Simplified seismic assessment of detached, single-family, wood-frame dwellings

Transcription

1 Simplified seismic assessment of detached, single-family, wood-frame dwellings ATC 50 Applied Technology Council Funded by City of Los Angeles, Department of Building and Safety California Governor s Office of Emergency Services

2 ATC-50 Simplified Seismic Assessment of Detached, Single-Family, Wood-Frame Dwellings by APPLIED TECHNOLOGY COUNCIL 201 Redwood Shores Parkway, Suite 240 Redwood City, California Funded by CITY OF LOS ANGELES DEPARTMENT OF BUILDING AND SAFETY Nick Delli Quadri, Project Officer and CALIFORNIA GOVERNOR S OFFICE OF EMERGENCY SERVICES (Federal Emergency Management Agency Hazard Mitigation Grant) John Coil Edward F. Diekmann Susan Dowty 1 Laurence M. Kornfield Onder Kustu Prepared for ATC by BREIHOLZ QAZI ENGINEERING, INC. Lomita, California PROJECT MANAGER Christopher Rojahn PROJECT DIRECTOR Ronald T. Eguchi PROJECT ENGINEERING PANEL Timothy McCormick Doc X. Nghiem James Russell Robin Shepherd John G. Shipp 2 1 Steering Committee Representative 2 ATC Board Contact 2002, 2007 Second Printing

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4 Preface In September 1998 the Applied Technology Council (ATC) was awarded a contract by the City of Los Angeles, Department of Building and Safety, to develop and test a series of standardized procedures for the voluntary seismic evaluation and rehabilitation of detached single-family wood-frame dwellings (ATC-50 project). The project was prompted by high economic losses resulting from damage to single-family wood-frame dwellings during the 1994 Northridge earthquake. The concept for the project, however, had been conceived in the early 1990s by the Financial Services Subcommittee of the City of Los Angeles Mayor s Blue Ribbon Panel for Seismic Hazard Reduction, with input from the banking and insurance industries. The project contract called for ATC to: (1) develop and test a seismic performance grading system for detached single-family wood-frame dwellings that reflected expected economic losses in future damaging earthquakes; (2) develop and test seismic rehabilitation guidelines for detached single-family wood-frame dwellings that would enable a homeowner to improve the seismic grade; (3) prepare a set of examinations to certify that inspectors, contractors, and design professionals have been trained to implement the procedures developed under the project, and (4) plan and conduct a seminar to introduce the ATC-50 project products to home inspectors, contractors, and design professionals in the Los Angeles area. As part of the project, ATC was also asked to provide guidance on how to develop and implement a market-driven, incentives-based program to encourage homeowner use of the seismic assessment and seismic rehabilitation procedures developed under the project. This report, ATC-50: Simplified Seismic Assessment of Detached, Single-Family, Wood- Frame Dwellings, provides inspection procedures and a four-page Simplified Seismic Assessment Form to evaluate detached single-family woodframed dwellings and to assign to each a seismic performance grade. The procedure considers the potential for damage or collapse in a manner that is consistent and useful to owners, purchasers, insurers, lenders, contractors, design professionals, and regulatory officials. The report development effort included a pilot testing program whereby 500 dwellings in the City of Los Angeles were evaluated using a preliminary version of the Simplified Seismic Assessment Form. The purpose of the Pilot Seismic Assessment Testing Program was to evaluate the utility and ease-of-use of the Form, and results from the pilot program were used to update the Form. The companion ATC-50-1 report, Seismic Rehabilitation Guidelines for Detached, Single- Family, Wood-Frame Dwellings, contains prescriptive methods, simplified engineering methods, and fully engineered methods that, if implemented, allow a homeowner to improve the seismic performance grade. Another companion report, the ATC-50-2 report, Safer at Home in Earthquakes: A Proposed Earthquake Safety Program, provides recommendations for the development and implementation of a market-driven, incentives-based program to encourage homeowners to have their homes evaluated and rehabilitated, if necessary, using the procedures developed under the ATC-50 project. All three reports were initially completed in 2002 in time to be made available for an introductory training seminar in Los Angeles on the ATC- 50 project products. The second printing of this report in 2007 contains minor edits (e.g., corrected web addresses and phone numbers for various references) and Appendices D, E, and F, which were not available in Funding for the ATC-50 report and the Pilot Seismic Assessment Testing Program was provided by the City of Los Angeles and by a Federal Emergency Management Agency Hazards Mitigation Grant from the California Governor s Office of Emergency Services. The ATC-50 report was developed under the direction of Ronald T. Eguchi, who served as ATC-50 Project Director. Breiholz Qazi Engineering (BQE), Inc., a California-based engineering company, served as the ATC-50 Report Development Subcontractor. Shafat Qazi, and David Breiholz of BQE served as principals in charge, and Nels Roselund, Craig Taylor, and John Wiggins were technical consultants. The Pilot Seismic Assessment Testing Program was carried out by personnel from four engineering firms: ABS Consulting (formerly EQE International); BQE; Thornton-Tomasetti/Coil & Welsh, LZA; and ATC-50 Preface iii

