Prepared For San Francisco Community College District 33 Gough Street San Francisco, California Prepared By

Transcription

1 Project Structural Conditions Survey and Seismic Vulnerability Assessment For SFCC Civic Center Campus 750 Eddy Street San Francisco, California Prepared For San Francisco Community College District 33 Gough Street San Francisco, California Prepared By Thornton Tomasetti 650 California Street, Suite 1400 San Francisco, CA Phone: Fax: Project No. U February 17, 2015

2 TABLE OF CONTENTS Executive Summary Description of Structure/Seismic Characteristics Seismic Vulnerability Assessment Evaluation Criteria Computer Model Summary of Results Anticipated Seismic Performance Summary of Findings and Recommendations Disclaimer APPENDICES Appendix A1: Risk Acceptability Table Appendix A2: Risk Level Description... 17

3 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 The building was seismically strengthened in 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 17

4 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 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 Page 2 of 17

5 Roof Framing: Damage due to water intrusion Roof Framing: Damage due to water intrusion Page 3 of 17

6 Steel Roof Trusses Steel Roof Trusses Page 4 of 17

7 Unreinforced Masonry Wall Ties Unreinforced Masonry Wall Ties Page 5 of 17

8 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 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 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, Structural drawings for seismic strengthening by Erle L. Cope Structural Engineer, dated March 17, 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 Page 6 of 17

9 the Tier 1 phase are analyzed often referred to as a Tier 2 Deficiency Only Evaluation. ASCE 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 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 Site specific spectral acceleration values were generated using Hazard Map Analysis Tools developed by the U.S. Geological Survey (USGS). Page 7 of 17

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14 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 nd Floor st Floor 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 nd Floor st Floor 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 17

15 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- effects may occur. Page 13 of 17

16 4.00 SUMMARY OF FINDINGS AND RECOMMENDATIONS 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. Seismic deficiencies and deteriorated conditions have been identified for the building and a Seismic Priority has been assigned to each deficiency. Additionally, the building has been assigned a Seismic Risk Level. The following provides further explanation of the key aspects of the findings and a summary of the findings and recommendations. Seismic Deficiencies Seismic deficiencies have been identified as part of an in-depth seismic vulnerability assessment of the building and observations made during the site walk. Seismic Priority The seismic deficiencies are expected to result in varying levels of structural damage due to a moderate to severe earthquake, as represented by the Priorities assigned to each deficiency. The Priorities are defined as follows: Priority 1: Priority 2: Priority 3: Priority 4: Seismic deficiency expected to cause substantial structural damage and partial collapse, posing a major life-safety hazard. Seismic deficiency expected to cause substantial structural damage, posing a moderate life-safety hazard. Seismic deficiency expected to cause moderate structural damage. Item is not a seismic deficiency but structure would benefit from mitigation. Seismic Risk Level A Seismic Risk Level, as defined in the Risk Acceptability Table of the State Building Seismic Program developed by Division of the State Architect (DSA) in April 1994, has been assigned to the subject buildings based on the site visit observations and the review of the existing drawings. The Seismic Risk Level defines the expected hazard to life safety in the event of strong ground motion. Refer to Appendix A "Risk Acceptability Table and Risk Level Descriptions" for more information regarding the Seismic Risk Level rating system. The following tables provide a summary of the Seismic Deficiencies, Seismic Priorities, and recommendations for repair or retrofit to mitigate the seismic deficiencies. Page 14 of 17

17 Deficiency or Deteriorated Condition 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. Water damage to roof framing members Seismic Recommended Remediation Priority 2 Reinforce existing horizontal steel truss diaphragm members using new steel members. 1 Strengthen wall anchorage connection with new wall anchors and new diaphragm ties. 1 Provide new steel strongbacks with dowels into the wall. 2 Provide new concrete wall in-fill between existing columns at locations of overstressed masonry walls. Provide new dowels into the existing, masonry walls. 3 Further inspect and repair framing The building has been assigned to Seismic Risk Level V 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. The ground shaking induced hazards can be mitigated through a comprehensive retrofit to the structure DISCLAIMER Users of this report are advised that deficiencies may exist in the structures that were not observed in this evaluation. Our services have consisted of providing professional opinions, conclusions, and recommendations based on generally accepted structural engineering principles and practices. Page 15 of 17

18 Appendix A1: Risk Acceptability Table Risk Acceptability Table RISK LEVEL Hospitals Essential Facilities Hazardous Materials Public Schools Nursing, Prisons University, Research Offices, Courts Other Occupancies I II III IV V VI VII Acceptable Key Questionable Unacceptable CCSF Civic Center Campus February 11, 2015 Page 16 of 17 Project # U

19 Appendix A2: Risk Level Descriptions Risk Level Descriptions RISK LEVEL ASPECT ANTICIPATED SEISMIC PERFORMANCE I Building: Risk to Life: Systems: Occupancy Potentially no structural damage; immediately repairable, if any. Negligible non-structural damage; repairable. Negligible. Probably remain operational. Immediate, with only negligible disruption during clean-up. II Building: Risk to Life: Systems: Occupancy Negligible structural damage; repairable. Minor non-structural damage; repairable. Negligible. Minor disruptions for hours to days Minor disruptions during clean-up. III Building: Risk to Life: Systems: Occupancy Minor structural damage; repairable. Moderate non-structural damage; extensive repair. Minor. Disruption of systems for days to months. Within weeks, with minor disruptions. IV Building: Risk to Life: Systems: Occupancy Moderate structural damage; substantial repair. Substantial non-structural damage; extensive repair. Moderate. Disruption of systems for months to years. Partially to totally vacated during repairs. V Building: Risk to Life: Systems: Occupancy Substantial structural damage; repair may not be cost effective. Extensive non-structural damage; repair may not be cost effective. Substantial. Total disruption of systems; repair may not be cost effective. Totally vacated during repairs. VI Building: Risk to Life: Systems: Occupancy Extensive structural damage, collapse likely; repair probably not cost effective. Extensive non-structural damage; repair may not be cost effective. Extensive, but not imminent. Extrication protracted and difficult. Total disruption of systems; repair probably not cost effective. Totally vacated during repairs (if repairable). VII Building: Risk to Life: Systems: Occupancy: Unstable under existing vertical loads or earthquake. Imminent threat to occupants and / or adjacent property. Total disruption of systems; repair probably not cost effective. Should be vacated until structural upgrading is accomplished. Page 17 of 17