZS 1170
4-2
Figure 3-2: Building Acceleration Spectra Comparison NZSS 1900 versus NZS 1170
4-5
Figure 3-3: Timber lath and plaster walls brace the upper roof of the Northern Building in the
transverse direction.
4-6
Figure 3-4: Load path to the critical in-plane longitudinal wall.
4-7
Figure 3-5: Details of the concrete block walls with unreinforced veneers.
4-8
Figure 3-6: Building Acceleration Spectra Comparison NZSS 1900 versus NZS 1170
4-11
Figure 3-7: Example Seismic Response Spectra
4-13
Figure 3-8: Two Dimensional Computer Model of the Transverse Western Concrete Moment
Resisting Frame
4-14
Figure 3-9: Two Dimensional Computer Model of the Eastern Concrete Moment Resisting Frame4-15
Figure 3-10: Western Elevation Showing Full Height Blockwall in Middle Bay
4-16
Figure 3-11: Building Acceleration Spectra Comparison NZSS 1900 versus NZS 1170
4-19
Figure 3-12: Example Seismic Response Spectra
4-22
Figure 3-13: Plan view of Southern extension showing transverse lateral load resisting elements4-23
Figure 3-14: Photos showing Reidbrace proprietary connectors which failed in the Canterbury
earthquakes
4-23
Figure 3-15: Plan view of Southern extension showing longitudinal lateral load resisting elements4-24
Figure 3-16: Building Acceleration Spectra Comparison NZS 4203 versus NZS 1170
4-27
Figure 3-17: Civil Defence Office and east Victory room section
4-30
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v
EXECUTIVE SUMMARY
SCOPE
Holmes Consulting Group have been commissioned to evaluate the likely seismic performance
of Nelson Trafalgar Centre, a multi purpose events facility located at 7 Paru Paru Road, Nelson.
A previous qualitative study identified a number of potential seismic vulnerabilities, for which
further evaluation was recommended. This work was undertaken in conjunction with a
quantitative study to determine the overall seismic capacity of the building in terms of the new
building standard (%NBS).
BACKGROUND
Nelson Trafalgar Centre currently consists of essentially four structures. Each of the four
structures are linked, however these have separate structural systems to support gravity and
lateral loads. These structures are: the Northern Build, the Main Hall, the Southern Addition
and the Civil Defence Office.
At the northern end of the building is an early 1970’s one storey structure which is mainly used
for functions and as an entrance to the central building (Main Hall). The Northern Building is
constructed of timber walls and reinforced concrete block walls which resist lateral loads. The
roof is supported by long spanning steel trusses which are inturn supported by the reinforced
concrete block walls.
The Main Hall was constructed in the early 1970’s. The roof is supported by large glulam
timber roof arch beams with the remaining structure primarily constructed of reinforced
concrete frames/walls. The lateral load resisting system consists of reinforced concrete frames
in the transverse direction. The lateral load resisting system in the longitudinal direction is
reinforced blockwork on the western side of the building and reinforced concrete frame on the
eastern side of the building. The Northern Building and Main Hall were originally designed in
1970 by Sanders and Lane consulting engineers.
The Southern Addition is located at the southern end of the building and is a new addition that
was built in 2008. This new addition is a mix of timber, structural steel, and reinforced
concrete wall construction. The lateral load resisting system for the building is crossed braced
frames supported on reinforced concrete precast panels. The new addition was designed in
approximately 2005.
The Civil Defence Office is a lightweight, single storey structure built as an addition to the
Nelson Trafalgar Centre. This building was constructed in 1980 and is attached to the northeastern corner of the Northern Building The Civil Defence is mostly constructed from timber
framing and GIB plaster board but also incorporates a reinforced concrete block wall at the
northern end of the building.. The Civil Defence Office and Northern Building are built
integral to each other and share a common wall between them.
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ES-1
F igu r e E S-1 -1: N els o n Tr a fa lga r C e nt r e
The buildings’ foundations are typically supported on a network of ground beams transferring
loads to piles.
All four buildings have been have been assessed against the current loadings standard
NZS1170.5:2004 [1]. The Main Hall and Southern Addition and Northern Building have been
assessed as Importance Level 3 buildings as they can contain crowds of people. The Civil
Defence Office has been assessed as an Importance Level 4 building as it is required for postdisaster function.
Based on a review of the 1970 loadings code (NZSS1900:1965 Chapter 8) [4] to which the Main
Hall building was originally designed, it is likely that the structure was classified as a public
building. A comparison of the response spectra from the two standards and comparison of the
resulting code loads indicates that the original design base shear was approximately 20% of that
for a similar new building on this site.
The Northern Building was likely designed to the same loading code as the Main Hall.
Although the Northern Building is a significantly smaller structure than the Main Hall and does
not have the capacity to accommodate large crowds of people, it is one of the main egress
routes for the Main Hall. Therefore, in accordance with NZS1170.5:2004 it must be considered
an Importance Level 3 building. A comparison of the response spectra from the two standards
and comparison of the resulting code loads indicates that the original design base shear was
approximately 30% of that for a similar new building on this site.
The new Southern Addition was designed in 2005. At this time the loadings code approved for
use in the New Zealand Building Code was NZS4203:1992 [5]. As the building is joined to the
main hall we would expect the building to have an elevated Importance factor for crowds using
this building. The building has been assessed against the current loadings standard
NZS1170.5:2004. A comparison of the response spectra from the two standards and
comparison of the resulting code loads indicates that the original design base shear was
approximately 90% of that for a similar new building on this site.
The Civil Defence Office was designed in the 1980. At this time the loadings code approved
for use in the New Zealand Building Code was NZS4203:1976 [7]. The Civil Defence Office
will be required for post disaster services and is therefore considered an Importance Level 4
building in accordance with AS/NZS1170. A comparison of the response spectra from the two
standards and comparison of the resulting code loads indicates that the original design base
shear was approximately 65% of that for a similar new building on this site.
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ES-2
Simple hand calculations and two dimensional linear elastic models based on the equivalent
static method were used to determine the seismic capacity of the building in terms of new
building standards.
NORTHERN BUILDING
Based on a series of analysis, it was determined that the existing Northern Building has a
strength equivalent to less than 15% NBS for an Importance Level 3 building.
The analysis included checks of the roof diaphragms, upper roof bracing, concrete block walls
(in-plane and out-of-plane), steel truss capacity and steel truss to concrete beam connections.
The building was analysed in both north-south and east-west directions to determine the
capacity of the structure resisting lateral loads in each direction of loading. The Northern
Building’s overall strength determination is based on the lowest capacity of any one of these
primary elements which make up the lateral load resisting system.
MAIN HALL
Based on a series of analysis, it was determined that the existing main building has a strength
equivalent to approximately 20-25% NBS for an Importance Level 3 building.
The analysis included checks on the roof bracing, gallery moment frames, gallery concrete
masonry walls and the foundations to the building. The building was analysed in both northsouth and east-west directions to determine the capacity of the structure resisting lateral loads
in each direction of loading. The Main Hall’s overall strength determination is based on the
lowest capacity of any one of these primary elements which make up the lateral load resisting
system.
SOUTHERN ADDITION
Based on a series of analysis, it was determined that the existing southern addition has a
strength equivalent to approximately 25-30% NBS for an Importance Level 3 building.
The analysis included checks on the roof bracing, steel braced frames, reinforced concrete walls
and the foundations to the building. The building was analysed in both north-south and eastwest directions to determine the capacity of the structure resisting lateral loads in each direction
of loading. The Southern Addition’s overall strength determination is based on the lowest
capacity of any one of these main elements which make up the lateral load resisting system.
CIVIL DEFENCE OFFICE
Based on an analysis of the structural drawings and through undertaking a site visit it was
determined that the capacity of the Civil Defence Office is less than 15% NBS of the ULS
requirements for an Importance Level 4 Building.
The capacity was limited by the lack of a structural roof diaphragm to distribute lateral loads to
the resisting wall elements and the removal of the internal timber bracing walls shown on the
original construction drawings.
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ES-3
The SLS 2 requirements of an Importance Level 4 building (immediate occupancies after a
1/500 year earthquake) have not been explicitly considered. However, this requirement will not
likely be an issue as the building is small and lightweight.
RECOMMENDATIONS
The detailed assessment shows that the Trafalgar Centre is an earthquake prone building
defined as less than 33% of the current loading standard. As such strengthening is required in
accordance with the statutory requirements.
We recommend that a specific strengthening design be carried out on the building to increase
its capacity up to a minimum of 67% of new building standard.
Strengthening was not part of the scope of this detailed engineering evaluation; however
indicative strengthening options have been provided based on the level of acceptable risk to the
building.
The geotechnical engineer has indicated that the site is susceptible to liquefaction and lateral
spreading issues. As part of any strengthening program for the buildings on this site, these
issues will need to be addressed sufficiently that the buildings’ foundations can perform in a
satisfactory manner.
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ES-4
1.
1.1
INTRODUCTION
THE BUILDING
Located at 7 Paru Paru road, Nelson the Northern Building and Main Hall of the Trafalgar
Centre are a multi purpose facility designed in 1970 by Sanders and Lane Consulting Engineers
as the structural engineers. In 1980 the Nelson City Council commissioned a small Civil
Defence Office to be built as an addition to the Northern Building. In 2008 a new structure
was built to the south of the main hall. This new structure was designed in 2005.
Figure 1-1 below shows the building viewed from the carpark looking south.
