Download Detailed Seismic Assessment Report

Detailed Seismic
Assessment Report
Islington Substation
August 2011
Seismic Review of Essential Buildings
Detailed Seismic Assessment Report
Islington Substation
Detailed Seismic Assessment Report
Islington Substation
Prepared for:
Transpower New Zealand
Prepared by:
Opus International Consultants
Reference: 5-C1929.01
Date: 8th August 2011
Status: Draft 1
Report prepared by:
Jack Shepherd
Reviewed by:
Robert Davey
Approved for Release:
Mark Ryburn
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Contents
Executive Summary ...................................................................................................................... 1
1
Introduction .......................................................................................................................... 2
1.1 Purpose of the report .................................................................................................... 2
1.2 Scope of work ............................................................................................................... 2
1.3 Sources of information .................................................................................................. 2
1.4 Minimum Seismic Ratings for Existing Structures ......................................................... 2
2
Site Conditions .................................................................................................................... 4
3
Performance in the Darfield and Christchurch Earthquakes ............................................ 4
4
Oil Filter/Compressor House .............................................................................................. 7
4.1 General Description ...................................................................................................... 7
4.2 Initial Seismic Assessment Results ............................................................................... 7
4.3 Material Strengths......................................................................................................... 7
4.4 Response Mechanisms and Critical Structural Weaknesses ......................................... 7
4.5 Method of Analysis ....................................................................................................... 7
4.6 Analysis Results ........................................................................................................... 7
4.7 Proposed Improvements ............................................................................................... 8
4.8 Cost Estimates.............................................................................................................. 8
5
Condenser Building ............................................................................................................. 9
5.1 General Description ...................................................................................................... 9
5.2 Initial Seismic Assessment Results ............................................................................... 9
5.3 Material Strengths......................................................................................................... 9
5.4 Response Mechanisms and Critical Structural Weaknesses ......................................... 9
5.5 Method of Analysis ..................................................................................................... 10
5.6 Analysis Results ......................................................................................................... 10
5.7 Proposed Improvements ............................................................................................. 11
5.8 Cost Estimates............................................................................................................ 11
6
Control Block ..................................................................................................................... 13
6.1 General Description .................................................................................................... 13
6.2 Initial Seismic Assessment Results ............................................................................. 13
6.3 Material Strengths....................................................................................................... 13
6.4 Response Mechanisms and Critical Structural Weaknesses ....................................... 13
6.5 Method of Analysis ..................................................................................................... 13
6.6 Analysis Results ......................................................................................................... 13
6.7 Proposed Improvements ............................................................................................. 14
6.8 Rough order of costs .................................................................................................. 14
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7
Other Structural Issues Identified on Site ........................................................................ 15
8
Conclusions ....................................................................................................................... 15
9
References ......................................................................................................................... 17
Appendix 1: ISA Report .............................................................................................................. 18
Appendix 2: Sketches ................................................................................................................. 19
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Executive Summary
Building
Importance
Level
Seismic Policy
Minimum
Standard
Detailed
Assessment
Standard
Improvements
Required
Oil Filter/
Compressor
House
4
75% NBS
100%NBS
Nil
Condenser
Building
4
75% NBS
38%NBS
Yes
Control Block
4
75% NBS
28%NBS
Yes
A detailed assessment has been made of the earthquake performance of the Oil
Filter/Compression House, Condenser Building and Control Block at Islington substation to
establish whether or not they meet Transpower‟s minimum requirements for its essential buildings.
This followed an initial screening assessment that indicated they potentially did not comply and
therefore required further investigation.
Transpower‟s minimum standard is 75% of the current Building Code requirement for a new
building on the site (i.e. 75%NBS). The assessed ratings of the Condenser Building and Control
Block are <75%NBS. Improvements are therefore required to these buildings to meet the minimum
requirements.
The Oil Filter/Compressor House assessed rating is 100%NBS therefore no improvements are
necessary to meet the minimum requirements.
