Recommended
Administrative
Bulletin
on the
Seismic Design & Review of
Tall Buildings Using NonPrescriptive Procedures
April 2007
PREPARED FOR
City of San Francisco Department of Building Inspection
PREPARED BY
Structural Engineers Association of
Northern California (SEAONC)
AB-083 Tall Buildings Task Group
Joe Maffei, S.E., Ph.D. (Chair)
Michael Gemmill, S.E.
Ronald Hamburger, S.E.
Neville Mathias, S.E.
Jack Moehle, P.E., Ph.D.
Mark Moore, S.E. (Chair, SEAONC Seismology)
Jeffrey Taner, S.E. (Chair, SEAONC Professional Practices)
Marko Schotanus, Ph.D. (Chair’s Assistant, non-voting)
APPROVED BY
SEAONC Board of Directors
Doug Hohbach, S.E. (President)
William Andrews, S.E.
Grace Kang, S.E.
Peter Lee, S.E.
Bret Lizundia, S.E.
Andrew Merovich, S.E.
Gary Mochizuki, S.E.
David Murphy, S.E.
Peter Revelli, S.E.
STRUCTURAL ENGINEERS ASSOCIATION
OF NORTHERN CALIFORNIA
23 April 2007
2006-2007
PRESIDENT
Douglas Hohbach, SE
Mr. Hanson Tom
Principal Engineer
Department of Building Inspection
City & County of San Francisco
1660 Mission Street, Sixth Floor
San Francisco, California 94103
VICE PRESIDENT
Bret Lizundia, SE
SECRETARY
Gary Mochizuki, SE
TREASURER
William Andrews, SE
Subject:
DIRECTORS
Grace King SE
Peter Lee, SE
David Murphy, SE
Peter Revelli, SE
Dear Hanson:
In response to your request to SEAONC in your letter of 29 September 2006, SEAONC is pleased to provide you the attached document
“Recommended Administrative Bulletin on the Seismic Design &
Review of High-Rise Buildings using Non-Prescriptive Procedures”.
The document has been prepared by a specially convened SEAONC
Task Group, brought together to develop consensus recommendations
that reflect input from our Seismology and Structural Standards Committee (SSSC), from our Professional Practices Committee (PPC), and
from representatives from our membership who are active in tall building design and peer review. The Task Group has met repeatedly over
the intervening months and has built upon previous work undertaken
by SFDBI and our SSSC and PPC during 2006. It has carefully considered every portion of the recommended bulletin and has incorporated input from many sources. The document includes a commentary
and a preface. The preface summarizes the process by which the document was developed and it includes a brief narrative that describes the
basis for some of the document’s contents and rationale. A comment
log has been created that lists comments the Task Group received during the bulletin’s development phase, and the Task Group’s responses
to the comments. The SEAONC Board of Directors reviewed and approved the recommended bulletin attached herewith on 3 April 2007.
PAST PRESIDENT
Andrew Merovich, SE
EXECUTIVE SECRETARY
Ken Miles
BUILDING
AB-083: Guidelines for the Structural Review
of New Tall Buildings
A
SAFER FUTURE
THROUGH
STRUCTURAL ENGINEERING
575 Market Street Suite 2125 • San Francisco, California 94105-2870
Phone 415/974-5147 • Fax 415/764-4915 • E-mail: [email protected] • Website: http://www.seaonc.org
Mr. Hanson Tom
Department of Building Inspection
City & County of San Francisco
23 April 2007
Page 2
As indicated in the commentary to Section 1, aspects of the Administrative Bulletin should
be revisited before San Francisco adopts a new building code based on the 2006 International Building Code. Additionally, the bulletin should be revisited to incorporate the
findings of future research on ground motion characterization and tall building seismic
performance, such as is currently underway with the PEER Tall Buildings Initiative.
SEAONC recommends incorporation of the Recommended Administrative Bulletin into an SFDBI Administrative Bulletin with as little modification as possible since the bulletin represents the product of a comprehensive, carefully coordinated effort by the Task Group. SEAONC will be publishing this document
as a service to its membership. The document is expected to serve as a useful resource to not only our membership, but to the larger community SEAONC serves.
