Performance Based Seismic Design Guidelines for Tall
Buildings and their Applications
Farzad Naeim
1
What is a Tall Building?
• Overall height as a measure ü Some codes such as ASCE 7 impose limits on
lateral systems to be used based on height • Aspect ratio as a measure • Vibration period as a measure • Prevalence of higher modes in response
as a measure • No universally accepted definition exists
but you know one when you see one!
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Should tall buildings be treated like other buildings?
• Tall buildings are occupied by hundreds if not thousands of people
• The consequence of failure of tall buildings is much more severe than an ordinary building
• Codes provide a “one size fits all” approach to seismic design.
• Tall buildings as small class of specialized structures will perform better during earthquakes if special attention is afforded to their individual characteristics.
• Prescriptive codes are not equipped with means to distinguish these differences.
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Why prescriptive codes are not suitable?
• Because they simply cannot give you what you need. • Linear analysis is incapable of accurately predicting
collapse and failure which are inherently nonlinear • The overwhelming majority of construction in United
States and worldwide consists of low-rise buildings
• Prescriptive provisions are not generally written with tall buildings in mind.
1 to 3 Stories (93%)
14 Stories and Taller (1%)
4 to 13 Stories (6%)
We will examine two guidelines.
ASCE 41and Tall Building Design Guidelines
• ASCE41 is officially intended for seismic rehabilitation of existing structures
• However, its component-based performance limits for NDP are routinely referenced by guidelines for performance based design of tall buildings
• Engineers who believe ASCE 41limits are too conservative, or are not applicable to their project, conduct tests to establish appropriate limits
• Peer review approval is always necessary for any deviation from ASCE 41
6
Common Performance Objectives
• SEAOC-99
• ASCE 41 – Similar objectives permitted. Emphasis on two events:
• 475 years (10% in 50 years), and • 2,475 years (2% in 50 years)
• Tall Building Design Guidelines – Serviceability: 43 years – Collapse Prevention: 2,475 years
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Analytical Procedures
• ASCE-41 permits four types of analyses: 1. Linear elastic static procedure (LSP) 2. Linear dynamic procedure (LDP) or response
spectrum analysis 3. Non-linear static procedure (NSP) commonly
referred to as the push-over analysis, and 4. Dynamic nonlinear response analysis (NDP).
• Tall Building Design Guidelines permit only
two: 1. 3D LDP or NDP for serviceability check 2. 3D NDP for all other checks
8
PEER-TBI & LATBSDC Performance Objectives
1. Serviceable behavior under events having a 50% probability of being exceeded in 30 years (43 year return period) • building structural and nonstructural components retain
their general functionality during and after earthquake • Repairs, if necessary, are expected to be minor and
could be performed without substantially affecting the normal use and functionality of the building
2. A low probability of collapse under events having a 2% probability of being exceeded in 50 years (2,475 year return period) • Demands are checked for all structural members (lateral
as well as gravity system) • Claddings and their connections to the structure must
accommodate MCE displacements without failure
1. Use 2.5% damping instead of 5% damping but permit DCR = 1.5 for deformation controlled members for serviceability.
2. 2011 LATBSDC limits DCR to 0.70 for force controlled members in serviceability check.
3. 2010 PEER requirements for collapse prevention are more elaborate and detailed than 2011 LATBSDC
4. No minimum base shear capacity requirement
PEER-TBI & LATBSDC Provisions
Design Procedures
• None of the guidelines tell you how to design • For example, 2011 LATBSDC states:
ü Use Capacity Design Techniques ü Develop Project-specific Design Criteria, and ü Clearly define where nonlinearity can occur and make sure it
does not occur elsewhere ü Recommends preferred zones of nonlinearity
• But they do not explain how the engineer is supposed to achieve this design.
