design philosphies and design considerations

7/23/2019 Design Philosphies and Design Considerations

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Introduction:

A structure refers to a system of connected parts

used to support forces (loads). Buildings, bridgesand towers are examples for structures in civilengineering. In buildings, structure consists of wallsoors, roofs and foundation.

A structure can be broadly classied as(i) sub structure

(ii) super structure

oundation is sub structure and plint!, walls,

columns, oor slabs wit! or wit!out beams, stairs,roof slabs wit! or wit!out beams etc are superstructure.



Introduction:

"!ereas, #oncrete is generally strengt!ened using

steel bars or rods $nown as rebar%s in tension &one.'uc! elements are reinforced concrete concretecan be moulded to any complex s!ape using suitableform wor$ and it !as !ig! durability, better

appearance, re resistance and economical. or astrong, ductile and durable construction t!ereinforcement s!all !ave !ig! strengt!, !ig! tensilestrain and good bond to concrete and t!ermalcompatibility. Building components li$e slab walls,

beams, columns foundation * frames are constructedwit! reinforced concrete. +einforced concreted canbe insitu concreted or precast concrete.



Designer has to ensure the structures, hedesigns are:

it for t!eir purpose

'afe

-conomical and durable



Following Uncertainties affect the safety of astructure

about loading

about material strengt! and

about structural dimensions

about be!aviour under load



In any method of design, thefollowing are the common steps to

be followed: (i) o assess t!e dead loads and ot!er external loads

and forces li$ely to be applied on t!e structure,

(ii) o determine t!e design loads from di/erentcombinations of loads,

(iii) o estimate structural responses (bending moment,s!ear force, axial t!rust etc.) due to t!e design loads,

(iv) o determine t!e crosssectional areas of concretesections and amounts of reinforcement needed



"or$ing 'tress 0esign (Allowable 'tress0esign),

widely $nown as (A'0) 1 used for over 233years.

4imited 'tates 0esign (4oad *+esistance actor 0esign),

also $nown as (4+0) 1 rst introduced in2567.



WORKING SR!SS "!#O$

!e wor$ing stress met!od, t!oug! ensuressatisfactory performance at t!e wor$ing loads, isunrealistic and irrational at ultimate load and it doesnot guarantee t!e satisfactory performance of t!e

structure at service loads.

!e wor$ing stress met!od is logically notapplicable to concrete structures because t!is

met!od assumes t!at t!e materials of w!ic! t!estructure is made up, namely concrete and steel,bot! obey 8oo$9s law. !e applicability of t!e8oo$9s law is rat!er limited in respect of concretestructures.



%roperties of Wor&ing Stress"ethod:'

!e 'tresses in an element is obtained from t!ewor$ing loads and compared wit! permissiblestresses. !e met!od follows linear stressstrain

be!aviour of bot! t!e materials.:odular ratio can be used to determineallowable stresses.:aterial capabilities are under estimated tolarge extent.

actor of safety are used in wor$ing stressmet!od. !e member is considered as wor$ing stress.;ltimate load carrying capacity cannot bepredicted accurately.

!e main drawbac$ of t!is met!od is t!at it



(I"I S)! $!SIGN

4imit 'tate<

'tate at w!ic! one of t!e conditionspertaining to t!e structure !as reac!ed a limitingvalue.

4imit state of collapse was found = detailed inseveral countries in continent fty years ago. In2573 'oviet #ode recogni&ed t!ree limit states<

(i)0eformation(ii)#rac$ing

(iii)#ollapse



(I"I S)! $!SIGN

ypes of 4imit 'tate<



All relevant limit states !ave to be considered in

t!e design to ensure ade>uate degree of safetyand serviceability. !e structure s!all be designedon t!e basis of t!e most critical limit state ands!all be c!ec$ed for ot!er limit states

(I"I S)! $!SIGN



4I:I 'A-' 0-'I?@

Basis of 4imit 'tates 0esign

Probability distribution of the safety margin R-Q

R-Q R-Q<0 R-Q>0

(R-Q)m

f(R-Q)

σ (R-Q)



0esign loads are calculated as,

(0esign load d) (#!aracteristic load ) (Cartial safety factor for load

Df)

+espective values of Df for loads in t!e twolimit states as given in able 26 of I' EF7 fordi/erent combinations of loads arefurnis!ed in able G.2.

