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akjhacpwd Silver Sponsor
Joined: 26 Jan 2003 Posts: 147

Posted: Thu Jan 13, 2011 8:21 am Post subject: Deflection of waffle slab 


Dear friends
I am designing a waffle slab 20 m by 20 m which will house library. Ribs are at 1 m spacing in both directions . I have taken ribs width as 250mm. The depth of rib including topping is 900 mm. The waffle slab is supported on peripherial beams which in turn are supported on columns of 400 mm by 900 mm . The columns are spaced at 5 m in one direction and at 4m in the other. The size of peripherial beam is 400 by 900. I am facing problem of deflection . Short term deflection is 62 mm and total defn incl creep is 112 mm. deflection has been calculated considering discontinuous ends by Rankin theory ( taking w/2 in each direction and using 5/384 wl^4/EI). Effective MIhas been calculated according to IS 456. Deflection by plate formula is also coming almost same.
Now permissible deflection is 80 mm (Span/250). If I increase the depth of ribs, headroom is getting reduced as storey ht is 3.6m. What to do? shall I provide camber? Varyani Saheb has recommended camber in his book.
Best Wishes,
A K JHA
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3157

Posted: Thu Jan 13, 2011 9:59 am Post subject: Deflection of waffle slab 


I think Camber can be provided in all long span structures.An Arch is nothing but a
heavily cambered beam .However additional effect of providing camber need to be
taken in the supporting beams and columns assuming hipped structure with small
rise(=camber) at centre especially for this 20mX20m waffle.
best regards
vikramjeet
 ญญ
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gpsarathyy ...
Joined: 28 Jun 2010 Posts: 487 Location: chennai

Posted: Thu Jan 13, 2011 10:03 am Post subject: 


Dear Sefian,
Deflection is also a function of stress in steel, hence increase the Tension steel and compression steel and check as per the IS 456 code. The allowable deflection shall be modified with some factor based on the stress in steel.
Regards,
G.Parthasarathy.
Chennai
Email: gpsarathyy@gmail.com 

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hemal ...
Joined: 01 Apr 2008 Posts: 129

Posted: Thu Jan 13, 2011 10:15 am Post subject: Deflection of waffle slab 


Dear Mr. JHA,
You can try Posttensioning.
Regards
 On Thu, 13/1/11, akjhacpwd <forum@sefindia.org> wrote:
Quote: 
From: akjhacpwd <forum@sefindia.org>
Subject: [SEFI] Deflection of waffle slab
To: general@sefindia.org
Date: Thursday, 13 January, 2011, 2:30 AM
Dear friends
I am designing a waffle slab 20 m by 20 m which will house library. Ribs are at 1 m spacing in both directions . I have taken ribs width as 250mm. The depth of rib including topping is 900 mm. The waffle slab is supported on peripherial beams which in turn are supported on columns of 400 mm by 900 mm . The columns are spaced at 5 m in one direction and at 4m in the other. The size of peripherial beam is 400 by 900. I am facing problem of deflection . Short term deflection is 62 mm and total defn incl creep is 112 mm. deflection has been calculated considering discontinuous ends by Rankin theory ( taking w/2 in each direction and using 5/384 wl^4/EI). Effective MIhas been calculated according to IS 456. Deflection by plate formula is also coming almost same.
Now permissible deflection is 80 mm (Span/250). If I increase the depth of ribs, headroom is getting reduced as storey ht is 3.6m. What to do? shall I provide camber? Varyani Saheb has recommended camber in his book.
Best Wishes,
A K JHA

Posted via Email 

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B.V.Harsoda General Sponsor
Joined: 19 Jan 2009 Posts: 2325 Location: RAJKOT,GUJARAT, INDIA


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anshugoel ...
Joined: 22 Jan 2010 Posts: 341

Posted: Thu Jan 13, 2011 4:28 pm Post subject: Re: Deflection of waffle slab 


akjhacpwd wrote:  Dear friends
I am designing a waffle slab 20 m by 20 m which will house library. Ribs are at 1 m spacing in both directions . I have taken ribs width as 250mm. The depth of rib including topping is 900 mm. The waffle slab is supported on peripherial beams which in turn are supported on columns of 400 mm by 900 mm . The columns are spaced at 5 m in one direction and at 4m in the other. The size of peripherial beam is 400 by 900. I am facing problem of deflection . Short term deflection is 62 mm and total defn incl creep is 112 mm. deflection has been calculated considering discontinuous ends by Rankin theory ( taking w/2 in each direction and using 5/384 wl^4/EI). Effective MIhas been calculated according to IS 456. Deflection by plate formula is also coming almost same.
Now permissible deflection is 80 mm (Span/250). If I increase the depth of ribs, headroom is getting reduced as storey ht is 3.6m. What to do? shall I provide camber? Varyani Saheb has recommended camber in his book.
Best Wishes,
A K JHA
Posted via Email 
Why not increase the rib width and reduce the spacing  that won't hurt.
May be increase the slab depth by couple of inches (50 mm)
The depth of 900 mm for span of 20 metre seems to be less.
If that does not work, may be use higher strength concrete  to get the E up.
Prepost tensioning will be little expensive, I think.
On another note  providing camber to take care of deflection  would it not introduce horizontal thrust at supporting columns ? 

