Advanced Structural Design - Slab Design

8/20/2019 Advanced Structural Design - Slab Design

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Prepared by: Eng. Chamil Duminda

Mahagamage B.Sc.Eng (Hons), C Eng,1

Design of Reinforced Concrete Slabs

Code o" Prac#ice $ EC% Design o" concre#e s#ruc#ures

IntroductionReinforced concrete slabs are used in oors, roofs and walls inbuildings and as the deck of bridges. The oor syste of a structurecan take any fors such as in situ solid slabs, ribbed slabs or !re"cast units. #labs ay s!an in one direction or two directions and they

ay be su!!orted on onolithic concrete beas, steel beas, walls ordirectly by the structure$s coluns.

%ontinuous slabs should in !rinci!le be designed to withstand the ostunfa&ourable arrangeents of loads, in the sae anner as beas.#labs ay be analysed using following ethods.

" using bending oent and shear force coe'cients" yield line ethod" (nite eleent ethod


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Prepared by: Eng. Chamil Duminda Mahagamage B.Sc.Eng (Hons), C Eng,ME(S!)

One way spanning slabs

*f these coe'cients are used the reinforceent ust be of ductilityclass + or % and the neutral ais de!th - .0d and le&er ar 2.0d to allow for oent re"distribution incor!orated in the &aluesgi&en 4which ay be u! to 5). These coe'cients should only beused when there are at least 3 s!ans that do not dier in length by

ore than 15, and k - 8in 91.:k, k;/


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Two way spanning slabs

The oents in slabs s!anning in two directions can also bedeterined using tabulated coe'cients. #labs which are notrectangular in !lan or which su!!ort an irregular loading arrangeentay be analysed by techni=ues such as the yield line ethod or the>ileborg stri! ethod.%oncrete slabs are de(ned as ebers where the breadth is not lessthan ties the o&erall de!th and beha&e !riarily as euralebers with the design siilar to that of beas and soewhatsi!ler because,

" the breadth of the slab is already (ed and a unit breadth of 1is used in the calculations.

" the shear stresses are usually low in a slab ece!t when there arehea&y concentrated loads

" co!ression reinforceent is seldo re=uired

De&ec#ion re'uiremen#s are usually domina#e


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?


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Shear in slabs


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%o!ared with the beas, shallow slabs fail at slightly high shear andthis is incor!orated into the &alues of the ultiate concrete shearresistance , @Rd,c. #ince shear stresses in slabs subAect to uniforly

distributed loads are generally sall, shear reinforceent will seldobe re=uired and it would be usual to design the slab such that thedesign ultiate shear force @Bd is less than the shear strength of the

unreinforced section, @Rd,c. *n this case it is not necessary to !ro&ide

any shear reinforceent.


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C

Dunching shear E analysis and designF concentrated load on a slab causes shearing stresses on a sectionaround the loadG this eect is referred to as !unching shear. The

critical surface for checking !unching shear is located at .d fro theloaded area.


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*f reinforceent is re=uired to resist shear around the control!erieter it should be !laced between not ore than .d fro theloaded area and a distance 1.d inside the outer control !erieter at

which shear reinforceent is no longer re=uired.

Hength of outer !erieter I

*f this length is less than 3d fro the face of the loaded area , thenreinforceent should be !laced in the one between .3d and 1.d

fro loaded face.@ertical links will norally be used and !ro&ided around at least two!erieters not ore than .7d a!art. Hink s!acing around a!erieter within d of the face of the loaded area should not begreater than 1.d, increasing to a liit of .d at greater !erieters.Dro&ided that the slab is greater than thick o&erall then the

aount of reinforceent re=uired is gi&en as followsJ


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Span-eective depth ratiosBcessi&e deections of slabs will cause daage to the ceiling, oor(nishes or other architectural (nishes. To a&oid this, liits are set onthe s!an"de!th ratio. Fs a slab is usually a slender eber, therestrictions on the s!an"de!th ratio becoe ore i!ortant and this

can often control the de!th of slab re=uired.

