asce 7 seismic loads srr

7/29/2019 ASCE 7 Seismic Loads SRR

1/6

Shake,Rattle&Rollwith

ASCE7TheSeismicChapters

AsseenthroughthebreakdownofIBC2006Section1613EarthquakeLoads,wefoundthat

favoritefriendoftheIBC2006,"designedinaccordancewithASCE7"showupoverandover.

Theinformationtheyreferencedcouldn'tbethatbad,right?Unfortunately,onceyoufindthe

truth,you'll

pray

afault

opens

below

you

and

ends

your

suffering.

It

is

bad.

Very

bad.

Thesecretsbehindseismicloadsand"designedinaccordancewithASCE7"arenotrelegatedto

onesimplechapter.No,sir.Here'swhatyou'llfind:

1) Chapter11SeismicDesignCriteria2) Chapter12SeismicDesignRequirementsforBuildingStructures3) Chapter13SeismicDesignRequirementsforNonstructuralComponents4) Chapter14MaterialSpecificSeismicDesignandDetailingRequirements(thankfully,this

isexcludedfromIBC2006seeIBC2006Section1613.1Scope)

5) Chapter15SeismicDesignRequirementsforNonbuildingStructures6) Chapter16SeismicResponseHistoryProcedures7) Chapter17SeismicDesignRequirementsforSeismicallyIsolatedStructures8) Chapter18SeismicDesignRequirementsforStructureswithDampingSystems9) Chapter19SoilStructureInteractionforSeismicDesign10)Chapter20SiteClassificationProcedureforSeismicDesign11)Chapter21SitespecificGroundMotionProceduresforSeismicDesign12)Chapter22SeismicGroundMotionandLongPeriodTransitionMaps13)Appendix11AQualityAssuranceProvisions(thisisalsoexcludedfromIBC2006)14)Appendix11BExisitingBuildingProvisions

Hadenoughyet?I'llpausewhileyougovomit....backalready?Doesn'tthislookjustdandy?

Before

you

panic,

all

this

isn't

required

reading

for

the

Lateral

Forces

Exam.

In

fact,

I'd

hazard

a

guessthatonlyastructuralengineerspecificallytrainedinseismicdesignwouldeverlookatall

thismaterial(theygettousebooksandgraphingcalculators,afterall).

Sowheredowebegin?Howdowemakesense?Allhopeisn'tlost.

Chapter11SeismicDesignCriteria

Thisiswherethingsstart,andyou'llquicklyrecognizesomeofthevariablesandcoefficientsin

the21/2pages(twocolumnsperpage)ofnotation.TherearemanyandtheASCEemphasizes

thatthenotationpresentedonlyappliestoseismicdesign.Sincethescopeoftheinformationis

vast,I'mgoingtodivergefromnormalpresentationandjustverybrieflylistvarioustables,a

few

formulas

and

their

corresponding

variables

and

coefficients.

Section11.4.2SiteClass

ThisisdeterminedfromaTableinChapter20bywayofaseriesofcalculations,butthemost

relevantinformationistheSiteClassanddefinition:

SiteClass Definition SiteClass Definition

A HardRock B Rock

C Verydensesoil&softrock D Stillsoil

E Softclaysoil F Soilsrequiringsiteresponseanalysis

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Section11.4.3SiteCoefficientsandAdjustedMaximumConsideredEarthquake(MCE)

SpectralResponseAccelerationParameters

Hereisanacronymthatyouseeofteninreadingonseismicdesign:MCE.Thismeans

"maximumconsideredearthquake"andisusedinconjunctionwith"maximumconsidered

earthquake(MCE)spectralresponseacceleration."Quiteamouthful.Section11.4.3bringsout

thespectralresponsecoefficientsinformulasandsometables,whichI'mgoingtoreproduce

justforcontext,butarecertainlynotrequiredtomemorize.Also,thissectiontiesinexclusively

withthe

following

Section

11.4.4.

If

there

is

anything

information

to

keep

in

mind

for

the

LateralForcesexamasitrelatestothesesections,rememberthefollowing:

1) "S"=spectralresponseacceleration2) "SubscriptS"=shortperiods(0.2seconds)3) "Subscript1"=1secondperiods4) "SubscriptM"=maximum5) "SubscriptD"=design

TheASCEthrowsafewcurveballslater,andyouhavetorefertothenotationchartto

understandtheiruseofthesubscriptinthecontextthesectionitsbeingused.Restassured

though,ifyouareatacocktailpartyandyouseeageekedoutengineerscribble"SDS=0.25"on

anapkin,

you

can

wow

your

friends

by

saying,

"He's

working

on

the

design

earthquake

spectral

responseaccelerationparametersatshortperiods.It'saseismicdesigncategoryBorCbased

ontheoccupancy."Headimmediatelytowardthealcoholafterwards.

Thefollowingaretheformulas,tables,anddefinitionsfoundinSection11.4.3andSection

11.4.4.Ignoreatyourleisure.