(I"I S)! $!SIGN



%artial Safety *actor +f for loads)ctions- Combina

tion

Limit State of Strength Limit state of Serviceability

DL

LL WL /EL

AL DL

LLWL /ELLead

ingAccompa

NyingLeadi

ngAccompan

ying

DL+LL+CL 1! 1! 1"!

1" 1" 1"

DL+LL+CL+WL/EL

1#1#

1#1#

1"!"!$

"%1#

1" "& "& "&

DL+WL/EL1!

'"()*

1!

1"

1"

DL+E1#

'"()1#

DL+LL+AL 1" "$! "$!

1"



%artial Safety *actor +f for loadsStrength-

Sl

No

Definition ,artial Safety -actor

1 esistance. governed by

yieldingmo

11

# esistance of member to

bc0lingmo

11

$ esistance. governed by

ltimate stressm1

1#!

esistance of connectionm1

2olts3-riction 4ype2olts32earing 4ype

ivetsWelds

Shop-abrication

s

-ield-abricatio

ns

1#!1#!1#!1#!

1#!1#!1#!1!"



General %rinciples of (imitStates $esign

'tructure to be designed for t!e 4imit 'tates atw!ic! t!ey would become unt for t!eir intendedpurpose by c!oosing, appropriate partial safetyfactors, based on probabilistic met!ods.

wo partial safety factors, one applied to loading(γ f) and anot!er to t!e material strengt! (γ m) s!allbe employed.



Cossible deviation of t!e actual be!aviour of t!e structure

from t!e analysis model0eviation of loads from specied values and +educed probability t!at t!e various loads acting toget!er will

simultaneously reac! t!e c!aracteristic value.

f allows for.



(I"I S)!S $!SIGN

Σ(Load * Load Factor) ≤Resistance-

Resistance*actor-

∀

m ta&es account.

Cossible deviation of t!e material in t!estructure from t!at assumed in design

Cossible reduction in t!e strengt! of t!ematerial from its c!aracteristic value

:anufacturing tolerances.:ode of failure (ductile or brittle)



)((OW)/(! SR!SS $!SIGN)S$-

'tresses caused by t!e c!aracteristic loadsmust be less t!an an allowable stress,w!ic! is a fraction of t!e yield strengt!

Allowable stress may be dened in terms of afactor of safetyH w!ic! represents a marginfor overload and ot!er un$nown factorsw!ic! could be tolerated by t!e structure

CharacteristicLoad Effects

Characteristic Strength-actor of Safety

≤



)((OW)/(! SR!SS $!SIGN)S$-

Allowable stress (ield stress) = (actor of safety)

(imitations

:aterial nonlinearity

@onlinear be!aviour in t!e postbuc$led state andt!e property of steel to tolerate !ig! stresses byyielding locally and redistributing t!e loads not

accounted for.

@o allowance for redistribution of loads in staticallyindeterminate members



(I"I S)!S $!SIGN

4imit 'tatesH are various conditions in w!ic! astructure would be considered to !ave failed to fullt!e purpose for w!ic! it was built.

;ltimate 4imit 'tates are t!ose catastrop!icstates,w!ic! re>uire a larger reliability in order toreduce t!e probability of its occurrence to a very lowlevel.

'erviceability 4imit 'tateH refers to t!e limits onacceptable performance of t!e structure duringservice.



Introduction !e procedure for analysis and design o f a

given building will depend on t!e type ofbuilding, its complexity, t!e number of storiesetc.

!ere are two type of building systems<

(a) 4oad Bearing :asonry Buildings .

(b ) ramed Buildings.



(oad /earing "asonry/uildings

'mall buildings li$e !ouses wit! small spansof beams , slabs generally constructed as loadbearing bric$ walls wit! reinforced concreteslab beams. !is system is suitable for

building up to four or less stories. In suc!buildings crus!ing strengt! o f bric$s s!all be233 $ g= cmG minimum for four stories. !issystem is ade>uate for vertical loads it also

serves to resists !ori&ontal loads li$e win d *eart!>ua$e by box action.



Framed Buildings

In t!ese types of buildings reinforced

concrete frames are provided in bot! principaldirections to resist vertical loads and t!e verticalloads are transmitted to vertical framing systemi.e columns and oundations. !is type ofsystem is e/ective in resisting bot! vertical *!ori&ontal loads.



