overview of the basic seismic resistant design...

CEE 227 - Earthquake Engineering

U.C. Berkeley Spring 2003 ©UC Regents 3-1

Overview of the Basic SeismicResistant Design Process

vA quick look at the overalldesign process.

vThen look at some basic reasons for usingnonlinear response to help us achieveperformance objectives



Basic Steps in the Design Process

Site Evaluation

Select Structural System

Engineering

Quality Control /Assurance

Functional Requirements



Functional /Operational Requirementsv Loading - Dead, live, etc.v Spatial requirements (minimum area,

height limits, interior open space, etc.)v Operating restrictions (deflection,

settlement, vibration limits)v Expected life of structure

v Time available for constructionv Local cost of materials/laborv Value of moneyv Assess owner’s aversion to riskv Establish performance levels and key

response parameters for frequent, rareand very rare seismic events and windstorms.

Functional Requirements Site Evaluation


Engineering




Site Evaluationv Settlement potentialv Thermal settingv Windv Mechanical vibration (trains, etc.)v Seismic¸ Fault rupture¸Differential ground movement¸ Liquefaction potential¸Slope instability¸ Tsunami and seiche¸ Fire and flood¸Ground shaking

Functional Requirements

SiteEvaluation


Engineering




Selection of Structural SystemTypically, several types of structures arepicked and investigated at least throughthe preliminary or schematic design stage.Consideration to alternative:

• Materials for structural and non-structural elements

• Connection types (fixed, partiallyrestrained, etc.)

• Configuration (regular or not?)• Load carrying system (complete

vertical and horizontal load systems)• Type of foundation

Assess seismic and functionalperformance, and economic costs.


SelectStructuralSystem

Engineering

Quality Control /

Assurance



EngineeringPerformance-Based Format: establish thatdesign satisfies performance criteria set forvery rare, rare and frequent earthquakes.Preliminary Design - Simple methods todevelop structure that performs well.Detailing - Detail elements, connections ofstructural elements, attachments of non-structural elements to achieve desiredcapacities and performance.Final Design - More refined methods usedto fine tune building design to achieveperformance goals (physical and economic).Probabilistic Performance Evaluation -Assess sensitivity of final structure to varioussources of uncertainty and randomness.



Engineering




Generally most important design stage.Focus on simple methods to developstructure that performs well(proportions, load path completion,stiffness, strength, etc.)Iterative Process:¸ Immediate Occupancy Goal: Elastic Analysis, very

simple models and elastic spectrum¸ “Life Safe” Goal: Elastic analysis, and “equiv. elastic”

spectrum.¸ Collapse Prevention Goal: Simplified nonlinear

analysis, nonlinear design response spectrum.

vIntegrate design requirementsvEvaluate designsvRe-do, if needed

Engineering: Preliminary Design



Engineering




Current codes do not attempt to relatedetails quantitatively to predicteddeformation capacities. Nominal “ductile”details typically used -- relatively non-deteriorative and insensitive to loadinghistory effects.

Tend is towards identifying qualitativecatagories. such as “special,”“intermediate” and “ordinary” detailswhich possess different abilities to deforminelastically

Engineering: Detailing



Engineering




PBEE format: Design using computedestimates of demands.

‘ Immediate Occupancy Goal: Focus onstrength. Some damage permitted.

‘ Life Safe Goal: Focus on triggers ofexpensive/hazardous behavior (spalling,buckling, falling hazards, etc.)

‘ Collapse Prevention Goal: Focus onultimate deformation capacity ofmember, connection and system thatwould lead to partial or global collapseof structure.

‘Integrate and evaluate

Engineering: Detailing



Engineering




Generally most analysis-oriented step.Focus on refined methods topredict/evaluate demands on nearlycompleted structural design.Iterative Process:

‘ Immediate Occupancy Goal: Elastic Analysis,refined models and elastic spectrum, 2D vs 3D

‘ “Life Safe” Goal: Elastic analysis, and “equiv.elastic” or nonlinear spectrum.

‘ Collapse Prevention Goal: Nonlinear analysis(static push-over with nonlinear design responsespectrum, or dynamic time history analysis).

‘ Integrate design requirements‘ Evaluate designs (details, goals)‘ Re-do if needed

Engineering: Final Design



Engineering




Infrequently done at presentFocus on refined methods to assessreliability of structure being able toachieve performance objectivesIterative Process: (see FEMA 350-352)

‘ Immediate Occupancy Goal: Elastic Analysis,refined models and elastic spectrum, 2D vs 3D

‘ “Life Safe” Goal: Elastic analyses, and “equiv.elastic” or nonlinear spectrum (+1s).

‘ Collapse Prevention Goal: Nonlinear analysis(static push-over with nonlinear responsespectrum, dynamic time history analyses, etc.).

‘ Integrate performance information‘ Evaluate confidence in achieving objectives

Engineering: Reliability Assessment



Engineering




Quality Control/AssuranceEngineering QualityVision 2000 RecommendsEngineering Peer Review¸ Safety Critical and Essential/Hazardous

performance objectives.¸ Advanced or unusual technologies¸ Rehabilitation projects involving

untested details¸ Complex or important structures

Should include all stages of design:establishment of project goals andprocedures, Schematic DesignPhase, Design Development Phase,and Construction Document Phase



Engineering

Quality Control /

Assurance



Quality Control/AssuranceConstruction QC/QA ProgramsCritical to achievement of performance goalsv Contractor should have in-house quality control program.v Owner should have independent quality assurance program,¸ Includes on-site and shop inspection.¸ Preconstruction conference with engineer, contractor and inspectors. Local

building departments are not set up to do this.

