Download Project Structural Conditions Survey and Seismic Vulnerability

Project
Structural Conditions Survey and
Seismic Vulnerability Assessment
For
SFCC Civic Center Campus
750 Eddy Street
San Francisco, California 94109
Prepared For
San Francisco Community College District
33 Gough Street
San Francisco, California 94103
Prepared By
Thornton Tomasetti
650 California Street, Suite 1400
San Francisco, CA 94108
Phone: 415.365.6900
Fax: 415.365.6901
Project No. U15039.00
February 11, 2015
TABLE OF CONTENTS
Executive Summary ............................................................................................................................. 1
1.00
Description of Structure/Seismic Characteristics................................................................ 2
2.00
2.01
2.02
2.03
Seismic Vulnerability Assessment ........................................................................................ 6
Evaluation Criteria ..................................................................................................................... 6
Computer Model ........................................................................................................................ 7
Summary of Results ................................................................................................................ 12
3.00
Anticipated Seismic Performance ....................................................................................... 13
4.00
Recommendations ................................................................................................................ 14
5.00
Summary of Findings ............................................................................................................ 14
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Executive Summary
The purpose of this study is to conduct an in-depth seismic vulnerability assessment of the
750 Eddy Street Building at Civic Center Campus for CCSF to identify life safety hazards and
other deficiencies and to address possible remediations. The “Tier 2” procedures of ASCE
31-03, “Seismic Evaluation of Existing Buildings” have been employed to identify life safety
hazards and seismic deficiencies as well as collapse potential.
This report provides a description of the building and the structural features essential to this
study, the detailed criteria and procedure employed, the computer model of the lateral force
resisting system and the findings of this assessment. Also included are TT’s professional
opinions on possible remediations.
750 Eddy Street is a three-story, steel frame/wood floor hybrid structure with unreinforced
masonry and reinforced concrete infill shear wall building which was reportedly built in 1910.
The building was seismically strengthened in 1934.
The roof framing system consists of 2x8 wood joists spaced at 16” o.c., supporting straight
sheathing. The floor framing consists of 3x18 wood joists spaced at 12” and 16” o.c.
supporting straight sheathing. The wood joists are supported on steel beams and columns.
The steel columns are supported on spread footings.
Lateral resistance is provided by unreinforced masonry walls at the exterior and interior
reinforced concrete walls. Steel horizontal trusses serve as horizontal diaphragm to
distribute seismic inertial loads to the vertical shear resisting line of resistance. Concrete
walls and unreinforced masonry walls are supported on continuous footings.
The major identified seismic deficiencies of the lateral force resisting system which are
hazards to life safety are:
1. The unreinforced masonry infill walls have large height-to-thickness ratio. Masonry infill
walls with large height-to-thickness ratios have potential for damage due to out-of-plane
forces. Failure of these walls out-of-plane will result in falling hazards and degradation of
the strength and stiffness of the lateral force resisting system.
2. Unreinforced masonry walls out-of-plane wall anchorage to the roof and the floors are
inadequate and lacking cross ties.
3. Reinforced concrete walls out-of-plane wall anchorage to the roof and the floor
diaphragms are inadequate and lacking cross ties.
4. Unreinforced masonry shear walls are overstressed in shear.
5. Some of the horizontal steel truss members at the roof and the floors are overstressed.
These deficiencies are expected to have significant impact on the performance of the building
structure in a moderate to severe earthquake. In TT’s opinion, the structure could experience
significant damage in a moderate earthquake causing strong ground shaking. In a severe
earthquake with intense ground shaking, it is likely that these deficiencies will pose significant
life safety hazards and localized collapse in several locations of the building.
Page 1 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
1.00
DESCRIPTION OF STRUCTURE/SEISMIC CHARACTERISTICS
According to the original construction documents, the building was reportedly constructed in
1910 and was seismically strengthened in 1934. The building is a three-story steel
framed/wood floor hybrid structure with unreinforced masonry and reinforced concrete shear
wall building.
The roof framing system consists of straight sheathing supported by 2x8 wood joists spaced
at 16” o.c. The wood joists are supported on steel beams and columns.
At high roof, steel trusses spaced at 15’-0” o.c. form the ridge and support the wood joists.
The typical floor framing consists of 3x18 wood joists spaced at 12” to 16” o.c. supporting
straight sheathing. The wood floor joists are supported on steel beams and columns.
In 1934, the building was seismically strengthened by the addition of reinforced concrete
walls and horizontal steel trusses. Lateral resistance is provided by unreinforced masonry
walls at the building perimeter and interior reinforced concrete walls. The steel horizontal
trusses serve as horizontal diaphragms to distribute seismic inertial loads to the vertical shear
resisting lines of resistance. The unreinforced masonry and the reinforced concrete shear
walls transmit the seismic forces into the soil through continuous footing foundation.