The costs of proposed improvements are estimated to be $1,407,500.
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1
Introduction
This report covers the detailed seismic assessment of the Oil Filter/Compressor House,
Condenser Building and Control Block at Islington substation.
1.1
Purpose of the report
The purpose of this report is to:
Provide a structural description of the structures under consideration;
Explain the methodology used for assessing the seismic performance of the structures;
Assess the seismic performance of the structures and equipment contained within
them;
Identify areas where potential damage is most likely to occur based on the strength
hierarchy established within the structural systems;
Present the results obtained from the structural review;
Make recommendations on required strengthening measures; and
Provide a rough order of costs for any proposed strengthening schemes
1.2
Scope of work
We carried out the following activities in order to complete this report:
Site inspection of the Oil Filter/Compressor House, Condenser Building and Control
Block;
Review of the available documentation;
Initial seismic evaluation of the structures to check preliminary risk classifications;
Detailed seismic assessment of the high risk structures; and
Concept design and rough order of cost estimate of improvement work if necessary.
1.3
Sources of information
The seismic assessment of the Oil Filter/Compressor House, Condenser Building and
Control Block structures are based on information obtained from the existing structural
drawings provided by Transpower. A site inspection was carried out in May 2011 to identify
the structures, check the currency of the drawings and identify potential site hazards.
Other building data such as the classification of the buildings by importance level in
accordance with NZS 1170.0:2002 Standards [1] was obtained from Opus report dated
September 2008 [2]. This classification was decided upon by Transpower following the
Seismic Policy Guidelines [2].
1.4
Minimum Seismic Ratings for Existing Structures
The recommended minimum seismic requirements for Transpower substation structures
are stated in the draft seismic policy prepared in September 2008 [2].
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This draft policy takes into account the requirements for earthquake prone buildings
prescribed in the Building Act 2004 and the requirements for lifeline facilities prescribed in
the Civil Defence and Emergency Management Act 2002. The policy also incorporates the
recommendations of the New Zealand Society for Earthquake Engineering (NZSEE) [3] to
mitigate the risk to buildings that are essential to the functioning of the National Grid.
Table 2 summarises the policy requirements for existing buildings. The minimum seismic
standards of Territorial Authorities are deemed acceptable for less critical buildings
(importance level 1, 2, and 3). These minimum requirements correspond to the Building Act
requirements. For essential buildings (importance level 4), a higher seismic standard has
been selected to ensure resilience of critical assets and meet Emergency Management Act
obligations and NZSEE recommendations.
Table 2. Proposed seismic policy requirements for existing Transpower substation
buildings
Importance
level
Limit state
Annual
probability of
exceedance
%NBS
Driving documentation
Ultimate
1/1000
(74%)
Serviceability
1/250
(75%)
Civil Defence and Emergency Management Act 2002
NZSEE Recommendations
Ultimate
-
34%
4
1, 2 and 3
Territorial Authority minimum requirements
Building Act 2004
Note that %NBS = the percentage ratio of the subject building seismic strength to the New
Zealand Building Code strength requirement for a new building at the site. A building with
100%NBS for example meets current requirements for new buildings.
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2
Site Conditions
No geotechnical data were available for the site. According to geological maps the site soils
are likely to be postglacial alluvium. The soils are not expected to be prone to liquefaction
or lateral spreading, however this should be confirmed by a geological investigation.
3
Performance in the Darfield and Christchurch Earthquakes
The performance of the buildings in recent large earthquakes could provide useful imperical
indications of their earthquake resistances depending upon intensity of load relative to the
assessment criteria.
The epicentres of the September 2010 magnitude 7.1 Darfield and February 2011
magnitude 6.3 Christchurch Earthquakes were located 27km West and 16 km South-East
respectively from the Islington substation (see Figure 1). Strong motion recordings of the
ground motions from these earthquake was made by the Geonet network at a station
(TPLC) that is located approximately 3.4 km from the substation. While some distance
apart, the ground motions recorded at TPLC station are likely to be similar to those
experience at the substation site (actually higher for Darfield and lower for Christchurch).
