PCI 6th Edition

1
PCI 6th Edition

• Building Systems
• (Seismic)

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Presentation Outline

• Building System Loads
• Seismic
• Structural Integrity
• LFRS Walls
• LFRS Frames
• Diaphragms

3
Seismic Changes

• Based on new changes to ASCE 7 and ACI 318
• Based current seismic research and observations

4
Seismic Changes

• Some of these changes are
• Recognition of jointed panel construction
• Recognition of strong and ductile connections in
  precast frames
• Recognition and requirements for connections in
  precast walls

5
Seismic Changes

• Additional changes are
• Modification of drift computation and limiting
  drift
• Deformation compatibility of elements
• Additional soil type classifications
• Special considerations locations near seismic
  faults
• Consideration of redundancy and reliability in
  strength design requirements

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Seismic Changes

• Design Forces are Based on Risk
• Previous codes based on 10 chance of exceedance
  in 50 years
• IBC 2000, 2003 codes based on 2 chance of
  exceedance in 50 years

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Seismic Risk

• Soil factors
• Other regions of high seismic risk - not just
  west coast anymore

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• Practically every precast, prestressed concrete
  structure designed under IBC 2000 will require
  some consideration of seismic effects.

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Seismic Performance Objectives

• Current design - minor damage for moderate
  earthquakes
• Accepts major damage for severe earthquakes
• Collapse is prevented of severe events

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Seismic Performance Objectives

• In order to achieve the design objectives, the
  current code approach requires details capable of
  undergoing large inelastic deformations for
  energy dissipation.

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Seismic Design Approach

• Emulation
• No special requirements for low seismic risk
• Chapter 21 requirements for moderate and high
  seismic risk
• Non-emulative design
• PRESSS
• Acceptance criteria for frames

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Earthquake Loads Equivalent Lateral Force
Method

• Base Shear, V
• V CsW
• Where
• Cs - Seismic Response Coefficient
• W - Total Weight

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Equivalent Lateral Force Method Limitations

• This method may not apply to buildings with
  irregularities in Seismic Design Categories D, E,
  or F

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Earthquake Loads Total Weight, W

• Dead Load of structure plus
• 25 of reduced floor live load in storage areas
• live load in parking structures not included
• Partition load if included in gravity dead
• Total weight of permanent equipment
• 20 of flat roof snow load, pf
• where pf gt 30 psf

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Seismic Response Coefficient, Cs

• Function of
• Spectral response acceleration
• Site soil factors
• Building Period
• Response modification factors
• Importance factor

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Seismic Response Coefficient, Cs

• Step 1 - Determine SS and S1
• Step 2 - Determine site Soil Classification
• Step 3 - Calculate Response Accelerations
• Step 4 - Calculate the 5 Damped Design
  Spectral Response Accelerations
• Step 5 - Determine the Seismic Design Category
• Step 6 - Determine the Fundamental Period
• Step 7 - Calculate Seismic Response Coefficient

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Step 1 Determine SS and S1

• From IBC Map
• From local building codes
• IBC 2003 CD-ROM
• Based on
• Longitude / Latitude
• Zip Code

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Step 2 Determine Site Soil Classification

• If site soils are not known use Site Class D
• Figure 3.10.7 (a) (page 3-111)
• From soil reports

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Step 3 Calculate Response Accelerations

• SMS FaSS
• SM1 FvS1
• Where
• Fa and Fv are site coefficients from Figure
  3.10.7 (b) and (c) (page 3-111)
• SS spectral accelerations for short periods
• S1 spectral accelerations for 1-second period
• All values based on IBC 2003

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Step 4 Calculate the 5-Damped Design Spectral
Response Accelerations

• SDS (2/3)SMS
• SD1 (2/3)SM1

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Step 5 Determine the Seismic Design Category

• Table 3.2.4.1.
• Sometimes this restricts the type of Seismic
  Force Resisting System (SFRS) used (see Figure
  3.10.8) (page 3-112)

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Step 6 (Approximate Period) Determine the
Buildings Fundamental Period

