Seismic Performance and Design

Requirements for High-Rise Concrete

Buildings
In recent years there has been a resurgence of
high-rise construction in the major cities along
the West Coast of the U.S. Unlike previous
high-rise booms, most of the new and proposed
tall buildings are for residential or mixed use
rather than for offices. Concrete construction is
often favored, and many of the new high-rises
use concrete core-wall construction without
supplemental moment frames in the seismic-
force-resisting system.
Concrete core-wall construction can offer
advantages of lower costs, faster construction,
and more open and flexible architecture. Cost
and schedule savings are realized because core-
wall buildings withstand seismic forces and
deformations without the moment frames that
are used in traditional high-rise construction. By
eliminating the need for moment frames,
smaller framing members or flat slabs can be
used for the building floors, and the framing
depth of floors can be reduced.
In a core wall building, resistance to seismic
forces is provided by a reinforced concrete core
that surrounds the elevator banks. Stairs,
restrooms, and mechanical/service uses may
also be located within the core. For buildings
300 feet or taller, the concrete core usually has a
minimum dimension of 30 feet in each plan
direction, with walls that are 18 to 30 inches
thick (Figure 1). Regular openings are used in
the core walls, and the coupling beams above
the openings are reinforced and detailed to
dissipate earthquake energy.
Figure 1: Concrete core-wall building under
construction, the Washington Mutual/Seattle Art
Museum, Magnusson Klemencic Associates,
Structural Engineers.
Code Acceptance of Non-Prescriptive Designs
In high seismic zones, prescriptive provisions of
U.S. building codes do not permit the core-wall
structural system for buildings over 240 feet
tall; however, under building code provisions
that permit alternative systems, building
authorities have granted approval to core-wall
buildings greater than 240 feet tall using the
process of Seismic Peer Review. (See sidebar.)
The Engineer of Record is required to identify
any exceptions being taken to prescriptive
requirements, and to demonstrate to an expert
reviewer that the building provides at least
equivalent seismic performance to that implied
or resulting from the prescriptive requirements
of the building code.
The task of the Engineer of Record is to show
that a building satisfies the equivalent
performance criteria defined in IBC Section
104.11:
104.11 Alternate materials, design and methods
of construction and equipment. The provisions
of this code are not intended to prevent the
installation of any material or to prohibit any
design or method of construction not
specifically prescribed by this code, provided
that any such alternative has been approved. An
alternative material, design or method of
construction shall be approved where the
building official finds that the proposed design
is satisfactory and complies with the intent of
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 quality, strength,
effectiveness, fire resistance, durability and
safety.
For non-prescriptive seismic designs, the
performance is evaluated with respect to
strength, effectiveness, and safety. Alternative or
non-prescriptive seismic designs are also
accepted in the building code by ASCE 7-05,
Section 12.1.1, paragraph 3:
Seismic force-resisting systems that are not
contained in Table 12.2-1 shall be permitted if
analytical and test data are submitted that
establish the dynamic characteristics and
demonstrate the lateral force resistance and
energy dissipation capacity to be equivalent to
the structural systems listed in Table 12.2-1 for
equivalent response modification coefficient, R,
system overstrength coefficient, o, and
deflection amplification factor, Cd, values.
Although Table 12.2-1 of ASCE 7-05 lists a
number of types of concrete wall seismic-force-
resisting systems, none of the design rules for
such systems are as stringent as the capacity-
design requirements typically applied to the
design of core-wall high-rise buildings. Thus,
based on expected seismic performance,
capacity-designed and flexure-governed
concrete wall buildings can be considered a
distinct type of seismic-force-resisting system.
This distinction currently exists in building
codes outside the US, and has been discussed as
a potential change to upcoming US building
codes by the American Concrete Institute and
National Earthquake Hazards Reduction
Program.
Figure 2: The typical nonlinear action for a
cantilever wall (left) is a flexural plastic hinge at
the base of the wall. For a coupled wall (right)
nonlinear actions are flexure-yielding coupling
beams and a flexural plastic hinge at the base of
the wall.
Capacity Design
The capacity-design approach to seismic design
requires that the structural engineer:
1) Select a desirable mechanism of non-linear
lateral deformation for the structure, which
identifies those structural elements and actions
that are intended to undergo nonlinear response.