Figure 1: Locations of the September 2010 Darfield and February 2011 Christchurch
Earthquake Epicentres Relative to Islington Substation and Geonet Station TPLC
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The earthquake accelerations produced by these ground motions are shown as response
spectra in Figures 2 and 3. The Transpower minimum accelerations (75%NBS) used for the
seismic assessment at the Islington site are also shown for comparison.
1.40
Darfield N-S
1.20
Acceleration Cd (g)
Darfield E-W
1.00
Transpower 75%NBS
0.80
0.60
0.40
0.20
0.00
0
1
2
3
4
Period T (secs)
Figure 2: Comparison of Darfield Earthquake Response Spectra (5% Damped) with
Transpower Minimum Requirements at Islington
1.40
Acceleration Cd (g)
Christchurch N-S
1.20
Christchurch E-W
1.00
Transpower 75%NBS
0.80
0.60
0.40
0.20
0.00
0.00
1.00
2.00
3.00
4.00
Period T (secs)
Figure 3: Comparison of Christchurch Earthquake Response Spectra (5% Damped) with
Transpower Minimum Requirements at Islington
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In both instances the earthquake forces imposed on the structures at Islington substation
were significantly less than forces used for this detailed assessment. Their good
performances in these earthquakes cannot therefore be used as an indication that the
buildings meet Transpower minimum seismic rating requirements.
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4
Oil Filter/Compressor House
4.1
General Description
The Islington Oil Filter/Compressor House is a single storey, concrete walled building with a
concrete roof. The approximate dimensions of the building on plan are 5.5m x 8.1m and
3.6m high.
It was designed in 1953 by Ministry of Works New Zealand. The building was assessed to
be in good condition in the May 2011 inspection.
Drawings and photographs of the structure are included in the ISAR in Appendix 1.
It was classified as an “essential”, importance level 4 building by Transpower.
4.2
Initial Seismic Assessment Results
An initial screening seismic assessment was made based on a simple comparison of
seismic design standards at time of construction with current Building Code new building
standard [Appendix 1] which indicated that the building was potentially high risk.
4.3
Material Strengths
The following material strengths have been adopted based on NZSEE [3] guidelines:
Concrete compressive strength: 30MPa
Reinforcement tensile strength: 270MPa
4.4
Response Mechanisms and Critical Structural Weaknesses
Earthquake forces are resisted in both planes by reinforced concrete walls. The critical
elements were identified as a „short column‟ between windows at high level.
4.5
Method of Analysis
The detailed assessment followed the requirements of the NZSEE guidelines [3], using the
force-based procedures. ULS forces were based on µ = 1.25 for shear. An equivalent static
method of analysis was used in accordance with requirements of NZS 1170.5 [4].
4.6
Analysis Results
The analysis results are summarised in Table 3.
Table 3: Analysis Results
August 2011
Building Element
Action
Limit State
Capacity
Side walls
Shear
SLS2
>100%NBS
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Side walls
Shear
ULS
>100%NBS
The capacity of the longitudinal walls is controlled by the shear strength at window opening
level. The walls at this level act as „short columns‟ and therefore will be governed by shear
mode of failure.
These results indicate that the walls of the Oil Filter/Compression House meet Transpowers
minimum requirements for earthquake collapse avoidance (ULS) and damage avoidance
(SLS2).
4.7
Proposed Improvements
Not required.
4.8
Cost Estimates
Not applicable.