Our organization remains deeply committed to the protection of public safety and we hope we
can continue to help in the development of anAB that is enforceable, cogent and reflective of the
level of safety and protection the citizens of San Francisco deserve. We stand ready to provide
further explanation of this document as the administrative process of adopting theAB proceeds.
Sincerely,
Doug Hohbach, S.E.
SEAONC President
cc:
Isam Hasenein, Director, SFDBI
Raymond Lui, Structural Engineer, SFDBI
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
Preface
The attached recommended administrative bulletin Requirements and Guidelines for the Seismic
Design and Review of New Tall Buildings using Non-Prescriptive Seismic-Design Procedures
was prepared by a specially convened volunteer task group of the Structural Engineers
Association of Northern California (SEAONC), in response to a request from the San Francisco
Department of Building Inspection (SFDBI). The objective of the task group was to provide
recommendations to SFDBI on how to evaluate the acceptability of seismic designs for proposed
new tall buildings — in particular those buildings that use non-prescriptive seismic procedures.
(ackgr+,nd
The idea that San Francisco should have an administrative bulletin for the seismic design of tall
buildings was brought forward by Laurence Kornfield of SFDBI in November 2005. On 3 March
2006, an administrative bulletin labeled AB-xxx Draft was prepared by Ray Lui of SFDBI and
sent for comment to SEAONC and a number of engineers with expertise on the topic. The draft
bulletin was partly based on the product of a Los Angeles Tall Buildings Structural Design
Council workshop led by Farzad Naeim in September of 2005. The AB-xxx Draft also considered
input from a number of engineers experienced in the seismic design and peer review of tall
buildings. Through its Seismology and Structural Standards Committee (SSSC), SEAONC
provided comments to SFDBI on this draft. Considering the comments received through
SEAONC and from the other engineers who had been asked by SFDBI to comment, Ray Lui
prepared an updated version labeled AB-083 Draft #2, dated 29 September 2006.
This AB-083 Draft #2 was the starting point for the SEAONC task group. In its instructions from
the SEAONC Board of Directors, the task group was “charged with the task of incorporating
SEAONC member input into the administrative bulletin such that it reflects SEAONC consensus
opinion.” In its newsletter and through its SSSC and professional practice committees, SEAONC
solicited comments on Draft #2 from the structural engineering community. The task group was
charged with considering these new comments along with all previous written comments and
their own expertise in developing a revised administrative bulletin.
Task 1r+,p Activities
SEAONC’s AB-083 Task Group was composed of seven voting members: the chairpersons of the
organization’s SSSC and Professional Practice committees plus five members chosen by the
SEAONC Board of Directors for their expertise in the seismic design and seismic peer review of
tall buildings.
The task group convened nine half-day meetings, roughly every two weeks starting 3 November
2006 and finishing 26 February 2007. Meeting agendas, approved meeting minutes, and progress
drafts of the bulletin were made available to all SEAONC members through the SEAONC
website. A teleconference on ground motion issues and minimum base shear was also included in
the task group’s scope of work.
The overall approach of the task group was to first address peer review requirements followed by
MCE-level seismic requirements, and then code- and serviceability- level seismic requirements.
During the first two or three meetings it became clear that when the task group discussed
comments applicable to a section of Draft #2, the group ended up modifying the section line-byline, often leading to a finished product that was completely revised from the Draft #2 starting
April 2007
i
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
point. Eventually, the group tended toward an approach where in advance of a meeting, a
selected task group member would take a first pass at revising a section based on comments to
date, and this proposed revision would be discussed, modified, and provisionally approved,
paragraph-by-paragraph, at the task group meeting. After the task group itself reached consensus
on a section, two task group members were assigned to review each submitted comment related to
that section.
At around the fifth task group meeting, the group decided that it was important to distinguish
commentary from requirements and guidelines, and to provide new commentary where needed.