Basic Codes for Design.

!e design s!ould be carried so as toconform to t!e following Indian code forreinforced concrete design, publis!ed by t!eBureau o f Indian 'tandards , @ew 0el!i

Loading Standards: 0ead loads

Imposed (live) loads

"ind loads

'now loads

'pecial loads and load combinations



)I" O* $!SIGN: !e aim of design is ac!ievement of an

acceptable probability t!at structures beingdesigned s!all, wit! an appropriate degree ofsafety .

Cerform satisfactorily during t!eir intended life.

'ustain all loads and deformations of normalconstruction * use

8ave ade>uate durability

8ave ade>uate resistance to t!e e/ects ofmisuse and re



"!#O$ O* $!SIGN1. 'tructure and structural elements s!all

normally be designed by 4imit 'tate :et!od.G. "!ere t!e 4imit 'tate :et!od cannot be

conveniently adopted, "or$ing 'tress :et!odmay be used.

Nominal Cover

@ominal cover is t!e design dept! of concretecover to all steel reinforcements, includinglin$s. It is t!e dimension used in design and

indicated in t!e drawings. It s!all be not lesst!an t!e diameter of t!e bar.



!0posure Nominal1oncrete 1o2er

in mm not (esshan

:ild G3

:oderate J3

'evere EF

Kery severe F3

-xtreme LF



or main reinforcement up to 2G mm diameterbar for mild exposure t!e nominal cover may be

reduced by F mm. ;nless specied ot!erwise, actual concrete

cover s!ould not deviate from t!e re>uirednominal cover by M 23 mm

or exposure condition Nsevere9 and Nverysevere9, reduction of F mm may be made, w!ereconcrete grade is :JF and above.



Relati2e Sti3ness: !e relative sti/ness of t!e members may be

based on t!e moment of inertia of t!e sectiondetermined on t!e basis of any one of t!efollowing denitions<

GrossSection

he cross'section ofthe member ignoring

reinforcement ransformed'ection

!e concrete crosssectionplus t!e area ofreinforcement transformedon t!e basis of modularratio

#rac$ed'ection

!e area of concrete incompression plus t!e areaof reinforcementtransformed on t!e basisof modularratio



Structural frames : Arrangement Of Live Load

It9s t!e combination of design dead load on allspans wit! full design live load on two adOacentspans * design dead load on all spans wit! fulldesign live load on alternate spans.

"!en design live load does not exceed t!reefourt!s of t!e design dead load, t!e loadarrangement may be design dead load and designlive load on all t!e spans.



Substitute *rame:or determining t!e moments and s!ears at any

oor or roof level due to gravity loads, t!ebeams at t!at level toget!er wit! columns

above and below wit! t!eir far ends xed maybe considered to constitute t!e frame.

or lateral loads simplied met!ods are used forsymmetrical structures and rigorous met!ods

for unsymmetrical structures



"oment and shear coe4cients

for continuous beams :;nless more exact estimates are made, for beams ofuniform crosssection w!ic! support substantiallyuniformly distributed load over t!ree or more

spans w!ic! do not di/er by more t!an 2F percent oft!e longest, t!e bending moments and s!ear forcesused in design may be obtained using t!e coePcientsgiven in ables below. or moments at supports w!ere two une>ual spansmeet or in case w!ere t!e spans are not e>uallyloaded, t!e average of t!e two values for t!e negativemoment at t!e support may be ta$en for design. "!ere coePcients given in able below are used forcalculation of bending moments, redistribution.



/eams O2er *ree !nd Supports:

"!ere a member is built into a masonry wallw!ic! develops only partial restraint, t!e member

s!all be designed to resist a negative moment att!e face of t!e support of "2=GE w!ere " is t!etotal design load and 2 is t!e e/ective span, or suc!ot!er restraining moment as may be s!own to beapplicable. or suc! a condition s!ear coePcientgiven in able below at t!e end support may be

increased by 3.3F.



/ending "oment 1o4cients



Shear *orce 1oe4cients:



1ritical Sections *or "oment

)nd Shear :

or monolit!ic construction, t!e momentscomputed at t!e face of t!e supports s!all be used

in t!e design of t!e members at t!ose sections. !e s!ears computed at t!e face of t!e 'upports!all be used in t!e design of t!e member at t!atsection except as in above cases"!en t!e reaction in t!e direction of t!e applied

s!ear introduces compression into t!e end regionof t!e member, sections located at a distance lesst!an d from t!e face of t!e support may bedesigned for t!e same s!ear as t!at computed atdistance d.