• Where
• Ct 0.016 for moment resisting frame systems of
  reinforced concrete
• 0.020 for other concrete structural systems
• x 0.9 for concrete moment resisting frames
• 0.75 for other concrete structural systems
• hn distance from base to highest level (in
  feet)

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Step 6 (Exact Period) Determine the Buildings
Fundamental Period

• Rayleighs formula
• Where
• wi dead load weight at Floor i
• di elastic displacement at Floor i
• Fi lateral force at Floor i
• g acceleration of gravity
• n total number of floors

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Step 7 Determine Seismic Response Coefficient,
Cs

• Lesser of
• Where
• R Response Modification Factor
• Figure 3.10.8 (page 3-112)
• ? Seismic Importance Factor

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Step 7 Determine Cs

• Minimum Value of Cs
• Special Cases In Seismic Design Categories E and
  F

Cs 0.044SDS?
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Vertical Distribution of Lateral Force

• Where
• Fx Force per floor
• Cvx Vertical distribution factor
• V Base shear
• k 1 - buildings with a period 0.5 sec
• 2 - buildings with a period gt 2.5 sec
• hi and hx height from base to Level i or x
• wi and wx Level i or x portion of total
  gravity load

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Location of Force in Plane

• Accidental Torsion
• calculated by assuming that the center of mass is
  located a distance of 5 of the plan dimension
  perpendicular to the applied load on either side
  of the actual center of mass
• Total torsion sum of the actual torsion plus
  the accidental torsion

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Seismic Drift Requirements

• Elastic Displacement Amplification
  Factor, dx
• Stability Coefficient Limits, q
• P-D Effects

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Drift Limits

• Figure 3.10.9 (page 3-113)

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Drift Amplification Factor, dx

• Where
• dx Amplified deflection of Level x
• dxe Deflection of Level x determined from
  elastic analysis, includes consideration of
  cracking
• Cd Deflection amplification factor
• (Figure 3.10.8)
• ? Seismic Importance Factor

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Stability Coefficient, ?

• Where
• Px Total vertical unfactored load including
  and above Level x
• ? Difference of deflections between levels x
  and x-1
• Vx Seismic shear force acting between levels x
  and x-1
• hsx Story height below Level x
• Cd Deflection amplification factor

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Stability Coefficient, ?

• The stability coefficient is limited to
• Where
• ß ratio of shear demand to shear capacity
  between Levels x and x-1

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P-D Effects

• To account for P-? effects, the design story
  drift is increased by
• (1- ?)-1
• If ? lt 0.10, P-? effects may be neglected

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Reliability Factor, ri

• Required in High Seismic Design Categories D, E,
  and F
• The Earthquake Force is increase by a Reliability
  Factor, ri
• 1.5 Maximum Required Value
• ri 1.0 for structures in Seismic Design
  Categories A, B and C

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Reliability Factor, ri For Moment Frames

• Where, for each level
• Ai floor area
• rmaxi For moment frames, the maximum of the
  sum of the shears in any two adjacent columns
  divided by the story shear. For columns common to
  two bays with moment-resisting connections on
  opposite sides, 70 of the shear in that column
  may be used in the column shear summary.

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Reliability Factor, ri For Shear Walls

• Where, for each level
• Ai floor area
• rmaxi For shear walls, the maximum value of
  the product of the shear in the wall and 10/lw
  divided by the story shear.

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Load Combinations

• U 1.4(DF)
• U 1.2(DFT) 1.6(LH)
• U 1.2D 1.6(Lr or S or R) (1.0L or 0.8W)
• U 1.2D 1.6W 1.0L 0.5(Lr or S or R)
• U 1.2D 1.0E f1L 0.2S
• U 0.9D 1.6W 1.6H
• U 0.9D 1.0E 1.6H
• f1 1.0 Parking garages
• 1.0 Live load 100 psf on public assembly
  floors
• 0.5 All others

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Modification for Vertical Acceleration

• E ?QE 0.2SDSD
• Seismic Load Combinations Become
• U (1.2 0.2SDS)D ?QE f1L 0.2S
• U (0.9 0.2SDS)D ?QE 1.6H

Where QE Horizontal Seismic Force
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Modification for Vertical Acceleration