As a result, commentary was provided throughout the bulletin.
7ey Iss,es Regarding Sc+pe and Pr+cess
At the fourth task group meeting the task group voted to narrow the scope of the document to
apply only to non-prescriptive designs. This came about because some task group members and
commenters felt that prescriptively designed tall buildings should not be subject to a mandatory
seismic peer review. Other task group members and commenters were of the opinion that a
prescriptively designed tall building would not necessarily provide desirable seismic performance
without additional checks, which could be encouraged through peer review. All task group
members agreed that the scope of seismic peer review would be different for a prescriptively
designed building because code acceptance is based on compliance with code requirements rather
than by demonstrating at least equivalent performance.
Requirements for the peer review process were discussed at length. Language from the SEAOC
peer review guidelines [1999b]1 was adapted into the bulletin, tailored to SFDBI’s typical process
for peer review. Language requiring peer reviewers “to be selected by the Director based on their
qualifications …” was in the end chosen over ideas for more specific requirements for the
qualifications and selection of peer reviewers.
Some commenters expressed a desire for more consistency between projects in the technical
requirements that different peer review panels might recommend. The task group considered a
number of different options for how such consistency could be encouraged, but eventually
decided against all such proposals. One task group member made the point that consistency
between peer reviews may not in fact be desirable as consistent requirements would not
automatically mean consistently correct or consistently appropriate requirements.
Some commenters requested a process for appeal or supervision of peer reviewer opinions. After
considering a number of specific ideas and finding them unsatisfactory, the task group in the end
decided on a simple statement that “The Director shall address differences of opinion between the
SPRP and the EOR.”
7ey Technical Iss,es
In the code-level evaluation of bulletin Section 4.1, the task group carefully considered whether
the calculated drift should include the minimum base shear limitation of Formula 30-7 of the San
Francisco Building Code (SFBC). To investigate this issue, Jack Moehle, through the Tall
Buildings Initiative of the Pacific Earthquake Engineering Research Center, convened a meeting
via internet and teleconference among experts on the topic. In addition to all task group
members, participants in the teleconference included Norman Abrahamson, Yousef Bozorgnia,
Greg Deierlein, Dave Fields, Sig Freeman, Helmut Krawinkler, Marshall Lew, Steve Mahin,
1
See references in the recommended bulletin
April 2007
ii
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
Farzad Naeim, Gary Searer, and Paul Somerville. After detailed presentations and discussion, the
unanimous consensus of the teleconference participants was that Formula 30-7 need not be
applied to the drift calculation if MCE response-history analyses are carried out according to the
requirements of the bulletin. This position was incorporated into Section 4.1 of the bulletin.
The serviceability level of seismic evaluation was debated at length by the task group, and many
different options were considered. In the end, the task group agreed that because Section 4.1 of
the bulletin requires the application of code lateral-force levels and drift limits, the serviceability
performance would in typical cases be equivalent to that for code-prescriptive designs. A more
detailed serviceability evaluation is required only for special cases identified in Section 4.2.
Early in the process the task group discussed comments that recommended deleting the
serviceability evaluation because such an evaluation is not done for prescriptive designs and
because it can become a duplication of the code-level evaluation. An approximately equal
number of comments recommended the opposite: deleting the code-level evaluation and instead
using a serviceability evaluation to define the structure’s minimum strength and stiffness. The
final bulletin falls closer to the former viewpoint, as a majority of task group members believed
that keeping the code-level evaluation was a useful benchmark in the aim for at least equivalent
seismic performance to code prescriptive designs.
Commenters on the earlier drafts of the bulletin noted an emphasis on analysis rather than design
requirements, even though both are essential to seismic performance. Section 4 of the final
bulletin contains relatively concise requirements for capacity design, modeling, analysis, and
basic acceptability criteria. The bulletin recognizes that, in addition, specific design criteria are
important, and Sections 2 and 3 require the submittal and peer review of the criteria and details of
the seismic design. Such criteria and details depend on the type of structure or structural element
being designed and often depend on the specific features of the project. Thus the task group
decided against trying to spell out more specific seismic design requirements in a bulletin that
covers tall buildings of any proposed structure type.