5 Wiss Janney Elstner Associates. Overview and guidance were provided by the Project Engineering Panel, consisting of John Coil, Edward F. Diekmann, Susan Dowty (Steering Committee representative), Laurence M. Kornfield, Onder Kustu, Timothy McCormick, Doc X. Nghiem, James Russell, Robin Shepherd, and John G. Shipp (ATC Board representative). Nancy Sauer served as technical editor, and Gerald Brady, Peter Mork, Bernadette Mosby, and Michelle Schwartzbach were responsible for final editing and report production. The affiliations of these individuals and the personnel who led the Pilot Seismic Assessment Testing program are provided in the list of Project Participants. ATC gratefully acknowledges the patience, input and guidance provided by Nick Delli Quadri, who served as Project Officer for the Department of Building and Safety, City of Los Angeles. ATC also acknowledges the encouragement and assistance provided by other City of Los Angeles personnel, including Scott McGill (Department of Building and Safety) and Perry Singerman (Mayor s Office), who nurtured the project from the outset, and Thomas Grant and Ann Ormiston, who guided ATC through the Department of Building and Safety s contractual requirements. The support of Andrew Adelman, General Manager of the Department of Building and Safety, is also gratefully acknowledged. Special thanks also go to the members of the Steering Committee who provided overarching guidance for the ATC-50 project: Susan Dowty, (Chair), S.K. Ghosh Associates, Inc.; John Brown, California Real Estate Inspection Association; Dave Carey, Fannie Mae (Washington DC); Kenneth Cooley, State Farm Insurance; Karl Deppe (retired), Los Angeles Department of Building and Safety; Mike Edwards, California Department of Insurance; Mike Grottkau, California Earthquake Authority; Do Kim, Institute for Business and Home Safety; Eugene Lecomte (deceased), Insurance Industry Consultant; Richard McCarthy, California Seismic Safety Commission; Tim McCormick, City of Santa Monica; Duane McCutcheon, California Real Estate Inspection Association; Dave Nelson, International Conference of Building Officials; Jeff Paggi, Farmers Insurance; William Petak, University of Southern California; Patricia Schumate, Freddie Mac; Earl Schwartz, Structural Engineers Association of California; and Barbara Zeidman, Fannie Mae (Los Angeles). Christopher Rojahn (ATC-50 Project Manager) ATC Executive Director iv Preface ATC-50

6 Contents Preface... iii List of Figures... ix List of Tables... xiii 1. Introduction Background Purpose and Scope of the ATC-50 Report Intended Users of the Report Relationship to Other Documents Contents and Organization of the Report Simplified Seismic Assessment Form Overview of the Form Applicability Major Sections of the Form Sections A through E: Assessment of the Dwelling Structure and Site Section F: Identification of Regional Seismic Hazards Section G: Determination of the Seismic Performance Grade Section H: Improving the Seismic Performance Grade Disclaimer Scoring Approach for the Structural System The Ten Major Determinants of Seismic Performance Development of Scoring Procedure Penalty vs. Reward Development of Weighting Procedure Output of Evaluation Seismic Performance Grade Definitions (A Grade A (including A+, A, (B Grade B (including B+, B, (C Grade C (including C+, C, (D Grade D (including D+, D, 2.5 Limitations of Simplified Assessment Assessment of Structural Elements Assessment of Foundation System (Section A) Footings and Reinforcement Slab on Grade vs. Raised Floor Sloping Lot Assessment of Superstructure Framing and Configuration (Section B) Spilt Level and Other Irregularities Exterior Walls Interior Walls Soft Stories Hillside Conditions Cripple Walls General Condition Assessment (Section C) Construction Quality Rehabilitated Structural Elements ATC-50 Contents v

