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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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