AB-099 Post-Earthquake Repair and Retrofit Requirements for Concrete Buildings

 

NO. AB-099   :
DATE   :	July 2, 2012 (Updated 01/01/2023 for code references)
SUBJECT   :	Permit Review and Operations
TITLE   :	Post-Earthquake Repair and Retrofit Requirements for Concrete Buildings
PURPOSE   :	The purpose of this Bulletin is to establish policy for interpreting the San Francisco Building Code regarding post-earthquake damage retrofit triggers for concrete buildings constructed before May 21, 1973 and to detail the scope and criteria for such triggered retrofits and other repairs.
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 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 FEMA 273: NHERP Guidelines for the Seismic Rehabilitation of Buildings FEMA 306: Evaluation of Earthquake Damaged Concrete and Masonry Wall Buildings: Basic Procedures Manual FEMA 308: The Repair of Earthquake Damaged Concrete and Masonry Wall Buildings FEMA 356: Prestandard and Commentary for the Seismic Rehabilitation of Buildings (FEMA, 2000)
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 Section 202 as, in essence, a loss of lateral capacity of 20 percent or more in any horizontal direction. The code gives no specific rules for identifying a 20-percent loss or 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 for concrete buildings constructed before May 21, 1973.

In addition to substantial structural damage, San Francisco Existing Building Code, Section 405.2 triggers seismic evaluation, and possibly retrofit, when earthquake-related damage reaches the level of disproportionate damage, 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 for concrete buildings constructed before May 21, 1973.

For concrete shear wall and infill buildings, the evaluation procedures developed in FEMA 306 and the simplified version of the methodology in FEMA 308 are used determine whether a building with substantial structural damage or disproportionate damage needs to be restored to its pre-earthquake capacity or retrofitted. Substantial structural damage or disproportionate damage may also be deemed to exist when damage to specific building components or conditions reaches the severity of “earthquake triggering damage” based on visual observation and classification.

For concrete moment-frame buildings, repair and retrofit requirements are based only on visual observation and classification of specific components 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 cast-in-place concrete bearing walls or cast-in-place concrete frames, and

   B.   The building has at least one floor diaphragm constructed with cast-in-place concrete.

Buildings of other construction types 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 standards do not result in seismic performance less than the evaluation and retrofit engineering criteria detailed in this Bulletin.

DEFINITIONS:

For the purpose of this bulletin, the following definitions shall apply:

   •   CONCRETE SHEAR WALL: A concrete wall which resists lateral forces applied parallel to the plane of the wall.

   •   CONCRETE MOMENT FRAME: A building frame system in which seismic shear forces are resisted by shear and flexure in members and joints of the frame, including slab-column moment frames.

   •   CONCRETE INFILL FRAME: A concrete moment frame having panel(s) of masonry that participate in resisting lateral forces that are placed within the frame members.

   •   NONSTRUCTURAL REPAIR: Repairs that improve the visual appearance of damage to a component. These repairs may also restore the nonstructural properties of a component, such as weather protection. Any structural benefit is negligible. This is defined as “Cosmetic Repair” in FEMA 308.

EVALUATION PROCEDURE AND RETROFIT SCOPE

Concrete Shear Wall and Infill Frame Buildings

General

Substantial structural damage to elements of the lateral force-resisting system shall be deemed to exist when the results of a FEMA 306 evaluation shows that capacity loss exceeds 20% for a concrete shear wall or infill frame building, or when any of the “triggering damage” criteria for substantial structural damage described in Table 1 is observed in an eligible building.

Additionally, disproportionate damage shall be deemed to exist when a FEMA 306 evaluation shows a capacity loss exceeding 10%, or when any of the “triggering damage” for disproportionate damage described in Table 1 is observed in an eligible building.

Overview

The flowchart in Figure 1 shall be followed to determine the post-earthquake damage repair or retrofit requirements related to substantial structural damage to concrete shear wall and infill frame buildings.