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5
Condenser Building
5.1
General Description
The Islington Condenser Building is a large single storey, concrete framed building with
reinforced concrete walls and a mezzanine floor which is located across 2No. bays. The
plan dimensions of the building are approximately 18.5 x 61.1m, with a maximum height of
16.95m. In the longitudinal direction, reinforced concrete walls with significant structural
openings provide stability whilst in the transverse direction this is catered for by reinforced
concrete cantilever columns. The roof is supported by pinned base steel portal frames
which sit atop the concrete columns and is constructed of lightweight metal decking
overlaying steel and timber purlins. The concrete columns also support deep steel crane
girders which spans the full length of the building supporting a large gantry crane. The
foundations are reinforced pad foundations with ground beams tying them together in the
transverse direction and supporting the walls in the longitudinal direction. The condensers
are located at the level of the mezzanine floors and have their own foundations.
It was designed in 1952 by Ministry of Works New Zealand. The building was assessed as
in good condition in the May 2011 inspection. There was minimal earthquake damage at
the link between the Condenser Building and the Control Room.
Drawings and photographs of the structure are included in the ISAR in Appendix 1.
It was classified as an “essential”, importance level 4 building by Transpower.
5.2
Initial Seismic Assessment Results
An initial screening seismic assessment was made based on a simple comparison of
seismic design standards at time of construction with current Building Code new building
standard [Appendix 1] which indicated that the building was potentially high risk.
5.3
Material Strengths
The following material strengths have been adopted based on NZSEE [3] guidelines:
Concrete compressive strength: 30MPa
Reinforcement tensile strength: 270MPa
5.4
Response Mechanisms and Critical Structural Weaknesses
Earthquake forces in the longitudinal direction are resisted by the external reinforced
concrete walls. These walls have many openings which are spaced at regular horizontal
and vertical centres therefore forming frames with deep and short beams and columns.
These components are vulnerable to shear failure.
Forces in the transverse direction are resisted by the reinforced concrete cantilever
columns and foundation beams. These components appear to be relatively well detailed for
ductility for a 1950‟s structure. It is expected then that plastic hinges could develop in the
base of the column. In this case the anchorages of the beam and column reinforcing into
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the joints are relatively good, so that the capacity of the structure will be based on the beam
and column rather than joint strengths. The major potential weaknesses are poor shear
strength and poor confinement of the concrete and longitudinal reinforcement as a result of
widely spaced ties (305mm centres).
The foundations of the condenser units are stiff and strong enough to cater for the seismic
forces which will be transferred from the mezzanine floor. For this reason the analysis of the
transverse frames and longitudinal walls exclude mass from the mezzanine floor.
5.5
Method of Analysis
The detailed assessment followed the requirements of the NZSEE guidelines [3], using the
force-based procedures for the longitudinal direction and displacement-based procedures
for the transverse frames. The criteria shown in Table 2 were used to determine the
earthquake actions on the structure.
Table 2: Criteria Used to Determine the Magnitude of the EQ Actions on the Building
Components
Building Element
Action
Limit State
SLS2
Transverse frames
Flexure
Criteria
Ductility factor µ = 2.0
ULS
Concrete compressive strain at
peak displacement response
Transverse frames
Shear
SLS2 & ULS
Shear force at member flexural
capacities
Side walls
Shear
SLS2 & ULS
Ductility factor µ = 1.25
The analysis is based on the assumption that the crane will be parked at the end wall
position when not in use.
5.6
Analysis Results
The analysis results are summarised in Table 3.
Table 3: Analysis Results
August 2011
Building Element
Action
Limit State
Capacity
Transverse frames
Flexure
SLS2
72%NBS
Transverse frames
Flexure
ULS
89%NBS
Transverse frames
Shear
ULS & SLS2
>100%NBS
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Side walls
Shear
SLS2
79%NBS
Side walls
Shear
ULS
38%NBS
The capacity of the longitudinal walls is controlled by the shear strength at window opening
level. The transverse frames will respond in a column flexural mode as the shear strength
of the beams and columns is sufficient to prevent shear failure. The displacement response
of the frames is limited by concrete spalling strain.