!3ecti2e $epth : -/ective dept! of a beam is t!e distance

between t!e centroid of t!e area of tensionreinforcement and t!e maximum

compression bre, excluding t!e t!ic$nessof nis!ing material not placedmonolit!ically wit! t!e member and t!et!ic$ness of any concrete provided to allowfor wear. !is will not apply to deep beams.



1ontrol Of $e5ection : !e deection s!all generally be limited to t!e

following< !e nal deection due to all loads including

t!e e/ects of temperature, creep ands!rin$age and measured from t!e ascast levelof t!e supports of oors, roofs and all ot!er!ori&ontal members, s!ould not normallyexceed span=GF3.

!e deection including t!e e/ects oftemperature, creep and s!rin$age occurringafter erection of partitions and t!e applicationof nis!es s!ould not normally exceedspan=JF3 or G3mm w!ic!ever is less.



For beams, the vertical deflection limits may generally be assumedto be satisfied provided that the span to depth ratio are not greater

than the value obtained as belo!a) Basic values of span to e/ective dept! ratiosfor spans up to 23m

#antilever L

'imply supported G3 #ontinuous G7

b) or spans above 23m, t!e values in (a) may bemultiplied by 23=span in meters, except for

cantilever in w!ic! case deection calculationss!ould be made.

@ote< "!en deections are re>uired to becalculated, t!e met!od given Annexure N#9 of I'EF7G333 may be used.



0epending on t!e area and t!e type of steelfor tension reinforcement, t!e value in (a)

or (b) s!all be modied as per ig



Depending on the area of compressionreinforcement, the value of span to depthratio be further modied as per Fig



Control Of Deection – Solid Slabsor slabs spanning in two directions, t!e s!orter

of t!e two spans s!ould be used for calculatingt!e span to e/ective dept! rations.

or twoway slabs of s!orter spans (up to J.Fm) wit! mild steel reinforcement, t!e span tooverall dept! rations given below may generallybe assumed to satisfy vertical deection limitsfor loading class up to J $@=mG.

'imply supported slab JF #ontinuousslabs E3or !ig! strengt! deformed bars of grade e

E2F,t!e values given above s!ould be multipliedby 3.6.



Slabs Continuous Over Supports:'labs spanning in one direction and continuous over

supports s!all be designed according to t!eprovisions applicable to continuous beams.

'labs :onolit!ic wit! 'upports Bending moments inslabs (except at slabs) constructed monolit!icallywit! t!e supports s!all be calculated by ta$ing suc!slabs eit!er as continuous over supports andcapable of free, or as members of a continuousframe wor$ wit! t!e supports, ta$ing into accountt!e sti/ness of suc! support

If suc! supports are formed due to beams w!ic! Oustify xity at t!e support of slabs, t!en t!e e/ectson t!e supporting beam, suc! as, t!e bending of t!eweb in t!e transverse direction of t!e beam,w!erever applicable, s!all also be considered in t!edesign of t!e beams.

or t!e purpose of calculation of moment in slabs ina monolit!ic structure, it will generally besuPciently accurate to assumed direct membersconnected to t!e ends of suc! slab are xed inposition and direction at t!e end remote from t!eirconnection wit! t!e slab.



+-Q;I+-:-@ R +-I@R+#-:-@R+ '+;#;+A4 :-:B-+

/eams:

ension reinforcement:"inimum reinforcement:' !e minimum area of

tension reinforcement s!all not be less t!an t!at given

by t!e following<



w!ereAs minimum area of tension reinforcement.b breadt! of beam or t!e breadt! of t!e

web of beam.d e/ective dept!, andfy c!aracteristic strengt! of reinforcement

in :=mmG

(b) "a0imum reinforcement:' t!emaximum area of tension reinforcement s!allnot exceed 3.3Eb0.

1ompression reinforcement: !e maximum area of comparison

reinforcement s!all not exceed 3.3E bd.#omparison reinforcement in beams s!all beenclosed by stirrups for e/ective lateralrestraint.



rans2erse reinforcement in beam forshear torsion

!e transverse reinforcement in beam s!all be

ta$en around t!e outer most tension *compression bars. In beams and Ibeams,suc! reinforcement s!all pass aroundlongitudinal bars located close to t!e outerface of t!e ange.