• E ?QE 0.2SDSD
• Seismic Load Combinations Become
• U (1.2 0.2SDS)D ?QE f1L 0.2S
• U (0.9 0.2SDS)D ?QE 1.6H

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Overstrength Factor, Wo

• Components within the Diaphragm
• Chord ties
• Shear Steel
• Connectors
• ?o 2.0 - Seismic Design Categories C, D, E
  and F
• ?o 1.0 - Seismic Design Categories A and B

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Special Load Combinations

• U 1.2D fiL Em
• U 0.9D E
• Where
• Em WoQE 0.2SDSD
• and
• Wo Overstrength Factor

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Overstrength Factor, Wo

• Connections from Diaphragms to Seismic Force
  Resisting System (SFRS)
• ?o Seismic Design Categories C and higher
• Figure 3.10.8 (page 3-112)

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Structural Integrity Requirements

• All members must be connected to the Lateral
  Force Resisting System (LFRS)
• Tension ties must be provided in all directions
• The LFRS is continuous to the foundation
• A diaphragm must be provided with
• Connections between diaphragm elements
• Tension ties around its perimeter
• Perimeter ties provided
• Nominal strength of at least 16 kips
• Within 4 ft of the edge
• Column splices and column base connections must
  have a nominal tensile strength not less than
  200Ag in pounds

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Structural Integrity Requirements

• Precast vertical panels connected by a minimum of
  two connections
• Each connection is to have a nominal strength of
  10 kips
• Precast diaphragm connections to members being
  laterally supported must have a nominal tensile
  strength not less than 300 lbs per linear ft
• Connection details allow volume change strains
• Connection details that rely solely on friction
  caused by gravity loads are not to be used

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Lateral Force Resisting Systems (LFRS)

• Rigid frames and shear walls exhibit different
  responses to lateral loads

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Influential Factors

• The supporting soil and footings
• The stiffness of the diaphragm
• The stiffness LFRS elements and connections
• Lateral load eccentricity with respect to center
  of rigidity of the shear walls or frames

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Shear Wall Systems

• Most common lateral force resisting systems
• Design typically follows principles used for
  cast-in-place structures

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International Building Code(IBC) Requirements

• Two categories of shear walls
• Ordinary
• Special

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ACI 318-02 Requirements

• Created an additional intermediate category, but
  has assigned no distinct R, ?o and Cd

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ACI 318-02 Wall Definitions

• Defines all shear walls as structural walls
• Three levels of definition
• Ordinary structural (shear) wall
• Intermediate precast structural (shear) wall
• Special precast structural (shear) wall

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Ordinary Structural (Shear) Wall

• Wall complying with the requirements of Chapters
  1 through 18
• No special seismic detailing

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Intermediate Precast Structural(Shear) Wall

• Wall complying with all applicable requirements
  of Chapters 1 through 18
• Added requirements of Section 21.13
• Ductile connections with steel yielding
• 1.5 factor for non-yielding elements
• IBC imposes restriction that yielding be in the
  reinforcing

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Special Precast Structural (Shear) Wall

• Precast wall complying with the requirements of
  21.8.
• Meeting the requirements for ordinary structural
  walls and the requirements of 21.2
• Requires precast walls to be designed and
  detailed like cast-in-place walls, emulative
  design
• Meet the connection requirements of Section 21.13.

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Design Guidelines for Shear Wall Structures

• Evaluation of building function and applicable
  precast frame
• Preliminary development of shear wall system
• Determination of vertical and lateral loads

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Design Guidelines for Shear Wall Structures

• Preliminary load analysis
• Selection of shear walls
• Final load analysis
• Final shear wall design
• Diaphragm design

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Moment Frame Classifications

• Three Classifications
• Ordinary Moment Frame
• Intermediate Moment Frames
• Special Moment Frames
• Based on Detailing
• Seismic Design Categories

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Ordinary Moment Frames

• Seismic Performance Categories A B
• ACI 318 Chapters 1 to 18
• Response modification factor, R 3

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Intermediate Moment Frames

• Seismic Performance Category C
• ACI 318 only defines intermediate as
  cast-in-place
• Response modification factor, R 5

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Special Moment Frames

• Seismic Performance Categories D, E, and F
• Yielding will be concentrated in the beam, Strong
  column -weak beam behavior
• Special Moment frames
• ACI 318 Sections 21.2 through 21.6
• Response modification factor, R 8

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Diaphragms

• A diaphragm is classified as rigid if it can
  distribute the horizontal forces to the vertical
  lateral load resisting elements in proportion to
  their relative stiffness
• Long-span applications suggest that many precast
  diaphragms may in fact be flexible

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Diaphragm Design

• The distinction between rigid and flexible
  diaphragms is important not just for diaphragm
  design, but also for the design of the entire
  lateral force resisting system.