Upper roof
Lower roof
F igu r e 1 -1: T ra f alg ar C e nt r e
Main Hall
Southern Addition
Civil Defence Office
Existing Northern building
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1-1
F igu r e 1 -2: P la n o f C u r re nt L a yo u t o f Bu i ld in gs
1.2
INFORMATION USED FOR THE EVALUATION
The information used for the analysis of the Northern Building and Main Hall was a
construction set of the original structural drawings generally dated May 1970. An Initial
Evaluation Procedure (IEP) for the Main Hall structure was made available to us which was
completed by W R Andrews Ltd dated April 2012.
The information used for the analysis of the Southern Addition was a construction set of the
original structural drawings generally dated May 2007. We also used construction precast panel
shop drawings dated March 2008 and steel shop drawings dated May 2008. We used part of
the structural specification for the project. The construction set of architectural drawings used
for review were dated May 2007.
The information used for the analysis of the Civil Defence building was a construction set of
drawing prepared in November 1980 by the Nelson City Council City Engineers Department.
Also a construction set of the original structural drawings generally dated May 1970 for the
Northern Building of the Trafalgar Centre was utilised.
1.3
LIMITATIONS
Findings presented as a part of this project are for the sole use of Nelson City Council. The
findings are not intended for use by other parties, and may not contain sufficient information
for the purposes of other parties or other uses. Our professional services are performed using
a degree of care and skill normally exercised, under similar circumstances, by reputable
consultants practising in this field at this time. No other warranty, expressed or implied, is
made as to the professional advice presented in this report.
Conclusions relate to the structural performance of the building under earthquake loads. We
have not assessed the live load capacity of the floors, nor have we assessed the performance of
non-structural components or building contents under earthquake loads.
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1-2
2. BUILDING DESCRIPTIONS
2.1
NORTHERN BUILDING
The building plan is approximately 31 m long in the North-South direction and 22 m wide in
the East-West direction. The main access to this building is from the west side of the building,
which is used during events to provide the primary access to the Main Hall. Access to the
Northern Building is also possible through the north and east sides of the building, though
these are not public entrances. The Northern Building and Main Hall have been built integral to
each other which may cause adverse behaviour during a seismic event.
2.1. 1 Lat e ral Lo ad R es is t i ng S ys t e m
The Northern Building lateral load resisting system is complex in each direction. The northsouth direction is considered from hereon as the longitudinal direction of the building and eastwest direction is considered the transverse direction.
2.1. 1.1 Tr a ns ve rs e di re ct i o n
In the transverse direction, the lateral load system typically consists of walls constructed from
slender reinforced concrete frames with reinforced concrete blockwork infill. These walls act to
transfer the lateral loads from the roof down to the foundations through cantilever action. The
roof in the Northern Building exists at two separate heights (as can be seen in Figure 1-1) and
will be referred to from hereon as the lower roof and upper roof. The upper roof is braced in
the transverse direction by timber, lath and plaster walls. Seismic loads are transferred to these
walls through diaphragm action in the timber, lath and plaster ceiling. These walls in turn
transfer the forces to the concrete block walls below. Figure 1 – 3 below shows the lateral load
resisting system layout.
2.1. 1.2 L on git u di n al di r ect io n
In the longitudinal direction, the lateral load resisting system consists of walls constructed from
slender reinforced concrete frames with reinforced concrete blockwork infill. The lateral load
resisting system in the longitudinal direction acts in much the same way as in the transverse
direction. The main dissimilarity is the steel trusses that span longitudinally and act to support
the upper roof and lower roof. The steel trusses, along with the lower roof diaphragms, act to
restrain the reinforced concrete block walls when loaded out-of-plane. These elements allow
the out-of-plane forces to be transferred to in-plane concrete block walls. Figure 1 – 3 below
shows the lateral load resisting system layout.
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2-3
F igu r e 2 -1: P la n o f N or t h er n Bu i ld i ng s h ow i ng lat er al l oa d re s is t in g s y s t e ms
The connections in this building will be critical for lateral load resistance. Specifically the
connections from the timber roofing and steel trusses to the concrete bond beams atop the
concrete block walls. These connections determine the extent to which the concrete block walls
can be utilised in-plane and provide support to other walls loaded out-of-plane.
No seismic gap exists between the Northern Building and Main Hall and therefore the seismic
response of each building cannot be considered in isolation. There may be adverse interaction
between the buildings and increased damage during a seismic event.
2.1. 2 Gra v it y L oa d R es is t i ng S ys t e m
The roof structure of the Northern Building consists of timber purlins connected to the top
(upper roof) or bottom (lower roof) of the steel trusses. The steel trusses are supported at three
points by reinforced concrete bond beams that run along the tops of reinforced concrete block
walls. A slightly different system exists along the two outer longitudinal walls. The upper roof
above the east wall is supported for approximately half its length by a steel truss and for the
other half by a timber lath and plaster wall. The upper roof along the west wall is supported for
the entire length by a timber lath and plaster wall. The steel trusses are not needed in these
locations as support is provided by the concrete block walls and timber lath and plaster walls
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2-4
directly below. Figure 1 – 4 below shows the upper and lower roofs and a supporting timber
lath and plaster wall.
Upper roof
Lower roof
Upper roof
support by a lath
and plaster wall
framing onto a
concrete block
wall.
Figure 2-2: Gra vit y L oa d Res is t i n g S ys t e m ins id e t h e N o rt h e r n B u i ld in g
2.1. 3 F ou nd at i o ns
The foundations consist of a network of reinforced concrete foundation beams supported by
driven reinforced concrete piles. Figure 1 – 5 below shows a typical wall and the foundations
below.
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2-5
Reinforced concrete
bond beam
Reinforced concrete
block wall
Reinforced concrete
foundation beam
Driven reinforced
concrete pile
F igu r e 2 -3: T y pic al wa ll a nd f ou nd at i o n i n t he No rt he r n B u i ld in g
2.2
MAIN HALL
The building plan is approximately 49 m long in the North-South direction and 52 m wide in
the East-West direction. The building can be accessed by either the new addition (2008)
building to the south or the proposed new building in the north at ground floor level. Steel
stairs are located on the western and eastern sides as secondary entries to the first floor
galleries.
The relatively new southern addition structure was intended to be independent of the main hall,
but it was found upon inspection that the seismic gap has been neglected on one side of the
building. The existing structure to the north has been built integral to the main hall structure.
This may result in adverse interactions between the buildings during an earthquake.
2.2. 1 Lat e ral Lo ad R es is t i ng S ys t e m
The main hall lateral load resisting system is complex in each direction. The north-south
direction is considered from hereon as the longitudinal direction of the building and east-west
direction is considered the transverse direction.
2.2. 1.1 Tr a ns ve rs e di re ct i o n
In the transverse direction the lateral load system typically consists of reinforced concrete
frames on evenly spaced grids at approximately ten metre centres. The lateral load from the
roof is transferred to these frames through axial compression and tension in the glulam timber
arch beams. The western frames are connected directly to the arch beam through a corbel type
of support. The timber arch requires a reinforced concrete column to transfer the lateral load
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2-6
down to the frame through cantilever action. The reinforced concrete frames transfer the
lateral loads through frame action to the piled foundations.
On the northern end of the building the lateral load resisting system is different for the last half
bay/end wall. The lateral loads from the roof are transferred initially by axial compression and
tension in concrete beams. These concrete beams transfer the lateral loads to concrete frames
on each side of the building. These frames will be demolished as part of the proposed
development at the northern end of the building and the replacement structure to fit in with the
new development will be designed to the current loading standards.
2.2. 1.2 L on git u di n al di r ect io n
In the longitudinal direction the lateral load resisting system consists of reinforced concrete
blockwork on the western side of the building and a reinforced concrete frame on the eastern
side. The lateral load from the roof is transferred through two bays of cross braced steel rod
horizontal trusses in the roof to collector beams at the sides of the building as shown in Figure
2-4.
F igu r e 2 -4: P la n o f R o o f Br ac in g
On the western side of the building the load is transferred down to the gallery level through
cantilever action of reinforced concrete columns on the gridlines. The load is transferred to the
foundations through cantilever reinforced concrete blockwork walls. The central ten metre bay
is the only full height reinforced blockwork wall and therefore provides the main shear
connection for the remaining bays of the blockwalls resisting the lateral loads.
On the eastern side of the building the load is transferred down the foundations through a two
storey five bay reinforced concrete frame.
Review of the existing structural drawings reveals detailing that indicates capacity design
principles for reinforced concrete frames had not been implemented into the loadings and
material standards in 1970. Therefore for the purposes of this review we have adopted a
comparison to new building standard with elastic response (µ=1.0).
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2-7
2.2. 2 Gra v it y L oa d R es is t i ng S ys t e m
The building roof structure consists of steel purlins supported on glulam timber arch beams
that span unsupported across the building, Refer Figure 2-5. The arch beams are connected to
the main gallery structure by a single large steel pin connection at each end. The arch beam is
connected directly to the reinforced concrete gallery frame on the western side of the building
at level one. The arch beam is connected to a reinforced concrete column on the eastern side
of the building which transfers the load down to the reinforced concrete gallery frame.
Eastern
suppor
Western
suppor
F igu r e 2 -5: E l e vat io n o f Glu la m Ti m be r A rc h B ea m
East
West
F igu r e 2 -6: P la n o f Ga l le r y S eat in g
The gallery seating shown in Figure 2-6 consists of precast concrete units that span between the
reinforced concrete frames located on the gridlines. The reinforced concrete frames transfer
the loads from the roof and gallery to the foundations. A network of reinforced concrete
ground beams support the concrete flat slab at ground level.
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2-8
The main central area of the hall at ground level is a timber floor supported on a timber joist
and bearer system.
