CIV530 LECTURE 203 Design manual by nura

Design by NURUDDEEN M MUSA

1. CIV5304:
Design of Structural Elements III
Engr Dr Nuruddeen Muhammad Musa
1

2. Design of Ribbed Slab
2

3. Design of Ribbed Slab
Ribbedslabsaremoreeconomicalthansolidslabsforlongspanswithrelativelylightloads.
TheymaybeconstructedinavarietyofwaysasdiscussedinBS8110:Part1,section3.6.
Twoprincipalmethodsofconstructionare
1.ribbedslabswithoutpermanentblocks
2.ribbedslabswithpermanentholloworsolidblocks
Thetoppingorconcretefloorpanelsbetweenribsmayormaynotbeconsideredto
contributetothestrengthoftheslab.Theholloworsolidblocksmayalsobecountedin
assessingthestrengthusingrulesgiveninthecode.
3

4. Design of Ribbed Slab
Ribbed slab proportions
Proportionsforribbedslabswithoutpermanentblocksaresetoutin
section3.6ofthecode.Themainrequirementsareasfollows:
1.Thecentresofribsshouldnotexceed1.5m;
2.Thedepthofribsexcludingtoppingshouldnotexceedfourtimes
theiraveragewidth;
3.Theminimumribwidthshouldbedeterminedbyconsiderationof
cover,barspacingandfireresistance.ReferringtoFig.3.2inthecode,
theminimumribwidthis125mm;
4.Thethicknessofstructuraltoppingorflangeshouldnotbelessthan
50mmoronetenthofthecleardistancebetweenribs(Table3.18in
thecode).
4

5. Design of Ribbed Slab
Design procedure and reinforcement
(a)Shearforcesandmoments
Shearforcesandmomentsforcontinuousslabscanbeobtainedby
analysisassetoutforsolidslabs.
(b)Designformomentandmomentreinforcement
Themid-spansectionisdesignedasaT-beamwithflangewidthequal
tothedistancebetweenribs.Thesupportsectionisdesignedasa
rectangularbeam.Theslabmaybemadesolidnearthesupportto
increaseshearresistance.Momentreinforcementconsistingofoneor
morebarsisprovidedinthetopandbottomoftheribs.Ifappropriate,
barscanbecurtailedinasimilarwaytobarsinsolidslabs.
5

6. Design of Ribbed Slab
(c) Shear resistance and shear reinforcement
Thedesignshearstressisgiveninclause3.6.4.2by
v=V/bvd
whereVistheultimateshearforceonawidthofslabequaltothedistancebetweenribs,
bvistheaveragewidthofaribanddistheeffectivedepth.
•Innocaseshouldthemaximumshearstressvexceed0.8fcu
1/2
or5N/mm2.
•NoshearreinforcementisrequiredwhenvislessthanthevalueofvcgiveninTable3.9
ofthecode.Shearreinforcementisrequiredwhenvexceedsvc.
(d)Reinforcementinthetopping
Thecodestatesinclause3.6.6.2thatfabricwithacross-sectionalareaofnotlessthan
0.12%oftheareaofthetoppingineachdirectionshouldbeprovided.Thespacingofwires
shouldnotexceedone-halfthecentre-to-centredistanceoftheribs.
•Deflection
Thedeflectioncanbecheckedusingthespan-to-effectivedepthrulesgiveninthecode.
6

7. Design of Ribbed Slab
7

8. Design of Ribbed Slab
8

9. Design of Stairs
9

10. Design of Stairs
Stairsconsistofrises,runs(ortreads),andlandings.Thetotalstepsandlandings
arecalledastaircase.Theriseisdefinedastheverticaldistancebetweentwosteps,
andtherunisthedepthofthestep.Thelandingisthehorizontalpartofthe
staircasewithoutrises.
10

11. Design of Stairs
Stairways are sloping one-way spanning slabs. Two methods of construction are
used.
(a) Transverse spanning stair slabs
Transversespanningstairslabsspanbetweenwalls,awallandstringer(anedge
beam),orbetweentwostringers.Thestairslabmayalsobecantileveredfroma
wall.
Thestairslabisdesignedasaseriesofbeamsconsistingofonestepwithassumed
breadthandeffectivedepth.Themomentreinforcementisgenerallyonebarper
step.Secondaryreinforcementisplacedlongitudinallyalongtheflight.
11

12. Design of Stairs
(b) Longitudinal spanning stair slab
The stair slab spans between supports at the top and bottom of the flight. The
supports may be beams, walls or landing slabs.
Theeffectivespanlliesbetweenthetoplandingbeamandthecentreofsupportin
thewall.Thelongitudinalstairaredesignedasaslabwiththewaistasthedepth.
Andtheself-weightoftheslabasequivalenttotheweightofthestairsonthe
slope.
12

13. Design of Stairs
Sincetrulyhorizontaldistancewillbeused,theloadmustbeconvertedby
multiplyingallinclinedloads(waistselfweightandfinishes)by:
WhereRistheriseofthestepandTthegoing.
Whensuchflightspanbetweentwobeams,themomentiscalculatedas:
M=
Andwhenitspansbetweentwolandings,designedorthogonaltothedirectionof
theflight,
M=
Wherew=theflightuniformlydistributedloadandLtheslab.
13
2
22
)(
T
TR
8
2
wL
10
2
wL

14. Design of Stairs
Fordeflection,thecoderecognizesthataflightofstairsisstifferthanaslabof
thicknessequaltothewaistofthestairs.Wherethestairflightoccupiesatleast
60%ofthespan,theallowablespan/effectivedepthratiocanbeincreasedby15%.
This,ofcourse,onlyappliestostaircasesspanninginthedirectionoftheflightand
withoutstringerbeams.
14

15. Design of Stairs
.
15

16. Design of Stairs
.
16

17. Design of Stairs
.
17

18. Design of Stairs
.
18

19. Design of Stairs
.
19