AB-100 Post-Earthquake Repair and Retrofit Requirements for One- and Two-Family Units

 

NO. AB-100   :
DATE   :	July 12, 2012 (Updated 01/01/2023 for code references)
SUBJECT   :	Permit Review and Operation
TITLE   :	Post-Earthquake Repair and Retrofit Requirements for One- and Two-Family 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 one- and two-family dwellings of wood-frame construction and to detail the scope and criteria for such triggered retrofits. The Bulletin also provides guidance on the scope of required building repair if retrofits are not triggered.
REFERENCES   :	Current edition of the San Francisco Existing Building Code    - Section 304.4, Minimum Lateral force for existing buildings    - Section 202, Definition of Disproportionate Earthquake Damage    - Section 202, Definition of Substantial Structural Damage    - Chapter 4, Repairs Current edition of the California Historical Building Code, C.C.R. Title Part 8 Current edition of the California Existing Building Code, Appendix A - Chapter A4, or the current edition of the International Existing Building Code, Appendix A - Chapter A4 with NCSEA/SEAOC amendments ASCE/SEI Standard 31, Seismic Evaluation of Existing Buildings ASCE/SEI Standard 41, 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), https://sfgov.org/sfc/sites/default/files/ESIP/FileCenter/Documents/9761- atc524_compressed.pdf CUREE EDA-2: General Guidelines for the Assessment and Repair of Earthquake Damage in Residential Woodframe Buildings FEMA 306: Evaluation of Earthquake Damaged Concrete and Masonry Wall Buildings: Basic Procedures Manual
DISCUSSION   :	San Francisco Existing Building Code, Section 405.2.3 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 San Francisco Existing Building Code, 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 a 20-percent lateral capacity loss based on visual indicators of such damage, and details the evaluation procedure and retrofit scope for buildings that exhibit earthquake-induced substantial structural damage. The Bulletin also provides guidance on the scope of required repair of building components or assemblies if such retrofits are not triggered.

 

In addition to substantial structural damage, San Francisco Existing Building Code, Section 405.2.2 triggers structural evaluation and possibly retrofit when earthquake-related damage reaches the level of disproportionate damage, which is defined in San Francisco Existing Building Code, 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 of Building Inspection’s interpretation of a 10 percent capacity loss based on visual indicators of such damage and provides evaluation procedures 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 includes at least one story in which the seismic force-resisting system consists of a wood light-frame system in at least one direction, and

   B.   The building has only wood floor and roof diaphragms, and

   C.   The building contains a residential occupancy group R-3 as defined in San Francisco Building Code, Section 310. At the discretion of the Department of Building Inspection, a building in this group may be evaluated and repaired or retrofitted using the criteria for a residential building with three or more units under AB-098 if the building is structurally and architecturally similar to that group of buildings.

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 also be allowed at the discretion of the Department.

Qualified buildings may be permitted to be evaluated or retrofitted using the provisions in the California Historical Building Code provided that such provisions do not result in seismic performance that is less than the evaluation and retrofit engineering provisions detailed in this Bulletin.

EVALUATION PROCEDURES AND RETROFIT SCOPE

For the purpose of determining if a building has incurred substantial structural damage or disproportionate damage per San Francisco Building Code, visual observation and classification of damage and severity may be used in lieu of a calculation of percentage loss of capacity. All determinations of substantial structural damage or disproportionate damage, including visual observation and classification of damage and severity, shall be made by a licensed design professional, and evaluation 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 original strength or condition by methods acceptable to the Department of Building Inspection.

Buildings with Substantial Structural Damage

Earthquake-induced substantial structural damage to elements of lateral force-resisting system of a building shall be deemed to exist when any of the components and conditions is observed to reach the severity of “triggering damage” given in Table 1. For buildings with such substantial structural damage, evaluation and retrofit, where required, shall proceed in accordance with the “Action Required” column shown in Table 1 and the “Further Evaluation and Retrofit Engineering Criteria” section.

Buildings with Disproportionate Damage

Disproportionate damage to elements of the lateral force-resisting system of a building shall be deemed to exist when any of the components and conditions is observed to reach the severity of “triggering damage” given in Table 1. For buildings with such disproportionate damage, evaluation and retrofit, where required, shall proceed in accordance with the “Action Required” column shown in Table 1 and the “Further Evaluation and Retrofit Engineering Criteria” section.