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rg.gupta ...
Joined: 26 Jan 2003 Posts: 92

Posted: Fri Jan 14, 2011 1:36 pm Post subject: deflection in waffle slab 


Dear A K Jha
Your problem has been understood and a solution is within the
preview of clause 23.2.1 of IS456:2000 to control the deflection
by the increase in either tension steel or compression steel is explained
as under:
For the explanation of your problem,The following tentative datas
have been worked out and taken up assumingly. The design of waffle
slab is no other than a slab except hollow portions surrounded by ribs.
As well as toppings.
DL+LL for slab =2.0 t/m2(unfactored)
FRCK=25 n/mm2
EFFECTIVE COVER=4.0 cms
SUPPORT CONDITION =9 (AS PER IS456:2000)
D:\FORTRAN\BINB>slabrg
A PROGRAM FOR ONE WAY , TWO WAY & CANTILEVER SLABS
BY Er R.G.GUPTA *BASED ON LIMIT STATE METHOD*
FOR FY=415.0 N/MM2 AND FCK IN N/MM2
LX =SHORT SPAN IN MTRS
LY =LONG SPAN IN MTRS
OD =OVERALL DEPTH IN CMS
W =LOAD IN TONNES/METER2(UNFACTORED)
N =SUPPORT CONDITION AS PER IS456
IF SUPPORT CONDITION IS CORRECT THEN C=1 OTHERWISE C=0
FOR ONE WAY SLABS
USE K =1,IF SLAB IS DISCONTINUIOUS OVER SUPPORTS, OTHERWISE
USE K =2, FOR SLAB IS CONTINUIOUS OVER SUPPORTS
USE K =3, FOR SLAB IS CANTILEVER
USE J = 0 IF DESIGN OF SLABS ARE TO BE CONTINUED OTHERWISE USE J= 1
FCK =25.
LX =20.0
LY =20.0
OD =90.
W =2.
ECOVER=4.
SLAB IS TWO WAY SLAB
N =9
SELECTED CONDITION IS FOUR EDGES DISCONTINUED
C =1
SLAB IS SAFE IN DEFLECTION & AMF= 1.57 RMF= 1.16
DIA1 = 20.
DIA2 = 20.
.056 44.74 22.61 DIA= 20. SPACING= 14.
.056 44.80 22.64 DIA= 20. SPACING= 14.
AS PER ABOVE RESULTS
MAX. MOMENT IN WAFFLE SLAB=44.80 tm
D:\FORTRAN\BINB>beamrg
A PROGRAM FOR SINGLY & DOUBLY REINFORCED BEAMS
BY Er R.G.GUPTA *BASED ON LIMIT STATE METHOD*
FOR FY IN N/MM2 AND FCK IN N/MM2
B =WIDTH IN CMS
OD =TOTAL DEPTH IN CMS
D1 =EFFECTIVE COVER IN CMS
M =MOMENT IN TONNES METERS
V =SHEAR IN TONNES
DIA=DIA. OF V/STIRRUP IN MM
USE J1 = 0 IF DESIGN OF BEAMS TO BE DISCONTINUED FOR SHEAR
OTHERWISE USE J1= 1
USE J = 0 IF DESIGN OF BEAMS ARE TO BE CONTINUED OTHERWISE USE J= 1
FY = 415.
FCK = 25.
B = 25.
OD = 90.
D1 = 4.
M = 44.80
VALUES OF M IS UNFACTORED,SAY YES OTHERWISE NO
yes
k= 3.634 Pt= 1.211 Pc= .265
At= 26.032 Ac= 5.707
AS PER ABOVE RESULTS
TENSION STEEL =26.03 cm2
COMPRESSION STEEL= 5.70 cm2
RELATIVE MODULAR RATIO =112/80=1.40(AS ACTUALLY REQUIRED)
D:\FORTRAN\BINB>defrg
A PROGRAMME FOR FINDING MODIFICATION FACTOR FOR STEEL
IF N = 0, Modification for TENSION reinforcement
IF N = 1, Modification for COMPRESSION reinforcement
ALFA = Modification Factor
D = Overall Depth in Cms.
B = Width in Cms.
COVER = EFFECTIVE COVER
AT = Area of Tension Steel
AC = Area of Compression Steel
N =0
AT =26.03
COVER=4.
B =25.
D =90.
% STEEL AREA OF STEEL SER.STRESS MOD. FACTOR
1.211 26.030 240.700 .956
1.311 28.180 222.336 1.014
1.411 30.330 206.575 1.069
1.511 32.480 192.901 1.121
1.611 34.630 180.925 1.172
1.711 36.780 170.349 1.221
1.811 38.930 160.941 1.268
1.911 41.080 152.518 1.313
2.011 43.230 144.932 1.356
2.111 45.380 138.066 1.398
2.211 47.530 131.820 1.439
2.311 49.680 126.116 1.478
2.411 51.830 120.884 1.515
2.511 53.980 116.069 1.552
2.611 56.130 111.623 1.587
2.711 58.280 107.506 1.621
2.811 60.430 103.681 1.654
2.911 62.580 100.119 1.686
Stop  Program terminated.
OPTION 1
FROM ABOVE RESULTS
TENSION STEEL REQUIRED TO OVER COME THE DEFLECTION OF 112mm
MF=(1.3131+1.356)/(2X0.956)=1.40
AT=1.95X25X(904)/100 =41.92 cm2 AGAIST 26.03
D:\FORTRAN\BINB>defrg
A PROGRAMME FOR FINDING MODIFICATION FACTOR FOR STEEL
IF N = 0, Modification for TENSION reinforcement
IF N = 1, Modification for COMPRESSION reinforcement
ALFA = Modification Factor
D = Overall Depth in Cms.
B = Width in Cms.
COVER = EFFECTIVE COVER
AT = Area of Tension Steel
AC = Area of Compression Steel
N =1
AC=5.70
COVER=4.0
B =25.
D =90.
% STEEL AREA OF STEEL MOD. FACTOR
.265 5.700 1.078
.365 7.850 1.102
.465 10.000 1.139
.565 12.150 1.166
.665 14.300 1.185
.765 16.450 1.203
.865 18.600 1.222
.965 20.750 1.243
1.065 22.900 1.264
1.165 25.050 1.284
1.265 27.200 1.303
1.365 29.350 1.320
1.465 31.500 1.335
1.565 33.650 1.349
1.665 35.800 1.361
1.765 37.950 1.373
1.865 40.100 1.386
1.965 42.250 1.401
2.065 44.400 1.419
2.165 46.550 1.443
2.265 48.700 1.470
2.365 50.850 1.499
Stop  Program terminated.
OPTION 2
FROM ABOVE RESULTS
COMPRESSION STEEL REQUIRED TO OVER COME THE DEFLECTION OF 112mm
MR=1.51/1.078=1.40
AC=2.37X25X(904)/100 =50.96cm2 AGAIST 5.70
OPTION 3
FROM ABOVE RESULTS
BUT IF WE PROVIDE 10 NOS 20 TOR AT TOP & BOTTOM
AREA OF STEEL AT TOP & BOTTOM=31.40cm2
MF1 DUE TO TENSION STEEL=(1.069+1.121)/(2X0.956)=1.145
MF2 DUE TO COMPRESSION STEEL=1.335/1.078=1.238
MF=MF1XMF2=1.145X1.238 =1.418 