A site visit was undertaken on February 6, 2015 to observe existing conditions. No evidence
of significant structural deterioration was observed except at the roof framing, several
locations showed sign of damage due to water intrusion. The building found generally
conform to the drawings used for the evaluation.
Figure 1: CCSF District 750 Eddy Street Building Site Map
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750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Roof Framing: Damage due to water intrusion
Roof Framing: Damage due to water intrusion
Page 3 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Steel Roof Trusses
Steel Roof Trusses
Page 4 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Unreinforced Masonry Wall Ties
Unreinforced Masonry Wall Ties
Page 5 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Slender Unreinforced Masonry Wall
2.00
SEISMIC VULNERABILITY ASSESSMENT
2.01
EVALUATION CRITERIA
The seismic evaluation guideline ASCE 31-03, Seismic Evaluation of Existing Buildings, was
utilized to identify the structural element deficiencies supplemented with experience and
engineering judgment. The seismic performance objectives that provided for in ASCE 31-03
are “Life Safety” and “Immediate Occupancy”. Thornton Tomasetti has based its evaluation of
this building on the “Life Safety” Performance objectives which is defined as “the building
performance that includes significant damage to both structural and nonstructural
components during a design earthquake, though at least some margin against either partial
or total collapse remains. Injuries may occur, but the level of risk for life-threatening injury and
entrapment is low.”
ASCE 31-03 consists of checklists defining building types and potential “weak links” in
buildings. These “weak links” are identified in Tier 1 phase where quick checks and
evaluations are performed.
The Tier 1 checklists were completed from a review of the following:


Original structural drawings by Bureau of Architecture Board of Public Works, dated June
14, 1910.
Structural drawings for seismic strengthening by Erle L. Cope Structural Engineer, dated
March 17, 1934.
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750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Deficiencies identified in Tier 1 are further evaluated in a Tier 2 evaluation which requires
detailed quantitative analyses. For most buildings, only the deficiencies that are identified in
the Tier 1 phase are analyzed often referred to as a “Tier 2 Deficiency Only Evaluation”.
ASCE 31-03 uses the performance based methodology of pseudo lateral forces originally
developed for FEMA 273, NEHRP Guidelines for the Seismic Rehabilitation of Buildings
rather than use of the equivalent lateral force methodology which is common in current
codes. The building is evaluated at the expected displacement of the structure during the
demand earthquake. The force demands and associated capacities for each component are
determined and evaluated based on the ductility of the element.
The Tier 2 evaluation includes construction of a three-dimensional computer model of the
building for analysis. The results of these analyses were employed in the more detailed
evaluation of the previously identified Tier 1 deficiencies.
2.02
COMPUTER MODEL
Thornton Tomasetti selected SAP 2000 computer software to model the 750 Eddy Street
Building at Civic Center Campus.
The SAP 2000 model includes all elements of the lateral and gravity force resisting system
including horizontal steel trusses, unreinforced masonry shear walls, concrete shear walls,
steel columns and beams with the exception of wood floor joists. The walls were set to have
pinned base because of the foundation type.
Concrete and masonry structural infill walls were modeled using membrane elements, which
only have in-plane stiffness. The concrete compressive strength used is 2000 psi and yield
strength of the reinforcing steel is assumed to be 33ksi for Intermediate grade per the seismic
evaluation guideline ASCE 31-03.
Site specific spectral acceleration values were generated using “Hazard Map Analysis Tools”
developed by the U.S. Geological Survey (USGS).
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750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
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750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Page 9 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Page 10 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
Page 11 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
2.03
SUMMARY OF RESULTS
Results from the Tier 2 Linear Static Analysis, as specified by ASCE 31 and described in
Section 3.01 of this report, for each critical element or connection of the lateral force resisting
system are presented in Tables 3-1 through 3-8. The results are presented using demand-tocapacity ratios (DCRs). Overstressed elements are indicated by a DCR greater than 1.0 (and
are shown in the following tables in bold, red text). The most critical DCR’s are shaded.
Elevations, provided in Section 2.02, designate the locations in the building of the individual
structural elements referenced in each of the following tables.
Table 2 – 1 Diaphragm – Tension Rods of Horizontal Steel Truss
LEVEL
DCR: TENSION
Roof
2nd Floor
1st Floor
9.62
10.78
5.28
Table 2 – 2 Out-of-Plane Wall Anchorage
LEVEL
DCR: TENSION
Typical
5.44
Table 2 – 3 Masonry Shear Stress
LEVEL
DCR: TENSION
Roof
2nd Floor
1st Floor
0.58
2.26
2.52
Table 2 – 4 Masonry Out-of-Plane Capacity
LEVEL
DCR: FLEXURE
Typical
1.33
The DCR results in these tables indicate that the “weakest links” in the lateral system are
likely to be diaphragms, wall out-of-plane anchorages, and slender unreinforced masonry
shear walls. In a moderate to severe earthquake, it appears that the diaphragms could
initially fail. The failure of the diaphragms and out-of-plane wall anchorages could result in
partial wall collapses.