2

3

23

s) ts d=

A

=themappedMCEspectralresponseaccelerationatshortperiods(basedonthemaps)

=themappedMCEspectralresponseaccelerationat1secondperiod(basedonthemap

=MCEspectralresponseaccelerationatshortperiods,adjustedforSiteClasseffects=MCEspectralresponseaccelerationat1secondperiod,adjustedforSiteClasseffec

=thedesignearthquakespectralresponseaccelerationparametersatshortperiods

=thedesignearthquakespectralresponseaccelerationparametersat1secondperio

shortperiodsitecoefficientbasedonTable11.4.1

=1secondperiodsitecoefficientbasedonTable11.4.2

nimportantnoteaboutthemaps:whenyouseethecontoursorselectedpoints,theyarea%

ofg,accelerationduetogravity.Youdon'twanttojustgrabanumberanduseitasyourS or

tomakesuretocheckyoursubscript.

s

S1.

ThecoefficientFcanbetricky.Fnormallymeans"force"ofsometypeatsomepoint,butyou

have

2


3/6

Table11.4.1SiteCoefficient,Fa

CESpectralResponseAccelerationParameteratShortPeriodSiteClass MappedM

Ss 5 0.25 Ss=0.5 Ss=0.75 Ss=1.0 Ss 1.2A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.2 1.2 1.1 1.0 1.0

D 1.6 1.4 1.2 1.1 1.0

E 2.5 1.7 1.2 0.9 0.9

F Callyo ngineerure

Table11.4.1Sit

SpectralResponseAccelerationParameterat1secondPeriod

eCoefficient,Fv

SiteClass MappedMCE

S 1 0.1 S1=0.2 S1=0.3 S1=0.4 S1 0.5A 0.8 0.8 0.8 0.8 0.8

B

1.0

1.0

1.0

1.0

1.0

C 1.7 1.6 1.5 1.4 1.3

D 2.4 2.0 1.8 1.6 1.5

E 3.5 3.2 2.8 2.4 2.4

F Callyo ngineerure

(I'vehighlighted s)

trum

ndardandsitespecificprocedurescan'tbeused,

acceleration(g)

= insecondsIn

thebaselinevalue

Section11.4.5DesignResponseSpec

Thissectionismostlyforinstanceswhensta

andseveral

formulas

are

involved

as

well

as

graphing.

In

this

section,

only

two

variables

are

relevantforouruse:

=spectralresponsefundamentalperiodofthestructure

terestingnote:Ifyouarebored,masochisticorhavealreadycountedthesocksinyoursock

drawer,theUSGShasaprettyniftypieceofsoftwarethatfiguresallthevaluesinSection11.4

se

rtancefactorinCodes,youseethevariable,I.Also,

importanceisalmostalwaysavaluebasedonoccupancytype(thecommonIBCtable).

OccupancyIII=1.25

OccupancyIV=1.5

anddrawsthedesignresponsespectrumgraphbymerelytypinginazipcode.Youcaneven

choosewhatCodestandardyouareusing.It'sactuallykindoffuntoplaywith(canyouusethe

word"fun"whentalkingaboutseismicdesign?).Alotofthedesignbabblestartstomakesen

onceyoumesswiththissoftware.Youcandownloadithere:

http://earthquake.usgs.gov/research/hazmaps/design/

Section11.5.1ImportanceFactor

Anytimeyouseereferencetoimpo

Importanceisverysimpletoremembersincethereareonlythreevalues:

OccupancyIorII=1.0

3


4/6

Section tegory

Thissec tainsseveralconditionalsandthe"ifthen"languagegets

convolu unately,thesectionischartdriven.Hereisthemostimportant

seismicdesigncategorychosenwillbeusedinmany

l

11.6SeismicDesignCa

tion,whileshort,con

tedquickly,butfort

thingtorememberaboutthissection:the

chartsfordefiningtheconstraintsofthestructureanditscomponents.

I'mgoingtoreproducethechartsandincludemyownasStep1,whichillustratesanoddbal

conditional.

Step1(basedon1secondperiod)

ValueofS1 OccupancyCategory

I,II,orIII IV

S 0.75 E F1

otherwise,useoneofthefollowingtables(thesearetheprimarycategories):

11.6.11SeismicDesignCategoryBasedonShortPeriodResponseAcceleration

a meter

Table

P raValueofSDS OccupancyCategory

IorII III IV

S 0.167 A A ADS0.1 SDS0.33 B B C670.3 .50 C C D3 SDS 00.5 D D D0 SDSTab . 1SeismicDesignCategoryBasedon1secondPeriodResponseAcceleration

