asce

Fundamentalsof Earthquake Engineering

developed by

Finley A. Charney, Ph.D., P.E

Virginia Polytechnic Institute and State UniversityBlacksburg, Virginia

Center for Extreme Load Effect on Structures

Introduction 1Revised 3/09/06

Purpose of The Course

• The purpose of this course is to introduce theFUNDAMENTAL CONCEPTS of earthquakeengineering.

• This is done by providing a strong theoreticalbasis, rooted in seismic hazard development,structural dynamics, and structural behavior.

• While building code concepts will be discussed, thisis NOT a design course.

Introduction 2

NEHRP RecommendedProvisions

Building CodeDevelopment Cycle

ASCE 7-05

Introduction 3

International Building Code

Introduction 4

U. S. Seismic Design Practice(Prequil)

• Seismic requirements provide minimum standards for use in building design to maintain public safety in an extreme earthquake.

• Seismic requirements safeguard against major failures and loss of life -- NOT to limit damage, maintain function, or provide for easy repair.

• Design forces are based on the assumption that a significant amount of inelastic behavior will take place in the structure during a design earthquake.

Introduction 5

U. S. Seismic Design Practice(Prequil)

• For reasons of economy and affordability, the design forces are much lower than those that would be required if thestructure were to remain elastic.

• In contrast, wind resistant structures are designed toremain elastic under factored forces

• Specified code requirements are intended to provide forthe necessary inelastic seismic behavior.

• In nearly all buildings designed today, survival in largeearthquakes depends directly on the ability of their framingsystems to dissipate energy hysteretically while undergoinglarge inelastic deformations.

Introduction 6

The Difference Between Wind Resistant Designand Earthquake Resistant Design

Wind:Excitation is an applied pressure or FORCE on the façadeLoading is dynamic, but (for most structures) response is nearly STATICStructure deforms due to applied forceDeformations are MONOTONIC (unidirectional)Structure is designed to respond ELASTICALLY under factored loadsThe controlling life safety limit state is STRENGTHProvide enough strength to resist forces elastically

Introduction 7

BEHAVIOR UNDER WIND EXCITATION

Introduction 8

Time

Pre

ssur

e

F

F

δ

δ

Factored 50 yr WindUnfactored 50 yr Wind

10 yr Wind

First SignificantYield

The Difference Between Wind Resistant Designand Earthquake Resistant Design

Earthquake:Excitation is an applied DISPLACEMENT at the baseLoading and response are truly DYNAMICStructural system deforms as a result of INERTIAL FORCESDeformations are fully REVERSEDThe structure is designed to respond INELASTICALLY under factored loadsThe controlling life safety limit state is DEFORMABILITYProvide enough strength to assure that deformation demands do

not exceed deformation capacity

Introduction 9

BEHAVIOR UNDER SEISMIC EXCITATION(Elastic Response)

Gro

und

Dis

p.

F

Introduction 10

Time

δ

Factored SeismicElastic StrengthDemand

Factored Wind

F

δ

δG In general, it is not economicallyfeasible to design structures torespond elastically to earthquakeground motions.

F

BEHAVIOR UNDER SEISMIC EXCITATION(Inelastic Response)

Gro

und

Dis

p.

Introduction 11

Time

Loading

F

δ

δG

δ

Introduction 12

Time

Gro

und

Dis

p.

F


F

δ

δ

δG

Unloading

DeformationReversal

Introduction 13

Time

Gro

und

Dis

p.

F


δ

F

δ Reloading

δG

Definition of Ductility, µ

y

u

δδµ =Stress or Force or Moment

δuδy

Strainor Displacementor (Curvature or Rotation)

HysteresisCurve

Introduction 14

Definition of Energy Dissipation, Θ

Stress or Force or Moment

Strainor Displacementor Rotation

Area = Θ = Energy DissipatedUnits = Force x Displacement

Introduction 15

Basic Earthquake EngineeringPerformance Objective

An adequate design is accomplished when a structureis dimensioned and detailed in such a way that thelocal ductility supply is greater than the correspondingdemand.

DemandSupplied µµ ≥

DemandSupplied Θ≥Θ

Introduction 16

The Role of Design

The role of “Design” is to estimate the strength of the structure that is required to limit the ductility demand to the available supply, and to provide thedesired engineering economy.