2.2. 3 F ou nd at i o ns
The concrete frames incorporate ground beams to span between bored reinforced concrete
piles. A pile plan indicates the design loads for the piles. The main area of timber floor is
supported on a regular grid of concrete piles on shallow concrete pad footings as shown in
Figure 2-7 below.
Timber
joists
Timber
bearer
Concrete
pile
Pad footing
not visiblle
F igu r e 2 -7: T i mb e r J o is t t o B ea r er t o Co nc r et e P il e S ys t e m
2.3
SOUTHERN ADDITION
The building plan is 70m long in the East-West direction and 19m wide in the North-South
direction. The building’s has an unusual shape resembling a segment of a circle as shown in
Figure 1-2. The building can be accessed either at the north through the main hall or through
doors on the west, south and east at ground floor level.
The building consists of four main levels including the roof, storage level (2nd floor), 1st floor
and ground floor level. The storage and 1st floor levels are split into eastern and western areas
with a large void between them.
We understand this building was originally designed to be independent of the main hall
structure incorporating a seismic gap into the intersection with the Main Hall. Upon inspection
no seismic gap can be seen between the Southern Addition and Main Hall on the west side of
the structure. Although a seismic gap is present on the east of the building, the buildings cannot
be considered seperate.
2.3. 1 Lat e ral Lo ad R es is t i ng S ys t e m
The Southern Addition building’s lateral load resisting system is similar in each direction. The
North-South direction is considered from hereon as the transverse direction of the building and
the East-West direction is considered the longitudinal direction.
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2.3. 1.1 Tr a ns ve rs e Di re ct i o n
In the transverse direction the lateral load resisting system consists of a mixed system of steel
rod braced frames and reinforced concrete shear walls.
The roof lateral load is distributed to the braced frames and concrete shear walls via a
horizontal steel rod braced truss at both the top and bottom chord truss levels. The storage
level lateral load appears to be distributed to the lateral load resisting elements via a non
specifically designed timber diaphragm. The 1st floor level lateral load is distributed to the
lateral load resisting elements via a reinforced concrete diaphragm in the topping slab.
There are two steel cross braced frames total, one braced frame located on either side of main
void space. These steel rod tension only braced frames are both supported by reinforced
concrete shear walls at 1st floor level. The reinforced concrete shear walls carry the lateral load
transferred from the braced frames down to the foundations.
There are two reinforced concrete shear walls total, one located just inside the extreme east and
west ends of the building. These reinforced concrete shear walls continue from the top chord
of the roof truss all the way to the foundation system. These reinforced concrete shear walls
carry the seismic load through cantilever action.
2.3. 1.2 L on git u di n al Di r ect io n
In the longitudinal direction the lateral load resisting system consists of steel rod tension only
cross braced frames.
The roof lateral load is distributed to the braced frames via a horizontal steel rod braced truss at
both the top and bottom chord truss levels. The storage level lateral load appears to be
distributed to the lateral load resisting elements via a non specifically designed timber
diaphragm. The 1st floor level lateral load is distributed to the lateral load resisting elements via
a reinforced concrete diaphragm in the topping slab.
There are two steel rod cross braced frames in total on the southern side of the building and
four steel rod cross braced frames in total around the perimeter of the northern side of the
building.
2.3. 2 Gra v it y L oa d R es is t i ng S ys t e m
The building roof structure consists of roof membrane system supported on timber plywood
sheathing. The plywood is supported by timber purlins which span between steel secondary
trusses. The secondary trusses span between a main truss and columns. The main truss is
supported by two reinforced concrete columns. A smaller triangle of roof has its timber purlins
supported by steel beams which are supported by the main truss and steel columns.
The storage level floor consists of timber joists supported by steel beams. These steel beams
are supported by steel columns.
The first floor level floor is a precast prestressed concrete rib and timber infill system with a
90mm cast insitu topping. These ribs are supported by precast concrete panels which transfer
the loads to ground level and the foundations.
The ground floor level is typically a 150mm slab on grade.
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2.3. 3 F ou nd at i o ns
The foundation consists of a network of ground beams supported on steel screw piles. The
pile plan in the structural drawings indicates the design loads for the piles.
2.4
CIVIL DEFENCE OFFICE
The Civil Defence Office building is approximately 12m long in the north-south direction and
4m wide in the east-west direction. The main entrance to the building is an exterior door on the
east of the building but access is also possible from the Victory room of the Northern Building.
The Civil Defence Office has been built integral to the Northern Building. The buildings share
the internal wall on the west of the Civil Defence Office. The concrete block wall on the north
side of the Civil Defence Office is attached via epoxied steel rods to the north wall of the
Northern Building. Figure 2-8 below shows a detailed plan of the Civil Defence Office; note
the internal walls have been removed.
Shared wall
North
Connected
concrete
block wall
F igu r e 2 -8: D et a il ed pl an o f t h e C i vi l De f e nc e O f f ic e
2.4. 1 Lat e ral Lo ad R es is t i ng S ys t e m
The lateral load resisting system consists of GIB plaster board on timber framing on the east,
south and west walls and of reinforced concrete block on the north wall.
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2.4. 1.1 Tr a ns ve rs e di re ct i o n
The transverse lateral load resisting system consists of timber framed GIB plaster board walls
on the southern end of the building and a reinforced concrete block wall on the northern end.
2.4. 1.2 L on git u di n al di r ect io n
The longitudinal load resisting system consists of timber framed GIB plaster board walls on
both the east and west of the building. The west wall is shared with the victory room of the
Northern Building.
2.4. 2 Gra v it y L oa d R es is t i ng S ys t e m
Gravity loads are transferred to the foundations through the walls of the structure. Purlins carry
the roof loads to the timber and concrete block walls which frame into the reinforced concrete
foundations below them.
2.4. 3 F ou nd at i o ns
The foundations of the Civil Defence Office are shallow reinforced concrete footings that run
along the east and west boundaries of the building. On the west side the building the existing
foundations under the east wall to the Northern Building are utilised.
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3. STATUTORY REQUIREMENTS
3.1 BUILDING ACT
When dealing with existing buildings there are a number of relevant sections of the Building
Act that need to be considered in relation to the building’s structure and strength.
Section 112 - Alterations to Existing Buildings
Section 112 of the Building Act requires that a building subject to an alteration
continue to comply with the relevant provisions of the Building Code to at least the
same extent as before the alteration.
Essentially this section means that the building may not be made any weaker than it
was, as a result of any alteration.
Section 122 – Meaning of Earthquake Prone Building
Section 122 of the Building Act 2004 deems a building to be earthquake prone if its
ultimate capacity (strength) would be exceeded in a “moderate earthquake” and it
would be likely to collapse causing injury or death, or damage to other property.
The Building Regulations (2005) define a moderate earthquake as one that would
generate loads 33% as strong as those used to design an equivalent new building.
Section 124 – Powers of Territorial Authorities
If a building is found to be earthquake prone, the territorial authority has the power
under section 124 of the Building Act to require strengthening work to be carried out,
or to close the building and prevent occupancy.
Section 131 – Earthquake Prone Building Policy
Section 131 of the Building Act requires all territorial authorities to adopt a specific
policy on dangerous, earthquake prone, and unsanitary buildings.
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3-1
NELSON CITY COUNCIL EARTHQUAKE PRONE POLICY
Ta bl e 3-1: P r i or it i es a n d Ti m e fr a m es – E a rt h qu ak e P ro n e Bu i ld i ng P o li cy
Priority
Type of Building
Example
Timeline for risk
reduction
1
Special post-disaster
functions
Hospital, civil
defence
15 years
2
Crowds or high
value contents
School, stadium
20 years
3
Heritage
classification A or B
Historically
significant buildings
25 years
4
Normal or low
hazard
Most buildings
30 years
The Northern Building, Main Hall and Southern Extension falls into the Priority 2 category
according to the Nelson City Council “Timeline for risk reduction” in Table 3-1 and therefore
has a 20 year timeframe for reducing the seismic risk of the building.
The Civil Defence Office falls into the Priority 1 category according to the Nelson City Council
“Timeline for risk reduction” in Table 3-1 and therefore has a 15 year timeframe for reducing
the seismic risk of the building.
3.2 BUILDING CODE
The Building Act requires all new building work to comply with the New Zealand Building
Code which outlines the performance standards required for new building work. The Ministry
of Business, Innovation and Employment also publishes Compliance Documents which may
be used to establish compliance with the Building Code.
3.3
SCOPE OF THIS REPORT
The evaluation is restricted to a detailed assessment of the lateral load resisting system and does
not consider the gravity load capacity of the floors or the performance of non-structural
components and contents.
Stages involved in completing this scope of work are:
1. Use of available existing documentation and qualitative site survey information to create a
structural model of the existing buildings. The structural models were created and the
equivalent static method used for the review.
2. Evaluate the seismic response of the buildings in terms of the requirements of NZS
1170.5:2004 [1] (loadings standard), and NZS3101: 2006 [2](Concrete Structures Standard),
and NZS3404:1997 [3] (Steel Structures Standard), and NZS3603:1993 (Timber Structures
Standard), and NZS 4230:2004 (Reinforced Concrete Masonry Structures).
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3. Determine the existing seismic performance of the structures as a percentage of the New
Building Standard requirements (%NBS).
Prepare a detailed seismic analysis report. This report summarises the technical aspects of the
assessment and any assumptions made. The report also includes modelling parameters, results
and discussion of any vulnerabilities identified.
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4. BUILDING EVALUATION
This section summarises the detailed engineering evaluation of the three buildings on the
Trafalgar Centre site.