The mechanism should not lead to concentrated
nonlinear deformations such as occurs, for
example, with a story mechanism.
2) Ensure that the detailing of the designated
nonlinear elements provides adequate ductility
capacity, i.e., allows the elements to deform
well beyond yield without significant strength
degradation.
3) Design all other elements and actions of the
structure for elastic, or nearly elastic, response.
For a concrete core-wall building under
earthquake lateral displacement, the desired
mechanism consists of flexural plastic hinging
near the base of the core wall and flexural
yielding of coupling beams, as shown in Figure
2. Some core-wall buildings have coupling
beams only in one plan direction, with walls in
the other plan direction acting as cantilever
walls, as shown in Figure 2. The cantilever wall
is designed to develop a single plastic hinge at
its base. In each plan direction, the wall flanges,
typically including the entire core-wall section,
contribute to global moment capacity.
The nonlinear elements of the structure -
coupling beams and the base plastic hinge - are
detailed for ductile response. Other elements
and actions of the structure - such as wall shear,
wall moment outside the hinge zone, floor and
roof diaphragms, and foundations - are given
sufficient strength that their behavior will be
essentially elastic. Table 1 lists structural
elements and actions for a core-wall building
that are typically designed for nonlinear
behavior and those that are designed for elastic,
"capacity-protected" behavior.
Figure 3: Concrete wall failing in shear in the
1995 Kobe earthquake. Capacity design aims to
protect against such a failure mode.
Flexure-Governed Design
A critical consideration in the design of the
concrete wall system is to protect against shear
failure in the wall. A wall governed by flexural
yielding will maintain its lateral-force resistance
through large displacements and will deform in
a way that distributes deformation over the
height of the building. A wall shear failure, by
contrast, leads to a degradation of strength and
can cause a concentration of deformation and
damage over a limited height (Figure 3).
Flexure-governed response provides a greater
assurance against collapse in a severe
earthquake.
The seismic design process for concrete core-
wall buildings is based on methods that were
established in the New Zealand and Canadian
building codes beginning in the 1970s. A large
number of core-wall high-rises were built in
Vancouver before the methodology was applied,
with Seismic Peer Review, to high-rise
buildings in the Seattle area and elsewhere in
the U.S.
Capacity Design using Nonlinear Response-
History Analyses
The capacity design approach was principally
developed and promoted by researchers and
practicing engineers in New Zealand, at a time
when computer analysis capabilities were
limited. Nonlinear response-history (NLRH)
analyses were only feasible on large university
computers using two-dimensional models of
simplified structures. Researchers used such
analyses to derive detailed requirements for
capacity design that could be applied to simpler
static and linear analysis and design practices.
These detailed capacity-design requirements,
such as dynamic shear amplification factors, are
still useful, particularly for regular structures
less than 20 stories and for the preliminary
design of taller structures. Today, thanks to
recent advances and availability in structural
analysis software, the capacity design approach
can be combined with building-specific NLRH
analyses to design high-rise buildings and verify
acceptable seismic performance.
Table 1: Typical nonlinear and capacity-
protected elements for a core-wall building with
concrete flat slabs.
Two-Stage Design Process
Core-wall high-rise buildings can be designed
according to a two-stage process that follows
the capacity-design approach and assesses
seismic performance under severe earthquake
ground motions.
The first stage of the process is to design the
building to comply with all code provisions
(except for identified exceptions such as the
height limit). This means that the designated
yielding elements of the building, namely the
flexural design of the core-wall hinge zone and
the coupling beams, are designed for code-level
demands including the code R factor. For tall
buildings with long periods, this code-level
demand is typically governed by minimum base
shear requirements (Figure 4).
The second stage is to analyze the structure
using an NLRH analysis at the Maximum
Considered Earthquake (MCE) level of ground
motion. The MCE level is currently defined in
building codes to correspond to a 975-year
return period in California and about a 2500-
year return period elsewhere. The purpose of
this analysis is to:
1) Verify that the expected seismic behavior of
the structure is governed by the intended
mechanism, with nonlinear behavior occurring
only in the designated structural elements.
2) Verify that all other potential mechanisms
and actions remain essentially elastic. When
evaluating actions designed to remain elastic,
the design should consider the dispersion of the
NLRH results, rather than just the average
response.