(asic Appr+ach
From discussion of the bulletin among the profession, it became clear that there is a strong
interest among some structural engineers to create more purely “performance-based” guidelines
for seismic design, including for tall buildings. Such an approach would dispense entirely with a
code-level check and instead base designs on more explicit performance levels such as immediate
occupancy. Those with a passing familiarity with the bulletin effort often assume that it adopts
such an approach. The recommended bulletin does not.
The task group instead followed an approach tied as closely as practical to the SFBC, judging that
this was the most appropriate and expeditious way to address the immediate question at hand:
how to evaluate whether a non-prescriptive design meets the SFBC standard of “at least
equivalent” seismic performance.
Development of requirements for more purely performance-based design methods has been
under investigation in the earthquake engineering research community, including the Pacific
Earthquake Engineering Research Center (www.peer.berkeley.edu) which recently began a Tall
Buildings Initiative.
April 2007
iii
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
Appr+val
By an e-mail vote on 1 March 2007, the task group voted unanimously to approve the
recommended administrative bulletin, subject to a final review for editorial corrections. On
3 April 2007, the SEAONC Board of Directors unanimously approved the bulletin, subject to
final editorial review by the board’s executive committee.
The final editorial review of the bulletin by the task group included votes on three relatively
minor but non-editorial changes and was completed on 16 April 2007, using consensus voting
procedures. The Executive Committee of the SEAONC Board of Directors approved the final
camera-ready document, including the bulletin, cover letter, and preface, on 23 April 2007.
Res+l,ti+n +f C+mments
Over 190 written comments on the 3 March 2006 and 29 September 2007 drafts of the
administrative bulletin were reviewed and considered by the task group in the development of the
final recommended bulletin. For each comment, a task group action was written that indicates
that either the comment was addressed by the bulletin or that the task group considered the
comment and decided against its recommendations. Because sections of the bulletin were revised
and re-revised throughout the duration of the task group process, the final resolution for most
comments was written at the end of the process. In addition to the earlier comments, thirty three
written comments by the SEAONC Board of Directors on near-final drafts of the bulletin were
considered by the task group. The complete log of comments and the task group action regarding
them is available to SEAONC members at www.seaoc.org/member/committees /ab083tg.asp.
Ackn+wledgments
As task group chair, I would like to thank Amy Lee, Hanson Tom, Ray Lui, and
Laurence Kornfield of SFDBI who initiated this project and who took the time to understand the
key issues and seek SEAONC’s expertise. I would also like to thank Isam Hasenin, the new
director of SFDBI, who has quickly brought himself up to speed on the topic. I express my
gratitude to all those who contributed written comments on the bulletin, to those who participated
in the teleconference on minimum base shear, and to the SEAONC Board of Directors and
SEAONC president Doug Hohbach for directing this time-sensitive project.
Above all, I would like to thank the task group members who made an extraordinary dedication
of their time and expertise over six months to complete the recommended bulletin. I estimate that
among the seven members of our task group, plus our reliable organizer Marko Schotanus, we
devoted about 700 person-hours of effort to producing the attached document, all of it volunteer.
This is in addition to the efforts of the SEAONC Board of Directors, SSSC committee, and those
who sent in comments.
I believe that the magnitude of this endeavor and the quality of our final product are a credit to
SEAONC and to the dedication of our profession to creating a safe and functional built
environment.
Joe Maffei
Chair, SEAONC AB-083 Task Group
23 April 2007
April 2007
iv
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
SEAOCC RECODDECDED ADDICISTRATIVE (ULLETIC FOR SAC FRACCISCO
COJ A(K
DATE
L
April 200O, FICAL SEAOCC RECODDECDED VERSIOC
SU(QECT
L
Seismic Design and Review Pr+ced,res f+r Cew Tall
(,ildings
TITLE
L
ReR,irements and 1,idelines f+r the Seismic Design and
Review +f Cew Tall (,ildings ,sing C+nKPrescriptive SeismicK
Design Pr+ced,res
PURPOSE
L
The purpose of this administrative bulletin is to
requirements and guidelines for the seismic structural
Seismic Peer Review, and building permit submittals for
buildings in San Francisco that use non-prescriptive
design procedures.