7 4. Assessment of Nonstructural Elements, Dwelling Age and Size Chimneys Water Heater Gas Shutoff Exterior Veneer Age of Dwelling Floor Plan Area Local Site Conditions Slope Conditions Soil Instability Drainage Regional Seismic Hazards Use of Zip Codes Ground Shaking Intensity Surface Fault Rupture Liquefaction Susceptibility Tsunami Run-Up Total Regional Seismic Hazard Points Hazards Not Considered Seismic Performance Grade Determination of Structural Score Determination of Seismic Hazard Score Determination of Seismic Performance Grade Improving the Seismic Performance Grade Effect of Seismic Rehabilitation on the Structural Score Effect of Rehabilitation on the Seismic Performance Grade Implementing ATC Role of Simplified Assessment in Overall Seismic Loss Reduction Incentives for Voluntary Seismic Assessment and Rehabilitation Qualifications of Inspectors Field Equipment Needed Future Development of the Methodology Updating to Incorporate New Seismic Hazard Data and Dwelling Seismic Performance Data Enhancing Utilization through New Technologies, e.g., Hand-Held Computers Development of Commentary Application to Other Regions of the United States Application to Other Wood-Frame Buildings Appendix A: Simplified Seismic Assessment Form Appendix B: Pilot Test of Simplified Seismic Assessment Form B.1 Purpose of Pilot Test Program B.2 Approach for Selecting 500 Evaluations B.3 Results from Initial 100 Evaluations B.4 Results from the Second Set of Evaluations (400 Total) Appendix C: Example Evaluations C.1 Hillside Home C.2 Seismically Rehabilitated Dwelling vi Contents ATC-50

8 C.3 Home with Unbraced Cripple Wall C.4 Slab-on-Grade Dwelling C.5 Split-Level Home Appendix D: Seismic Performance of Wood-Frame Structures D.1 Introduction D.2 Characteristics and Seismic Vulnerabilities of Wood-Frame Dwellings D.2.1 Cripple-Wall Houses D.2.2 Slab-on-Grade Houses D.2.3 Basement Houses (Perimeter Foundation with No Cripple Wall) D.2.4 Perimeter Post-and-Pier Foundation Houses D.2.5 Split-Level Houses, Multi-Level Hillside Houses D.3 Studies of the Performance of Wood-Frame Dwellings in Recent California Earthquakes D San Fernando Earthquake D Loma Prieta Earthquake D Northridge Earthquake D.4 Comparative Analysis and Interpretation of Data from the 1971 San Fernando and 1994 Northridge Earthquakes D.4.1 Effects of Number of Stories D.4.2 Effects of Age of Construction D.4.3 Loss Ratio as a Function of MMI, Year Built and Building Square Footage D.4.4 Summary of Findings: Factors that Significantly Affect Damageability D.4.5 Recommendations for Improved Collection of Dwelling Earthquake Performance Data Appendix E: Derivation of Expected Shaking Intensity E.1 Data Sources E.2 Use of Dynamic Amplification Factors (DAF) E.3 Conversion of Surface Ground Acceleration to MMI E.4 Results Appendix F: Basis of Structural Score Methodology F.1 Categorization F.2 Application of Past Earthquake Data F.3 Initial Weighting Assessments F.4 Final Weighting Assignments F.5 Incorporation of Regional Seismic Influences in Scoring References and Bibliography Project Participants Applied Technology Council Projects and Report Information Applied Technology Council Directors ATC-50 Contents vii

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10 List of Figures Figure 2-1 Page 1 of Simplified Seismic Assessment Form... 3 Figure 2-2 Page 2 of Simplified Seismic Assessment Form... 4 Figure 2-3 Page 3 of Simplified Seismic Assessment Form... 5 Figure 2-4 Page 4 of Simplified Seismic Assessment Form... 6 Figure 3-1 Section A of the Simplified Seismic Assessment Form... 9 Figure 3-2 Section B of the Simplified Seismic Assessment Form Figure 3-3 Section C of the Simplified Seismic Assessment Form Figure 4-1 Section D of the Simplified Seismic Assessment Form Figure 5-1 Section E of the Simplified Seismic Assessment Form Figure 6-1 Section F of the Simplified Seismic Assessment Form Figure 6-2 Active and potentially active faults in the City of Los Angeles overlaid onto zip code boundaries Figure 6-3 Liquefaction-prone areas in the City of Los Angeles Figure 6-4 Zip codes affected by potential liquefaction hazards Figure 6-5 Run-up potential for tsunamis with 100-year and 500-year return periods (Ziony, 1985) Figure 7-1 Section G of the Simplified Seismic Assessment Form Figure 8-1 Section H of the Simplified Seismic Assessment Form Figure 9-1 A sample two-column format for comments on Question A Figure B-1 Homeowners who applied for the ATC-50 Seismic Evaluation Program (over 2000 applications) Figure B-2 Initial 105 evaluations shared by the subcontractor who developed the form and the subcontractor who developed the seismic rehabilitation guidelines Figure B-3 All evaluations completed under the ATC-50 Pilot Test Program (517, in total) Figure B-4 Distribution of final scores in the first 105 evaluations Figure B-5 Distribution of grades in the first 105 evaluations Figure B-6 Distribution of final scores in the 204 evaluations by the first subcontractor, part of the second set of evaluations (400+, total) ATC-50 List of Figures ix

