PCI 6th EditionPCI 6th Edition

Building Systems

(Seismic)

Presentation OutlinePresentation Outline

• Building System Loads– Seismic

• Structural Integrity• LFRS – Walls• LFRS – Frames• Diaphragms

Seismic ChangesSeismic Changes

• Based on new changes to ASCE 7 and ACI 318

• Based current seismic research and observations

Seismic ChangesSeismic 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

Seismic ChangesSeismic 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

Seismic ChangesSeismic 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

Seismic RiskSeismic Risk

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

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

Seismic Performance Objectives Seismic Performance Objectives

• Current design - minor damage for moderate earthquakes

• Accepts major damage for severe earthquakes

• Collapse is prevented of severe events

Seismic Performance Objectives 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.

Seismic Design ApproachSeismic 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

Earthquake Loads – Equivalent Lateral Force Method

Earthquake Loads – Equivalent Lateral Force Method

• Base Shear, V

V= Cs·W

Where:

Cs - Seismic Response CoefficientW - Total Weight

Equivalent Lateral Force Method LimitationsEquivalent Lateral Force Method Limitations

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

Earthquake Loads – Total Weight, W 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 > 30 psf

Seismic Response Coefficient, Cs Seismic Response Coefficient, Cs

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

Seismic Response Coefficient, CsSeismic 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

Step 1 – Determine SS and S1 Step 1 – Determine SS and S1

• From IBC Map• From local building

codes• IBC 2003 CD-ROM

– Based on• Longitude / Latitude• Zip Code

Step 2 – Determine Site Soil Classification 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

Step 3 – Calculate Response AccelerationsStep 3 – Calculate Response Accelerations

• SMS = Fa·SS

• SM1 = Fv·S1

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

Step 4 – Calculate the 5%-Damped Design Spectral Response Accelerations

Step 4 – Calculate the 5%-Damped Design Spectral Response Accelerations

• SDS = (2/3)SMS

• SD1 = (2/3)SM1

Step 5 – Determine the Seismic Design CategoryStep 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)

Step 6 – (Approximate Period) Determine the Buildings Fundamental Period

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 systemshn = distance from base to highest level (in feet)

TaC

th

n

x

Step 6 – (Exact Period) Determine the Buildings Fundamental Period

Step 6 – (Exact Period) Determine the Buildings Fundamental Period

Rayleigh’s formula

Where:wi = dead load weight at Floor iδi = elastic displacement at Floor iFi = lateral force at Floor ig = acceleration of gravityn = total number of floors

T 2w

i

i

2

i1

n

g Fi

ii1

n

Step 7 – Determine Seismic Response Coefficient, Cs

Step 7 – Determine Seismic Response Coefficient, Cs

Lesser of

Where:R = Response Modification

Factor Figure 3.10.8 (page 3-112)

Ι = Seismic Importance Factor

Cs

SDS

RI

or Cs

SD1

T RI

Step 7 – Determine CsStep 7 – Determine Cs

Minimum Value of Cs

Special Cases In Seismic Design Categories E and F

Cs = 0.044·SDS·Ι

Cs

0.5S1

RI

Vertical Distribution of Lateral ForceVertical Distribution of Lateral Force

Where:

Fx = Force per floor

Cvx = Vertical distribution factorV = Base sheark = 1 - buildings with a period ≤ 0.5 sec = 2 - buildings with a period > 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

FxC

vxV C

vx

wxh

x

k

wih

i

k

i1

n

Location of Force in PlaneLocation 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

Seismic Drift RequirementsSeismic Drift Requirements

• Elastic Displacement Amplification Factor, x

• Stability Coefficient Limits, • PEffects

Drift LimitsDrift Limits

• Figure 3.10.9 (page 3-113)

Drift Amplification Factor, xDrift Amplification Factor, x

Where:δx = Amplified deflection of Level xδxe = Deflection of Level x determined from elastic

analysis, includes consideration of crackingCd = Deflection amplification factor

(Figure 3.10.8)Ι = Seismic Importance Factor

x

Cd

xe

I

Stability Coefficient, θStability Coefficient, θ

Where:Px = Total vertical unfactored load including and above

Level x∆ = Difference of deflections between levels x and x-1Vx = Seismic shear force acting between levels x and x-1hsx = Story height below Level xCd = Deflection amplification factor

P

x

Vxh

sxC

d

Stability Coefficient, θStability Coefficient, θ

The stability coefficient is limited to:

Where:β = ratio of shear demand to shear capacity between Levels x and x-1

max

0.5

Cd

0.25

P- EffectsP- Effects

• To account for P-∆ effects, the design story drift is increased by

(1− θ)-1

• If θ < 0.10, P-∆ effects may be neglected

Reliability Factor, iReliability Factor, i

• Required in High Seismic Design Categories D, E, and F

• The Earthquake Force is increase by a Reliability Factor, i

• 1.5 Maximum Required Value

i = 1.0 for structures in Seismic Design Categories A, B and C

Reliability Factor, i For Moment FramesReliability Factor, i 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.

i2

20

rmaxi

Ai

Reliability Factor, i For Shear WallsReliability Factor, i 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.