The process begins with a determination of whether the damaged building has sufficient pre-earthquake capacity to satisfy San Francisco Existing Building Code, Section 304.4, which references the May 21, 1973 milestone requirements of San Francisco Existing Building Code, Section 304.4. If the building in its pre-damaged state satisfies this code provision, then the building need not be retrofitted regardless of the level of damage, and restoration to pre-earthquake capacity is sufficient. Alternatively, if a full FEMA 306 evaluation (labeled as “Optional FEMA 306 Evaluation” in Figure 1) shows a capacity loss of less than 5 percent, nonstructural repair to the building instead of restoration to pre-earthquake capacity shall be permitted, except that individual component damage or condition repair per Table 1 is also required.

If the building does not meet the San Francisco Existing Building Code, Section 304.4 requirements per San Francisco Existing Building Code Section 304.4, then a full FEMA 306 evaluation (labeled as “Mandatory FEMA 306 Evaluation” in Figure 1) is required, and nonstructural repair, restoration to pre-earthquake capacity, or retrofit requirements may be triggered as shown in Figure 1. In addition, individual component damage or condition repair per Table 1 is required.

Figure 1: Flowchart for Post-Earthquake Repair and Retrofit of Concrete Shear Wall and Infill Frame Buildings

FEMA 306 Evaluation Process

A FEMA 306 evaluation process for the pre-event structure and the damaged structure shall be performed using the guidelines below:

   1.   The evaluation shall use the nonlinear static procedures defined in FEMA 306 to determine the capacity for pre-event and damaged conditions. FEMA 306 was developed at the time FEMA 273 was also in development, prior to the publication of FEMA 356. Since then, additional research and development effort was incorporated into FEMA 356 and later into ASCE 41. Therefore, the comparable, more current equations in ASCE 41 shall be used in performing a FEMA 306 evaluation rather than the FEMA 273 equivalents.

   2.   The global displacement demand shall be determined in accordance with ASCE 41.

   3.   The performance objective shall meet the requirements of ASCE 41 for Life Safety Structural Performance Level (S-3) at 75% of the spectral demand associated with the current code value at the building site.

Simplified FEMA 308 Evaluation Process

The following simplified version of the FEMA 308 approach, based on loss in performance, L, may be used for the purpose of determining threshold triggers for restoration to pre-earthquake capacity and retrofit. The process is similar to that outlined in Figure 1. The FEMA 308 parameters shall be determined using the guidelines as follows. For further definition of the FEMA 308 parameters needed for the evaluation.

   1.   To use this method, first determine the following performance capacity and loss indices:

   2.   •   Pre-event (Undamaged) Performance Index:

   P = dc / dd,

where dc is the global displacement capacity for the selected performance objective and dd is the maximum global displacement demand for the selected ground motion. This performance index is calculated using component properties for the pre-event conditions in accordance with the methodology outlined in FEMA 306.

      •   Damaged Performance Index:

   P’ = d’c / d’d,

where the prime symbol (’) denotes that the global displacement capacity and demand, d’c and d’d, respectively, are determined for the components in their damaged state using FEMA 306.

•   Loss:

   L = 1 – (P’ / P),

where L is the performance loss of a building due to earthquake damage, and is given by the ratio of the damaged performance index, P’, to the undamaged performance index, P, for a specific performance objective. L ranges between 0 and 1.