Occupan tegory

le1

ValueofS

1 6.1

Parameter

D1 cyCa

IorII III IV

S 0.067 A A AD10.0 SD10.133 B B C670.1 0.20 C C D33 D1 S 0 D D D.20 SD1

r 1 SeismicDesignCriteriaChapt e 2

o, aren'tgoinghereindetail,ortheotherchaptersforthatmatter.Thischapteriswhere

comestogether,butitisvastandcomplex.Asananalogy,imaginetheKaplan

asheetoficecoveringacesspool.Chapter12isthe

N

everything

we

materialfortheLateralForcestestbeing

waistdeepsludgeyousinkintowhentheicebreaks,andthefollowingchaptershaveyouin

overyourhead(literallyandfiguratively).ThereafewthingstomakenoteofinChapter12,

though,mainlytoseethecomparisontotheoldUBCseveralitemsareeasilyrecognizable

4


5/6

First,Chapter12isimmediatelytabledrivenbywayofTable12.21DesignCoefficientsand

FactorsforSeismicForceResistingSystems.Rememberhowwewentthroughthemazein

Chapter11togettheSeismicDesignCategory?Well,hereiswhereyouuseit.Inverysimplifi

steps,youbasicallydothefollowing:

1) PickyourSeismicforceresistingsystem2) Checkyourlimitationsandper

ed

missibilitybasedontheSeismicDesignCategory

ient,systemoverstrengthfactorand

ngfromthetable.Acommonquestionis"Whatisthis

2.1.1:

That'sre

andvar

eitis.Forthesakeofcomparison,herearetheoldformulasfromthe

hemeaningofthecoefficients):

3) Recordtheresponsemodificationcoefficdeflection

amplification

factor

4) HeadovertoSection12.8EquivalentLateralForceProcedureLetmestepbackandmentionsomethi

mysterious"R"value?"TheanswerisfoundinChapter12andTable1

=theResponseModificationCoefficientwhichisanumericalvaluebasedonthetypeofseismicforceresistingsystemused.

allyallyouneedtoknowaboutRsincetheexactvaluemustbederivedfromatable

iesconsiderably.

Section12.8.1SeismicBaseShear

Goodoldbaseshear.Her

1994UBC(I'mnotgoingtorehasht

1994UBCformulafordeterminingbaseshear

withC=anumericalcoefficient

1.25

/

andTisapproximatedbythefollowingformula:

/

TheASCE705version,whichisalsotheIBC2006version,comesintwoflavors:equivalentand

simplified:

ASCE705/IBC2006Equivalentformulafordeterminingbaseshear

=istheseismicresponsecoefficient=istheeffectiveseismicweight

determinedbytheformula:

where:

withCs

5


6/6

6

where:

=thedesignearthquakespectralresponseaccelerationparametersatshortperiods

=theresponsemodificationfactor

ccupancyimportancefactor

Acoupleadditiona

andtheASCEn tCsshallnotbelessthan

0.01.Themostimportantitemto

recognizeis

the ve

not

shown,

and

it

means:

=thefundame minedinSection12.82

lformula to

If whichisvery

similar

=theo

lformulasforCsfollow,buttheymainlyprovideandupperandlowerlimits,

otestha

interjection

of

Tin

those

formulas

I'

ntalperiodofthestructure(s)deter

JustlikedeterminingTinthe1994UBC,determiningTinASCE7requiresaccesstoatable,with

theexceptionthattheASCErequiresanadditiona thatreferencesanothertable

providearangefortheupperlimitandapproximatefundamentalperiod.

certaincriteriaaremet(mainlyheight),thesimplifiedversioncanbeused,

tothe1994UBC:

ASCE705/IBC2006Simplifiedformulafordeterminingbaseshear

where:

23

=1.0foronestorybuildings,1.1for2storybuildings,and1.2forthreestorybuildings

modificationfactor

=effectiveseismicweightofstructurethatshallincludethetotaldeadandotherloads(ashortlistfollows)

=thedesignearthquakespectralresponseaccelerationparametersatshortp

appedMCEspectralresponseaccelerationatshortperiods(basedon

th

And,thatisitfol

schoolbellring usly,whatwehavediscussedisnomorethan

thatfrostyfilmo l

theASCE7seis ters.Thatisfinewithme:Vn,vd,vc(Icame;Isaw;Iconquered...or

atleastmadeaf

Howmuch

of

t

see

on

the

Lateral

Forces

Exam?

Probably

nothing

or

maybe

afew

=theresponse

eriods

=thememaps)

ksfortheunderpinningsofASCE7TheSeismicChapters.IthinkIhearthe

ing,soclassmustbeover.Obvio

nyourwindshieldinthemorningwhencomparedtothe"biblical"scopeofal

micchap

inemessofthings).

hiswill

you

coefficients,butatleastyouhavethe"secret"knowledgebehindthosemysteriouswordsin

theIBC2006:"designedinaccordancewithASCE7).

Pleasewarnmeinadvanceifmytextcausesyoutofindyourselfsittinginadarkcloset,rocking

backandforth,mumbling"Die,IW.DieIW.....IthinkIcancatchthenextflighttoBelize.

IWthebluehairedfreak

asce 7 seismic loads srr

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