Properly applied, the NLRH analysis takes the
place of applying the code-prescribed over-
strength factor, W0,to actions designed to
remain elastic.

Figure 4: Minimum base shear equations for

recent building codes, as a function of the
ground motion parameter S1.
Semi-Performance-Based Design
The design approach could be considered a
"semi-performance-based". The Code Level
evaluation aims to have the design meet all
prescriptive code requirements with which it is
logical that the design comply, without
evaluating seismic performance. The MCE
Level evaluation explicitly considers the
performance of the structure at a level for which
the structure should not collapse. This
evaluation uses state-of-the-art methods of
analysis, and structural force and deformation
capacities based on expected rather than
nominal values. Story drift limitations can be
checked at the Code Level, and also at the MCE
Level; for example, using the average of the
response-history runs and taking acceptable drift
as 1.5 times that in the building code.
A performance-based evaluation of
serviceability in moderate earthquake ground
motions can also be added to the design
approach. For core-wall buildings the
serviceability evaluation could include an
explicit evaluation of the level of ground motion
for which coupling beam damage affects the
post-earthquake occupancy of the building. A
determination about the significance of various
levels of coupling-beam damage, based on
research results, would be necessary for such an
evaluation.
Interaction with the Gravity System
In customary seismic design practice, the
structural engineer designates certain elements
to be part of the Seismic-Force-Resisting
System. For concrete buildings, these are
typically structural walls and moment frames.
Gravity framing is usually not included in the
lateral analysis for earthquake resistance, but is
instead evaluated for its ability to sustain the
imposed seismic deformations. In reality,
gravity framing systems contribute to some
degree to lateral-force resistance, and this
contribution should be considered in the design
of high-rise buildings, particularly at the MCE-
level evaluation.
For core-wall buildings with concrete flat-slab
floors, the gravity structural system consists of
the floor slabs and supporting columns. Lateral
displacement of the core wall and columns of
the building induces moments and shears in the
floor slabs, which act as unintentional
"outriggers" that increase the buildings lateral
resistance. Often, the lateral displacement under
MCE-level ground motions is enough to cause
flexural yielding in the slabs. Yielding of the
floor slabs is typically acceptable, while other
failure modes such as punching shear from the
induced deformations must be prevented (Table
1).
Two other aspects of this slab-outrigger effect
are important for engineers to evaluate. The first
is that shear in the core wall is increased, and
the second is that earthquake axial forces are
generated in the "gravity" columns. These
demands should be included in the shear design
of the core wall and in the design of the
columns.
Table 2: Differences between Seismic Peer
Review and Structural Plan Check
Defining Equivalent Seismic Performance
The IBCs equivalence criterion requires that the
buildings seismic performance be "at least the
equivalent of that prescribed in this code." In
assessing seismic performance, the Engineer of
Record and Peer Reviewer should consider both
the intentions of the building code, and the
performance that results from a code-
prescriptive design with good seismic
performance.
A problematic issue is that the building codes
intended seismic performance is defined only in
general terms (SEAOC Blue Book Section
C101.1), and it may be impossible to ever more
specifically define the seismic performance
intent of the building code. Part of the reason is
that current design rules for different seismic
systems in the building code may result in quite
different levels of seismic performance from
one system to another. Another part of the
reason is that the assumptions used in
attempting to define seismic performance -
ground motion, soil and structure properties,
non-linear demands, deformation capacity, etc. -
all include significant uncertainty. This
uncertainty is related to both the inherent
variability of earthquake and material
phenomena, and to the limitations in our
knowledge of the best methods and assumptions
to use in all the steps of predicting seismic
performance.
For the reasons noted above, predicting seismic
performance is complex and uncertain, and
hence code intentions are defined only in
general terms. Thus, if one only considers code
intentions, judging whether the seismic
performance of a non-prescriptive design is
"equivalent to code" can be difficult.
Accordingly, it can be helpful if one considers,
in addition to code intentions, the seismic
performance that is expected to result from the
code-prescriptive design of a building similar to
the non-prescriptive design being considered.