3.   To determine whether earthquake damage is acceptable and neither restoration to pre-earthquake capacity nor retrofit is triggered, the performance loss, L, is compared against the FEMA 308 Table 3-1 threshold parameters defined below:

•   Lr(min): The Loss threshold below which neither restoration to pre-earthquake capacity nor retrofit is triggered, shall be defined as follows:

   Lr(min) = 0.05 for earthquake event with Sa0.3 ≤ 0.4g

   Lr(min) = 0.05 for earthquake event with Sa0.3 > 0.4g

•   Lr(max): The Loss threshold above which either restoration to pre-earthquake capacity or retrofit is triggered. For this simplified procedure, Lr(max) may be taken to be the same as Lr(min) since Lr does not vary:

   Lr(max) = 0.05 for earthquake event with Sa0.3 ≤ 0.4g

   Lr(max) = 0.05 for earthquake event with Sa0.3 > 0.4g

Alternatively, the Damaged Performance Index, P’ may be used to determine whether earthquake damage is acceptable and neither restoration to pre-earthquake capacity nor retrofit is triggered by comparing P’ against the FEMA 308 Table 3-1 limit parameters defined below:

•   P’min: The Damage Performance Index limit below which restoration to pre-earthquake capacity or retrofit is triggered, is not used since Lr does not vary for the simplified method.

•   P’max: The Damage Performance Index limit above which neither restoration to pre-earthquake capacity nor retrofit is triggered regardless of the value of Loss, L, shall be defined as 1.0.

4.   If a building is required to be restored to its pre-earthquake capacity or retrofitted per step 2, the Performance Loss, L, is compared against the FEMA 308 Table 3-2 threshold parameters defined below to determine if retrofit is triggered:

•   Lu(min): The Loss threshold below which earthquake damage does not trigger retrofit but requires restoration to pre-earthquake capacity, shall be defined as below:

   Lu(min) = 0.10 for earthquake event with Sa0.3 ≤ 0.4g

   Lu(min) = 0.20 for earthquake event with Sa0.3 > 0.4g

•   Lu(max): The Loss threshold above which earthquake damage triggers retrofit, shall be taken to be the same as Lu(min) since Lu does not vary for the simplified method:

   Lu(max) = 0.10 for earthquake with Sa0.3 ≤ 0.4g

   Lu(max) = 0.20 for earthquake with Sa0.3 > 0.4g

Alternatively, the Undamaged Performance Index, P may be used to determine whether restoration to pre-earthquake capacity or retrofit is triggered by comparing P against the FEMA 308 Table 3-2 limit parameters defined below:

•   Pmin: The Pre-event Performance Index limit below which existing earthquake damage triggers retrofit, is not used since Lu does not vary for the purpose of the simplified method.

•   Pmax: The Pre-event Performance Index limit above which existing earthquake damage does not trigger retrofit and restoration to pre-earthquake capacity is sufficient regardless of the value of Loss, L, shall be taken as 1.0.

Retrofit Triggers due to Specific Component Damage or Conditions

In addition to the retrofit triggers per the FEMA 306 and 308 methodologies described above, damage to any of the specific components or other conditions noted in Table 1 below shall trigger retrofit shown in the “Action Required” column of the table if damage is observed to reach the severity of “triggering damage.” The conditions noted in Table 1 are primarily related to gravity-load-carrying component damage, load path failures, or significant damage in individual components. For damage less than the “triggering damage,” repairs shall be made to return the building to original strength or condition by methods acceptable to the Department of Building Inspection.

