NO. AB-098 : | |
DATE : | July 2, 2012 (Updated 01/01/2023 for code references) |
SUBJECT : | Permit Review and Operations |
TITLE : | Post-Earthquake Repair and Retrofit Requirements for Wood-Frame Residential Buildings with Three or More Dwelling Units |
PURPOSE : | The purpose of this Bulletin is to establish policy for interpreting the San Francisco Building Code regarding post-earthquake damage retrofit triggers for wood-frame residential buildings with three or more dwelling units and to detail the scope and criteria for such triggered retrofits. |
REFERENCES : | Current edition of the San Francisco Existing Building Code - Section 202, Definition of Disproportionate Earthquake Damage - Section 202, Definition of Substantial Structural Damage - Section 304.4 Minimum Lateral Force for Existing Buildings - Chapter 4, Repairs Current edition of the California Historical Building Code, CCR Title Part 8 Current edition of the California Existing Building Code, Appendix A- Chapter A4, or Current edition of the International Existing Building Code, Appendix A- Chapter A4 with NCSEA/SEAOC amendments ASCE/SEI Standard 31-03, Seismic Evaluation of Existing Buildings ASCE/SEI Standard 41-06, Seismic Rehabilitation of Existing Buildings, with Supplement 1 California Health and Safety Code, Section 17920.3 CAPSS Report, Here Today—Here Tomorrow: The Road to Earthquake Resilience in San Francisco, Post-Earthquake Repair and Retrofit Requirements (ATC-52-4 Report), http://www.sfcapss.org/PDFs/PostQuakeRepair.pdf |
DISCUSSION : | San Francisco Existing Building Code Section 405.2.3.1 triggers seismic evaluation, and possibly retrofit of buildings, when earthquake-related damage reaches the level of “substantial structural damage to vertical elements of the lateral-force-resisting system.” Substantial structural damage is defined in Section 202 as, in essence, a loss of lateral capacity of 20 percent or more in any horizontal direction. The code does not give specific rules for identifying a 20-percent capacity loss nor guidance as to how to calculate capacity loss, so implementation of these code provisions relies on interpretation by the Department of Building Inspection. This Bulletin presents the Department’s interpretation of 20-percent lateral capacity loss in terms based on visual indicators of such damage, and details the scope of required retrofit for buildings that exhibit earthquake-induced substantial structural damage. |
In addition to substantial structural damage, San Francisco Existing Building Code Section 405.2 triggers structural evaluation and possibly retrofit when earthquake-related damage reaches the level of disproportionate damage, which is defined in Section 202 as, in essence, a lateral capacity loss of 10 percent or more in an earthquake of limited intensity. This Bulletin presents the Department’s interpretation of a 10-percent capacity loss based on visual indicators of such damage and provides evaluation and retrofit scope for buildings with such earthquake induced disproportionate damage.
Residential buildings that incur substantial structural damage or disproportionate damage as detailed in this Bulletin are considered to be “substandard” per California Health and Safety Code Section 17920.3 (b) Structural hazards and (o) Inadequate structural resistance to horizontal forces.
APPLICABILITY
A building is eligible to apply the interpretations and provisions of this Bulletin if all of the following criteria are met:
A. The building has at least one story in which the seismic force-resisting system is a wood light-frame system in at least one direction, and
B. The building has only wood floor and wood roof diaphragms, and
C. The building has a continuous foundation, and
D. The building contains a residential occupancy group R-1, R-2, R-3.1, or R-4 as defined in San Francisco Building Code Section 310. At the discretion of the Department of Inspection, a building in occupancy group R-3 with one or two residential units may be deemed eligible if it is structurally and architecturally similar to the typical residential buildings with three or more units addressed in this Bulletin.
Buildings of other construction types and occupancies may also apply the provisions of this Bulletin on a case-by-case basis when approved by the Department of Building Inspection. Other methods of determining capacity loss based on analysis, testing, or other objective data may be allowed at the discretion of the Department.
Qualified historic buildings are permitted to be evaluated or retrofitted using the provisions in the California Historical Building Code, provided that such standards do not result in seismic performance less than the evaluation and retrofit engineering criteria detailed in this Bulletin.
EVALUATION PROCEDURES
For the purpose of determining if a building has incurred substantial structural or disproportionate damage, visual observation and classification of damage patterns may be used in lieu of a calculation of percentage loss of capacity. All determinations of substantial structural or disproportionate damage, including visual observation and classification of damage, shall be made by a licensed design professional and shall be submitted in accordance with San Francisco Existing Building Code Section 405.2.3.1. For damage not deemed to be either substantial structural damage or disproportionate damage, repairs shall restore the building to its permitted pre-earthquake condition by methods acceptable to the Department.
