ASCE

Fundamentals of Earthquake Engineering developed by

Finley A. Charney, Ph.D., P.E Virginia Polytechnic Institute and State University Blacksburg, Virginia

Center for Extreme Load Effect on Structures

Introduction Revised 3/09/06

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Purpose of The Course •

The purpose of this course is to introduce the FUNDAMENTAL CONCEPTS of earthquake engineering.

•

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

•

While building code concepts will be discussed, this is NOT a design course.

Introduction

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Building Code Development Cycle

NEHRP Recommended Provisions

ASCE 7-05 International Building Code Introduction

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Introduction

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U. S. Seismic Design Practice (Prequil) •

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

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

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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

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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 the structure were to remain elastic. In contrast, wind resistant structures are designed to remain elastic under factored forces

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Specified code requirements are intended to provide for the necessary inelastic seismic behavior.

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In nearly all buildings designed today, survival in large earthquakes depends directly on the ability of their framing systems to dissipate energy hysteretically while undergoing large inelastic deformations.

Introduction

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The Difference Between Wind Resistant Design and Earthquake Resistant Design Wind: Excitation is an applied pressure or FORCE on the façade Loading is dynamic, but (for most structures) response is nearly STATIC Structure deforms due to applied force Deformations are MONOTONIC (unidirectional) Structure is designed to respond ELASTICALLY under factored loads The controlling life safety limit state is STRENGTH Provide enough strength to resist forces elastically

Introduction

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Pressure

BEHAVIOR UNDER WIND EXCITATION F Time

δ F

Factored 50 yr Wind Unfactored 50 yr Wind 10 yr Wind

First Significant Yield

δ

Introduction

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The Difference Between Wind Resistant Design and Earthquake Resistant Design

Earthquake: Excitation is an applied DISPLACEMENT at the base Loading and response are truly DYNAMIC Structural system deforms as a result of INERTIAL FORCES Deformations are fully REVERSED The structure is designed to respond INELASTICALLY under factored loads The controlling life safety limit state is DEFORMABILITY Provide enough strength to assure that deformation demands do not exceed deformation capacity Introduction

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Ground Disp.

BEHAVIOR UNDER SEISMIC EXCITATION (Elastic Response) Time

Factored Seismic Elastic Strength Demand

F

δ Factored Wind

δ

δG F

In general, it is not economically feasible to design structures to respond elastically to earthquake ground motions. Introduction 10

Ground Disp.

BEHAVIOR UNDER SEISMIC EXCITATION (Inelastic Response) F Time

δ Loading

δ

δG F Introduction 11

Ground Disp.

BEHAVIOR UNDER SEISMIC EXCITATION (Inelastic Response) F Time

δ Unloading

δ Deformation Reversal

δG F Introduction 12

Ground Disp.

BEHAVIOR UNDER SEISMIC EXCITATION (Inelastic Response) F Time

δ

Reloading

δ

δG F Introduction 13

Definition of Ductility, µ Stress or Force or Moment

δy

δu

δu µ= δy Strain or Displacement or (Curvature or Rotation)

Hysteresis Curve Introduction 14

Definition of Energy Dissipation, Θ Stress or Force or Moment

Area = Θ = Energy Dissipated Units = Force x Displacement

Strain or Displacement or Rotation

Introduction 15

Basic Earthquake Engineering Performance Objective An adequate design is accomplished when a structure is dimensioned and detailed in such a way that the local ductility supply is greater than the corresponding demand.

µ Supplied ≥ µ Demand Θ Supplied ≥ Θ Demand 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 the desired engineering economy.

Introduction 17

Another View of Ductility Demand (Definitions) Inherent Capacity That capacity provided by the gravity system or by gravity plus wind. Affordable Capacity The capacity governed by reasonable (ordinary) building costs in the geographic area of interest. Seismic Premium The ratio of the (reduced) seismic strength demand to the Inherent Capacity. Introduction 18

Another View of Ductility Demand Elastic Seismic Demand Ductility Demand =

Affordable Capacity

Strength Elastic Seismic Demand

Affordable Capacity

Yield Deformation

Deformation Demand

Def. Introduction 19

Another View of Ductility Demand If “Affordable Capacity” is relatively constant, then ductility demand is primarily a function of elastic seismic demand. Because elastic seismic demand is a function of local seismicity, ductility demand is directly proportional to local seismicity. Hence, Berkeley California, which has higher seismicity than (say) Austin Texas, has a higher inherent ductility demand than does Austin.

Introduction 20

Ductility Demand vs Seismicity Elastic Demand Berkeley

Boston Austin Affordable Strength

1.0Y 1.8Y

3.0Y

5.0Y

Def. Introduction 21

Limitation The ductility demand can not exceed the ductility supply. Moment Frame Ductility Supply Ordinary Detailing Intermediate Detailing Special Detailing

1.5 2.5 5.0

In California, the high seismicity dictates a high ductility demand (typically > 3) hence, only moment frames with Special Detailing may be used. Introduction 22

Limitation (continued) In Austin, the relatively low seismicity dictates a low ductility demand (typically < 2) hence, Intermediate and special Special Detailing may be used. However, there is no motivation to use Special Detailing if the resulting design forces fall below the inherent capacity.

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 Demand Base Isolation Added Damping Introduction 24

Basic ASCE-7 Equations for Predicting Strength Demand of Buildings

V = C SW S DS CS = R/I

S D1 CS = T (R / I ) Introduction 25

Important Concepts to Understand: S D1 CS = T (R / I )

S DS CS = R/I

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.

p.m.

Day 2 a.m.

p.m.

Introduction Earthquakes: Cause and Effect SDOF Structural Dynamics SDOF Structural Dynamics (continued) Seismic Hazard Analysis

MDOF Structural Dynamics Inelastic Behavior of Structures Structural Design Philosophy Structural Systems Example

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