4.1 NORTHERN BUILDING
4.1. 1 Bu i ld in g S eis m ic P a ra m et ers
The Northern Building was designed to predecessor standards of the current NZ Building
Code, likely comprising principally NZSS1900:1965. A comparison of the acceleration the
building is likely to have been designed for and the current design standards is shown in Figure
3-1 below.
Seismic loads are currently based on the requirements of NZS1170.5:2004. The base shear
coefficient is a function of building period, structure ductility and the site geology, including
proximity to known fault lines. The assumed seismic parameters for the Northern Building site
are as listed in Table 4-1.
Ta bl e 4-1: Se is m ic P ar a m et ers
Design Code :
NZS1170.5:2004
Soil Category :
C
R:
1.3
Z:
0.27
Sp :
0.925
D:
>100km
Figure 4-1 is a plot of the design response spectra for the Trafalgar Centre at varying levels of
current loadings standard NZS1170 (%NBS). The vertical dashed lines on the figure indicate
the fundamental period of the building in each of the principal axes. These lines can be used to
determine the minimum design base shear coefficient in each direction of loading. The base
shear coefficient / acceleration is expressed as a percentage of gravity.
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0.7
NZS 1170 Spectra, Z=0.27, Soil Class C,
Ductility = 1.25, Sp = 0.925
NZS S1990 (1965), Zone A, Public, K=1,
Converted to ULS by factor = 1.3
Building Fundamental Period
0.6
Acceleration (g)
0.5
33% NZS1170 (NBS)
0.4
67% NZS1170 (NBS)
0.3
0.2
0.1
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Period (Seconds)
F igu r e 4 -1: Bu il di ng A c ce le rat i on Sp ect ra C o m par is o n N Z S S 19 00 v er s u s N Z S
1170
4.1. 1.1 I mp o rt a nc e L e ve ls
The Northern Building is not capable of containing more than 300 people in a single space,
unlike the Main Hall and Southern Addition. However, as it provides the main egress route to
the Main Hall it must be considered an Importance Level 3 building in accordance with
AS/NZS1170.
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Table 4-2 shown below shows the levels of risk posed to new buildings depending on the
proposed uses.
Ta bl e 4-2: I m p ort an c e L ev e ls
Importance
Risk of
Exceedance in
50 Year Design
Life
Risk Factor
Level
Earthquake
Annual
Exceedance
Probability
Comment
Examples
1
1/100
40%
0.5
Structures
representing a low
degree of hazard to
life and property.
Small structures,
farm buildings,
fences, masts, walls
1/500
10%
1.0
“Normal”
structures and
structures not in
other importance
levels.
Hotels, offices,
apartments
1/1000
5%
1.3
Structures that may
contain people in
crowds or contents
of high value to the
community.
Schools, emergency
medical and other
emergency facilities
but not essential
post-disaster
healthcare facilities.
1/2500
2%
1.8
Structures with
special postdisaster functions.
Designated civilian
emergency facilities,
medical emergency
facilities with post
disaster functions.
( IL1 )
2
( IL2 )
3
( IL3 )
4
( IL4 )
4.1. 1.2 St r u ct u r e D u ct i lit y
Ductility is a measure of a building or its individual components ability to undergo sustainable
inelastic displacements whilst maintaining sufficient residual strength to carry load. The term
“inelastic” refers to actions beyond the base yield strength of the building or component being
considered. The more ductile a building, the more energy it is able to dissipate. Since ductility
inherently requires building structural components to be stressed beyond yield, there will be
some permanent damage associated with this form of energy dissipation.
By considering available building ductility, the magnitude of the seismic forces for which the
building is being assessed is able to be reduced to capture the effect of the energy dissipation.
Structural ductility is highly dependent on the type of building and the individual member
detailing. Highly ductile concrete members, for example, need to be well confined with closely
spaced reinforcing ties in order to maintain the residual strength as these hinge or become
damaged.
Member detailing for ductility is a relatively modern concept. As such many older structures
such as the Northern Building of the Trafalgar Centre have little to no inherent ductility and are
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therefore considered elastic or nominally ductile and will not be expected to perform as well
under higher levels of load.
For the purpose of this assessment a structural ductility factor of 1.25 (nominally ductile) for
the building as a whole (limited by the response of the reinforced concrete block walls) has
been adopted. However, a ductility factor of 2.0 (limited ductility) has been used when
considering the response of the lath and plaster walls in the upper roof of the structure.
4.1. 2 Mat e ri al P r op e rt i es
Material properties were calculated from a largely complete set of original drawings and the ‘era’
material strengths of New Zealand Society for Earthquake Engineering ‘Assessment and
Improvement of the Structural Performance of Buildings in Earthquakes’
The concrete compressive strength was not specified on the drawings but was assumed
to be 20 MPa, as was typical for construction in the early 1970s. For analysis purposes a
strength of 30 MPa (1.5 times original compressive strength) was assumed for
calculations. This is based on testing of existing structures concrete which shows an
average 150% increase in the concrete strength over time.
The concrete compressive strength of the concrete blockwork was not specified but was
assumed to have a 28 day strength of 17 MPa as was typical for construction in the early
1970s. For analysis purposes a strength of 25 MPa (1.5 times original compressive
strength) was assumed for calculations. This is based on testing of existing structures
concrete which shows an average 150% increase in the concrete strength over time.
For concrete reinforcing steel and tie rod steel bars probable yield strength of 300 MPa
was used in the calculations. It is assumed that the longitudinal reinforcing steel used
was deformed bar which was common for the ‘era’ the building was built in.
4.1. 3 M od e ll in g A s s u mpt i ons
The seismic evaluation utilises a lumped mass model based on the equivalent static method set
out in NZS1170.5:2004 to simulate the effects of horizontal earthquake forces. This model is
developed using information from the original structural drawings of the building and
properties of materials used at the time of construction.
For the seismic load case, the amplitude of the acceleration is dictated by the fundamental
period of vibration of the building along its two principal axes. Tall flexible structures have
long periods, whilst short stiff buildings have short periods. As can be seen in Figure 4-2
below, the longer the period of a structure is, the lower the corresponding horizontal
acceleration. For a building period of 0.4 seconds the corresponding horizontal ground
acceleration is 0.67g, compared to a building period of 2.0 seconds where the horizontal
acceleration is approximately 0.17g.
The building’s fundamental period in both the transverse and longitudinal directions was
estimated to be less than 0.4 seconds. This assumption is based on the response of the
structure being essentially elastic and it being a low height structure.
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0.80
NZS1170 Spectra, Z=0.27, Soil Class C,
Ductility = 1.25, Sp = 0.925
0.70
NZSS1900 (1965), Zone A, Public, K = 1,
Converted to ULS by factor = 1.3
0.60
Acceleration (g)
Building Fundamental Periods
0.50
0.40
0.30
0.20
0.10
0.00
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Period (Seconds)
F igu r e 4 -2: Bu il di ng A c ce le rat i on Sp ect ra C o m par is o n N Z S S 19 00 v er s u s N Z S
1170
It is clear from the Figure 4-2 above, that at the building’s fundamental period (T<0.4 seconds),
the building’s acceleration is much more now than when the building was originally designed.
When distributing lateral load between the lateral load resisting systems, a tributary areas
method was typically used due to the flexibility of the roof structure.
Vertical distribution of the lateral loads applied to the structure is based on the equivalent static
base shear distribution recommended in NZS 1170.
4.1. 4 E s t i mat i on of B u i ldi n g S t re n gt h
As part of this review, hand calculations were used to calculate actions in the load resisting
elements.
4.1. 4.1 Tr a ns ve rs e Di re ct i o n A n al ys i s
In the transverse direction, timber lath and plaster walls transfer load from the upper roof
down to the lower roof. These walls were considered to have a ductility of 2.0 in accordance
with ASCE 41. The capacity of these walls to resist the lateral loads is 100% NBS. These walls
also act to brace the longitudinal trusses out-of-plane for which they have sufficient capacity.
Figure 4-3 shows an example of a steel truss and a transverse lath and plaster wall.
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The lower roof frames onto reinforced concrete bond beams which are supported by
reinforced concrete block walls. The reinforced concrete block walls transfer the load to the
foundation through cantilever action. When the building is loaded in the transverse direction
the critical wall acting in this direction has the capacity equivalent to 100% NBS.
The critical longitudinal wall loaded out-of-plane, when considering all restraints, was found to
have capacity equivalent to approximately 60% NBS.
Upper roof
Longitudinal
Steel roof truss
Transverse
Lath and plaster wall bracing
the upper roof in the
transverse direction
F igu r e 4 -3: I nt er na l v ie w of t he r o o f s u pp o rt s y s t e m i n N o rt he r n B u i ld i ng .
4.1. 4.2 L on git u di n al Di r ect io n A na l ys is
The lateral load resisting system in the longitudinal direction is similar to that of the transverse
direction. The main difference being the steel roof trusses which span longitudinally.
The transverse concrete block walls have very little capacity to resist loading out-of-plane
(longitudinally). Additionally, the small reinforced concrete bond beams atop these walls have
little capacity to restrain them and redistribute loads. Therefore, the steel trusses and roof
diaphragms provide the strongest load path to the in-plane walls. The critical weakness in this
load path comes from the connections of the steel trusses and the lower roof diaphragms into
the bond beams above the concrete block walls. The less stiff roof diaphragm will provide the
greatest restraint to the concrete block walls out-of-plane and thus determines the out-of-plane
capacity. The capacity of the longitudinal lateral load resisting system to redistribute the out-ofplane forces from the concrete block walls is approximately 30% NBS. This capacity is
determined from considering the worst case out-of-plane load redistribution shown in Figure
4-4 below.
