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| March 25, 2008 (Updated 01/01/2020 for code references) |
| Permit Processing and Issuance |
| Requirements and Guidelines for the Seismic Design of New Tall Buildings using Non-Prescriptive Seismic-Design Procedures |
| The purpose of this Administrative Bulletin is to present requirements and guidelines for the seismic structural design and submittal documents for building permits for new tall buildings in San Francisco that use non-prescriptive seismic design procedures. |
| 2019 San Francisco Building Code, Section 104A.2.8 Alternate materials, design, and methods of construction SEAONC, 2007, Recommended Administrative Bulletin on the Seismic Design & Review of Tall Buildings Using Non-Prescriptive Procedures, prepared by Structural Engineers Association of Northern California (SEAONC) AB-083 Tall Buildings Task Group ASCE, 2016, Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-16, Prepared by the Structural Engineering Institute of the American Society of Civil Engineers 2003 NEHRP Recommended Provisions For New Buildings And Other Structures Part 1: Provisions and Part 2: Commentary (FEMA 450) SEAONC, 1999, Contractual Provisions to Address the Engineer’s Liability when Using Performance-Based Seismic Design, Structural Engineers Association of Northern California SEAOC, 2001, “Seismology Committee Background and Position Regarding 1997 UBCEq. 30-7 and Drift,” Structural Engineers Association of California () |
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This bulletin presents requirements and guidelines for seismic structural design and submittal documents for building permit for new tall buildings in San Francisco that use non-prescriptive seismic design procedures.
Commentary: It is intended that buildings designed to the requirements and guidelines of this bulletin will have seismic performance at least equivalent to that intended of code-prescriptive seismic designs, consistent with the San Francisco Building Code sections indicated below. To demonstrate that a building design is capable of providing code equivalent seismic performance, a three-step procedure shall be performed as specified in Section 4 of this Administrative Bulletin. Intended code seismic performance can be found in the commentary of FEMA 450. |
This bulletin intentionally contains both requirements, which are stated in mandatory language (e.g., “shall”) and guidelines, which use non-mandatory language.
This bulletin is not written to cover essential facilities.
For the purposes of this Administrative Bulletin, a non-prescriptive seismic design is one that takes exception to one or more of the prescriptive requirements of the San Francisco Building Code and
Chapter 12 of ASCE/SEI 7-16 and the standards referenced therein, by invoking San Francisco Building Code, Section
104A.2.8, which allows alternative materials and methods of construction as approved by the Building Official.
For the purposes of this bulletin, tall buildings are defined as those with hn greater than 160 feet above average adjacent ground surface.
The height, hn is defined in the San Francisco Building Code as the height of Level n above the average level of the ground surface adjacent to the structure. Level n is permitted to be taken as the roof of the structure, excluding mechanical penthouses and other projections above the roof whose mass is small compared with the mass of the roof.
Procedures other than those presented herein may be acceptable pursuant to the approval of the Director of the Department of Building Inspection.
Commentary: ASCE/SEI 7-16 Sections that discuss non-prescriptive or “alternative” seismic design procedures are reproduced below: |
11.1.4 Alternate Materials and Methods of Construction. Alternate materials and methods of construction to those prescribed in the seismic requirements of this standard shall not be used unless approved by the authority having jurisdiction. Substantiating evidence shall be submitted demonstrating that the proposed alternate, for the purpose intended, will be at least equal in strength, durability, and seismic resistance.
12.1.1 Basic Requirements. …An approved alternative procedure shall not be used to establish the seismic forces and their distribution unless the corresponding internal forces and deformations in the members are determined using a model consistent with the procedure adopted.
San Francisco Building Code sections that discuss non-prescriptive or “alternative” seismic design procedures are reproduced below:
104A.2.8 Alternate materials, design and methods of construction. The provisions of this code are not intended to prevent the use of any material, alternate design or method of construction not specifically prescribed by this code, provided any alternate has been approved and its use authorized by the building official.
The building official may approve any such alternate, provided the building official finds that the proposed design is satisfactory and complies with the provisions of this code and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in suitability, strength, effectiveness, fire resistance, durability, safety and sanitation.
The building official shall require that sufficient evidence or proof be submitted to substantiate any claims that may be made regarding its use. The details of any action granting approval of an alternate shall be recorded and entered in the files of the code enforcement agency.
