topic Re: Deflection and displacement in slabs in Robot Structural Analysis Forum

Deflection and displacement in slabs

Anonymous — Thu, 15 Mar 2018 18:11:20 GMT

Hi,

Kindly, Can someone explain to me the difference between displacement and deflection in slabs. I checked the displacement for the slab shown in the attached photo , its 0.8 cm but the deflection for the same slab in the other photo is 7.6 cm !! and why i have such a big deflection for a slab have a thickness 25 cm !!!

i would be thanks full for an explication and a solution to this problem .

Thank You

Re: Deflection and displacement in slabs

Muhammed.OPERA — Thu, 15 Mar 2018 18:22:03 GMT

Hi @Anonymous

The deflection values you get after designing the slab (RFT.) is the long term deflection taking the other factors like creep, ....etc.

But the one you get from ordinary slab maps is the static one (linear deflection).

Re: Deflection and displacement in slabs

Muhammed.OPERA — Thu, 15 Mar 2018 18:38:04 GMT

The results (displacement map) from static analysis of the model is based on assumption of linear behavior of the material (no crack, no reinforcement).

The calculations or verification of cracked deflection always include creep and includes cracks and reinforcement for both elastic and stiffness update methods. This is long term deflection.

See the 5th webinair it may explain that much better:

https://www.youtube.com/watch?v=YvVRNFH8_cc&index=5&list=PLY-ggSrSwbZqow_60fiqJwS69mg1nQMzk&t=0s

Re: Deflection and displacement in slabs

Anonymous — Thu, 15 Mar 2018 18:46:19 GMT

Hi Mr ,

Thank you, but why i have such a big deflection ? for this Flat slab the minimum thickness is 6/31 = 20 cm, i put it 25 cm and the permissible deflection 2.5 cm, so how can i fix this problem?

Re: Deflection and displacement in slabs

Muhammed.OPERA — Thu, 15 Mar 2018 18:52:50 GMT

Hi @Anonymous

Are you sure you are using the right material & creep parameters?

You can attach the model to check (If you want).

Re: Deflection and displacement in slabs

Anonymous — Thu, 15 Mar 2018 19:16:54 GMT

Hi Mr @Muhammed.OPERA,

I tried to upload the file but its larger then the maximum size allowed.

I will be thanks full if you can send me a picture how to verify the Creep parameters .

Sincerely

Re: Deflection and displacement in slabs

Muhammed.OPERA — Thu, 15 Mar 2018 20:02:24 GMT

Hi @Anonymous

Use mediafire

Re: Deflection and displacement in slabs

Anonymous — Thu, 15 Mar 2018 20:43:01 GMT

Mr @Muhammed.OPERA,

Here is the file.

Thank you.

Re: Deflection and displacement in slabs

Muhammed.OPERA — Thu, 15 Mar 2018 21:26:18 GMT

Hi @Anonymous

Which story slab to check?

Re: Deflection and displacement in slabs

Anonymous — Thu, 15 Mar 2018 21:35:21 GMT

Hi Mr MuHammed.Opera,

I want to check the Level 0, Z=00m Slab ( number 581) .
Sincerely

Re: Deflection and displacement in slabs

Muhammed.OPERA — Thu, 15 Mar 2018 22:20:20 GMT

Hi @Anonymous

If you want to design any plate, it's preferred to take it into 2D model to avoid the problem of the cumulative deflection.

Check that post:

https://forums.autodesk.com/t5/robot-structural-analysis-forum/differential-deflection/td-p/7834170

So,

I took the story +0.00 into as substructure and run calculations to that stor, then designed the slab with these parameters and the deflection (long term one) is now in range safe.

Re: Deflection and displacement in slabs

Anonymous — Thu, 15 Mar 2018 22:24:21 GMT

Hi Mr Muhammed.Opera,
Thaankkk you soo much .

Re: Deflection and displacement in slabs

Refaat — Thu, 15 Mar 2018 22:38:22 GMT

Hi @Anonymous

Beside the provided solution of @Muhammed.OPERA

You have to be care about the live load percentage which should be considered for long term deflection.
IMHO that for RC slab mostly like (Bending shell element) should be considered instead of (Bending + axial).
I think, there is lack for reinforcement data. You should reset this data before starting your own calculation.
I highly recommend you to read this thread. It’s very important for those people using ACI 318.

https://forums.autodesk.com/t5/robot-structural-analysis-forum/immediate-and-long-term-deflection-as-per-aci-code-on-robot/m-p/7084551/highlight/true#M53475

Refaat

Re: Deflection and displacement in slabs

Muhammed.OPERA — Thu, 15 Mar 2018 22:48:21 GMT

Yup Mr. @Refaat

I have used the ACI RFT.

Re: Deflection and displacement in slabs

Refaat — Thu, 15 Mar 2018 23:22:19 GMT

@Muhammed.OPERA

Yes , I see

Refaat

Re: Deflection and displacement in slabs

Muhammed.OPERA — Fri, 16 Mar 2018 03:04:38 GMT

Hi @Anonymous

That is the exact parameters i have used:

First of all, save the story as sub structure, open, put supports and define which Reinforcing bars' database you want to use reinforcing this slab in: Tools > Job Preferences. I myself used (ACI 318-08 metric).

3- Make a mesh with element size = 0.25 and run calculations.

4- See the picture of (Plate and shell reinforcement type):

Finally, thanks Mr. @Refaat for your extremely useful remarks for me .

Re: Deflection and displacement in slabs

Anonymous — Fri, 16 Mar 2018 05:45:21 GMT

Hi Mr Muhammed and Mr Refaat,

Thank you for all these information, it was so helpful.

Sincerely

Re: Deflection and displacement in slabs

tx36 — Tue, 31 Mar 2026 03:41:57 GMT

Hi Muhammed,

Just want to make things clear.
so the deflection from slab required calculation tab is already considering creep and shrinkage? should I use the stiffness update method for the deflection verification?
or do i need to calculate manually the long term combination to be added and used during the calculation?
for information, I am using ACI for my RC Slab Calculation