i2

20

rmaxi

Ai

Load CombinationsLoad Combinations

• U = 1.4(D+F)• U = 1.2(D+F+T) + 1.6(L+H)• 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

Modification for Vertical Acceleration Modification for Vertical Acceleration

• E = ρ·QE ± 0.2·SDS·D

Seismic Load Combinations Become• U = (1.2 + 0.2·SDS)D + ρ·QE + f1L + 0.2S• U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H

Notice Building weight increase as Ground move UpWhereWhere

QQEE = Horizontal Seismic Force = Horizontal Seismic Force

Modification for Vertical Acceleration Modification for Vertical Acceleration

• E = ρ·QE ± 0.2·SDS·D

Seismic Load Combinations Become• U = (1.2 + 0.2·SDS)D + ρ·QE + f1L + 0.2S• U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H

Notice Building weight decreases as Ground move Down

Overstrength Factor, oOverstrength Factor, o

• Components within the Diaphragm – Chord ties– Shear Steel– Connectors

• Ωo = 2.0 - Seismic Design Categories C, D, E and F

• Ωo = 1.0 - Seismic Design Categories A and B

Special Load CombinationsSpecial Load Combinations

• U = 1.2D + fi·L + Em

• U = 0.9D + E

Where:Em = ·QE + 0.2·SDS·D

and = Overstrength Factor

Overstrength Factor, oOverstrength Factor, o

• Connections from Diaphragms to Seismic Force Resisting System (SFRS) – Ωo = Seismic Design Categories C and higher

Figure 3.10.8 (page 3-112)

Structural Integrity RequirementsStructural 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

Structural Integrity RequirementsStructural 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

Lateral Force Resisting Systems (LFRS)Lateral Force Resisting Systems (LFRS)

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

Influential FactorsInfluential 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

Shear Wall SystemsShear Wall Systems

• Most common lateral force resisting systems

• Design typically follows principles used for cast-in-place structures

International Building Code(IBC) Requirements

International Building Code(IBC) Requirements

• Two categories of shear walls– Ordinary– Special

ACI 318-02 RequirementsACI 318-02 Requirements

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

ACI 318-02 Wall DefinitionsACI 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

Ordinary Structural (Shear) WallOrdinary Structural (Shear) Wall

• Wall complying with the requirements of Chapters 1 through 18

• No special seismic detailing

Intermediate Precast Structural(Shear) Wall

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

Special Precast Structural (Shear) WallSpecial 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.

Design Guidelines for Shear Wall StructuresDesign 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

Design Guidelines for Shear Wall StructuresDesign Guidelines for Shear Wall Structures

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

Moment Frame ClassificationsMoment Frame Classifications

• Three Classifications– Ordinary Moment Frame– Intermediate Moment Frames– Special Moment Frames

• Based on Detailing• Seismic Design Categories

Ordinary Moment FramesOrdinary Moment Frames

• Seismic Performance Categories A & B

• ACI 318 Chapters 1 to 18

• Response modification factor, R = 3

Intermediate Moment FramesIntermediate Moment Frames

• Seismic Performance Category C

• ACI 318 only defines intermediate as cast-in-place

• Response modification factor, R = 5

Special Moment FramesSpecial 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

DiaphragmsDiaphragms

• 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

Diaphragm DesignDiaphragm 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.

Diaphragm ClassificationDiaphragm 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

Steps in the Design MethodSteps 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 limitsStep 4 - Adjustments in vertical element

stiffness and placement to limit drift

Diaphragm Design ForcesDiaphragm 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

Seismic Diaphragm Design Forces Seismic Diaphragm Design Forces

• Separate calculations from the design of the LFRS• Diaphragm Design force, FP

• Seismic Design Categories B or C

Fp = 0.2·IE·SDS·Wp + Vpx

Where

Vpx – represents forces from above levels that must be transferred through the diaphragm due to vertical system offsets or changes in stiffness.

Seismic Diaphragm Design ForcesSeismic Diaphragm Design Forces

• Seismic Design Category D

0.2·IE·SDS·wpx< Fp < 0.4·IE·SDS·wpx

Fpx

F

iix

n

wi

ix

n

w

px

Diaphragm DetailingDiaphragm Detailing

• Wind and Low Seismic Hazards

• Moderate Seismic Hazards

• Seismic Design Category D - Topped Systems

• High Seismic Hazards - Untopped Systems

Wind and Low Seismic HazardWind 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, Ωo (Revised from IBC 2000)

Moderate Seismic HazardModerate 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

Moderate Seismic HazardModerate 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

Seismic Design Category (SDC) D 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

SDC D – Topped SystemsSDC 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

SDC D – Topped SystemsSDC 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 = Acv·n·fy

Where

Acv = thickness of the topping slab

ρn = steel ratio of the reinforcement

SDC D – Topped SystemsSDC D – Topped Systems

• ACI 318-02 – minimum spacing requirement of 10 in– Diaphragm -factor ≤ vertical element shear-

factor– May result in = 0.6, based on ACI 318-02

Section 9.3.4

Questions?Questions?