1604.4 Analysis. Any system or method of construction to be used shall be based on a rational analysis in accordance with well-established principles of mechanics. Such analysis shall result in a system that provides a complete load path capable of transferring all loads and forces from their point of origin to the load-resisting elements.
2. STRUCTURAL DESIGN REVIEW
Structural Design Review shall be in accordance with
AB-082. At the conclusion of the review, the Structural Design Reviewer shall provide a written statement that, in their professional opinion, the building elements under their review are equivalent in strength, durability, and seismic resistance of the building to those of a building designed according to the prescriptive provisions of the San Francisco Building Code.
3. SUBMITTAL REQUIREMENTS
Project submittal documents shall be in accordance with the San Francisco Building Code and Department of Building Inspection interpretations, Administrative Bulletins, and policies. In addition, documents relevant to the Structural Design Review shall be submitted by the Engineer of Record to the Director and to the Structural Design Reviewer.
As early as practicable, the Engineer of Record shall submit to the Director an initial Seismic Design Criteria along with a description and initial drawings of the structure. The Seismic Design Criteria shall be consistent with the requirements of this bulletin, and shall be updated to incorporate issues resolved during the Structural Design Review process.
The Seismic Design Criteria shall describe the proposed building and structural system, proposed analysis methodology, and acceptance criteria. The Seismic Design Criteria shall include any proposed exceptions to the prescriptive provisions of the San Francisco Building Code, modeling parameters, material properties, drift limits, element force capacities and deformation capacities. The Seismic Design Criteria shall identify all exceptions to the San Francisco Building Code prescriptive requirements that the Engineer of Record proposes. The Seismic Design Criteria shall be subject to review by the Structural Design Reviewer and approval by the Director. A summary of the Engineer of Record’s final Seismic Design Criteria shall be included in the general notes of the structural drawings.
4. SEISMIC DESIGN REQUIREMENTS
The Engineer of Record shall evaluate the structure at the levels of earthquake ground motion as indicated in the subsections below.
If nonlinear response is anticipated under any of the Maximum Considered Earthquake (MCE) ground motions specified in Section 4.3, the Engineer of Record shall apply capacity design principles and design the structure to have a suitable ductile yielding mechanism, or mechanisms, under nonlinear lateral deformation. The code-level analysis shall be used to determine the required strength of the yielding actions. The Engineer of Record shall include in the Seismic Design Criteria all assumptions and factors used in the application of capacity design principles.
Commentary: The purpose of each level of seismic evaluation is as follows: |
The code-level evaluation of Section 4.1 is used to identify the exceptions being taken to the prescriptive requirements of the San Francisco Building Code and to define the minimum required strength and stiffness for earthquake resistance. Minimum strength is defined according to San Francisco Building Code minimum base shear equations, with a response modification coefficient R, proposed by the Engineer of Record, reviewed by the Structural Design Reviewer, and approved by the Director. Minimum stiffness is defined by requiring the design to meet San Francisco Building Code-specified drift limits, using traditional assumptions for effective stiffness. Providing a non-prescriptive seismic design with minimum strength and stiffness comparable to code-prescriptive designs helps produce seismic performance at least equivalent to the code. Minimizing the number of exceptions to prescriptive requirements also helps achieve this aim.
As indicated in Section 4.2, a service-level evaluation is required by this bulletin to demonstrate acceptable seismic performance for moderate earthquakes.
The MCE-level evaluation of Section 4.3 is intended to verify that the structure has an acceptably low probability of collapse under severe earthquake ground motions. The evaluation uses nonlinear response-history analysis to demonstrate an acceptable mechanism of nonlinear lateral deformation and to determine the maximum forces to be considered for structural elements and actions designed to remain elastic.
4.1 Code-Level Evaluation
The seismic structural design shall be performed in accordance with the prescriptive provisions of the San Francisco Building Code, except for those provisions specifically identified by the Engineer of Record in the Seismic Design Criteria as Code Exceptions.
Commentary: Code exceptions that have typically been taken for non-prescriptive designs of tall buildings in high seismic design categories include exceeding the height limitations of ASCE/SEI 7-16 Table 12.2.1. Other exceptions, including provisions related to R,ρ,Ω0, limitations on T, and various detailing requirements, may be considered at the discretion of the Director. The Engineer of Record is required to justify all exceptions to prescriptive code provisions. The scope of structural design review shall include all proposed code exceptions. |
The lower limit of ASCE/SEI 7-16 Eq. 12.8-5 and 12.8-6 for the calculation of the Seismic Response Coefficient applies to the scaling process of ASCE/SEI 7-16 Section 12.9. The value of R used shall be indicated in the Seismic Design Criteria, and shall not be greater than 8.5.