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Source: 2011 LATBSDC ROSE School 2013 12
Evaluation Procedures
• All guidelines require a three-dimensional detailed mathematical model of the physical structure
• Realistic estimates of stiffness and damping
• Expected material properties for ductile elements
• Specified material properties for brittle elements
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Source: 2011 LATBSDC Performance Based Seismic Assessment of Tall Buildings – I 14
Effective Stiffness Values for Linear Analysis
Source: 2008 LATBSDC, 2010 PEER ROSE School 2013 15
Effective Stiffness Values for Linear Analysis
Source: 2011 LATBSDC ROSE School 2013 Performance Based Seismic Assessment of Tall Buildings – I 16
Analysis Methods
• Serviceability: ü Can use either
1. Linear Response Spectrum Analyses � CQC mode combination � 90% mass participation
2. Nonlinear Response History Analyses • For MCE (ultimate state) evaluation:
ü Must use � Nonlinear Response History Analyses
• Inherent torsional properties of the structural system should always be considered.
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P-Δ Inclusion
• P-Δ effects must be included in all analyses ROOF DRIFT ANGLE vs. NORMALIZED BASE SHEAR
Pushover (NEHRP '94 k=2 pattern); LA 20-Story
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ROOF DRIFT ANGLE vs. NORMALIZED BASE SHEARPushover (NEHRP '94 k=2 pattern); LA 20-Story
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Figure courtesy of Prof. Helmut Krawinkler
Modeling Nonlinear Behavior
Figure courtesy of Prof. Prof. Greg Deierlein
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Modeling Nonlinear Behavior
• Concentrated plasticity model for beams and columns and fiber elements for walls are most common
• All other elements and components that in combination significantly contribute to or affect the total or local stiffness of the building should be included in the mathematical model.
• Axial deformation of gravity columns in a core-wall system is one example of effects that should be considered in the structural model of the building
Figure courtesy of MKA
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Accidental Eccentricity (AE)
• 2011 LATBSDC ü Consider implications during serviceability evaluation ü Address if significant during MCE evaluation
• 2010 PEER TBI ü Do not need to consider
• Consideration of AE in nonlinear analyses requires multiple evaluations and little is gained by such time-consuming exercises.
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Modeling Strength / Stiffness Degradation
• 2010 PEER TBI ü Provides detailed guidelines on four
approved methods for modeling degradation
• 2011 LATBSDC ü Adopts the first two of the detailed
procedures contained in 2010 PEER.
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2010 PEER TBI Degradation Modeling Options
Figure courtesy of Prof. Helmut Krawinkler
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Upper Limit on Column Axial Forces
• Large axial forces reduce available column ductility
• 2011 LATBSDC ü MCE: P < 0.4f’cAg
• 2010 PEER TBI ü MCE: P < balanced load < 0.3f’cAg
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Soil-Foundation-Structure-Interaction (SFSI)
• Naeim & Stewart (2008) demonstrated the difficulties of realistic modeling of SFSI in a design environment.
• 2010 PEER TBI has two recommended modeling techniques
• 2011 LATBSDC recommends a single approach for this.
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2010 PEER TBI Suggested Modeling Techniques for SFSI
2011 LATBSDC
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Damping
• A particularly thorny issue ü In nonlinear analyses most of the damping is
represented by hysteretic behavior of the elements ü Some small additional viscous damping may be
justified for: � Energy dissipation provided by components
and systems not explicitly modeled � As necessary to avoid numerical instability
• 2011 LATBSDC ü Limits viscous damping to 2.5% for both
serviceability and MCE.
