Slabs Deflection

[Refaat]

Dear Colleagues

Could I get responses for the following Inquiries :

•  Why does the max. slab deflection value in the capture (01) is greater than in the capture (02) for the same slab parameters?

• Which deflection value shall I consider as " Long term deflection "?

• Is there any possibility to get the elastic slab deflection directly from ROBOT       ?

Note : If you use any capture for explanation . Kindly , send it as attached .

Thanks in advance.

Refaat

[Artur.Kosakowski]

•  Why does the max. slab deflection value in the capture (01) is greater than in the capture (02) for the same slab parameters?

I assume that in the first scenario you calculated the deflection for all of SLS combinations (case 40 being the compound case with many components) whereas for the second one only for a single selected one (apparently not the one that gives max. value of deflection).

• Which deflection value shall I consider as " Long term deflection "?

Which design code is concerned? 

• Is there any possibility to get the elastic slab deflection directly from ROBOT       ?

 If you mean uncracked then check the results of the static analysis instead.

[Refaat]

Dear Artur

Thanks for response , but may I clarify the followings :

- Do you mean that ROBOT in capture (01) doesn't calculate the deflection for the case (40) ? It means that I Can not specify  a case to calculate the long term deflection without running the " deflection verification " ?

- " ACI318-80M" is concerned for long term deflection calculation .

-  Elastic deflection  , I meant " Short term deflection  " for slab . if  possible to get in ROBOT . Kindly , send me a capture for explanation.

Thanks

Refaat

[Artur.Kosakowski]

- Do you mean that ROBOT in capture (01) doesn't calculate the deflection for the case (40) ? It means that I Can not specify  a case to calculate the long term deflection without running the " deflection verification " ?

If you have a case that include more than one component (e.g. SLS code combinations) and you use it for calculations of deflection than the result is sort of 'envelope' of deflections. On your screen captures you used case 40 for calculations of deflection and 37 for its verification. Mind that for the simplified approach it is important that you divide your slab into panels with 'clearly' defined boundary conditions (line of supports) as it based on the scaling off results from the static analysis. The description of the method is available in Help.

- " ACI318-80M" is concerned for long term deflection calculation .

For ACI displayed values of deflections are the total long-term ones. See the attached picture.

-  Elastic deflection  , I meant " Short term deflection  " for slab . if  possible to get in ROBOT .

No, these values are not available for display.

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[Refaat · Accepted solution]

Dear Artur

Thanks , well done

Refaat

[Mirko.Jurcevic]

I am trying to get "short term" (initial) deflection of a simple slab, I've made short video that displays what I am doing.

http://screencast.com/t/KCmKZ0JGS

I put those Φ12's first, then I tried to get deflection for characteristic SLS load combination.

I used "With stiffness update" verification type and I get 12,29mm deflection.

Then I done all that again but with Φ18's and I got smaller deflection.

Any way, Robot keeps warning me about quasipermanent combination.

Is this correct way around to get initial deflections for provided reinforcement?

Code: EN 1991-1-1

[Artur.Kosakowski]

Robot calculates the value of total displacement and there is no option for calculating the initial ones. This is done for QPR SLS combinations and when there are no such combinations defined Robot assumes that all defined SLS combinations are of such type.

[Mirko.Jurcevic]

😞

Anyway, thank you Artur!

[Anonymous]

Hi! I also made my own Design Using Robot, Placed Reinforcement to it and Run Deflection Checks. Cracking seems too much and Deflection is out of this world.. I used SLS Envelope for this case

And Also, What is the Meaning of this? how can I fix this issue?

[Artur.Kosakowski]

Hi! I also made my own Design Using Robot, Placed Reinforcement to it and Run Deflection Checks. Cracking seems too much and Deflection is out of this world.. I used SLS Envelope for this case

Check what values of displacements you obtained for static analysis of the model itself.

And Also, What is the Meaning of this? how can I fix this issue?

Please check: http://forums.autodesk.com/t5/Robot-Structural-Analysis/SLS-combination/m-p/3316923

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[t.sautierr]

Maybe I missed something but what difference do you make between "elastic deflection" and "short term deflection" ?

Considering the code, we always calculate short and long term deflection and sometimes "detrimental deflection" when slabs supports fragile element but to me elastic = short term ins't it?

I hope I'm not saying a big crap ..... 

[t.sautierr]

To be more accurate :

Elastic deflection with homogeneous section (rebars converted in an area of concrete) =  Short term deflection 

Pure elastic deflection = concrete only with short term E and rebars not taken into account.

[faisal.aaouatif]

Good Morning, Good Evening ,

I'm trying to calculate a slab deflections under ACI.

there is two conditions to satisfy accordingly :

1. Limit the deflections due to Live load only ( Immediate deflection ) according to 9.5.b table

2. Limit the deflections under Live load + part due to creep and shrinkage ( Long term deflection ) according to 9.5.b table.

According RSA HELP , Deflections are computed, displayed, and can be verified for a selected combinations, but On the post above , it is stated that the software display (may be calculate) only the long term deflections (under ACI) , which means deflection values displayed should be combinations independent or there is a misunderstanding from me ?  Need some clarifications please.

Then :

1.Is it possible to calculate Deflection under Live load only ? if so How please.

2.Under what combination to display long term deflection ?

Thank you.

[Artur.Kosakowski]

Robot calculates only the long term deflection. In case you want to see the values for a selected combination you should run the verification of the deflection for it.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[faisal.aaouatif]

Robot calculate only long term deflections for aci - Got it.

Please tell me if the following makes sens or not :

1.For a selected combination , after verification , Robot gives its long term deflection (even if this combination does not contains a live load case.)

2.short term and long term deflections should be the same if we assign zero to Ksi Factor (last textbox below) , or zero to ratio of loads,  in the sls tab in the reinforcement parameters to the panel we want to Verify short term deflection for.

3.If we want Live load short term deflection , follow up the point 2.

Thank you.

[Artur.Kosakowski]

I'm sorry but I can't see a way of checking the live load deflection for reinforcement calculated for long term one.

[Anonymous]

RSAP can only give Long Term Deflection Once you apply Reinforcing Steel on the Section. RSAP gives you Short Term Deflection or Elastic deflection during analysis

[Anonymous]

Hi @Artur.Kosakowski  does the deflection map in the slab design show the total long term deflection ?

1. Total long term deflection = Full Live Load + Deflection Due to Creep + Defelection due to Shrincage ?

2. By using the Qasi permanenet combination we use only 0.3 Live Load, does the map consider the full Live Load in addition to the creep and shrincage or just 0.3 Live Load?

3. Or is just due to creep and shrincage and we have manually to consider the LL in addition to this ?