11 Figure B-7 Figure B-8 Figure B-9 Figure C-1 Distribution of grades in the 204 evaluations by the first subcontractor, part of the second set of evaluations (400+, total) Distribution of final scores in the 208 evaluations by the second subcontractor, part of the second set of evaluations (400+, total) Distribution of grades in the 208 evaluations by the second subcontractor, part of the second set of evaluations (400+, total) The house shown in this picture earned a grade of only C-, largely because of the hillside conditions and irregularities in the superstructure Figure C-2 Form # 1558 Page Figure C-3 Form # 1558 Page Figure C-4 Form # 1558 Page Figure C-5 Form # 1558 Page Figure C-6 The house shown in this picture was built before 1977, but it has been seismically rehabilitated and received a grade of B Figure C-7 Form # 1343 Page Figure C-8 Form # 1343 Page Figure C-9 Form # 1343 Page Figure C-10 Form # 1343 Page Figure C-11 The home shown in this picture is supported on unbraced and unbolted cripple walls and insufficient length of wall between openings, resulting in a grade of D Figure C-12 Form # 1913 Page Figure C-13 Form # 1913 Page Figure C-14 Form # 1913 Page Figure C-15 Form # 1913 Page Figure C-16 This newer home with slab-on-grade floor construction received an A- grade Figure C-17 Form # 1406 Page Figure C-18 Form # 1406 Page Figure C-19 Form # 1406 Page Figure C-20 Form # 1406 Page Figure C-21 This older split-level home received a structural score of 77.5 and a B grade Figure C-22 Form # 1918 Page x List of Figures ATC-50

12 Figure C-23 Form # 1918 Page Figure C-24 Form # 1918 Page Figure C-25 Form # 1918 Page Figure D-1 Drawing of wood stud frame construction Figure D-2 Typical one-story cripple-wall crawl-space house Figure D-3 Typical older one and one-half-story cripple-wall crawl-space house Figure D-4 Section through typical cripple-wall crawl-space house Figure D-5 Typical earthquake damage to a cripple-wall house Figure D-6 Typical one-story slab-on-grade house Figure D-7 Section through typical one-story slab-on-grade house Figure D-8 Cracking damage to stucco first-floor walls of a two-story slab-on-grade house Figure D-9 Damage and racking displacement in the first floor walls of a two-story slab-on-grade house Figure D-10 Section through a typical one-story basement house Figure D-11 Figure D-12 Section through a variation of the basement house where the perimeter foundation walls enclose a crawl space Damage to a basement-type house where the sill plate slid on the top of the foundation wall Figure D-13 Post-and-pier foundation house on sloping site Figure D-14 Post-and-pier house on flat site Figure D-15 Earthquake-damaged perimeter post-and-pier foundation Figure D-16 Split-level house, where the section of flooring above the garage is at a lower level than the main second-floor level Figure D-17 Split-level house, with damage at the garage-level walls at two-story portion ATC-50 List of Figures xi

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14 List of Tables Table 6-1 Table 1 of the Simplified Seismic Assessment Form Table 6-2 Table 2 of the Simplified Seismic Assessment Form Table 6-3 Table 3 of the Simplified Seismic Assessment Form Table 7-1 Table 4 of the Simplified Seismic Assessment Form Table D-1 Contribution of Specific Building Characteristics to Damage Table D-2 Loss Ratios for Dwellings Damaged during the 1971 San Fernando Earthquake Table D-3 Number of Buildings in Damage Ranges Table D-4 Table D-5 Table D-6 Table D-7 Mean Loss Ratio for Dwellings in 1994 Northridge Earthquake as Function of MMI and Year Built Mean Loss Ratio for Dwellings in 1994 Northridge Earthquake as Function of MMI and Year Built (Dwellings under 1600 sq ft) Mean Loss Ratio for Dwellings in 1994 Northridge Earthquake as Function of MMI and Year Built (Dwellings between 1600 and 2500 sq ft) Mean Loss Ratio for Dwellings in 1994 Northridge Earthquake as Function of MMI and Year Built (Dwellings over 2500 sq ft) Table E-1 Scenario Earthquakes Considered Table E-2 Modified Mercalli Intensity Scale Table E-3 Seismicity and Damage Ratio by Zip Code for City of Los Angeles Table F-1 Initial Weights Assigned to Assessment Categories ATC-50 List of Tables xiii

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