Buildings with Substantial Structural Damage
Substantial structural damage to vertical elements of the lateral force-resisting system shall be deemed to exist when a “triggering damage pattern” is observed in any system or components listed in Table 1. Table 1 also includes earthquake-induced substantial structural damage indicators for gravity load-carrying components. These are defined in San Francisco Existing Building Code, Section 202 as “any component, or any group of such components, that supports more than 30 percent of the total area of the structure’s floor(s) and roof(s),” and the remaining capacity of any damaged components, “with respect to all dead and live loads, is less than 75 percent of that required by this code for new buildings of similar structure, purpose and location.” Per San Francisco Existing Building Code, Section 502.2, the provisions of this Bulletin apply to substantial structural damage to gravity load-carrying components only when that damage has been caused by earthquake.
Buildings with Disproportionate Damage
Disproportionate damage to vertical elements of the lateral force-resisting system shall be deemed to exist when any of the earthquake “triggering damage patterns” is observed in any system or component listed in Table 1. Table 1 also includes disproportionate damage indicators for gravity load-carrying systems, which include any component, or any group of components, that supports more than 10 percent of the total area of a structure’s floor(s) and roof(s), and in which the remaining capacity of any damaged components, with respect to all dead and live loads, is less than 75 percent of that required by this code for new buildings of similar structure, purpose and location. The provisions of this Bulletin apply to disproportionate damage to gravity load-carrying components only when such damage has been caused by earthquake, as SFEBC Section 405.2.2 notes that a building with disproportionate damage is subject to the provisions and requirements for substantial structural damage.
Table 1: Substantial Structural Damage and Disproportionate Damage Patterns for Wood Frame Residential Buildings with Three or More Dwelling Units | ||
Damage Pattern | Triggering Damage | |
Substantial Structural Damage | Disproportionate Damage | |
Wood-frame shear panels (wall segments or piers) and sheathing.
When any of the following damage patterns is observed: • Stucco or plaster loose at more than one wall stud, or • Nail pull-through at wood or gypsum board sheathing at more than one wall stud, or • Visible permanent in-plane racking, or • Diagonal shear cracking across half or more of a plaster or stucco panel, or • Horizontal flexural cracking across half or more of a plaster or stucco panel, or • Loss of nailing connection from sheathing to top plate or sole plate, or • Any other indicators of sheathing delamination or panel mechanism. | In any story, in any direction, where the sum of the length of all wall segments and piers with any of the listed damage patterns is 20 percent or more of the total length of wall segments and piers in that story and direction. Only wall segments or piers contributing significant strength or stiffness to each wall line of the pre-damaged structure shall be considered. Large openings do not count toward the summation or total length of wall segments and piers. | In any story, along any wall line, where the sum of the length of wall segments and piers with any of the listed damage patterns is 20 percent or more of the total length of wall segments and piers along that wall line in that story. Only wall segments or piers contributing significant strength or stiffness to each wall line of the pre-damaged structure shall be considered. Large openings do not count toward the summation or total length of wall segments and piers. |
Connections and load path elements.
When any of the following damage patterns is observed: • Hold-down pullout or stud fracture at hold-down, or • Sliding of sole plate at floor line, or • Sliding of sill plate at top of footing, or • Any other indicators of lateral load path failure. | Any | |
Connections and load path elements
When any of the following damage patterns is observed: • Failure of diaphragm-to-wall connections at rim joist or blocking, or • Collector or chord failure. | At any floor level, where the damage affects the load path to more than one pier or wall segment, or affects the load path to the only pier or wall segment along a wall line. | |
Damage Pattern | Triggering Damage | |
Substantial Structural Damage | Disproportionate Damage | |
Gravity load-carrying members, connections, and load path elements.