The Engineer of Record shall demonstrate that the structure meets the story drift ratio limitations of the San Francisco Building Code using a code-level response-spectrum analysis and the following requirements:
a) The design lateral forces used to determine the calculated drift need not include the minimum base shear limitation of ASCE/SEI 7-16 eq. 12.8-5 and 12.8-6.
b) Stiffness properties of non-prestressed concrete elements shall not exceed 0.5 times gross-section properties.
c) Foundation flexibility shall be considered, using recommendations provided by the Geotechnical Engineer of Record that are defined in the Seismic Design Criteria.
d) The analysis shall account for P-delta effects.
Commentary: ASCE/SEI 7-16 requires the consideration of the minimum base shear of Eq. 12.8-5 and 12.8-6 for checking design story drifts relative to allowable story drifts. However, the consensus of SEAONC’s AB-083 Task Group for this Administrative Bulletin, approved by the SEAONC Board, is that UBC Formula 30-7 (equivalent to ASCE/SEI 7-16 Eq. 12.8-6) need not be applied to the check of drift limits for tall buildings designed according to this bulletin, because the MCE-level Evaluation of Section 4.3 includes a check of drift for site-specific ground motions. Such ground motions are required to take account of near-fault and directivity effects. The consensus of the task group is that this is an appropriate and more explicit way of addressing the intended purpose of applying Formula 30-7 to the check of drift limits. |
Actual concrete stiffness properties may vary significantly from the value of 0.5 times gross-section properties referenced for the code-level check of story drift limits. This assumption is specified to provide a consistent requirement for minimum building stiffness. This requirement is intended to lead to earthquake serviceability performance related to story drift that is at least comparable to that expected of prescriptively-designed tall buildings designed to the San Francisco Building Code.
For the deformation compatibility evaluation of critical non-structural elements, such as exterior curtain wall and cladding systems and egress stairways, the drift ratio demand shall be calculated using the minimum base shear limitations of ASCE/SEI 7-16 Eq. 12.8-5 and 12.8-6. In lieu of this requirement, these critical non-structural elements may be designed for drift ratios at the MCE-level.
4.2 Service-Level Evaluation
A service-level evaluation of the primary structural system is required to demonstrate acceptable, essentially elastic seismic performance at the service-level ground motion.
Commentary: To ensure code-equivalent seismic performance, the Director is requiring a service-level evaluation for new tall buildings utilizing non-prescriptive design procedures. |
There are circumstances where there is a reason to believe that the serviceability performance of the design would be worse than that anticipated for a code-prescriptive design. Some of these circumstances have been identified as follows:
a) Where the Engineer of Record has taken any exception to code-prescriptive requirements for non-structural elements (ASCE/SEI 7-16, Chapter 13)
b) Where the stiffness representation of any structural element in the code-level evaluation is significantly less than the effective linear-elastic stiffness described in applicable research
c) For a structure that exhibits disproportionably large drift or accelerations for ground motions less than the San Francisco Building Code Design Basis Ground Motion (not reduced by R).
While this bulletin does not require checking all non-structural elements at the service-level evaluation, it is expected that the building cladding will remain undamaged and that egress from the building will not be impeded when the building is subjected to the service-level ground motion.
For the purposes of this bulletin, the service-level ground motion shall be that having a 43-year mean return period (50% probability of exceedance in 30 years).
Structural models used in the service-level evaluation shall incorporate realistic estimates of stiffness and damping considering the anticipated levels of excitation and damage. The evaluation shall demonstrate that the elements being evaluated exhibit serviceable behavior.