The connections of the steel trusses to the concrete bond beams will likely fail from the steel
connector bolt breaking out of the concrete. This is a brittle failure and if the steel trusses
undergo large longitudinal displacements, these may lose their seating causing a collapse of the
roof structure. This is a critical structural weakness with a capacity of less than 15% NBS.
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The critical longitudinal in-plane wall (shown in Figure 3-4 below has a capacity equivalent to
100% NBS.
The steel roof trusses when subjected to seismic and gravity loads have capacity equivalent to
100% NBS.
F igu r e 4 -4: L oa d pat h t o t h e c rit ic al i n- pla n e l o ng it u d in al wa ll .
The absence of a seismic gap could lead to adverse interactions between the North building and
Main Hall. Such interactions could be pounding between the structures or torsion effects. In
new building design, different buildings are separated with a physical gap called a “seismic gap”.
This gap ensures the buildings are unable to interact or collide with each other during a seismic
event causing damage or, in the worst case, premature failure.
The key structural weakness for the longitudinal lateral resisting system are the diaphragm and
truss connections. If the lateral loads are able to reach the in-plane concrete block walls the
system has the capacity to resist these loads.
The failure of the reinforced concrete block walls out-of-plane may cause the steel trusses to
lose the seating support leading to a collapse of the structure.
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The concrete block walls along the northern and western edges of the building have
unreinforced masonry veneers with unknown anchorage. Out-of-plane loading of these walls
could result in out-of-plane failure of the veneer walls presenting a danger to people in the
immediate vicinity. Figure 4-5 below shows a detail of these walls.
West wall
North wall
Unreinforced
blockwork
veneer
F igu r e 4 -5: D et a ils o f t h e c o nc ret e b lo ck wa lls wit h u n r ei n f or ce d v en e ers .
The steel truss connections and the behaviour of the concrete block walls out-of-plane are
considered to comprise brittle collapse mechanisms. This means that after the capacities are
reached theses cannot carry any more significant load. Ductile systems typically can carry 1.51.8 times the ultimate capacity and therefore have more resilience.
4.1. 5 F ou nd at i o n A na l ys is
The driven reinforced concrete piles supporting the reinforced concrete foundation beams have
capacity in bearing equivalent to 100% NBS. This was determined from analysing the critically
loaded foundations.
4.1. 6 Ge ot ec h nic al Co ns id e r at i o ns
The draft geotechnical report for the proposed Northern Extension building has identified
potential liquefaction and lateral spreading issues for the site. Lateral spreading and liquefaction
can cause the building’s foundations to differentially displace which can potentially result in
significant damage to and/or collapse of the building.
The draft geotechnical report has identified the “trigger” event for the liquefaction to be
approximately a 1 in 200 year event.
4.1. 7 Su m m ar y
The Northern Building is consider to have an Ultimate Limit State strength of less than 15%
NBS for an Importance Level 3 building.
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The Building Act also defines a building as earthquake prone if it will have its ultimate capacity
exceeded in a moderate earthquake (defined as a level of earthquake one-third that required for
an equivalent new building); and would be likely to collapse. The Northern Building is
considered to be an earthquake prone building.
4.1. 8 St r e ngt h e ni ng R equ ir e d
Strengthening to get above 67%NBS Importance level 3:
•
Improvement of the steel truss to reinforced concrete bond beam connections through
providing additional anchorage with greater concrete cover. This could be achieved
through installing steel plates that connect the trusses to additional epoxied bolts.
These bolts would have adequate embedment into the concrete bond beam to utilise
the reinforcing cage and therefore provide a stronger connection.
•
Improvement of the lower roof diaphragm to reinforced concrete bond beam
connections through providing additional anchorage. This could be achieved through
using bolted steel plates to connect the timber roof and concrete bond beam.
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4.2
MAIN HALL
4.2. 1 Bu i ld in g S eis m ic P a ra m et ers
The Main Hall was designed to predecessor standards of the current NZ Building Code, likely
comprising principally NZSS1900:1965. A comparison of the acceleration the building is likely
to have been designed for is shown in Figure 3-6 below.
Seismic loads are currently based on the requirements of NZS1170.5:2004. The base shear
coefficient is a function of building period, structure ductility and the site geology, including
proximity to known fault lines. The assumed seismic parameters for the Trafalgar Centre
building site are as listed in Table 4-3.
Ta bl e 4-3: Se is m ic P ar a m et ers
Design Code :
NZS1170.5:2004
Soil Category :
C
R:
1.3
Z:
0.27
Sp :
1.0
D:
>100km
Figure 4-6 is a plot of the design response spectra for the Trafalgar Centre at varying levels of
current loadings standard NZS1170 (%NBS). The vertical dashed lines on the figure indicate
the fundamental period of the building in each of the principal axes. These lines can be used to
determine the minimum design base shear coefficient in each direction of loading. The base
shear coefficient / acceleration is expressed as a percentage of gravity.
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0.90
NZS1170 Spectra, Z=0.27, Soil Class C,
Ductility = 1, Sp = 1
NZSS1900 (1965), Zone A, Public, K = 1,
Converted to ULS by factor = 1.3
Building Fundamental Periods
0.80
Acceleration (g)
0.70
0.60
33% NZS1170 (NBS)
0.50
67% NZS1170 (NBS)
0.40
0.30
0.20
0.10
0.00
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Period (Seconds)
F igu r e 4 -6: Bu il di ng A c ce le rat i on Sp ect ra C o m par is o n N Z S S 19 00 v er s u s N Z S
1170
4.2. 1.1 I mp o rt a nc e L e ve ls
The Main Hall is capable of containing more than 300 people in a single space, and is therefore
considered an Importance Level 3 building in accordance with AS/NZS1170. This
corresponds to the current and continued planned use for the building as an events centre
building.
Table 4-4 shown below shows the levels of risk posed to new buildings depending on their
proposed use.
Table 4-4: Importance Levels
Importance
Risk of
Exceedance in
50 Year Design
Life
Risk Factor
Level
Earthquake
Annual
Exceedance
Probability
1
1/100
40%
0.5
( IL1 )
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Comment
Examples
Structures
representing a low
degree of hazard to
life and property.
Small structures,
farm buildings,
fences, masts, walls
4-11
2
1/500
10%
1.0
“Normal”
structures and
structures not in
other importance
levels.
Hotels, offices,
apartments
1/1000
5%
1.3
Structures that may
contain people in
crowds or contents
of high value to the
community.
Schools, emergency
medical and other
emergency facilities
but not essential
post-disaster
healthcare facilities.
1/2500
2%
1.8
Structures with
special postdisaster functions.
Designated civilian
emergency facilities,
medical emergency
facilities with post
disaster functions.
( IL2 )
3
( IL3 )
4
( IL4 )
4.2. 1.2 St r u ct u r e D u ct i lit y
Ductility is a measure of a building or its individual components ability to undergo sustainable
inelastic displacements whilst maintaining sufficient residual strength to carry load. The term
“inelastic” refers to actions beyond the base yield strength of the building or component being
considered. The more ductile a building, the more energy it is able to dissipate. Since ductility
inherently requires building structural components to be stressed beyond yield, there will be
some permanent damage associated with this form of energy dissipation.
By considering available building ductility, the magnitude of the seismic forces for which the
building is being assessed are able to be reduced to capture the effect of the energy dissipation.
Structural ductility is highly dependent on the type of building and the individual member
detailing. Highly ductile concrete members, for example, need to be well confined with closely
spaced reinforcing ties in order to maintain their residual strength as they hinge or become
damaged.
Member detailing for ductility is a relatively modern concept. As such many older structures
such as the Main Hall of the Trafalgar Centre have little to no inherent ductility and are
therefore considered elastic or nominally ductile and will not be expected to perform as well
under higher levels of load.
For the purpose of this assessment a structure ductility factor of 1.0 (elastic) has been adopted.
This assumption is based on the general non-ductile detailing observed in the structural
drawings.
4.2. 2 Mat e ri al P r op e rt i es
Material properties were calculated from a largely complete set of original drawings and the ‘era’
material strengths of New Zealand Society for Earthquake Engineering ‘Assessment and
Improvement of the Structural Performance of Buildings in Earthquakes’
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Concrete compressive strength was typically specified on the drawings as 27.5 MPa. For
analysis purposes a strength of 41 MPa (1.5 times original compressive strength) was
assumed for calculations. This is based on testing of existing structures concrete which
shows an average 150% increase in the concrete strength over time.
For concrete reinforcing steel and tie rod steel bars probable yield strength of 300 MPa
was used in the calculations. It is assumed that the longitudinal reinforcing steel used
was deformed bar which was common for the ‘era’ the building was built in.
4.2. 3 M od e ll in g A s s u mpt i ons
The seismic evaluation utilises a lumped mass model based on the equivalent static method set
out in NZS1170.5:2004 to simulate the effects of horizontal earthquake forces. This model is
developed using information from the original structural drawings of the building and
properties of materials used at the time of construction.
For the seismic load case the amplitude of the acceleration is dictated by the fundamental
period of vibration of the building along its two principal axes. Tall flexible structures have
long periods, whilst short stiff buildings have short periods. As can be seen in Figure 4-7
below, the longer the period of a structure is, the lower the corresponding horizontal
acceleration. For a building period of 0.4 seconds the corresponding horizontal ground
acceleration is 0.83g, compared to a building period of 2.0 seconds where the horizontal
acceleration is approximately 0.23g.
