How to check beam & slab deflection in ETABS
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Author: budis
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How to check beam & slab deflection in ETABS
10-31-2010, 04:48 PM
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How to check beam & slab deflection in ETABS
How to check beam & slab deflection in ETABS ?
Tell me also the limit of the deflection of it
Thx
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10-31-2010, 04:54 PM
Post: #2
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RE: How to check beam & slab deflection in ETABS
You can check the nodal displacement in ETABS (global X, Y and Z direction). I do not recommend ETABS for checking slab deflection. Use SAFE.
The limit of deflection is depends on your code requirements.

Hope I answered your question.
Regards,
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10-31-2010, 05:01 PM
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RE: How to check beam & slab deflection in ETABS
Dear Visu
After I run the model, then how check the nodal displ ? Please explain more explicit
Btw, is there the print out output of it ? Where I look for it ?
For ACI-318 ( as far I remember, please correct me if wrong in remember it ) there only beam & slab deflection of simple supported ? Then for slab & beam in frame or frame - wall system which fomula I should use ?
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10-31-2010, 06:15 PM
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RE: How to check beam & slab deflection in ETABS
dear user...

for beam deflection, refer to ACI 318 chapter 9 section "CONTROL OF DEFLECTIONS"...or refer to PCA Notes on ACI 318.
calculate the desired section properties, moment of inertia, cracked moment of inertia, effective moment of inertia etc...and calculate deflection.

Now ETABS does not take cracked section properties in its analysis, that is why you will have to define FRAME PROPERTY MODIFIERS...
Also in your manual calculations do not calculate Icr (Cracket moment of inertia)...rather take Ieff (effective mom. of inertia) = some factor x Ig (Gross mom. of inertia...)....

for example if you have 12 inch by 12 inch rect. section...your Ig is bd^3/12 = 12*12^3 / 12 = 1728 in^4...now take your Ieff = some factor * Ig....

Most common factor for beams = 0.35 so your Ieff = 0.35*Ig = 0.35*1728 = 604.8

Now use this value in your hand calculations and to check it in ETABS do this:-

1) draw a simple beam and apply Uniformly dist. load on it...
2) make its end offsets value 0...by ASSIGN>FRAME LINE>END OFFSETS and select DEFINE and put 0 in x & y.
3) now again ASSIGN>FRAME LINE>FRAME PROPERTY MODIFERS and put 0.35 in mom. of inertia along 3 -axis (major axis) and press OK.
4) define the desired load comb on which you want deflection..e.g. 1.0D+ 1.0L etc
5) Run the analysis
6) Click DISPLAY>SHOW MEMBER FORCES/STRESSES> FRAME/PIER/FORCES and select desired combination and select Moment 3-3 then Ok.
7) Right click on the bending moment diagram and there its at the last...deflection...dont forget to click ABSOLUTE option (first one) at the bottom to check deflection...this will be the exact deflection you calculated manually...

if you have further doubts dont hesitate to ask....but for the actual cracked section properties without using this factor of 0.35 etc...USE SAFE...
to check the long term deflection...manually put the long term deflection combination e.g. 3SW+3DL+3LL etc...


WS
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11-01-2010, 01:02 AM
Post: #5
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RE: How to check beam & slab deflection in ETABS
Dear coolestbliss
value of 0.35 or 35% is a recommendation of aci or pca
can you give me an explanation please
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RANA WASEEM
11-01-2010, 08:37 AM
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RE: How to check beam & slab deflection in ETABS
Dear PANA2

1) actualy this 0.35 is the ratio (not exactly but nearly) of Mcr/Ma used in deflection calculations of ACI & PCA...(PCA is the same thing...its the explanation on ACI )....

2) Instead of doing detailed calculations of cracked moment of inertia for deflection calculations, we use reduced moment of inertia by using these factors....this 0.35 for beams is a common practice for beams in design industry...

3) just take an example of simply supported rect. beam as i said in my earlier post and calculate both deflections; by hand and by etabs...using 0.35 factor...they will give you same values with less than 1 or 2% error.


WS
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11-01-2010, 12:14 PM
Post: #7
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RE: How to check beam & slab deflection in ETABS
Dear coolestbliss
Very clear explanation on beam deflection checking ! Thanks
Btw, I didn't find output of beam deflection in etabs output file ? Where I can see it ?
Can you explain checking of slab deflection in etabs ?
Thx
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RANA WASEEM
11-01-2010, 01:56 PM
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RE: How to check beam & slab deflection in ETABS
try to learn etabs interface first
for slab deflection you can get values @ nodes...i mean you have to mesh the slab manually...better use SAFE


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