Project Seismic Vulnerability Assessment For SFCC District Building 33 Gough Street San Francisco, California 94103

Transcription

1 Project For SFCC District Building 33 Gough 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 January 28, 2015

2 TABLE OF CONTENTS 1.0 Executive Summary Introduction Site Observations Campus Site Map Reviewed Drawings Building Descriptions Findings Conclusions and Recommendations Limitations and Disclaimer... 6 APPENDICES Appendix A: Risk Acceptability Table and Risk Level Descriptions...A1

3 1.0 Executive Summary A limited seismic vulnerability assessment has been conducted on SFCC District 33 Gough Street Building in San Francisco, California. The purpose of the assessment was to identify seismic deficiencies based on Thornton Tomasetti s experience and knowledge with comparable construction, and to provide repair and retrofit measures to mitigate the seismic deficiencies. Additionally, the building has been assigned a Seismic Risk Level. From this evaluation, conclusions can be drawn regarding the feasibility of repair and retrofit measures for planned building occupancy. Based on our site observations and past experience with reinforced concrete buildings with wood diaphragms,the most common seismic deficiencies are: Inadequate or lack of wall anchorage, insufficient diagonal or straight sheathing, inadequate connections for chord and collector elements, inadequate length of shear walls at exterior walls with large openings, inadequate or missing crossties, and inadequate diaphragms. A 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. A Voluntary Seismic Upgrade (VSU) is recommended for buildings with a Seismic Risk Level of IV or greater. In Thornton Tomasetti s opinion, based on the visual evaluations, the 33 Gough Street Building has been assigned Seismic Risk Level V. Page 1 of 6

4 2.0 Introduction This report presents the findings of the limited structural conditions survey and seismic vulnerability assessment of SFCC District 33 Gough Street Building. The building is located at 33 Gough Street, San Francisco, California and is part of the San Francisco Community College District. The structural conditions survey and seismic vulnerability assessment included a visual evaluation of the building. It should be noted that no original structural drawings were available for the building, and thus only a visual evaluation has been included in the survey and assessment. Assumptions were made based on the vintage and type of construction to assist in the identification of seismic deficiencies for the building. Due to the lack of original drawings, a limited Tier 1 level evaluation according to the American Society of Civil Engineers document ASCE/SEI Seismic Evaluation of Existing Buildings was performed. It is Thornton Tomasetti s experience with concrete buildings that the most common seismic deficiencies are inadequate or lack of wall anchorage, insufficient diagonal or straight sheathing, inadequate connections for chord and collector elements, inadequate length of shear walls at exterior walls with large openings, inadequate or missing crossties, and inadequate diaphragms. 3.0 Site Observations Thornton Tomasetti visited the 33 Gough Street Building on January 27, The main purpose of the site visit was to verify the existing conditions, collect additional data, verify the vertical and lateral systems, and evaluate the physical condition of the structure. No finishes were removed during the site visit. The following items were confirmed and evaluated. Types and materials of construction Building use Types of ceiling Type of roofs, walls, and floors Type of finishes Visible cracks in superstructure, and slab-on-grade Visible water damage The building is shown on the Site Map included in Section 4.0. Findings of the observations and the corresponding seismic retrofit recommendations are discussed later in this report. Page 2 of 6

5 San Francisco Community College District 4.0 Building Site Map Figure 1: SFCC District 33 Gough Street Building Site Map 5.0 Reviewed Drawings There were no architectural or structural drawings available for review. Page 3 of 6

6 6.0 Building Description The following table provides descriptions of the assumed structural elements. No drawings were available. Assumptions have been made based on the observations made during the site walk and other buildings of similar vintage and type of construction. Foundation Basement Ground Floor Continuous footings, spread footings Slab-ongrade Reinforced concrete two-way slab Exterior Wall Reinforced concrete Roof Framing Diagonal sheathing over wood joists and wood trusses Vertical Load Resisting System Reinforced concrete bearing walls, steel and concrete columns Horizontal Load Resisting System Diagonal sheathing roof diaphragm, concrete shear walls and steel cantilever columns 7.0 Findings The following section provides the findings of the limited seismic vulnerability assessment of the SFCC District 33 Gough Street Building. Seismic deficiencies 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 limited qualitative seismic evaluation of the building and are based on age, type of structure, 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: Seismic deficiency expected to cause substantial structural damage and partial collapse, posing a major life-safety hazard. Priority 2: Seismic deficiency expected to cause substantial structural damage, posing a moderate life-safety hazard. Priority 3: Seismic deficiency expected to cause moderate structural damage. Priority 4: 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 Page 4 of 6

7 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. Seismic Deficiency or Deteriorated Condition Possible insufficient cantilever steel columns at high roof Missing insufficient concrete wall out of plane anchorage Roof diaphragm may be insufficient for current span Large exterior wall openings; possible insufficient concrete wall piers Possible insufficient collector beams and connections to shear walls Discontinuous or missing chord elements in low roof Seismic Priority Repair or Retrofit 1 Retrofit steel columns or provide bracing 1 Provide wall anchorage 2 Retrofit diaphragm or provide additional shear walls 2 Provide additional length of shear wall or retrofit shear walls 2 Retrofit collector beams and connections to shear walls 2 Retrofit chord elements Water damage at ground floor slab 3 Further inspect/repair cracks The Building has been assigned to Seismic Risk Level V 8.0 Conclusions and Recommendations This report represents the findings of the limited structural conditions survey and seismic vulnerability assessment of SFCC District 33 Gough Street Building. Given the vintage of the building, many elements of the construction do not meet the provisions of the current building code. However, in Thornton Tomasetti s opinion, based on the limited qualitative and/or visual evaluations, the buildings will not pose serious life safety hazards if the seismic deficiencies identified in Section 7.0 are corrected. Thornton Tomasetti recommends that the seismic deficiencies identified in Section 7.0 be addressed with quantitative analysis to define the required retrofit scope. It should be noted that: The seismic deficiencies indentified in this report are based on limited visual analysis. Deficiencies may be eliminated upon quantitative analysis. Items noted as damaged or decayed should be investigated further to determine the extent of the deterioration. Page 5 of 6

8 9.0 Limitations and Disclaimer This report includes a qualitative structural assessment of the existing building. Obvious gravity or seismic deficiencies that were identified visually during site visit are identified and documented in this report. Elements of the structure determined to be critical and which could pose life safety hazards are identified and documented. Users of this report are advised that deficiencies may exist in the structures that were not observed in this limited evaluation. Our services have consisted of providing professional opinions, conclusions, and recommendations based on generally accepted structural engineering principles and practices. Page 6 of 6

9 Appendix A: Risk Acceptability Table and Risk Level Descriptions 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 Page A1 January 28, 2015 Project No. U

10 Risk Level Descriptions RISK LEVEL I II III IV ASPECT ANTICIPATED SEISMIC PERFORMANCE 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. Negligible structural damage; repairable. Minor non-structural damage; repairable. Negligible. Minor disruptions for hours to days Minor disruptions during clean-up. Minor structural damage; repairable. Moderate non-structural damage; extensive repair. Minor. Disruption of systems for days to months. Within weeks, with minor disruptions. 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 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 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 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 A2 January 28, 2015 Project No. U