0.90
0.80
NZS1170 Spectra, Z=0.27, Soil Class C,
Ductility = 1, Sp = 1
0.70
NZSS1900 (1965), Zone A, Public, K = 1,
Converted to ULS by factor = 1.3
Building Fundamental Periods
Acceleration (g)
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.00
0.50
1.00
1.50
2.00
2.50
Period (Seconds)
3.00
3.50
4.00
F igu r e 4 -7: E x a m pl e S e is mi c R es po ns e S pe ct r a
The building’s fundamental period in both the transverse and longitudinal directions was
estimated to be less than 0.4 seconds. This assumption is based on the response of the
structure being essentially elastic and it being a low height structure.
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It is clear from the graph above, that at the building’s fundamental period (T<0.4 seconds), the
building’s acceleration is much more now than when the building was originally designed.
When distributing lateral load between the lateral load resisting systems, a tributary areas
method was typically used due to the flexibility of the structures diaphragms.
Vertical distribution of the lateral loads applied to the structure is based on the equivalent static
base shear distribution recommended in NZS 1170.
4.2. 4 E s t i mat i on of B u i ldi n g S t re n gt h
As part of this review, hand calculations and two dimensional elastic computer models were
used to calculate actions in the load resisting elements. Hand calculations were used to
calculate the capacity of the frames and concrete walls along the west and east elevations.
4.2. 4.1 Tr a ns ve rs e Di re ct i o n A n al ys i s
In the transverse direction, the arch beam distributes the lateral load from the roof into a
cantilever column on the eastern elevation and directly into the gallery frame on the western
elevation. Both frames were modelled in Microstran, a two dimensional linear analysis
program. The timber arch beam directly connects to the western moment resisting frame. The
two dimensional model representation is shown below in Figure 4-8.
F igu r e 4 -8: T wo D i me n s i on al C o m pu t er M od el o f t h e T ra ns v ers e W e s t er n
C o ncr et e M o m e nt R es is t in g F ra m e
The eastern transverse moment resisting frame is similar in representation. These frames have
an approximate capacity of 50% NBS.
The northern wall has a different lateral load resisting system consisting of a reinforced
concrete frame. This frame has an approximate lateral load capacity of 20%NBS. It is
proposed that this end wall will be demolished as part of the new development and in its place
a system which can carry 100% NBS installed.
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4.2. 4.2 L on git u di n al Di r ect io n A na l ys is
The lateral load from the roof and end walls needs to be transferred to the eastern and western
walls via two bays of steel rod roof bracing. The loads in these steel rod roof bracing were
calculated using simple hand calculations based on their geometry. The capacity of the steel rod
roof braces and their associated connections into the side frames was estimated to be
approximately 40%NBS.
The longitudinal lateral load from the steel rod braces is transferred into the systems on the
west and east sides of the building.
On the eastern side of the building the lateral load is resisted by a reinforced concrete moment
resisting frame. A two dimensional model representation was developed in the Microstran
software to estimate the distribution of the actions in the members. An elevation of the eastern
frame is shown below in Figure 4-9.
F igu r e 4 -9: T wo D i me n s i on al C o m pu t er M od el o f t h e E as t er n C o nc r et e M o m e nt
Res is t i n g F ra m e
The capacity of this moment resisting frame was estimated to be approximately 50%NBS.
On the western side of the building the lateral loads from the building are resisted by reinforced
concrete cantilever columns. The capacity of these columns in flexure was estimated to be
approximately 25% NBS.
These reinforced concrete cantilever columns transfer the lateral load down to the gallery level,
where both the roof lateral load and that from the gallery is transferred to reinforced blockwalls
along the western elevation. As shown in Figure 4-10 only one bay of blockwall continues
from ground level up to the underside of the gallery level. The capacity of this system was
estimated to be approximately 25%NBS limited by the connection of the blockwall to the level
1 gallery.
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F igu r e 4 -10: W es t er n E l e vat io n S h ow i ng F u l l H ei ght B l ock wa ll i n M idd le Ba y
Some of these elements are considered to comprise brittle collapse mechanisms. This means
that after their capacity is reached they cannot carry any more significant load. Ductile systems
typically can carry 1.5-1.8 times their ultimate capacity and therefore have more resilience.
4.2. 5 Ge ot ec h nic al Co ns id e r at i o ns
The draft geotechnical report for the proposed Northern Extension building has identified
potential liquefaction and lateral spreading issues for the site. As the Main Hall building
occupies the same site it is likely to be subject to that same issue. Lateral spreading and
liquefaction can cause the building’s foundations to differentially displace which can potentially
result in significant damage to and/or collapse of the building.
This lateral spreading issue is a particular concern for the building in the transverse direction
because the timber arch beams supporting the roof require consistent lateral restraint to
support the roof.
The draft geotechnical report has identified the “trigger” event for the liquefaction to be
approximately a 1 in 200 year event.
4.2. 6 Su m m ar y
The Main Hall is considered to have an Ultimate Limit State strength of approximately 20-25%
NBS for an Importance Level 3 building.
The Building Act also defines a building as earthquake prone if it will have its ultimate capacity
exceeded in a moderate earthquake (defined as a level of earthquake one-third that required for
an equivalent new building); and would be likely to collapse. The Main Hall is considered to be
an earthquake prone building.
4.2. 7 St r e ngt h e ni ng R equ ir e d
Strengthening to get above 33%NBS
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•
•
•
Removal of the Northern wall as part of the proposed new development. A new
portal frame structure installed in its place.
Wrapping reinforced concrete columns along the western elevation with Fibre
Reinforced Polymers (FRP’s) to increase their ductility and capacity to resist seismic
loads.
Improvement of the connection from the Level 1 gallery floor to the blockwall in the
central bay of the western elevation by steel plates or similar.
Strengthening to get above 67% NBS Importance Level 3 (As for 33% NBS above plus the
following)
•
•
•
•
•
•
Providing a secondary steel braced frame at the gridlines (~10 metre centres) plus
associated foundations if the existing foundations cannot carry loads.
Replace or provide additional bay(s) of the steel rod bracing in the roof.
Provide new connections for the existing steel rod braces in the roof where they meet
the perimeter frames.
Provide a new steel braced frame/reinforced concrete wall along the eastern side of the
building plus associated piled foundations if the existing piles cannot provide the load
capacity required.
Provide a new steel braced frame/reinforced concrete wall along the front of the
western gallery plus any associated foundations if the existing piles cannot provide the
load capacity required.
The liquefaction and lateral spreading issues as identified in the geotechnical report will
need to be mitigated such that the foundations to the building can cope with the
expected displacements.
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4.3 SOUTHERN ADDITION
4.3. 1 Bu i ld in g S eis m ic P a ra m et ers
The Southern Addition was designed to predecessor standards of the current NZ Building
Code, likely comprising principally NZS4203:1992. A comparison of the acceleration the
building is likely to have been designed to and New Building Standard (NZS1170.5:2004) is
shown in Figure 4-11below.
Seismic loads are currently based on the requirements of NZS1170.5:2004. The base shear
coefficient is a function of building period, structure ductility and the site geology, including
proximity to known fault lines. The assumed seismic parameters for the Trafalgar Centre
building site are as listed in Table 4-5.
Ta bl e 4-5: Se is m ic P ar a m et ers
Design Code :
NZS1170.5:2004
Soil Category :
C
R:
1.3
Z:
0.27
Sp :
1.0
D:
>100km
Figure 4-11 is a plot of the design response spectra for the Trafalgar Centre at varying levels of
current loadings standard NZS1170 (%NBS). The vertical dashed line on the figure indicates
the fundamental period of the building in each of the principal axes. These lines can be used to
determine the minimum design base shear coefficient in each direction of loading. The base
shear coefficient / acceleration is expressed as a percentage of gravity.
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0.90
NZS1170 Spectra, Z=0.27, Soil Class C,
Ductility = 1, Sp = 1
NZS4203 Spectra, Z=1.2, Soil Category b,
Ductility = 1, Sp = 0.67
Building Fundamental Periods
0.80
Acceleration (g)
0.70
0.60
67% NBS
0.50
33% NBS
0.40
0.30
0.20
0.10
0.00
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Period (Seconds)
F igu r e 4 -11: Bu il di n g A cce l er at i o n Sp e ct r a C o m pa ris o n N ZS S 19 00 v ers u s N Z S
1170
4.3. 1.1 I mp o rt a nc e L e ve ls
The Southern Addition is capable of containing more than 300 people in a single space, and is
therefore considered an Importance Level 3 building in accordance with AS/NZS1170. This
corresponds to the current and continued planned use for the building as an events centre
building.
Table 4-6 shown below shows the levels of risk posed to new buildings depending on their
proposed use.
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Ta bl e 4-6: I m p ort an c e L ev e ls
Importance
Risk of
Exceedance in
50 Year Design
Life
Risk Factor
Level
Earthquake
Annual
Exceedance
Probability
Comment
Examples
1
1/100
40%
0.5
Structures
representing a low
degree of hazard to
life and property.
Small structures,
farm buildings,
fences, masts, walls
1/500
10%
1.0
“Normal”
structures and
structures not in
other importance
levels.
Hotels, offices,
apartments
1/1000
5%
1.3
Structures that may
contain people in
crowds or contents
of high value to the
community.
Schools, emergency
medical and other
emergency facilities
but not essential
post-disaster
healthcare facilities.
1/2500
2%
1.8
Structures with
special postdisaster functions.
Designated civilian
emergency facilities,
medical emergency
facilities with post
disaster functions.
