ADAPT Builder 2019 showing lower deflections than ETABS 2016 1

[Shamiq]

Hello,

I am currently working on a large-span slab and the team was considering a PT slab and beam for a 65' by 75' hall span. We have a fully developed ETABS model in RC, but when I started developing the slab in ADAPT Builder I ran it as an RC slab with the same stiffness modifiers, loads, section thicknesses and material specifications as the model in the ETABS file just to compare Service deflections. And the results have surprised me, where the ETABS model is showing me a SL deflection of ~7" (ignore how high that number is as we will be going PT anyway) but the ADAPT Builder analysis only shows ~2.5" at the max deflection point. I have checked and double checked that all specifications are same across both models but there is still this huge difference.

I think this is a good exercise to verify the softwares we use and I have seen such discrepancies between analysis results between Builder and ETABS before, but never of THIS magnitude. Please help! What am i missing here?

 

[CURVEB]

It would be difficult to say why you are getting this discrepancy without seeing the details of both models . Just in case, here are a few things to check:
- Boundary conditions, column fixity at the slab and at the ends away from the slab
- Load combination factors
- Is one program applying long-term creep/shrinkage multipliers?
- Have you compared how both programs handle long-term deflections and cracked section properties, if those are being applied to your analysis.

 

[rapt]

Older Adapt software did not handle cracking and long term effects well. I would suggest you start there.

Not sure about ETABS, Safe has a long term deflection option.

 

[271828]

Have you created a model that is simple enough for hand calcs (or a textbook example) to compare with output from these programs?

If not, then I'd recommend starting there.

 

[ThomasH]

I think this is a good exercise to verify the softwares we use and I have seen such discrepancies between analysis results between Builder and ETABS before, but never of THIS magnitude. Please help! What am i missing here?

If you have managed to give ALL the settings in the two softwares the same value, all you can say is that at least one of them is wrong. Unfortunatly, both of them can be wrong.

I completely agree with 271828, test with something that you can verify "by hand".

 

[Celt83]

From my understanding Adapt Builder you have to define an analysis case for the stiffness modifiers and then also apply the modifiers to the elements, it’s pretty easy to miss a step in this process. Also if you ran their long term deflection analysis in Builder I think that performs its own separate analysis beginning with uncracked properties, check in with their support staff on the proper process.

Etabs I don’t recall performing a specific load history deflection analysis so if you are just globally applying stiffness reductions factors of say 0.25 to slabs then the deflection can be significantly larger than a more detailed load history analysis in which only portions of the slab experience cracking.

 

[271828]

If you have managed to give ALL the settings in the two softwares the same value, all you can say is that at least one of them is wrong. Unfortunatly, both of them can be wrong.

I completely agree with 271828, test with something that you can verify "by hand".

I feel like a broken record. I type that advice at least a dozen times per year.

People are in a hurry. One handy approach is to feed a textbook example into the program.

 

[rapt]

Quickest check for normal RC members would be expected Total Long Term Deflection is about 5 to 6 times Short Term Uncracked Deflection.