REFERECCES
L
2001 San Francisco Building Code (SFBC)
- Section 104.2.5 Alternate materials, alternate design and
methods of construction
- Section 1605.2 Rationality
- Section 1629.10 Alternative procedures
present
design,
new tall
seismic
ASCE, 2005, Minimum Design Loads for Buildings and Other
Structures (ASCE/SEI 7-05), Prepared by the Structural
Engineering Institute of the American Society of Civil
Engineers, Reston, Virginia.
SEAOC, 1999a, Recommended Lateral Force Re=uirements and
Commentary (Blue Boo?), Seismology Committee, Structural
Engineers Association of California, Sacramento California.
SEAOC, 1999b, “Project Design Peer Review” (Chapter 4,
October 1995) Recommended Auidelines for the Practice of
Structural Engineering in California, Structural Engineers
Association of California, Sacramento, California
SEAONC, 1999, Contractual Provisions to Address the EngineerFs
Liability when Ising Performance-Based Seismic Design,
Structural Engineers Association of Northern California, San
Francisco, California, 7 pages, June
SEAOC, 2001, “Seismology Committee Background and Position
Regarding 1997 UBC Eq. 30-7 and Drift,” Structural Engineers
Association of California, Sacramento California, September
(http://www.seaoc.org/seismpdfs/UBC/30_7.pdf)
DISCUSSIOC
April 2007
L
See Commentary sections throughout this document.
1
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
1. SCOPE
This bulletin presents requirements and guidelines for seismic structural design, Seismic
Peer Review, and building permit submittals, for new tall buildings in San Francisco that
use non-prescriptive seismic design procedures.
Commentary: This bulletin intentionally contains both requirements, which are stated in
mandatory language (e.g., “shall”) and guidelines, which use non-mandatory language.
The bulletin is applicable to the 2001 SFBC. In 2008, San Francisco is scheduled to adopt a new
building code based on the 2006 International Building Code. At such a time, applicability of
specific requirements of the bulletin to the new code should be reviewed, in particular with
respect to issues such as minimum base shear and construction quality assurance plans.
The requirements and guidelines of this bulletin are written specifically for San Francisco. Any
application to other locations and jurisdictions should consider appropriate modifications, for
example, to account for local seismic hazard issues (including near-fault effects), different
governing building codes, or different building authority practices.
This administrative bulletin is written to address only non-prescriptive seismic designs of tall
buildings. Should the Director deem it appropriate to require Seismic Peer Review of a different
building type or a code-prescriptive design, some sections of this bulletin may be applicable.
Additional recommendations for Seismic Peer Review are provided by SEAOC [1999b].
The bulletin is not written to cover essential facilities.
For the purposes of this bulletin, a non-prescriptive seismic design is one that takes
exception to one or more of the prescriptive requirements of the SFBC related to seismic
design by invoking Section 104.2.8, 1605.2, and/or 1629.10.1 of the SFBC, which permit
alternative (i.e., non-prescriptive) seismic design procedures.
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 SFBC as the height of Level n above the Base. 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. The Base is permitted to be taken at the average level of the ground surface
adjacent to the structure.
Procedures other than those presented herein may be acceptable pursuant to the
approval of the Director.
Commentary: SFBC sections that permit non-prescriptive or “alternative” seismic design
procedures are reproduced below:
104.2.8 Alternate materials, alternate 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.
April 2007
2
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
1605.2 Rationality. 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.
1629.10.1 [Alternative Procedures] General. Alternative lateral force [i.e., seismic design] procedures
using rational analyses based on well-established principles of mechanics may be used in lieu of those
prescribed in these provisions.