Page 12 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
3.00
ANTICIPATED SEISMIC PERFORMANCE
The seismic performance of a structure for ground shaking is dependent upon the behavior of
its critical components. The critical components can be defined as components that are
necessary for stability and complete seismic load path. The seismic performance of critical
components of the CCSF 750 Eddy Street Building has been identified in Section 3.03 of this
report.
The building has several potential failure modes that threaten the structure’s ability to sustain
vertical loads and maintain stable lateral behavior. In all buildings, seismic forces originate
by inertial excitation and are transferred through connections to horizontal diaphragms and
then via collectors to the vertical resisting elements and to the foundation. Failure of
individual members and connections within this system of elements creates a discontinuity in
the seismic load path and can lead to modes of response which precipitate damage and in
some cases, life safety hazards.
The primary deficiencies for the CCSF 750 Eddy Street Building are the inadequate
diaphragm strength at the roof and floor level, inadequate wall out-of-plane anchorage
strength, and inadequate slender unreinforced masonry walls.
The roof and the floor horizontal steel truss diaphragm members have inadequate tension
strength to transfer the anticipated seismic forces. In the event of a diaphragm failure, the
diaphragm inertial forces cannot be delivered to the shear walls. This can cause loss of outof-plane support for the concrete and masonry walls. This could lead to partial wall collapse.
The unreinforced masonry infill walls span between the roof and floor steel beams when
subject to out-of-plane forces. The masonry shear walls at each story have large height-tothickness ratio. Slender unreinforced masonry walls have a potential for damage due to outof-plane forces which may result in falling hazards and potential wall collapse.
The unreinforced masonry walls could be overstressed in shear in a moderate to severe
seismic event. Performance of frame buildings with masonry infill walls is dependent on the
interaction between the frame and infill panels. If the infill panels separate from the frame
due to out-of-plane forces, the strength and stiffness of the system will be determined by the
properties of the bare frame, which is not detailed to resist seismic forces. Severe damage or
partial wall collapse due to excessive drift and P-D effects may occur.
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750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
4.00
RECOMMENDATIONS
The ground shaking induced hazards can be mitigated through a comprehensive retrofit to
the structure. Some possible remediations are listed below;
Item
1
2
3
4
5.00
Deficiency
Roof and floor diaphragms lack
adequate strength to resist expected
seismic forces.
Wall anchorage connection lacks
strength to resist the expected out-ofplane seismic forces.
Unreinforced masonry walls have large
height-to-thickness ratio to resist the
expected out-of-plane seismic forces.
Unreinforced masonry walls
overstressed in shear.
Recommended Remediation
Reinforce existing horizontal steel truss
diaphragm members using new steel
members.
Strengthen wall anchorage connection
with new wall anchors and new diaphragm
ties.
Provide new steel strongbacks with
dowels into the wall.
Provide new concrete wall in-fill between
existing columns at locations of
overstressed masonry walls. Provide new
dowels into the existing, masonry walls.
SUMMARY OF FINDINGS
The purpose of this study is to conduct an in-depth seismic vulnerability assessment of the
750 Eddy Street Building at Civic Center Campus for CCSF to identify life safety hazards and
other deficiencies and to address possible remediations. The “Tier 2” procedures of ASCE
31-03, “Seismic Evaluation of Existing Buildings” have been employed to identify life safety
hazards and seismic deficiencies as well as collapse potential.
This report provides a description of the building and the structural features essential to this
study, the detailed criteria and procedure employed, the computer model of the lateral force
resisting system and the findings of this assessment. Also included are TT’s professional
opinions on possible remediations.
The major identified seismic deficiencies of the lateral force resisting system which are
hazards to life safety are:
1. The unreinforced masonry infill walls have large height-to-thickness ratio. Masonry infill
walls with large height-to-thickness ratios have potential for damage due to out-of-plane
forces. Failure of these walls out-of-plane will result in falling hazards and degradation of
the strength and stiffness of the lateral force resisting system.
2. Unreinforced masonry walls out-of-plane wall anchorage to the roof and the floors are
inadequate and lacking cross ties.
3. Reinforced concrete walls out-of-plane wall anchorage to the roof and the floor
diaphragms are inadequate and lacking cross ties.
4. Unreinforced masonry shear walls are overstressed in shear.
5. Some of the horizontal steel truss members at the roof and the floors are overstressed.
These deficiencies are expected to have significant impact on the performance of the building
structure in a moderate to severe earthquake. In TT’s opinion, the structure could experience
significant damage in a moderate earthquake causing strong ground shaking. In a severe
Page 14 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00
earthquake with intense ground shaking, it is likely that these deficiencies will pose significant
life safety hazards and localized collapse in several locations of the building.
The ground shaking induced hazards can be mitigated through a comprehensive retrofit to
the structure.
Page 15 of 15
750 Eddy Street – Seismic Analysis
CCSF Civic Center Campus
February 11, 2015 | Project # U15039.00