See: FEMA 353 for steel construction

Similarly, maintenance is a problem for many types of structure(corrosion, defeat of base isolation systems, etc.)



Approaches to Seismic Design

Some Issues to Consider:v General types of engineering demand

parametersvWhat design characteristic controls them?vWhere can we use inelastic behavior to our

advantage?v Design methods (formats that address seismic

design issues best)



Introduction - Design Approaches

For immediate occupancyand similar performancegoals, we generally willwant to:¸ Limit drifts (stiffness)¸ Limit accelerations (strength)¸ Limit yielding (strength and

proportioning)

Elastic design approachesgenerally suffice

For collapse prevention limitstates, we need to¸ limit drifts to prevent static and

dynamic instability (stiffness)¸ Prevent brittle failure modes

(limit forces)¸ Insure adequate deformation

capacity in ductile failure modes(detailing)¸ Limit deformation demands

(stiffness)

Inelastic approaches needed



Some Approaches for Rare Events

Elastic ApproachMake stronger & stifferWhat is maximum load?

Nonlinear Fuse Concepts

Seismic IsolationIsolator Deformations?

Structure Strength?

Energy DissipationDevice demands?

Structure Deformations?

Plastic HingesHinge Demands?




Nonlinear systems limit forces


Structure Strength?





Device or member properties

Deform.

Load

Structure properties

Displ.

Force



Nonlinear systems limit forces - 2


Structure Strength?





Nonlinear Structure’s Properties

Displ.

Force

Force Limit

Elastic Structure’s Properties

Displ.

Force

Force & Displacement Limited Only by Input



These systems allow us to controldamage distribution


Structure Strength?





Except for the isolatedsystem, these systemsare staticallydeterminant near-collapse:¸Know forces in:

• all members• foundations

¸Make all otherlocations stronger tocontrol damagedistribution!

Pick locations where localdamage is acceptable



Nonlinear systems limit forces - 3


Structure Strength?






Displ.

Force

Force Limit

Plastic Hinge

Yielding Limits Loads on Foundation



Nonlinear systems limit accelerations


Structure Strength?






Displ.

Force

Force Limit

Plastic Hinge

Mp

Fy = Mp/H

Fy

Ma + FD + FS = -Mag

FS = -M(ag+a) = -Matotal

atotal<Fy/M



So...What displacements do wedesign for?

This is the big question?¸Simple nonlinear spectral

displacement estimates¸Estimates based on elastic

demands (empiricalrelationships)¸Nonlinear analysis (static for

local, dynamic for global andresidual deformations)

A big part of this course

?



Common Design Methods

v Allowable Stress/Elastic Methods¸ older codes

v Capacity design methods¸New Zealand code since 1960s¸ various parts of concrete sections of UBC and hidden in some

parts of steel UBC provisions

v Damage Tolerant Design¸ Coming from blast and progressive collapse limit state

considerations. European and Japanese building practice.

v Plastic Design¸ Very useful, but often too complex for hand calculations.

v Displacement-based Design



Allowable Stress / Elastic Methods

H=12

6

Qy=5

Piles

Design Loadsand Dimensions

Qy=5W=20

P = 20/2 + 5*12/6 = 20P=20/2 - 5*12/6 = 0

Vcol = 5

Mmax = 5*12 = 60Vp= 6Mp= 78

Cp=30Tp = 0

Design Internal ForcesProvided Capacity

C ≥ D

Vp,p=4

Vpile = 2.5



Response of Strength Designed Structure

Element Load Q to ReachElement Capacity

ElementOvercapacity

Column shear 6 1.2ColumnMoment

78/12=6.5 1.3

PileCompression

(30-10)*6/12=10 2

Pile Tension (0+10)*6/12 = 5 1.0Pile shear 2*4 = 8 1.6 Provided Capacity

C ≥ D

Vp= 6Mp= 78

Cp=30Tp = 0

Q=?

Vp,p=4

Load structure until something fails.Assuming we are looking for firstdamage, structure is elastic



Capacity Design Approach

Minimum Capacitiesbeyond Plastic Hinge

Vp= 8.1Mp= 78

Cp=26.3Tp = -6.3Vp,p=1.6

Target Behavior

Mp= 78

Q=?Element Force in element

when columnplastic hinge forms

ElementOvercapacity

Column shear 6.5*1.25 = 8.1 1.25ColumnMoment

78/12=6.5 1.0

PileCompression

20/2+1.25(6.5)12/6 =26.3

1.25

Pile Tension 20/2 -1.25(6.5)12/6 = -6.3

1.25

Pile shear 1.25*6.5/2= 4.1 1.25

FIXED

1.25 factor used here to provide margin of safety against otherelements reaching capacity prior to column plastic hinge



Capacity Design Concept

Assumed Structure

Simple application of plastic analysis on an element-wise basis

Plastic Hinges

MpMp

Moment

Shear

Design for thesemoments and shears



Damage Tolerant Design

Poor qualityworkmanship,poor theory,bomb blast, orsoil failurereduces elementcapacity

Planned Structure Actual Structure

Explicit redistribution of load path possible in redundant system



Damage Tolerant Designv Many codes have incorporated Redundancy Factor

v FEMA 365 (NEHRP New Building Requirements)“The design of a structure shall consider the potentiallyadverse effect that the failure of a single member,connection or component of the seismic-force-resistingsystem would have on the stability of the structure”(Section 5.2.6.2.4)



Summary

Design Process¸ Many steps¸ Multiple objectives require tradeoffs¸ Iterative

Tools and methodologies¸ Trend to more quantitative approaches¸ Various formats and concepts to help us achieve a

structure likely to achieve performance objectives¸ Trend towards explicit reliability assessment

overview of the basic seismic resistant design...

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