2. SEISMIC PEER REVIEW PANEL
For each project, a Seismic Peer Review Panel (SPRP) shall be convened. The SPRP
is to provide an independent, objective, technical review of those aspects of the
structural design of the building that relate to seismic performance, according to the
requirements and guidelines described in this bulletin, and to advise the Director
whether the design generally conforms to the intent of the SFBC provisions referenced
in Part 1 of this bulletin.
The SPRP participation is not intended to replace quality assurance measures ordinarily
exercised by the EOR in the structural design of a building. Responsibility for the
structural design remains solely with the EOR, and the burden to demonstrate
conformance of the structural design to the intent of the SFBC provisions referenced in
this bulletin resides with the EOR. The responsibility for conducting Structural Plan
Check Review resides with the Director and any Plan Check Review consultants.
Qualifications and Selection of Panel Members
Except when determined otherwise by the Director, the SPRP should include a minimum
of three members with recognized expertise in relevant fields, such as structural
engineering, earthquake engineering research, performance-based earthquake
engineering, nonlinear response history analysis, tall building design, earthquake ground
motion, geotechnical engineering, geological engineering, and other such areas of
knowledge and experience relevant to the issues the project poses.
The SPRP members shall be selected by the Director based on their qualifications
applicable to the Seismic Peer Review of the project. The Director may request the
opinion of the Project Sponsor and EOR on proposed SPRP members, with the Director
making the final decision on the SPRP membership. SPRP members shall bear no
conflict of interest with respect to the project and shall not be part of the design team for
the project. The SPRP provides their professional opinion to and acts under the
instructions of the Director.
Peer Review Scope
The general scope of services for the SPRP shall be indicated by the Director. Based
on this, the SPRP, either individually or as a team, shall include a written scope of work
in their contract to provide engineering services. The scope of services should include
review of the following: earthquake hazard determination, ground motion
characterizations, seismic design methodology, seismic performance goals, acceptance
criteria, mathematical modeling and simulation, seismic design and results, drawings
and specifications.
The SPRP should be convened as early in the structural design phase as practicable to
afford the SPRP opportunity to evaluate fundamental design decisions that could disrupt
design development if addressed later in the design phase. Early in the design phase,
the EOR, DBI, and the SPRP should jointly establish the frequency and timing of SPRP
April 2007
3
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
review milestones, and the degree to which the EOR anticipates the design will be
developed for each milestone.
The SPRP shall provide written comments to the EOR and to the Director, and the EOR
shall prepare written responses thereto. The SPRP shall maintain a log that
summarizes SPRP comments, EOR responses to comments, and resolution of
comments. The SPRP shall make the log available to the EOR and to the director as
requested. At the conclusion of the review the SPRP shall submit to the Director a
written report that references the scope of the review, includes the comment log, and
indicates the professional opinions of the SPRP regarding the design’s general
conformance to the requirements and guidelines in this bulletin. The Director may
request interim reports from the SPRP at the time of interim permit reviews.
Commentary: None of the reports or documents from the SPRP are Construction Documents.
Under no circumstances should letters or other documents from the SPRP be put into the EOR’s
drawings or reproduced in any other way that makes SPRP documents appear to be part of
the Construction Contract Documents. The EOR is solely responsible for the Construction
Contract Documents.
The Director will address differences of opinion between the EOR and the SPRP.
The EOR shall inform the Director of significant changes to the structural design,
detailing, or materials made subsequent to the Peer Review, including during
construction. At the discretion of the Director, such changes shall be reviewed by the
SPRP and approved by the Director.
Compensation of the SPRP members shall be borne by the project sponsor. In the case
that SPRP members contract with the project sponsor, the scope of services in the
contract shall be approved by the Director. Any changes to the scope of services shall
be approved by the Director.
Commentary: A number of jurisdictions contract directly with Seismic Peer Reviewers and pass
the cost through to the Project Sponsor, an approach recommended by the Structural Engineers
Association of Northern California. Currently the City of San Francisco procedures for
procurement of professional services are not suited to this approach, so Seismic Peer Reviewers
instead contract directly with the project sponsor. Even so, the SPRP provides their professional
opinion to and acts under the instructions of the Director.