"a0imum spacing of shear reinforcement:aximum spacing of s!ear reinforcement

means long by axis of t!e member s!all notexceed 3.LF d for vertical stirrups and d for

inclined stirrups at EF w!ere d is t!e e/ectivedept! on t!e section under consideration. In nocase s!all be spacing exceed J33mm.



:inimum s!ear reinforcementMinimum shear reinforcement in the form of stirrups shall be

provided such that:

Where

Asv = total cross-sectional area of stirrups legs effective inshear.

Sv = stirrups spacing along the length of the member

B = breadth of the beam or breadth of the web of flange beam,and

f = characteristic strength of the stirrups reinforcement in

!"mm# which shall not ta$en greater than %&' !"mm#Where the ma(imum shear stress calculated is less than half

the permissible value in member of minor structure importancesuch as lintels, this provision need not to be complied with.



)istribution of torsion reinforcement

"!en a member is designed for torsion

reinforcement s!all be provided as below<a) t!e transverse reinforcement for torsion s!all

be rectangular closed stirrups placedperpendicular to t!e axis of t!e member. !espacing of t!e stirrups s!all not exceed t!e list of

x2, x2My2=E and J33 mm, w!ere x2, y2 arerespectively t!e s!ort * long dimensions of t!estirrup. b* 4ongitudinal reinforcement s!all be place as

closed as is practicable to t!e corner of t!e cross

section * in all cases, t!ere s!all be at least onelongitudinal bar in eac! corner of t!e ties. "!ent!e cross sectional dimension of t!e memberexceed EF3 mm additional longitudinal bar s!allbe provided to satised t!e re>uirement ofminimum reinforcement * spacing



MINIMUM DI!"N#$ %$!&$$N INDI'IDU" %"R:

!e !ori&ontal distance between two parallelmain reinforcing bars s!allusually be notless t!an t!e greatest of t!efollowing<

(i) 0ia of larger bar and(ii)F mm more t!an nominal maximum si&e ofcoarse aggregate."!en needle vibrators are used it may be

reduced to G=Jrd of nominal maximum si&e ofcoarse aggregate. "!ere t!ere are two or more rows of bars, barss!all be vertically in line and t!e minimum vertical

distance between bars s!all be 2F mm, G=Jrd of



"%

!e mild steel reinforcement in eit!er direction inslabs s!all not be less t!an 3.2Fpercent of t!e total crosssectional area.8owever, t!is value can be reduced to 3.2Gpercent w!en !ig! strengt! deformed bars orwelded wire fabric are used.

:aximum diameter !e diameter of reinforcing bars s!all not exceedone eig!t of t!e total t!ic$ness of

slab.



Mai*u* distance +etween +ars la+s:

!e !ori&ontal distance between parallel mainreinforcement bars s!all not be more t!an t!reetimes t!e e/ective dept! of solid slab or J33 mmw!ic!ever is smaller.

!e !ori&ontal distance between parallelreinforcement bars provided against s!rin$age andtemperature s!all not be more t!an ve times t!e

e/ective dept! of a solid slab or J33 mm w!ic!everis smaller.

D$'$.M$N! $N/!0 F %"R



D$'$.M$N! $N/!0 F %"R:

$e2elopment of Stress in Reinforcement:

!e calculated tension or compression in any bar atany section s!all be developed oneac! side of t!e section by an appropriatedevelopment lengt! or end anc!orage or by

a combination t!ereof 4d STst =EUbd

"!ere

S nominal diameter of bar, Ubd design bondstress

Tst stress in bar at t!e section considered atdesign load



hear reinforce*ent (!IRRU.):

0evelopment lengt! and anc!orage re>uirementis satised, in case of stirrups andtransverse ties, w!en Bar is bent 1V !roug! an angle of at least 53 degrees (round

a bar of at least its own dia) *is continued beyond for a lengt! of at least 6 W, orV !roug! an angle of 2JF degrees * is continuedbeyond for a lengt! of at least

7 W orV !roug! an angle of 263 degrees and iscontinued beyond for a lengt! of atleast E W

design philosphies and design considerations

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