When any of the following damage patterns is observed: • Floor framing-to-column/wall shear connection damage, or • Loss or substantial reduction of seat bearing, or • Crushing, fracture, or shortening of posts, wall studs, or similar components, or • Column, post, or pier damage due to deformation incompatibility, or • Subsidence or differential settlement of foundation, or • Any other indicators of member failure, load path failure, or loss of bearing capacity. | In gravity load-carrying components defined in San Francisco Building Code, as supporting, as a group, “more than 30 percent of the total area of the structure’s floor(s) and roof(s),” and the remaining capacity of any damaged components, “with respect to all dead and live loads, is less than 75 percent of that required by this code for new buildings of similar structure, purpose and location.” | In gravity load-carrying components supporting, as a group, more than 10 percent of the total area of the structure’s floor(s) and roof(s), and the remaining capacity of any damaged components, with respect to all dead and live loads, is less than 75 percent of that required by this code for new buildings of similar structure, purpose and location. |
Permanent lateral deformation indicating increased P-delta instability.
|
When permanent story drift of 2 percent or more is observed in any story. | • When permanent story drift of 1 percent or more is observed in any story, or • A pattern of jammed doors or windows repairable only by structural repair and not by minor adjustments or rehanging doors or windows. |
Damage indicating torsional instability.
|
• When there is significant stiffness loss on more than one perimeter wall line, or • Significant strength loss not balanced between wall lines on opposite sides of the building. | • When there is significant stiffness loss on more than one perimeter wall line, or • Significant strength loss not balanced between wall lines on opposite sides of the building, or • Any visually noticeable permanent torsional deformation. |
Further Evaluation and Retrofit Scope for Buildings with Substantial Structural Damage
For buildings with substantial structural damage, further evaluation and retrofit shall proceed in accordance with San Francisco Existing Building Code Section 405.2.3.1 subject to the following guidelines:
A. Evaluation and retrofit shall consider the entire structure, including all stories and all directions, regardless of where in the structure the triggering damage occurred, except that for certain building types and damage patterns, partial retrofit may be permitted under one of the alternative criteria documents given in the “Evaluation and Retrofit Engineering Criteria” section of this Bulletin.
B. Gravity load-carrying components need not be considered for evaluation and retrofit except when they are also part of the building’s seismic force-resisting system or are subject to San Francisco Existing Building Code Section 502.2.
C. Nonstructural components need not be considered for evaluation or retrofit unless subject to separate building code requirements, ordinances, or regulations.
D. Load combinations that include wind or earthquake effects shall be considered.
Further Evaluation and Retrofit Scope for Buildings with Disproportionate Damage
For buildings with disproportionate damage, further evaluation and retrofit shall proceed in accordance with San Francisco Existing Building Code Section 405.2.2 subject to the following guidelines:
A. Evaluation and retrofit shall consider the entire structure, including all stories and all directions, regardless of where in the structure the triggering damage occurred, except that for certain building types and damage patterns, partial retrofit may be permitted under one of the alternative criteria documents given in the “Evaluation and Retrofit Engineering Criteria” section of this Bulletin.
Exception: When the disproportionate damage is limited to connections or load path elements and does not affect wood-frame wall segments, piers, or other vertical elements of the lateral force-resisting system, the evaluation and retrofit scope may be limited to components throughout the building in directions similar to or performing the same function as the components with damage.
B. Gravity load-carrying components need not be considered for evaluation and retrofit except when they are also part of the building’s seismic force-resisting system or are subjected to San Francisco Existing Building Code Section 502.2.
C. Nonstructural components need not be considered for evaluation and retrofit unless subject to separate building code requirements, ordinances, or regulations.
D. Load combinations that include earthquake effects shall be considered. Load combinations that include wind effects may be ignored.
EVALUATION AND RETROFIT ENGINEERING CRITERIA
If, after evaluation, the pre-earthquake building is determined to satisfy the criteria of San Francisco Existing Building Code, Section 405.2.3, then the building need not be retrofitted, but shall be restored to its pre-earthquake capacity. When retrofit is triggered by earthquake damage at any level, the engineering criteria for retrofit shall be permitted to use earthquake loads that are 75 percent of those prescribed by the San Francisco Building Code for new construction, in accordance with SFEBC Section 405.2.3.
Alternatively, any of the following codes, standards, or guidelines may be used as alternative evaluation or retrofit criteria for qualifying buildings:
A. Meets ASCE 31-03 for the Life Safety Performance Level, or
B. Meets ASCE 41-06 for the Life Safety Performance Level (S-3) in a BSE-1 earthquake hazard level, or
C. If the triggering damage involves only a soft, weak, or open front first story, meets CEBC Appendix A - Chapter A4 or IEBC Appendix A - Chapter A4 with NCSEA/SEAOC amendments, or
D. Meets San Francisco Existing Building Code Section 304.4.
Originally Signed by:
Tom C. Hui, S.E. July 2, 2012
Acting Director
Department of Building Inspection
Approved by the Building Inspection Commission on June 20, 2012.