3. SUBMITTAL REQUIREMENTS
Project submittals shall be in accordance with the SFBC and SFDBI interpretations,
bulletins, and policies. In addition, documents relevant to the Seismic Peer Review shall
be submitted by the EOR to the Director and to the SPRP.
As early as practicable, the EOR 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 Seismic Peer 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 SFBC,
modeling parameters, material properties, drift limits, element force capacities and
deformation capacities. The Seismic Design Criteria shall identify all exceptions to the
SFBC prescriptive requirements the EOR proposes. The Seismic Design Criteria shall
be subject to review by the SPRP and approval by the Director. A summary of the
April 2007
4
Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
EOR’s final Seismic Design Criteria shall be included in the general notes of the
structural drawings.
4. SEISMIC DESIGN REQUIREMENTS
The EOR 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 MCE ground motions specified in
Section 4.3, the EOR 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 EOR 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 SFBC and to define the minimum required strength and stiffness
for earthquake resistance. Minimum strength is defined according to SFBC minimum base shear
equations, with a seismic design coefficient R, proposed by the EOR, reviewed by the SPRP, and
approved by the Director Minimum stiffness is defined by requiring the design to meet SFBCspecified drift limits, using traditional assumptions for effective stiffness. Providing a nonprescriptive 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 serviceability evaluation is required by this bulletin only in
circumstances where there is a reason to expect that serviceability performance of the design
would not be at least equivalent to that anticipated for a code-prescriptive design.
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.
SJ1 C+deKLevel Eval,ati+n
The seismic structural design shall be performed in accordance with the prescriptive
provisions of the SFBC, except for those provisions specifically identified by the EOR 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 zones include exceeding the height limitations of SFBC Table 16-N.
Other exceptions that have been taken include SFBC provisions related to R, !, "0, limitations on
T, various detailing requirements, and others. The EOR is required to justify all exceptions to
prescriptive code provisions.
The lower limits of SFBC Formulas 30-6 and 30-7 in the calculation of the Elastic
Response Base Shear apply to the scaling process of SFBC Section 1631.5.4. The
value of R used shall be indicated in the Seismic Design Criteria, and shall not be
greater than 8.5.
April 2007
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Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
Commentary: For buildings with a fundamental period, T greater than about 2 to 3 seconds, the
design base shear is likely to be governed by the lower limits specified in Formulas 30-6 and 307, in which case the seismic coefficient R only affects the design according to its application in
Formula 30-7. For R greater than about 7, Formula 30-6 governs over Formula 30-7, in which
case the design base shear is independent of R. The relationships that determine the significance
of R depend on the ground motion factors at the site and the soil profile type, and must be
evaluated by the EOR for the specific project under consideration.
The EOR shall demonstrate that the structure meets the story drift ratio limitations of the
SFBC 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 SFBC Formulas 30-6 and 30-7
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: The position statement by SEAOC [2001] gives background on the application of
Formula 30-7 to the check of story drift ratio. The position statement recommends, and parallel
code sections in ASCE/SEI 7-05 require, including the minimum forces of SFBC Formula 30-7 in
the check of drift limits. However, the consensus of SEAONC’s AB-083 Task Group for this
administrative bulletin, approved by the SEAONC Board, is that Formula 30-7 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 grosssection 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 SFBC.
SJ2 ServiceaUility Eval,ati+n
An evaluation of the building for the serviceability level of seismic performance is
required in the circumstances below:
a) Where the EOR has taken any exception to code-prescriptive requirements for
non-structural elements (SFBC 1632), the EOR shall submit an analysis and
evaluation of the seismic performance of non-structural elements and systems
for the serviceability ground motion.
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, the EOR shall submit an analysis of the structure for the
serviceability ground motion and an evaluation of the serviceability performance
of such elements.