These results indicate that the longitudinal walls of the Condenser Building do not meet
Transpowers minimum requirements for earthquake collapse avoidance (ULS).
5.7
Proposed Improvements
The proposed improvement method is to infill the lower three levels of window openings
with reinforced in-situ concrete. The infills must be dowelled into the existing structure. See
Appendix 2 for a drawing indicating which windows are to be filled in. An alternative method
would be to apply fibre reinforced epoxy coatings to the walls. Initial indications are that this
would be more costly than the window infill option. However, this could be considered in
more detail during design development stage.
5.8
Cost Estimates
A rough order of costs has been prepared for the improvement scheme outlined above.
Costs are exclusive of GST and construction management costs, and include the following:
Structural works relating to the seismic upgrade scheme
Site accessibility provision:
o Main centre 0%
o Provincial centre 15%
o Remote 50%
Enabling works provision:
o Minor (moving furniture, internal walls etc etc) 5%
o Medium (temporary screening to equipment, some coordination with Transpower
operations, medium safety risk) 25%
o Major (temporary removal or shutdown, relocation of equipment, building open to
weather, works within minimum approach distance, likely to require on site
Transpower representative, major safety risk involved) 100% or as assessed.
A design contingency sum of 25% to allow for uncertainties associated with the
conceptual status of the design
An allowance of 10% for architectural finishing
An allowance of 25% for preliminary and general costs, design costs, construction
observation and consent fees.
The current estimates are approximate, allowing for the most adverse conditions and
design uncertainties. The Table 3 below summarises the results.
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Table 3: Summary of Costs for Improvement Work
Item
Cost
Infill of panels; $7300 per window x 82 windows
$599,000
Enabling works (minor)
$30,000
Net costs
$629,000
Design contingency (25%)
$157,500
Architectural contingency (10%)
$63,000
Preliminary and general, design, consent fees (30%)
Rounded total
August 2011
$189,000
$1,038,500
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6
Control Block
6.1
General Description
The Islington Control Block is a two storey, reinforced concrete frame building with
reinforced concrete walls. The first floor consists of a precast concrete slab supported by
steel beams. Plan dimensions are approximately 23 x 34m and maximum height is 7.2m.
The foundations consist of RC ground beams and pad footings. A link building is
constructed between the Control Block and the Condenser Building. This structure is
constructed 25mm away from the Condenser Building. It was designed in 1952 by The
Ministry of Works. Its condition in the May 2010 inspection was assessed to be good
however the link building between the Control Block and the Condenser Building showed
signs of damage.
Drawings and photographs of the structure are included in the ISAR in Appendix 1.
It was classified as an “essential”, importance level 4, building by Transpower.
6.2
Initial Seismic Assessment Results
An initial screening seismic assessment was made based on a simple comparison of
seismic design standards at time of construction with current Building Code new building
standard [Appendix 1] which indicated that the building was potentially high risk.
6.3
Material Strengths
The following material strengths have been adopted:
Concrete compressive strength: 30MPa
Reinforcement tensile strength: 270MPa
6.4
Response Mechanisms and Critical Structural Weaknesses
Earthquake forces in both the transverse and longitudinal direction are resisted by the
external and internal reinforced concrete walls. These walls generally have many openings
which reduces their strength. The link building is potentially vulnerable to pounding forces
due to the displacement of the transverse frames of the Condenser Building.
6.5
Method of Analysis
The detailed assessment followed the requirements of the NZSEE guidelines [3], using the
force-based procedures. ULS forces were based on µ = 1.25 for shear. An equivalent static
method of analysis was used in accordance with requirements of NZS 1170.5 [4].
6.6
Analysis Results
The analysis results are summarised in Table 2.