( IL1 )
2
( IL2 )
3
( IL3 )
4
( IL4 )
4.3. 1.2 St r u ct u r e D u ct i lit y
Ductility is a measure of a building or its individual components ability to undergo sustainable
inelastic displacements whilst maintaining sufficient residual strength to carry load. The term
inelastic refers to actions beyond the base yield strength of the building or component being
considered. The more ductile a building, the more energy it is able to dissipate. Since ductility
inherently requires building structural components to be stressed beyond yield there will be
some permanent damage associated with this form of energy dissipation.
By considering available building ductility, the magnitude of the seismic forces for which the
building is being assessed are able to be reduced to capture the effect of the energy dissipation.
Structural ductility is highly dependent on the type of building and the individual member
detailing. Highly ductile concrete members for example need to be well confined with closely
spaced reinforcing ties in order to maintain their residual strength as they hinge or become
damaged.
Member detailing for ductility is a relatively modern concept. As such many older structures
have little to no inherent ductility and are therefore considered elastic or nominally ductile and
will not be expected to perform as well under higher levels of load.
For the purpose of this assessment a structure ductility factor of 1.0 (elastic) has been adopted.
This assumption is based on the primary lateral load carrying elements being Reidbrace “RB”
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steel rod cross brace frames. Reidbrace does not comply with the Material Requirements for a
ductile system (Category 1, 2 and 3) as outlined in Table 12.4 in the NZ Steel Structures
Standard NZS3404:1997 [3].
The Reidbrace steel rod bracing system is not considered a ductile system and needs to be
designed for elastic forces or be protected from elastic forces using capacity design principles in
another ductile system. We were not able to identify a ductile system in the building that is
capable of protecting these braces from elastic forces through capacity design.
4.3. 2 Mat e ri al St r en gt hs
Properties were calculated from a largely complete set of original drawings.
Concrete compressive strength was typically specified on the drawings as 30 MPa . For
analysis purposes a strength of 45 MPa (1.5 times original compressive strength) was
assumed for calculations for general concrete strength increase over time.
For concrete reinforcing steel, there were two different types specified in the original
structural drawings. Regular deformed bars (denoted “D”) have a probable yield strength
of 325 MPa. High strength deformed bars (denoted “HD”) have a probable yield
strength of 550 MPa.
For the purposes of evaluating existing buildings the probable yield strengths are allowed to be
used instead of the normally used lower characteristic value. This equates to approximately an
8% increase in the yield strength from their lower characteristic yield strength.
4.3. 3 M od e ll in g A s s u mpt i ons
The seismic evaluation utilises a lumped mass model based on the equivalent static method set
out in NZS1170.5:2004 to simulate the effects of horizontal earthquake forces. This model is
developed using information from the original structural drawings of the building and
properties of materials used at the time of construction.
For the seismic load case the amplitude of the acceleration is dictated by the fundamental
period of vibration of the building along its two principal axes. Tall flexible structures have
long periods, whilst short stiff buildings have short periods. As can be seen in Figure 4-12
below, the longer the period of a structure is, the lower the corresponding horizontal
acceleration. For a building period of 0.4 seconds the corresponding horizontal ground
acceleration is 0.83g, compared to a building period of 2.0 seconds where the horizontal
acceleration is approximately 0.23g.
The building’s fundamental period was estimated using the Raleigh Method which is an
acceptable method proposed by NZS1170.5:2004. The period in both directions was calculated
to be less than 0.4 seconds.
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0.90
0.80
NZS1170 Spectra, Z=0.27, Soil Class C,
Ductility = 1, Sp = 1
0.70
NZS4203 Spectra, Z=1.2, Soil Category b,
Ductility = 1, Sp = 0.67
Building Fundamental Periods
Acceleration (g)
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.00
0.50
1.00
1.50
2.00
2.50
Period (Seconds)
3.00
3.50
4.00
F igu r e 4 -12: E x a m pl e S eis m ic R es p o ns e Sp e ct r a
4.3. 4 E s t i mat i on of B u i ldi n g S t re n gt h
As part of this review, hand calculations and 2D elastic computer models were used to calculate
the actions in the load resisting elements. Hand calculations were used to estimate the capacity
of the steel cross braced frames and concrete walls. The loads from the equivalent static model
were followed through the structure with calculations of the % NBS strength estimated for a
number of the load resisting elements. The philosophy of the design was based on elastic loads
unless a system capable of reliable ductility allowed a reduction in the loading. We were not
able to show that any ductile mechanism in the building is activated in advance of braced
frames reaching their capacities.
4.3. 4.1 Tr a ns ve rs e Di re ct i o n an al ys i s
In the transverse direction steel rod (Reidbrace, “RB”) bracing in the top and bottom chord
truss levels distributes the lateral loads to the vertical lateral load resisting systems. The capacity
of the steel rod bracing in the roof was estimated to be approximately 100% NBS.
The lateral load from the roof is transferred into the vertical lateral load resisting systems. The
vertical lateral load resisting systems in the transverse direction consist of 4 vertical elements.
These consist of two steel rod braced frames either side of the void and two reinforced
concrete shear walls one at each extreme end of the building as shown in Figure 4-13 The roof
diaphragm is considered to be a ‘flexible’ because it has large deflections relative to the load
resisting system. It is appropriate for flexible diaphragms to distribute the seismic weights of
the roof/floor according to tributary areas. This means that the steel rod braced frames carry a
large portion of the roof seismic weight relative to the reinforced concrete shear walls.
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F igu r e 4 -13: P l an vi ew o f S ou t h er n ex t e ns i o n s h owi n g t ra ns v ers e lat e r al l oad
res is t in g el e m e nt s
The steel braced frames (RB25) capacity were calculated by simple hand calculations to be
approximately 30% NBS. We note that the Reidbrace rods used in these frames have not
performed well in the Canterbury earthquakes. The Reidbrace system was found to have failed
at the connector in some instances rather than through tension yielding of the rod. This is a
brittle failure mechanism and hence why ductility cannot be considered for this element. Refer
to Figure 4-14 for a photo of the brittle failure of the connector.
F igu r e 4 -14: P hot os s h ow in g Re id br ac e p r op ri et a r y c o n ne ct ors w hi ch fa il e d i n
t h e Ca nt er bu ry ea rt h qu ak es
The reinforced concrete shearwalls, supporting the lateral loads at each end of the building,
have between 80%-100% NBS capacity. As these elements are located in the extreme ends of
the building it is difficult to improve the building’s performance by trying to drag more lateral
load out to these walls.
The piles that support the transverse lateral load resisting systems have been found to have a
capacity of approximately 30%NBS. This capacity is based on the design loads provided on the
structural drawings.
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4.3. 4.2 L on git u di n al Di r ect io n A na l ys is
In the longitudinal direction steel rod (Reidbrace) bracing in the top and bottom chord truss
levels distributes the lateral loads to the vertical lateral load resisting systems. The capacity of
the steel rod bracing in the roof was estimated to be approximately 100% NBS.
The lateral load from the roof is transferred into the vertical lateral load resisting systems. The
vertical lateral load resisting systems in the longitudinal direction consist of 6 vertical elements.
These consist of two steel rod braced frames at the northern end and four steel rod braced
frames around the southern perimeter as shown in Figure
4-15
F igu r e 4 -15: P l an vi ew o f S ou t h er n ex t e ns i o n s h owi n g l o ng it u di na l lat er al l oa d
res is t in g el e m e nt s
The roof diaphragm is considered to be a ‘flexible’ because it has large deflections relative to
the load resisting system. It is appropriate for flexible diaphragms to distribute the seismic
weights of the roof/floor according to tributary areas.
Hand calculations indicated that the braced frames forming the primary lateral load resisting
system are likely to have an Ultimate Limit State capacity in the order of 25% NBS for an
Importance Level 3 building. This is limited by the two Reidbrace steel rod braced (RB20)
frames along the Northern side of the building.
4.3. 5 Ge ot ec h nic al Co ns id e r at i o ns
The draft geotechnical report for the proposed Northern Extension building has identified
potential liquefaction and lateral spreading issues for the site. As the Southern Extension
building occupies the same site it is likely to be subject to that same issue. Lateral spreading
and liquefaction can cause the building’s foundations to differentially displace which can result
in significant damage to and/or collapse of the building.
The draft geotechnical report has identified the “trigger” event for the liquefaction to be
approximately a 1 in 200 year event.
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4.3. 6 Su m m ar y
The Southern Addition is considered to have a strength of approximately 25-30% NBS for an
Importance Level 3 building.
The Southern Addition is considered to be an earthquake prone building.
4.3. 7 St r e ngt h e ni ng R equ ir e d
Strengthening to get above 33%NBS Importance Level 3
•
Installation of new tension braces in the braced frame locations.
•
Improve connections from braced frames to the supporting concrete walls.
The liquefaction and lateral spreading issues identified in the draft geotechnical report will need
to be mitigated such that the foundations can cope with the expected displacements.
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4.4 CIVIL DEFENCE OFFICE
4.4. 1 Bu i ld in g S eis m ic P a ra m et ers
The Civil Defence Office was designed to predecessor standards of the current NZ Building
Code, likely comprising principally NZS4203:1976. A comparison of the acceleration the
building is likely to have been designed for is shown in Figure 4-16 below.
Seismic loads are currently based on the requirements of NZS1170.5:2004. The base shear
coefficient is a function of building period, structure ductility and the site geology, including
proximity to known fault lines. The assumed seismic parameters for the Trafalgar Centre
building site are as listed in Table 4-7.