• 2010 PEER TBI ü 2.5% for linear serviceability evaluation ü Refers to ATC-72 for nonlinear evaluation
27
Ground Motion Selection and Scaling
• A minimum of 7 pairs is usually required • 2011 LATBSDC
ü Adopts by reference Chapter 21 of ASCE 7 • 2010 PEER TBI
ü More flexible ü Permits scaling, matching or CMS ü Multiple CMS required if CMS is used, making
this impractical for tall buildings • Most practicing engineers prefer matching
ü One must be careful as, matched motion contains less record to record dispersion
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Acceptance Criteria -- Maximum Drift
• Absolute Maximum Transient Drift Limit ü Serviceability:
� 2011 LATBSDC & 2010 PEER TBI: 0.005 overall
ü MCE: � 2011 LATBSDC & 2010 PEER TBI:
0.030 max average at any story 0.045 max. interstory drift at any story under any
record
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Acceptance Criteria -- Maximum Drift
• Absolute Maximum Residual Drift Limit ü Serviceability:
� 2011 LATBSDC 0.005 overall ü MCE:
� 2011 LATBSDC and 2010 PEER: 0.010 average max. of time histories
0.015 maximum from any
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Acceptance Criteria -- Serviceability
• 2011 LATBSDC ü Brittle Actions:
Strength Demand < 0.7*Capacity ü Ductile Actions:
� Linear Analysis Strength Demand < 1.50 Capacity
� Nonlinear Analysis Can use up to IO limit of ASCE 41
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Acceptance Criteria MCE
• 2010 PEER and 2011 LATBSDC ü Ductile Actions:
� Deformation Demand < ASCE 41-06 CP Deformation Capacity
� Continuous Load Path � Capacity exhausted when it drops below
80% of maximum strength
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Acceptance Criteria -- MCE
• 2010 PEER ü Brittle Actions:
� Two Groups: � Critical Actions
� failure mode pose severe consequences to structural stability under gravity and/or lateral loads
� Design for mean + 1.3 to 1.5 times SD � Noncritical Actions
� Design for mean values � Use ϕ = 0.75 for shear
• 2011 LATBSDC ü Essentially the same, except uses 1.5 times
mean and ϕ = 1.0
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R/C Specific Requirements
• None in 2010 PEER • Several in 2011
LATBSDC ü Detailing
� The spacing limit of 12 inches of ACI 318 §21.5.3.2 (d) is reduced to 6 inches.
ü High-Strength Concrete
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Peer Review Requirements
• Each project needs a Seismic Peer Review Panel (SPRP)
• SPRP is to provide an independent, objective, technical review of design
• Paid by the owner but reports to Building Official • Responsibility for the structural design remains
solely with the EOR • SPRP is not a plan checking entity • Minimum of three members with recognized
expertise in relevant fields such as: ü structural engineering ü earthquake engineering research ü performance-based earthquake engineering ü nonlinear response history analysis ü tall building design ü earthquake ground motions, geotechnical engineering,
geological engineering 35
Instrumentation Requirements
• 2010 PEER TBI ü No requirements
• 2011 LATBSDC ü Detailed requirements ü Consistent with CGS / CSMIP
36
A typical tall building instrumented by CSMIP
ROSE School 2013 Performance Based Seismic Assessment of Tall Buildings – I 37
Seismic Instrumentation
CSMIP sensor layout ROSE School 2013 38
Applications
• Many tall buildings have been designed using these guidelines in Los Angeles, San Francisco, San Diego, and elsewhere
• Here are some examples ü Los Angeles:
� 888 Olive � 1133 Olive � 1212 Flower Towers � Wilshire & Grand � Metropolis Tower
ü San Diego � 7th & Ash
ü San Francisco � Transbay Tower
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• 888 Olive Street in downtown Los Angeles ü 34 stories ü Core wall construction ü Podium ü Subterranean levels ü Basement walls ü Flat plates ü Gravity columns
Illustrations and drawings courtesy of Onni Group and Glotman-Simpson
Illustrations and drawings courtesy of Onni Group and Glotman-Simpson
LEVEL 1 0.00'
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CHRIS DIKEAKOSARCHITECTS INC.
SCALE: 1/32" = 1'-0"OLIVE ST. MIXED-USE DEVELOPMENT817 - 825 Hill St. & 820 S. Olive St., Los Angeles, CA
SECTIONAugust 26th, 2013 A306Design Development
1/32" = 1'-0"A2011 Section 9
42
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LEVEL 1 0.00'
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CHRIS DIKEAKOSARCHITECTS INC.
SCALE: 1/32" = 1'-0"FLOWER ST. MIXED-USE DEVELOPMENT1212 S. Flower Street, Los Angeles, CA
BUILDING SECTIONSAugust 26, 2013 A305DESIGN DEVELOPMENT
1/32" = 1'-0"A2011 TOWER 2 BUILDING SECTION
1/32" = 1'-0"A2012 TOWER 1 BUILDING SECTION
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Thank you!
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