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knsheth123 Silver Sponsor
Joined: 26 Jan 2003 Posts: 123

Posted: Sat Jan 15, 2011 3:34 pm Post subject: Deflection of waffle slab 


Provide Larger widths (400) for Ribs spanning between Columns & To carry out analysis as Space Frame.
Will result in reduce deflections.
Use of Higher Conc Grade, Some Addl., Imposed loads Loads applied after 3 to 6 month duration and provision of Camber 25mm.
All these put together should work to bring deflection under Control.
With Regards
K. N. Sheth
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umeshrao ...
Joined: 23 Aug 2010 Posts: 573 Location: Bangalore, India

Posted: Sun Jan 16, 2011 7:22 am Post subject: Re: Deflection of waffle slab 


akjhacpwd wrote:  Dear friends
I am designing a waffle slab 20 m by 20 m which will house library. Ribs are at 1 m spacing in both directions .
Now permissible deflection is 80 mm (Span/250). If I increase the depth of ribs, headroom is getting reduced as storey ht is 3.6m. What to do? shall I provide camber? Varyani Saheb has recommended camber in his book. Best Wishes, A K JHA Posted via Email 
Dear Mr. Jha,
I stand to be corrected.The floor being designed has to carry loading of library or is the roof of library block?
Eitherway. Is it necessary to provide orthogonal grid for the slab. Can it be diagonal? In case of diagonal grids maximum span of beam is 14.12M and not 20m,hence beam moments, shear and deflection will come down drastically. In that case instead of ribbed theory use grid theory and increase distance between ribs to 1.25m.
Regards Umesh Rao 

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abhio ...
Joined: 08 Mar 2010 Posts: 535

Posted: Sun Jan 16, 2011 8:45 am Post subject: 


Respected Er. Rao Sir,
I am not clear about how diagonal beams would have a smaller span than perpendicular ones. The maximum span of beam would be 20*sqrt(2) = 28.3m.
Regards,
A S Oundhakar 

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