Commentary: While essentially elastic performance is required in the service-level ground motion, it is not the intent of this bulletin to require that a structure remain fully linear and elastic. It is permissible for the analysis to indicate minor yielding of ductile elements of the primary structural system, provided such results do not suggest appreciable permanent deformation in the elements, strength degradation, or significant damage to the elements requiring more than minor repair. It is permissible for the analysis to indicate minor and repairable cracking of concrete elements. |
Where numerical analysis is used to demonstrate serviceability, the analysis model should represent element behavior that is reasonably consistent with the expected performance of the elements. In typical cases it may be suitable to use a linear response spectrum analysis, with appropriate stiffness and damping, and with the earthquake demands represented by a linear response spectrum corresponding to the service-level ground motion. Where response history analysis is used, the selection and scaling of ground motion time series should comply with the requirements of ASCE/SEI 7-16, Section 16.2, with the service-level response spectrum used instead of the design basis earthquake response spectrum, and with the design demand represented by the mean of calculated responses for not less than seven appropriately selected and scaled time series.
As expressed by SEAONC [1999], it should be understood “that the current state of knowledge and available technology is such that the design profession’s ability to accurately predict the earthquake performance of a specific building is limited and subject to a number of uncertainties.” Actual performance may differ from intended performance.
4.3 Maximum Considered Earthquake-Level Evaluation
Ground Motion: The ground motion representation for this evaluation shall be the Maximum Considered Earthquake (MCE) as defined in ASCE/SEI 7-16, Chapter 21.
A suite of not less than seven pairs of appropriate horizontal ground motion time series shall be used in the analyses. The selection and scaling of these ground motion time series shall comply with the requirements of ASCE/SEI 7-16, Chapter 16, with the following modifications:
a) The MCE response spectrum shall be the basis for ground motion time series scaling instead of the design response spectrum.
b) Either amplitude-scaling procedures or spectrum-matching procedures may be used.
c) Where applicable, an appropriate number of the ground motion time series shall include near fault and directivity effects such as velocity pulses producing relatively large spectral ordinates at relatively long periods.
Commentary: The procedures for selecting and scaling ground motion records, as presented here, represent the current state of practice. The procedures are written to retain some flexibility so that engineering judgment can be used to identify the best approach considering the unique characteristics of the site and the building. |
Selection and scaling of earthquake ground motion records for design purposes is a subject of much current research. The Engineer of Record may wish to consider alternative approaches recently proposed; however, some of the proposed approaches have not been adequately tested on tall buildings so their adoption should only be considered with caution. Aspects of particular concern include the long vibration period of many tall buildings and the contributions of multiple vibration “modes” to key response quantities.
At near-fault sites, the average fault-normal response spectrum usually is larger than the average fault-parallel response spectrum due to the presence of a rupture directivity pulse in the fault-normal component of the ground motion. It is important to include in the suite of ground motions an appropriate number of motions that include near-fault and directivity effects so that design drift demands are appropriately determined, especially considering that Section 4.1 permits the design to be exempt from applying Equations 12.8-5 and 12.8-6 to drift calculations. If spectral matching is used, individual ground motion components should account for the distinction between fault-normal and fault-parallel hazard.
Mathematical Model: The three-dimensional mathematical analysis model of the structure shall conform to ASCE/SEI 7-16 Section 12.7.3.
The analyses shall consider the interaction of all structural and non-structural elements that materially affect the linear and nonlinear response of the structure to earthquake motions, including elements not designated as part of the lateral-force-resisting system in the code-level analysis (Section 4.1).
Commentary: This requires explicit modeling of those parts of the structural and non-structural systems that affect the dynamic response of the building. In addition, the effect of building response on all materially affected parts of the building must be evaluated. |
The stiffness properties of reinforced concrete shall consider the effects of cracking and other phenomena on initial stiffness.
Commentary: In addition to cracking, effective stiffness can be affected by other phenomena. These include bond slip, yield penetration, tension-shift associated with shear cracking, panel zone deformations, and other effects. |
The effective initial stiffness of steel elements embedded in concrete shall include the effect of the embedded zone. For steel moment frame systems, the contribution of panel zone (beam-column joint) deformations shall be included.
The Engineer of Record shall identify any structural elements for which demands for any of the response-history runs are within a range for which significant strength degradation could occur, and shall demonstrate that these effects are appropriately considered in the dynamic analysis.
Commentary: For typical situations, element strength degradation of more than 20% of peak strength should be considered significant. |
P-Δ effects that include all the building dead load shall be included explicitly in the nonlinear response history analyses.
Documentation submitted for Structural Design Reviewer review shall clearly identify which elements are modeled linearly and which elements are modeled nonlinearly. For elements that are modeled as nonlinear elements, submitted documentation shall include suitable laboratory test results or analyses that justify the hysteretic properties represented in the model.