Introduction 17

(Definitions)Another View of Ductility Demand

Inherent CapacityThat capacity provided by the gravity system or bygravity plus wind.

Affordable CapacityThe capacity governed by reasonable (ordinary) building costs in the geographic area of interest.

Seismic PremiumThe ratio of the (reduced) seismic strength demand to theInherent Capacity.

Introduction 18

Ductility Demand = Elastic Seismic Demand

Affordable Capacity

Another View of Ductility Demand

StrengthElasticSeismicDemand

AffordableCapacity

Introduction 19

DeformationDemand

YieldDeformation

Def.

Another View of Ductility Demand

If “Affordable Capacity” is relatively constant, thenductility demand is primarily a function of elasticseismic demand.

Because elastic seismic demand is a functionof local seismicity, ductility demand is directlyproportional to local seismicity.

Hence, Berkeley California, which has higher seismicitythan (say) Austin Texas, has a higher inherent ductilitydemand than does Austin.

Introduction 20

Ductility Demand vs Seismicity

Berkeley

Introduction 21

Boston

Austin

Elastic Demand

AffordableStrength

Def.3.0Y 5.0Y1.0Y 1.8Y

LimitationThe ductility demand can not exceed the ductility supply.

Moment Frame Ductility SupplyOrdinary Detailing 1.5Intermediate Detailing 2.5Special Detailing 5.0

In California, the high seismicity dictates a highductility demand (typically > 3) hence, only momentframes with Special Detailing may be used.

Introduction 22

Limitation (continued)

In Austin, the relatively low seismicity dictates a lowductility demand (typically < 2) hence, Intermediateand special Special Detailing may be used.

However, there is no motivation to use Special Detailing ifthe resulting design forces fall below the inherentcapacity.

Introduction 23

What if Supplied Ductility can not meet the Demand?

Ductility Demand = Elastic Seismic Demand

Affordable Capacity

• Increase Affordable Capacity (Pay a higher seismic premium)

• Reduce Elastic Seismic DemandBase IsolationAdded Damping

Introduction 24

Basic ASCE-7 Equations for PredictingStrength Demand of Buildings

WCV S=

)/(1

IRTSC D

S =IRSC DS

S /=

Introduction 25

Important Concepts to Understand:

)/(1

IRTSC D

S = IRSC DS

S /=

1) The Cause and Effect of Earthquakes (SDS, SD1)2) Seismic Hazard Analysis (SDS, SD1)3) Structural Dynamics (T, SDS, SD1)4) Inelastic Behavior of Structures (R)5) Current Design Philosophy (SDS, SD1, R, I)6) Future Trends

Introduction 26

1) The Cause and Effect of Earthquakes

• Why Earthquakes Occur• How Earthquakes are Measured• Earthquake Effects• Mitigation Strategy• Earthquake Ground Motions

Introduction 27

2) Seismic Hazard Analysis

• Deterministic/Probabilistic Analysis• USGS Hazard Maps• ASCE 7-05 Hazard Maps• Site Amplification• Elastic Response Spectra• Near Source Effects

Introduction 28

3) Structural Dynamics (Linear Response)

• Equations of Motion for SDOF Systems• Response to Simple Loading• Response to Earthquake Loading• Elastic Response Spectra• Equations of Motion for MDOF Systems• Modal Analysis• Equivalent Lateral Force Analysis

Introduction 29

4) Inelastic Behavior of Structures

• Why Inelastic Behavior is Necessary• Inelastic Behavior of Components• Equal Displacement Concept• Basic Design Equation

Introduction 30

5) Current Design Philosophy

• ASCE 7-05 Philosophy• Seismic Resistant Structural Systems• Example Building Analysis

Introduction 31

Course Schedule

Day 1 a.m. IntroductionEarthquakes: Cause and EffectSDOF Structural Dynamics

p.m. SDOF Structural Dynamics (continued)Seismic Hazard Analysis

Day 2 a.m. MDOF Structural DynamicsInelastic Behavior of Structures

p.m. Structural Design PhilosophyStructural SystemsExample

Introduction 32

Course Materials

• Course Visuals

• References

•NONLIN Manual and CD

• FEMA 450 and 451 [(800) 480-2520]

http://filebox.vt.edu/users/fcharney/

Introduction 33