8iniu eecti&e de!th I s!an/4basic ratio correction factor)

The correction factors account for slab ty!e and su!!ort conditions aswell as cases of s!ans greater than 7 and for at slabs greater thanC.. The basis ratio ay also be corrected to account for grades ofsteel other than grade and for when ore reinforceent is!ro&ided than that re=uired for design at the ultiate liit state.*t ay norally be assued that, in using a&ailable tables, slabs arelightly stressed although a ore eact deterination can be ade

fro the gra!h when the !ercentage of tension reinforceent is know.*n the case of two"way s!anning slabs, the check on the s!an"eecti&ede!th ratio should be based on the shorter s!an length. This does nota!!ly to at slabs where the longer s!an should be checked.


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1


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Flat Slab FloorsF at slab oor is a reinforced concrete slab su!!orted directly byconcrete coluns without the use of interediary beas. The slabay be of constant thickness throughout or in the area of the colunit ay be thickened as a dro! !anel. The colun ay also be of

constant section or it ay be ared to for a colun head or ca!ital.


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The at slab oor has any ad&antages o&er the bea and slab oor. The si!li(ed forwork and the reduced storey heights ake it oreeconoical. The absence of shar! corners gi&es greater (re resistanceas there is less danger of the concrete s!alling and e!osing the

reinforceent. Keection re=uireents will generally go&ern slabthickness which should not norally be less than 1C for (reresistance as indicated in table below.


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The analysis of a at slab structure ay be carried out by di&iding thestructure into a series of e=ui&alent fraes. The oents in thesefraes ay be deterined by

-a ethod of frae analysis such as oent distribution, or thestiness ethod on a co!uterJ

- F si!li(ed ethod using the oent and shear coe'cients frothe tables subAect to the following re=uireentsG

i. the lateral stability is not de!endent on the slab"colunconnections.

ii. there are at least three rows of !anels a!!roiately e=ual s!anin the direction being considered.

iii. the bay sie eceeds 3

*nterior !anels of the at slab should be di&ided as shown in (gure

below. Kro! !anels should be ignored if their saller diension is lessthan one"third of the saller !anel diension. *f a !anel is not s=uare,stri! widths in both directions are based on saller !anel diension.8oents deterined fro a structural analysis or the coe'cients frothe !re&iously !resented tables are distributed between the stri!s asshown in following tables.


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%olun oents can be calculated fro the analysis of the e=ui&alentfrae. Darticular care is needed o&er the transfer of oents to edgecoluns. This is to ensure that there is ade=uate oent ca!acitywithin the slab adAacent to the colun since oents will only be able

to be transferred to the edge colun by a stri! of slab considerablynarrower than the noral internal !anel colun stri! width.he rein"orcemen# "or a &a# slab should generally be arrangedaccording #o #he simplied rules bu# a# leas# % bo##om bars in eachor#hogonal direc#ion should pass #hrough in#ernal columns #o enhancerobus#ness.

*n considering !unching shear, B% !laces additional re=uireents onthe aount and distribution of reinforceent around colun heads toensure that full !unching shear ca!acity is de&elo!ed.


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Stair Slab


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Prepared by: Eng. Chamil Duminda Mahagamage B.Sc.Eng (Hons), C Eng,

ME(S!)

1

roble! "F 6 thick slab of class %/3 concrete is reinforced by 1high yield bars at 1 centres in each direction. The slab is subAectto a dry en&ironent and ust be able to carry a localied

concentrated ultiate load of 6k; o&er a s=uare area of 3side. Keterine the shear reinforceent re=uired for fyk I ;/

roble! # The four s!an slab shown in (gure below su!!orts a &ariable load of3k;/ !lus oor (nishes and a ceiling load of 1.k;/. Kesign the

slab using fck I ;/, fyk I ;/.


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Prepared by: Eng. Chamil Duminda Mahagamage B.Sc.Eng (Hons), C Eng,

roble! $ The slab is thick and s!ans in two directions. The eecti&es!an in each direction is ?. and 6.3 and the slab su!!orts a&ariable load of 1kn/. Kesign the slab using fck I ;/, fyk I

;/.

roble! % The coluns are at 6. centres in each direction and the slabsu!!orts a &ariable load of k;/. The characteristic aterial

strengths are fckI;/ and fyk I ;/.*t is decided to use a oor slab as shown in (gure below with o&erall de!th, and dro! !anels . s=uare by 1 dee!. Thecolun heads are to be ade 1. diaeter.

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Advanced Structural Design - Slab Design