LECTURE 1 - Design Philosophies

7/21/2019 LECTURE 1 - Design Philosophies

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Conventionalmaterialinlocalbuildingconstructioncheaper(materialsand

construction)

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ART Useofjudgementbasedon:experience,designpractice,construction

practice

(Partof

the

designers

of

Burj

Khalifa

were

also

involved

in

the

design

of

Petronas

TwinTowersandTaipei101)

SCIENCE systematicmethodsofstudybasedon(a)structuralmechanics,(b)

materialbehavior,(c)sourceandeffectsofactions

BurjKhalifaprimarystructureisreinforcedconcrete.192piles;eachpileis1.5

metrediameterx43mlong,buriedmorethan50m

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Notes:

1.Computations(Science)serveonlyasaguidetodesigndecisions to

verifythat

the

structure

will

behave

as

intended

2.Designproblemsareinherentlyindeterminatesuchthatseveral

alternativesolutionsarepossible

3.Actualbehaviorofstructuresareverycomplex

a. Knowledgeisfarfromcomplete(uncertaintiesandrandomvariables)

b. Simplifyingassumptionsarerequired

c. Physicalmodellingservesasverification

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SAFE (STRENGTH) structureshouldnotposeanydangerofinjuryorfatality

(occupants,passers,andoccupantsofadjacentstructures

ECONOMICAL

SERVICEABILITY structuresaredesignedtoavoidconditionswhichmayinterfere

withnormaloperationsorfunctionsofthedifferentcomponents(e.g.excessive

deflections/drift,vibration,andcracking)

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Structuraldesigngivenitscomplicatednature,designersfindawaytoadaptasystem

thatwillgovernthestandardsofstructuraldesignbuildingcodes

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Specifications designrequirementwithrespectto[Strength]material,design

loadings,loadcombinations,designmethods,strengthevaluationand

[Serviceability]requirements

e.g.sidestory:HyattTerraces,BaguioCity

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Ultimate conditionswhichmakethestructureunsafeduetothecollapseofany

partoforthewholestructure

lossofequilibrium(slidingortipping),rupture(collapse),progressivecollapse

(collision,explosion,9/11attack),formationofamechanism(plastichinges),

instability(buckling),fatigue

Serviceabilityconditionswhichmakethestructureunusableduetothe

disruptionofthefunctionaluseofpartoforthewholestructure

Deflections(fear,additionalloads,transferofforces),crackwidths(leakage,

corrosionordeterioration),vibrations(MMDAbridge,occupantcomfort)

Q:Howdowenowtranslatethesetheoriesmathematically?

A:Afterknowingthelimitstates,we'llnowproceedtodesignourstructure

basedontheselimitstates

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LimitStatesDesign adesignprocedurewhereinthestructureorstructural

membersarearrangedandproportionedsuchthatnoapplicablelimitstateis

exceededwhen

subjected

to

all

appropriate

load

combinations

Basicprocess

Determineallpotentialorsignificantlimitstates

Determineacceptablelevelsofsafetyagainsttheoccurrenceofeachlimit

states

Design/proportionstructure/memberstoprovidetherequiredsafety

Example:designofmonorail(ultimate,serviceability)

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Fromthelawofstatics,wecanprovidethebendingmomentdiagram.This

meansthatatpoint(x)alongthebeam,thereisaninternalbendingmoment

equivalentto

the

values

on

the

bending

moment

diagram.

This

bending

moment

isconsideredas"loadeffect"

Otherloadeffectsincludeshearforce,axialforce,deflectionandvibration

Inbothexamples,thememberwillsupporttheloadssafely,ifateverysection,the

strengthofthemember(resistance)exceedstheloadeffects

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Remarks:

1.QandRarerandomvariables

In

reality,

the

most

one

can

learn

is

the

probability

of

the

strength

of

the

testspecimentakingcertainvalue

Randomvariableisavariablethatcannotbepredictedbutisgovernedby

probabilisticrules(withprobabilitydistribution,PDFe.g. Gaussian)

2.Variabilityinstrengthandloading

3.Incontrast,randomvariablevs.deterministicvariable(seeslides)

4.Toaddressvariability,weintroducesafetyfactorsShouldbesuchthat

thereisonlya"small"probabilityofthestructurebeingunsafe

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Note:otherdesignprocedureistheWorkingStressDesign (ES13?CE156?)

Asthenamesuggest,proportioningorfactoringinordertoavoidpossible

overloads

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1. Requiredstrength(factor*loadeffects)canbeacquiredfromthegiven

loading(externalloads),whilethedesignstrength(factor*nominal

strength)can

be

acquired

from

the

specified

concrete

and

steel

strengthsanddimensionsshownonthedrawings

2. Loadfactorsandstrengthfactor providesminimummarginofsafety

Safeandunsafecombinationsofloadsandresistance(Note:replaceSwithQfor

ourclassnotation)

Ifwehaveloadeffectwithresistance,wenotethat therefore

failure(comparewith, )

Ingeneral,foragivenloadeffectS,asweincrease thevalueofR,weare

moreintheSafeZone

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Safetymargin,probabilityoffailureandsafetyindex

Safetymargin, ,havingnormaldistribution

If

the

safety

margin

is

zero,

we

say

that

the

material

fail.

Therefore

the

shaded

portion(whichoccupies)istheprobabilityoffailure

Notingthatwehavethevalueofthemeanvalue,andstandarddeviation

,weintroducethesafetyindex,

Asweincreasethemeanvalue(meaning,Rincreases),wealsoincreasethe

valueofsafetyindex.Ifforexample, 3.5,fromthetableofnormal

distribution,willresulttoaprobabilityoffailureequalto1/9090orroughly1

inevery10,000structuralmembersdesignedonanygivenandvalue

satisfyingequalto3.5

Loadfactorsandstrengthfactorwereselectedinconsiderationofthesafety

index

Q:ifweapplynumericalfactorstoRandQ,whichquantitywouldbe

reducedandwhichquantitywouldbeamplified?

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Now,ifwearetalkingaboutloadeffects,forcertaininrealstructurestherewill

bedifferenttypeofloadswhichwillconstitutesucheffects(e.g.Wind,

Earthquake,Soil,

etc.)

.Moreover,

since

we

will

deal

with

load

factors,

it

would

bethenlogicaltoconsidertheeffectofeachtypeofloadsandapply

appropriateloadfactor.Ontopofthat,wewanttodesignfortheultimateload

orthemaximumrequiredstrengthinconsiderationofdifferenttypeofloads

appliedonourstructure.Hence:loadcombination(NSCP203.3.1andNSCP

409.3.1)

Askiftheyalreadyknowabouttheearthquakeandwindloads

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Remarks:

Thesymbolandsubscriptareusedtorefertothesumofthefactoredloads

representingthe

required

strength

or

maybe

expressed

in

terms

of

the

load

effects,, , withtheircorrespondingloads

Reasonsforspecifyingloadfactors

Magnitudemayvaryfromcalculated

Dead membersize,dimensions,materialdensitiesandestimates

forcomponentsystems(utilities,ceilings,walls)

Live statisticallydetermined

Uncertaintiesindistribution

Uncertaintiesarisingindeterminationofloadeffects

Assumptions/variationsinmaterialandsectionproperties(linear

analysis)

Modelling simplification

Degreeofcracking(nonlinear)

Changeinuseoroccupancy

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LECTURE 1 - Design Philosophies