This consideration can be useful in judging
equivalent performance for parts of a structural
design that are not closely related to those
prescriptive exceptions being taken using
alternative design methods. A point to remember
here is that, because building codes are not
perfect, it is possible to design a high-rise
building that meets all prescriptive code
requirements, and yet still leads to inadequate
performance in an earthquake. (For example a
shear failure in a wall along with a
concentration of nonlinear deformation over just
a few stories.) Such a benchmark would not be
accepted as equivalent performance, because it
does not meet the intent of the code. It is not
acceptable to provide equivalence to a poorly
performing, yet code compliant building.
Seismic Peer Review versus Structural Plan
Check
For both Seismic Peer Review and Structural
Plan Check, the work of the Engineer of Record
is subjected to an independent and objective
review by another licensed engineer. While
Structural Plan Check has long been part of the
permitting process for most buildings, the
additional step of Seismic Peer Review has
become more common in the past decade
because of an increased realization that good
seismic performance can depend on more than
conformance to building-code prescriptions.
Differences between Seismic Peer Review and
Structural Plan Check are summarized in Table
2. The Structural Engineer Association of
California has written professional practice
guidelines on Peer Review.
Neither a Seismic Peer Review nor a Structural
Plan Check relieves the Engineer of Record
from being fully responsible for the structural
design. Both Seismic Peer Review and
Structural Plan Check should be carried out with
the objective of providing an impartial and
independent review of the Engineer of Records
work.
Seismic Peer Review should start during the
early phases of a project and include an
examination of basic design concepts,
objectives, and criteria proposed for the project.
Major decisions affecting the seismic design are
reviewed throughout the project with a
consideration of the expected seismic
performance. Typically the Peer Reviewers
comments are documented in a comment log,
along with the Engineer of Records response,
references to associated follow-up comments,
and an indication whether each comment is
resolved.
Seismic Peer Review can be a voluntary process
that an owner chooses to employ, it can be
requested by a building authority, or it can be
required by the building code. The 2006
International Building Code requires Seismic
Peer Review (called Design Review) when the
nonlinear response-history method of structural
analysis is used, or when certain design
solutions, such as base-isolation or energy-
dissipation devices, are used. Building
authorities typically require a Seismic Peer
Review when an alternative (i.e., non-
prescriptive) method of seismic design is
proposed.
Structural Plan Check focuses on determining if
a set of construction documents conforms to the
structural requirements of the governing
building code. Structural Plan Check differs
from Seismic Peer Review in that it covers the
review of the structural design for gravity, wind,
and other loads in addition to seismic effects.
Structural Plan Check is typically a review of
final or near-final documents, and does not
focus on evaluating seismic performance, but
instead on reviewing a completed design for
code conformance.
A building authority can use Structural Plan
Check to approve or reject a building permit
application. In contrast, a Seismic Peer
Reviewer does not directly have the authority to
approve or reject a design. The responsibility of
the Peer Reviewer is to provide their
professional opinion, typically in a findings
letter, to the party requesting the Peer Review.
Structural Plan Check is typically paid for by
building permit fees, while Seismic Peer
Review is typically an added cost to the owner.
In the case where a building authority requests a
peer review, the peer reviewer often contracts
with the jurisdiction, which then passes on the
cost to the building owner.
Joe Maffei S.E., Ph.D. is a Principal and Noelle
Yuen S.E. is a senior technical consultant at
Rutherford & Chekene Consulting engineers in
San Francisco. From 1999 to the present, R & C
has carried out the Seismic Peer Review of more
than fifteen high-rise concrete wall buildings, in
Seattle, Bellevue, San Francisco, Sacramento,
San Jose, and San Diego.
REFERENCES
ICC, 2006, International Building Code 2006,
International Code Council, Falls Church
Virginia.
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, Virgina.
FIB, 2003, Seismic Design of Precast Concrete
Building Structures, State of the Art Report
prepared by Task Group 7.3, International
Federation for Structural Concrete (FIB),
Lausanne, Switzerland, October.
Paulay, T. and M. J. N. Priestley, 1992, Seismic
Design of Reinforced Concrete and Masonry
Buildings, John Wiley and Sons, New York.
SEAOC, 1999, Recommended Lateral Force
Requirements and Commentary, Seismology
Committee, Structural Engineers Association of
California, Sacramento California.
SEAOC, 1999, "Project Design Peer Review"
(Chapter 4, October 1995) Recommended
Guidelines for the practice of Structural
Engineering in California, Structural Engineers
Association of California, Sacramento,
California.