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Table 2: Analysis Results
Building Element
Action
Limit State
Capacity
Longitudinal side walls
Shear
ULS
56%NBS
Longitudinal side walls
Shear
SLS2
>100%NBS
Transverse side walls
Shear
ULS
28%NBS
Transverse side walls
Shear
SLS2
59%NBS
The longitudinal walls do not meet the 75%NBS requirement largely due to the fact that
there are many openings between ground floor and first floor along the north elevation. The
shear resistance in the longitudinal plane is therefore reliant on the south elevation wall
between ground floor and first. There is adequate strength provided in the longitudinal
direction at first floor level and above due to the reinforced concrete internal wall.
In the transverse direction there are many openings at both floor levels to the east and west
elevations of the building. The shear resisting components are not adequate to cater for the
seismic forces of 75%NBS.
6.7
Proposed Improvements
The recommended improvement method is to infill the ground floor window openings of the
North Wall elevation (9No. in total), the West Wall elevation (2No. in total) and the East wall
elevation (5No. in total). Additionally at 1st floor level, the West Wall elevation windows
(4No. in total) and the East wall elevation windows (5No. in total) are proposed to be infilled
with reinforced in-situ concrete. The infills must be dowelled into the existing structure. See
Appendix 2 for a drawing indicating which windows are to be filled in.
6.8
Rough order of costs
A rough order of costs has been prepared for the improvement scheme outlined above.
Costs are exclusive of GST and construction management costs, and include the following:
Structural works relating to the seismic upgrade scheme
Site accessibility provision:
o Main centre 0%
o Provincial centre 15%
o Remote 50%
Enabling works provision:
o Minor (moving furniture, internal walls etc etc) 5%
o Medium (temporary screening to equipment, some coordination with Transpower
operations, medium safety risk) 25%
o Major (temporary removal or shutdown, relocation of equipment, building open to
weather, works within minimum approach distance, likely to require on site
Transpower representative, major safety risk involved) 100% or as assessed.
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A design contingency sum of 25% to allow for uncertainties associated with the
conceptual status of the design
An allowance of 10% for architectural finishing
An allowance of 25% for preliminary and general costs, design costs, construction
observation and consent fees.
The current estimates are approximate, allowing for the most adverse conditions and
design uncertainties. The Table 3 below summarises the results.
Table 3: Summary of costs for strengthening schemes
Item
Cost
Infill of panels; $6300 per window x 25 windows
$201,500
Enabling works (medium)
$22,000
Site accessibility (main centre)
$0
Net costs
$223,500
Design contingency (25%)
$56,000
Architectural contingency (10%)
$22,500
Preliminary and general, design, consent fees (30%)
$67,000
Rounded total
$369,000
The figures above are those for the strengthening scheme we believe to be most feasible.
7
Other Structural Issues Identified on Site
Nil
8
Conclusions
The results of the detailed seismic assessment of the Islington Substation Control Building
are summarised in Table 2.
Building
Oil Filter/
Importance
Level
Seismic Policy
Minimum
Standard
Assessed
Standard
4
75% NBS
100%NBS
Compressor
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Improvements
Required
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Detailed Seismic Assessment Report
Islington Substation
House
Nil
Condenser
Building
4
75% NBS
38%NBS
Yes
Control Block
4
75% NBS
28%NBS
Yes
The Condenser Building and the Control Block were assessed to be below the current NZ
Building Code requirements for a new importance level 4 building on the site. Remedial
works as detailed in Section 4 of this report to improve the building‟s earthquake resistance
are required.
As noted in Section 5 the Oil Filter/Compression House is at 100%NBS and therefore
requires no improvements.
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9
References
[1]
AS/NZS 1170.0, Structural Design Actions, General Principles, SNZ, 2002.
[2]
Structural Review Programme for Transpower Substation Buildings, Opus, 2008.
[3]
Assessment and Improvement of the Structural Performance of Buildings in
Earthquakes, NZSEE, 2006.
[4]
NZS 1170.5, Structural Design Actions, Earthquake Actions, SNZ, 2004.
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Appendix 1: ISA Report
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Appendix 2: Sketches
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