Table 1: Substantial and Disproportionate Damage Triggers for Repair and Retrofit of One and Two-Family Dwellings
Components and Damage Condition	Triggering Damage		Action Required
	Substantial Structural Damage	Disproportionate Damage
Stone or masonry veneer, incidental URM wall (non-chimney)	•   Appearance similar to “Heavy Damage” as described in Section 7.5 of FEMA 306, or	•   Appearance similar to “Moderate Damage” as described in Section 7.5 of FEMA 306, or	Remove and replace damaged elements.
	•   Failure of anchorage to backing in over 20% of the wall area	•   Visible failure of anchorage to backing anywhere
•   URM foundation piers	•   “Moderate Damage” as described in Section 7.5 of FEMA 306, or		Retrofit crawl space or under-floor area.
•   Continuous footings with crawl space or under-floor area	•   Visible relative movement of supported joist or beams on support of 1" or more, or
	•   Permanent movement that results in inadequate bearing of supported member
Cracks in continuous footings without visible related soil failure or movement	Crack width of less than 0.25"		No retrofit required. Repair to original strength in accordance with Section 4A.3 of CUREE EDA-2.
	Crack width or offset of greater than 0.25"		No retrofit required. Obtain design professional guidance for repair.
Cracks in continuous footings with visible related soil failure or movement	Cracks and visible related soil failure or movement		•   Obtain design professional guidance for mitigation of soil movement and repair of footing, and
			•   Mitigate any soil issues as recommended by design professional.
•   Post-and-beam crawl space or under-floor area	•   Permanent lateral displacement of 2" anywhere, or	•   Permanent lateral displacement of 1" anywhere, or	Retrofit crawl space or under-floor area in accordance with IEBC Chapter A3.
•   Cripple wall with stud height not exceeding 4 feet	•   Visible relative movement of 1" or more between supported joists or beams and their supports, or	•   Visible relative movement for more than 50% of the supported joists or beams and their supports
	•   Permanent movement that results in inadequate bearing of over 50% of the supported members
Anchorage of floor/wall framing to foundations	Permanent movement of 1" anywhere		Retrofit crawl space or under-floor area in accordance with IEBC Chapter A3.
•   Hillside structure where height of supports from foundation to the point of bearing for the floor assembly above exceeds 4 feet	•   Permanent lateral displacement of 2" or 2% drift, whichever is greater, at downhill cripple wall stud in any direction, or	•   Permanent lateral displacement of 1" or 1% drift, whichever is greater, at downhill cripple wall stud in any direction, or	Retrofit from the foundation level to a level above with a full-plate diaphragm, specifically addressing the torsion created by walls of varying height, supports, or other causes.
•   Cripple wall with stud height exceeding 4 feet	•   Failure of connections in downhill supports if post-and-beam braced frame or moment frame, or	•   Signs of movement that could lead to failure of the downhill supports, or
	•   Separation of uphill framing from foundation support or indication of relative movement during shaking of 1" or more in the direction parallel to the slope	•   Visible relative movement of the uphill support in the direction parallel to the slope
Weak Story: when any story has less than 80% of the strength of the story above in either direction	•   Permanent lateral displacement of 2" or more, or		Retrofit soft story and any support system below.
	•   Indication of any lateral movement in story of 4" or more during shaking in any direction
Stories other than weak stories	•   Permanent lateral displacement of 2" or more anywhere in any direction, or		•   Retrofit from damaged story down to the foundation, and
	•   Permanent lateral displacement of 1" anywhere if torsional displacement is observed, or		•   Repair walls not part of the designated lateral force-resisting system in accordance with Section 5.8 of CUREE EDA-2.
	•   Indications of excessive response such as severe cracking of brittle walls nail fracture or pullout in wood, multiple jammed doors, and/or broken windows
Connection between two parts of a structure including wings, split levels, porches, and beam to post connections	•   Permanent separation or sliding at joint of 1" or more, or		Provide structural separation with independent gravity support for each structure or a seismic tie that will transfer 20% of the weight of the lighter portion across the joint.
	•   Permanent movement that results in inadequate bearing of a supported member
Unreinforced masonry chimneys	Damage patterns described in Chapter 7 of CUREE EDA-2 that require replacement of any chimney bricks or flue tiles or substantial extent of mortar	Earthquake caused horizontal cracking at roof line or at the top of fire box	Minimum retrofit/replacement according to Appendix 7A of Chapter 7 of CUREE EDA-2.
Any chimney	•   Earthquake induced separation of chimney from the surrounding or adjacent wood framing, or   •   Clear movement from a hand pushed “rock test” as described in Section 7.7.3 of CUREE EDA-2.		For Substantial Structural Damage: Repair/replace attic ties if present. If no tie to wood framing is evident, provide new engineered tie or replace chimney according to Appendix 7A of CUREE EDA-2
			For Disproportionate Damage: Repair/replace chimney according to Appendix 7A of CUREE EDA-2.
Ceiling plaster	Falling or delaminated ceiling plaster greater than 10% of area within any room.		Determine extent of delamination or deteriorated plaster and replace.
Ceiling material	Cracks in ceiling material indicating permanent movement or local crushing of ceiling material at crack.		If cracks are caused by movement of joists at their supports, provide tie across area of slippage. Otherwise, repair.
Roof tiles	Damage to anchorage of roof tiles, unanchored or slipped tiles.		Determine extent of missing or deteriorated anchorage and replace damaged tiles.

 

FURTHER EVALUATION AND RETROFIT ENGINEERING CRITERIA:

If, after an evaluation per San Francisco Existing Building Code, Section 405.2.3, the pre-earthquake building is determined to satisfy the criteria, 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 San Francisco Existing Building Code, 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 the requirements of ASCE 31-03 for the Life Safety Performance Level, or

   B.   Meets the requirements of ASCE 41-06 for the Life Safety Performance Level (S-3) in a BSE-1 earthquake hazard level, or

   C.   Meets the requirements of CEBC Appendix A - Chapter A4 or IEBC Appendix A - Chapter A4 with NCSEA/SEAOC amendments, or

   D.   Meets the San Francisco Existing Building Code, Sections 315, 320.

Originally Signed:

Tom C. Hui, S.E.         July 2, 2012

Acting Director

Department of Building Inspection

Approved by the Building Inspection Commission on 6/20/2012