April 2007
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Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
c) For a structure that exhibits disproportionably large drift or accelerations for
ground motions less than the SFBC Design Basis Ground Motion (not reduced
by R), the EOR shall submit an analysis and evaluation of the seismic
performance of all structural and non-structural elements for the serviceability
ground motion.
Commentary: Because Section 4.1 requires the application of code lateral-force levels and drift
limits, a serviceability-level evaluation is required by this bulletin only in 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. Three such circumstances are identified.
The first applies when exceptions are taken to code-prescriptive non-structural requirements.
The second applies to the case where the effective stiffness of a structural element, for example a
concrete coupling beam, is artificially reduced in the code-level analysis in order to lower the
design forces for the element. Such elements may be susceptible to excessive damage under the
serviceability ground motion and must be checked by analysis.
The third circumstance would apply to an unusual type of structure that is able to meet code-level
lateral force and drift limits per Section 4.1, but that experiences disproportionably large drift or
accelerations for lower levels of ground motions. This circumstance is identified in the bulletin to
address any potential new structural system, not currently covered in the SFBC, that might be
particularly vulnerable to damage in a serviceability ground motion.
For the purposes of this bulletin, the serviceability ground motion shall be that having a
43-year mean return period.
Structural models used in the serviceability 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: It is not the intent of this bulletin to require that a structure remain fully linearly
elastic for the serviceability ground motion. 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, 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 serviceability ground motion. Where response history
analysis is used, the selection and scaling of ground motion time series should comply with the
requirements of SFBC 1631.6.1 with the serviceability-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.
April 2007
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Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
SJ3 DCEKLevel Eval,ati+n
Ground Motion: The ground motion representation for this evaluation shall be the
Maximum Considered Earthquake (MCE) as defined in SFBC Section 1655.
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 SFBC 1631.6.1 with the
following modifications:
a) The MCE response spectrum shall be the basis for ground motion time series
scaling instead of the design-basis earthquake (DBE) 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 EOR 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 faultnormal 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 Equation 30-7 to drift calculations. If spectral matching is
used, individual ground motion components should account for the distinction between faultnormal and fault-parallel hazard.
Mathematical Model: The three-dimensional mathematical analysis model of the
structure shall conform to SFBC Section 1631.3.
The analyses shall consider the interaction of all structural and nonstructural 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.
April 2007
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Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
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 EOR 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-# effects that include all the building dead load shall be included explicitly in the
nonlinear response history analyses.
Documentation submitted for SPRP 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 = 0.1L, where L is the codeprescribed 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 EOR 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 EOR.
April 2007
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Structural Engineers Association
of Northern California
Recommended Administrative Bulletin on the Seismic Design
and Review of Tall Buildings Using Non-Prescriptive Procedures
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:
1.0D + Lexp + 1.0E
Alternative load combinations, if used, shall be adequately substantiated by the EOR.
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 codelevel analysis but deemed to be materially affected.
Commentary: Elements not designated as part of the lateral-force-resisting system in the codelevel analysis (gravity systems) may be subjected to substantial deformations and forces,
including axial forces accumulated over many stories, as they interact with the primary lateralforce-resisting system. Nonstructural elements such as cladding are evaluated according to code
requirements. This bulletin does not require checking nonstructural elements at the MCE level.
The EOR 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 are defined as nominal strengths multiplied by
strength reduction factors, where nominal strengths are calculated based on expected material
properties, and strength reduction factors are as prescribed in the SFBC for non-ductile actions
and are taken as $ =1.0 for ductile actions. Alternative approaches to demonstrating essentially
elastic response may be acceptable where appropriately substantiated by the EOR.
For structural elements or actions that are designed for nonlinear seismic response, the
EOR 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 SPRP and approval by the Director.
April 2007
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STRUCTURAL ENGINEERS ASSOCIATION OF NORTHERN CALIFORNIA
575 Market Street Suite 2125 • San Francisco, California 94105-2870
Phone 415/974-5147 • Fax 415/764-4915 • E-mail: office @seaonc.org • Website: http://www.seaonc.org