Ta bl e 4-7: Se is m ic P ar a m et ers
Design Code :
NZS1170.5:2004
Soil Category :
C
R:
1.8
Z:
0.27
Sp :
0.7
D:
>100km
Figure 4-16 is a plot of the design response spectra for the Civil Defence Office at current
NZS1170 loadings standard and at the NZS4203 loading standards that it was likely designed in
accordance with. The vertical dashed lines on the figure indicate the fundamental period of the
building in each of the principal axes. These lines can be used to determine the minimum
design base shear coefficient in each direction of loading. The base shear coefficient /
acceleration is expressed as a percentage of gravity.
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0.40
NZS1170 Spectra, Z=0.27, Soil Class C,
Ductility = 3, Sp = 0.7
0.35
NZS4203 (1976), Zone A, Class I, Flexible
soil, S = 1, M = 1
0.30
Acceleration (g)
Building Fundamental Periods
0.25
0.20
0.15
0.10
0.05
0.00
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Period (Seconds)
F igu r e 4 -16: Bu il di n g A cce l er at i o n Sp e ct r a C o m pa ris o n N ZS 42 03 v e r s u s N Z S
1170
4.4. 1.1 I mp o rt a nc e L e ve ls
The Civil Defence Office is required for post disaster services and is therefore considered an
IL4 building in accordance with AS/NZS1170. IL4 buildings in accordance with AS/NZS1170
have 2 key structural requirements. The ULS requirement (that the building maintain life safety
in a 1/2500 year earthquake event) was explicitly considered in this investigation. The SLS 2
requirement (immediate occupancy after a 1/500 year earthquake event) was not explicitly
considered. This requirement was not investigated because the building is lightweight and
constructed mostly of ductile materials. Therefore, the damage that may be expected after a
1/500 year event (cracking in GIB walls and possible dislodging of ceiling tiles) will not restrict
occupancy.
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Table 4-8 shown below shows the levels of risk posed to new buildings depending on their
proposed use.
Table 4-8: Importance Levels
Importance
Risk of
Exceedance in
50 Year Design
Life
Risk Factor
Level
Earthquake
Annual
Exceedance
Probability
Comment
Examples
1
1/100
40%
0.5
Structures
representing a low
degree of hazard to
life and property.
Small structures,
farm buildings,
fences, masts, walls
1/500
10%
1.0
“Normal”
structures and
structures not in
other importance
levels.
Hotels, offices,
apartments
1/1000
5%
1.3
Structures that may
contain people in
crowds or contents
of high value to the
community.
Schools, emergency
medical and other
emergency facilities
but not essential
post-disaster
healthcare facilities.
1/2500
2%
1.8
Structures with
special postdisaster functions.
Designated civilian
emergency facilities,
medical emergency
facilities with post
disaster functions.
( IL1 )
2
( IL2 )
3
( IL3 )
4
( IL4 )
4.4. 1.2 St r u ct u r e D u ct i lit y
Ductility is a measure of a building or its individual components ability to undergo sustainable
inelastic displacements whilst maintaining sufficient residual strength to carry load. The term
“inelastic” refers to actions beyond the base yield strength of the building or component being
considered. The more ductile a building, the more energy it is able to dissipate. Since ductility
inherently requires building structural components to be stressed beyond yield, there will be
some permanent damage associated with this form of energy dissipation.
By considering available building ductility, the magnitude of the seismic forces for which the
building is being assessed is able to be reduced to capture the effect of the energy dissipation.
Structural ductility is highly dependent on the type of building and the individual member
detailing. Highly ductile concrete members, for example, need to be well confined with closely
spaced reinforcing ties in order to maintain their residual strength as they hinge or become
damaged.
For the purpose of this assessment a structural ductility factor of 3 (ductile) will be adopted as
this is an acceptable ductility for GIB plaster board walls.
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4.4. 2 Mat e ri al P r op e rt i es
Material properties were calculated from a largely complete set of original drawings and the ‘era’
material strengths of New Zealand Society for Earthquake Engineering ‘Assessment and
Improvement of the Structural Performance of Buildings in Earthquakes’
The GIB walls were specified to be 9.6mm GIB plaster board. The type of GIB board is not
specified, therefore it is assumed to be “GIB Standard plasterboard” with a capacity of
5.8kN/m.
4.4. 3 M od e ll in g A s s u mpt i ons
The seismic evaluation utilises a lumped mass model based on the equivalent static method set
out in NZS1170.5:2004 to simulate the effects of horizontal earthquake forces. This model is
developed using information from the original structural drawings of the building and
properties of materials used at the time of construction.
For the seismic load case the amplitude of the acceleration is dictated by the fundamental
period of vibration of the building along its two principal axes. Tall flexible structures have
long periods, whilst short stiff buildings have short periods. As can be seen in Figure 4-16
above, the longer the period of a structure is, the lower the corresponding horizontal
acceleration. For a building period of 0.4 seconds the corresponding horizontal ground
acceleration is 0.37g, compared to a building period of 2.0 seconds where the horizontal
acceleration is approximately 0.08g.
The building’s fundamental period in both the transverse and longitudinal directions was
estimated to be less than 0.4 seconds. This assumption is based on the low height of the
structure.
When distributing lateral load between the lateral load resisting systems, a tributary areas
method was typically used due to the flexibility of the structures diaphragms.
Vertical distribution of the lateral loads applied to the structure is based on the equivalent static
base shear distribution recommended in NZS 1170.
4.4. 4 E s t i mat i on of B u i ldi n g S t re n gt h
As part of this review, hand calculations were used to calculate actions in the load resisting
elements.
4.4. 4.1 Lat e ral R es is t a nc e A n al ys is
There were three key finding from analysing the Civil Defence Office, they were:
•
The roof appears to have no structural diaphragm.
•
The internal walls, as shown on the construction drawings, do not exist. After contact
with the Nelson city council, the removal of the internal walls does not appear to have
been completed with consent.
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•
The office was built integral to the Northern Building of the Trafalgar centre.
The roof consists of light weight roofing iron supported by timber rafters and battens with a
layer of “Pinex” acoustic insulation below. This is not a structural diaphragm and a reliable
capacity cannot be attributed to it. There exists no reliable load path for the lateral actions on
the roof to reach the supporting walls. Although there is no reliable load path, it is likely the
secondary elements will still provide some capacity to transfer this load.
The timber frame, GIB board walls along the east and west of the building span 12m between
lateral restraining walls (with the internal walls this length would have only been 6m). The roof
diaphragm, as mentioned, cannot provide adequate out-of-plane restraint. Furthermore, the
roof above the Victory Room is at a higher level than the Civil Defence Office roof. The
connection between the Victory Room roof and the shared wall below consists of a steel truss
and light timber framing. This connection cannot be relied upon to transfer out-of-plane forces
and so the Northern Building roof cannot be considered to provide additional bracing to the
Civil Defence Office. Figure 4-17 below shows a typical section of the Civil Defence Office
and the Victory Room with then mentioned steel truss supporting the upper roof.
Upper roof of
Northern Building
Steel truss
F igu r e 4 -17: Ci v il D ef e nc e O f f ic e an d eas t V ic t or y r o o m s e ct i o n
As the Civil Defence Office is built integral to the Northern Building there will possibly be
adverse structural interactions during a seismic event. Such adverse interactions could involve
additional loads being transfer to the Civil Defence Office from the Northern Building.
From the above structural limitations the capacity of the structure is considered to be less than
15% NBS for an Importance Level 4 building.
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4.4. 5 Ge ot ec h nic al Co ns id e r at i o ns
The draft geotechnical report for the proposed Northern Extension building has identified
potential liquefaction and lateral spreading issues for the site. Lateral spreading and liquefaction
can cause the building’s foundations to differentially displace which can potentially result in
significant damage to and/or collapse of the building.
The draft geotechnical report has identified the “trigger” event for the liquefaction to be
approximately a 1 in 200 year event.
4.4. 6 Su m m ar y
The Civil Defence Office is considered to have an Ultimate Limit State strength of less than
15% NBS for an Importance Level 4 building.
The Building Act also defines a building as earthquake prone if it will have its ultimate capacity
exceeded in a moderate earthquake (defined as a level of earthquake one-third that required for
an equivalent new building); and would be likely to collapse. The Civil Defence Office is
considered to be an earthquake prone building.
4.4. 7 St r e ngt h e ni ng R equ ir e d
Strengthening to get above 67%NBS
•
Install a structural roof diaphragm by constructing an appropriate “GIB Braceline”
ceiling roof diaphragm.
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5. RECOMMENDATIONS - CONCLUSIONS
5.1
RECOMMENDATIONS
The detailed assessment shows that the Trafalgar Centre is an earthquake prone building
defined as less than 33% NBS. As such strengthening is required in accordance with the
statutory requirements.
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6. REFERENCES
[1]
Standards New Zealand, Structural Design Actions Part 5: Earthquake Actions
NZS1170.5:2004, Wellington, New Zealand, 2004.
[2]
Standards New Zealand, Concrete Structures Standard Part 1: The Design of Concrete Structures,
NZS 3101:Part 1:2006, Wellington, New Zealand, 2006.
[3]
Standards New Zealand, Steel Structures Standard: NZS 3404:Part 1:1997, Wellington, New
Zealand, 1997.
[4]
New Zealand Standards Institute, Chapter 8: Basic Design Load NZSS1900:1965, Wellington,
New Zealand, 1965.
[5]
Standards New Zealand, General Structural Design and Design Loadings for Buildings:
NZS4203:1992, Wellington, New Zealand, 1992.
[6]
New Zealand Society for Earthquake Engineering, Assessment and Improvement of
the Structural Performance of Buildings in Earthquakes, 2006.
[7]
New Zealand Standards Institute, Part 3: Earthquake Provisions NZSS1900:1976, Wellington,
New Zealand, 1976.
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