Deflection and displacement in slabs

[Anonymous]

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

[Muhammed.OPERA]

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).

[Muhammed.OPERA · Accepted solution]

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

[Anonymous]

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?

[Muhammed.OPERA]

Hi @Anonymous

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

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

[Anonymous]

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

[Muhammed.OPERA]

Hi @Anonymous

Use mediafire

[Anonymous]

Mr @Muhammed.OPERA,

Here is the file.

Thank you.

[Muhammed.OPERA]

Hi @Anonymous

Which story slab to check?

[Anonymous]

Hi Mr MuHammed.Opera,

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

[Muhammed.OPERA · Accepted solution]

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.

[Anonymous]

Hi Mr Muhammed.Opera,
Thaankkk you soo much .

[Refaat]

Hi @Anonymous

 Beside  the  provided solution  of  @Muhammed.OPERA

1. You have to be care about the live load percentage which should be considered for long term deflection.
2. IMHO that for RC slab mostly like (Bending shell element) should be considered instead of (Bending + axial).
3. I think, there is lack for reinforcement data. You should reset this data before starting your own calculation.
4. 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...

Refaat

[Muhammed.OPERA]

Yup Mr. @Refaat

I have used the ACI RFT.

[Refaat]

@Muhammed.OPERA

Yes , I see

Refaat

[Muhammed.OPERA]

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 .

[Anonymous]

Hi Mr Muhammed and Mr Refaat,

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

Sincerely

[tx36]

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