The properties of elements in the analysis model shall be determined considering earthquake plus expected gravity loads. In the absence of alternative information, gravity load shall be based on the load combination 1.0D + Lexp, where D is the service dead load and Lexp is the expected service live load.
Commentary: In typical cases it will be sufficient to take Lexp = 0.2L, where L is the code-prescribed live load without live load reduction. |
The foundation strength and stiffness contribution to the building seismic response shall be represented in the model. The foundation strength and stiffness characterization shall be consistent with the strength and stiffness properties of the soils at the site, considering both strain rate effects and soil deformation magnitude.
Analysis Procedure: Three-dimensional nonlinear response history (NLRH) analyses of the structure shall be performed. Inclusion of accidental torsion is not required. When the ground motion components represent site-specific fault-normal ground motions and fault-parallel ground motions, the components shall be applied to the three-dimensional mathematical analysis model according to the orientation of the fault with respect to the building. When the ground motion components represent random orientations, the components shall be applied to the model at orientation angles that are selected randomly; individual ground motion pairs need not be applied in multiple orientations.
Commentary: Three-dimensional analyses are required to represent the inherent torsional response of the building to earthquake ground shaking. This is done by including in the NLRH model the actual locations and distribution of the building mass, stiffness, and strength. Accidental torsion is not required to be included in the NLRH analyses. (Accidental torsion is required for the code-level analysis of Section 4.1.) |
The Engineer of Record shall report how damping effects are included in the NLRH analyses. The equivalent viscous damping level shall not exceed 5%, unless adequately substantiated by the Engineer of Record.
Commentary: The effects of damping in an analysis depend on the type of damping model implemented. Some models may over-damp higher modes or have other undesirable effects. |
For each horizontal ground motion pair, the structure shall be evaluated for the following load combination:
Alternative load combinations, if used, shall be adequately substantiated by the Engineer of Record.
Demands for ductile actions shall be taken not less than the mean value obtained from the NLRH. Demands for low-ductility actions (e.g., axial and shear response of columns and shear response of walls) shall consider the dispersion of the values obtained from the NLRH.
Commentary: In typical cases the demand for low-ductility actions can be defined as the mean plus one standard deviation of the values obtained from the NLRH. Procedures for selecting and scaling ground motions, and for defining the demands for low-ductility actions, should be defined and agreed to early in the review process. |
Acceptance Criteria: Calculated force and deformation demands on all elements required to resist lateral and gravity loads shall be checked to ensure they do not exceed element force and deformation capacities. This requirement applies to those elements designated as part of the lateral-force-resisting system in the code-level analysis (Section 4.1), as well as those elements not designated as part of the lateral-force-resisting system in the code-level analysis but deemed to be materially affected.
Commentary: Elements not designated as part of the lateral-force-resisting system in the code-level analysis (gravity systems) may be subjected to substantial deformations and forces, including axial forces accumulated over many stories, as they interact with the primary lateral-force-resisting system. Non-structural elements such as cladding are evaluated according to code requirements. This bulletin does not require checking non-structural elements at the MCE level. |
The Engineer of Record shall identify the structural elements or actions that are designed for nonlinear seismic response. All other elements and actions shall be demonstrated by analysis to remain essentially elastic.
Commentary: Essentially elastic response may be assumed for elements when force demands are less than design strengths. Design strengths for non-ductile behaviors (e.g., shear and compression) of these essentially elastic elements are defined as nominal strengths, based on specified material properties, multiplied by strength reduction factors as prescribed in the SFBC. Design strengths for ductile behaviors of these essentially elastic elements are defined as nominal strengths, based on expected material properties, multiplied by ø=1.0. Alternative approaches to demonstrating essentially elastic response may be acceptable where appropriately substantiated by the Engineer of Record. |
For structural elements or actions that are designed for nonlinear seismic response, the Engineer of Record shall evaluate the adequacy of individual elements and their connections to withstand the deformation demands. Force and deformation capacities shall be based on applicable documents or representative test results, or shall be substantiated by analyses using expected material properties.
The average result, over the NLRH analyses, of peak story drift ratio shall not exceed 0.03 for any story.
All procedures and values shall be included in the Seismic Design Criteria and are subject to review by the Structural Design Reviewer and approval by the Director.
Isam Hasenin, P.E., C.B.O.,
Department of Building Inspection
Approved by the Building Inspection Commission on March 19, 2008