Table 1: Triggers for Specific Components or Conditions in Concrete Buildings
Components or Conditions	Triggering Damage		Action Required
	Substantial Structure Damage	Disproportionate Damage
Shear cracks in gravity load-carrying columns or bearing walls supporting less than 30% of the area of a roof or an individual floor.	Preemptive diagonal tension crack meeting the “Moderate” or worse criteria of the RC2H component in Section 5.5 of FEMA 306 or any component with “Extreme” damage per Section 5.5 of FEMA 306.	Preemptive diagonal tension crack meeting “Moderate” criteria of the RC2H component in Section 5.5 of FEMA 306, except that inclined crack widths are to be taken as between 1/16" and 1/8".	Replace component.
Shear cracks in gravity- load-carrying columns or bearing walls supporting 30% or more of the area of a roof or an individual floor.	Preemptive diagonal tension crack meeting the “Moderate” or worse criteria of the RC2H component in Section 5.5 of FEMA 306 or any component with “Extreme” damage per Section 5.5 of FEMA 306.	Preemptive diagonal tension crack meeting “Moderate” criteria of the RC2H component in Section 5.5 of FEMA 306, except that inclined crack widths are to be taken as between 1/16" and 1/8".	Replace component and retrofit lateral system to SFEBC Section 405.2.4.
Leaning story (excessive drift) in a concrete moment-frame building.	Permanent lateral displacement of 1% of the story height or more resulting from earthquake damage.	Permanent lateral displacement of 0.5% of the story height or more resulting from earthquake damage.	Retrofit lateral system to SFEBC Section 405.2.4.
Beam-column joint shear at joints with at least one exterior face in columns supporting less than 30% of the area of a roof or individual floor.	Cracking representative of joint shear at the beam-column joint with cracks at least 1/8" wide or offset along the crack at least 1/16".		Replace component.
Beam-column joint shear at joints with at least one exterior face in columns supporting more than 30% of the area of a roof or individual floor.	Cracking representative of joint shear at the beam-column joint with cracks at least 1/8" wide or offset along the crack at least 1/16".		Replace component and retrofit lateral system to SFEBC Section 405.2.4.

 

Components or Conditions	Triggering Damage		Action Required
	Substantial Structure Damage	Disproportionate Damage
Punching shear damage at slab around columns without intersecting beams in columns supporting less than 30% of the area of a roof or individual floor.	Evidence representative of potential punching shear such as fresh circular cracking in the slab around a column with or without vertical offset at the crack.		Replace component.
Punching shear damage at slab around columns without intersecting beams in columns supporting more than 30% of the area of a roof or individual floor.	Evidence representative of potential punching shear such as fresh circular cracking in the slab around a column with or without vertical offset at the crack.		Replace component and retrofit lateral system to SFEBC Section 404.3.
Separation of floor-to-wall connections.	•   Permanent separation or sliding at joint of 1" or more, or		Retrofit connection using forces from SFEBC Section 404.3.
	•   Permanent movement that results in inadequate bearing of supported member.
Delamination of more than 30% of cast-in-place topping from precast floor or roof framing where topping serves as the diaphragm.	Permanent separation of topping from precast members.		Replace damaged topping slab and tie new slab to underlying precast members using SFEBC Section 404.3 forces and current detailing.
Fractured bars at diaphragm chords or collectors.	•   Permanent separation or sliding at joint of 1" or more, or		Replace damaged bars and tie or splice new components to surrounding structural elements using SFEBC Section 404.3 forces and current detailing.
	•   Permanent movement that results in inadequate bearing of supported member.

 

Concrete Moment-Frame Buildings

The process for determining whether repair or retrofit is triggered for a concrete moment-frame building begins with a determination of whether the damaged building has adequate pre-earthquake capacity to comply with San Francisco Existing Building Code, Section 304.4. If this is satisfied, then the building need not be retrofitted regardless of the level of damage, and restoration of the building to its pre-earthquake capacity shall be undertaken. Unlike concrete shear-wall and infill buildings, nonstructural repairs shall not be permitted even if a full FEMA 306 evaluation has determined that the capacity loss is below 5 percent. For non-complying buildings, if any of the component damage or conditions given in Table 1 is present, the building shall be retrofitted as required by Table 1.

For concrete frame buildings with any interacting walls, in addition to the Table 1 checks, the evaluation procedure and retrofit scope given above for concrete shear wall and infill frame buildings shall be applied. When a FEMA 306 analysis is used to determine loss of capacity outlined in this bulletin for concrete shear wall and infill frame buildings, the moment-frame capacity may not be included in development of the force-displacement pushover curve.

EVALUATION AND RETROFIT ENGINEERING CRITERIA

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.

In addition, any of the following alternative 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 the BSE-1 earthquake hazard level, or

   C.   Meets the requirements of San Francisco Existing Building Code 304.4.

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