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Diaphragm Classification

• Flexible diaphragm
• Lateral deflection twice average story drift
• Rigid diaphragm
• Not flexible
• Implies capability to distribute load based on
  relative stiffness of LFRS elements

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Steps in the Design Method

• Step 1 - Calculate and compare distribution and
  diaphragm forces
• Based on rigid diaphragm action
• Based on flexible diaphragm action
• Step 2 - Check of diaphragm deformation with
  respect to drift limits
• Step 3 - Check attached element drift limits
• Step 4 - Adjustments in vertical element
  stiffness and placement to limit drift

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Diaphragm Design Forces

• Based on Wind and Seismic Events
• Wind
• Combined windward and leeward wind pressures
• Act as uniform load on building perimeter
• Distributed to the LFRS based on diaphragm
  behavior

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Seismic Diaphragm Design Forces

• Separate calculations from the design of the LFRS
• Diaphragm Design force, FP
• Seismic Design Categories B or C
• Fp 0.2IESDSWp Vpx
• Where
• Vpx represents forces from above levels that
  must be transferred through the diaphragm due to
  vertical system offsets or changes in stiffness.

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Seismic Diaphragm Design Forces

• Seismic Design Category D
• 0.2IESDSwpxlt Fp lt 0.4IESDSwpx

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Diaphragm Detailing

• Wind and Low Seismic Hazards
• Moderate Seismic Hazards
• Seismic Design Category D - Topped Systems
• High Seismic Hazards - Untopped Systems

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Wind and Low Seismic Hazard

• Seismic Design Category A
• Strength requirements imposed by the applied
  forces, No Amplification
• Seismic Design Category B
• Requires the design of collector elements
• Does not require forces to be increased by over
  strength factor, ?o (Revised from IBC 2000)

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Moderate Seismic Hazard

• Topped and Pretopped Systems
• Seismic Design Category C
• Concrete wall systems have special requirements
  IBC 2003
• Diaphragm must include
• special continuous struts or ties between
  diaphragm chords for wall anchorage.
• use of Sub-Diaphragms, the aspect ratio of is
  limited to 2½ to 1

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Moderate Seismic Hazard

• Walls classified as Intermediate Precast Walls
• Collector elements, their connections based on
  special load combinations
• Need to include overstrength factor
• Ductile connections with wall interface
• The body of the connection must have sufficient
  strength to permit development of 1.5fy in the
  reinforcing steel

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Seismic Design Category (SDC) D

• Topped Systems
• Untopped Systems
• Not implicitly recognized in ACI 318 - 02
• Section 21.2.1.5
• permits a system to be used if it is shown by
  experimental evidence and analysis to be
  equivalent in strength and toughness to
  comparable monolithic cast-in-place systems

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SDC D Topped Systems

• High strain demand across the joints
• Reinforcing steel needs to be compatible with
  this demand
• Use of larger wire spacing or bars may be needed
• Mesh in the topping must take the entire shear
  across the joint.
• Correct lapping to maintain diaphragm integrity

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SDC D Topped Systems

• Specific provisions in ACI 318-02
• Chord steel determined from flexural analysis
• Shear strength based entirely on reinforcement
  crossing the joint
• Vn Acvrnfy
• Where
• Acv thickness of the topping slab
• ?n steel ratio of the reinforcement

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SDC D Topped Systems

• ACI 318-02
• minimum spacing requirement of 10 in
• Diaphragm f -factor vertical element fshear
  -factor
• May result in f 0.6, based on ACI 318-02
  Section 9.3.4

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Questions?