tngv752,
My apologies for the change in topic, but I would like to see this one through. And everyone apologies for the long post.
demayeng,
Please don't get me wrong, engineering programs are tools of the trade and when in the hands of a capable engineer they can perform satisfactorily. But a few hand cals or spreadsheets will always catch those minor glitches in the system, as asixth has point out.
To use an old quote don’t know whom started it but it is a golden one “If a designer knows what results to expect from a computer program (within 5-10%), then he should be ok to use it, If not he is more than likely to get is wrong”. I use hand cals to get the (5-10%), some engineers may be able to do this with experience alone.
As for Slabs and deflection problems, I will speak only in general terms of FEA products, as I have only used Slab once before and it was before they really got serious.
The idea is generally to take FEA deflection and multiply it by 2 or 3 (Kcs) and call it a total long term deflection. Unless it has changed very recently (Slabs may work out Ieff for based on reo, If so it may be getting better than the last time i used it), cracking is defined by the designer as a factor of gross inertia for deflections (ie 0.2 Igross for slabs and 0.4 Igross for beams), it is not calculated based on the actual stresses at each cross-section.
The kcs factor for long-term effects for your comparison is not very good because kcs for beams and slabs is actually quite different because of the difference in the creep and shrinkage for the different section shapes.
Thus is you don’t include cracking in your analysis correctly and use Kcs to factor up your deflections the real deflections could easily be 2 to 5 times higher. I am currently sitting in a building where all the walls are cracked from settlement of the slab over time due to this issue (the person whom did the analysis didn’t include cracked section properties for his beams and slabs in his FE model), everything is strong enough but the deflections are L/100 and getting worse by the year.
You will get your best deflection estimates for RC and PT slabs from Calculations which include full cracking analysis and long term creep and shrinkage analysis.
As for the other issue I will steal a few notes from Rapt (hope you don’t mind rapt).
“An FEA programs generally allows for Mxy moments in the analysis and then ignores them in design then the difference will be at least 15% and possibly up to 25% UNDER estimation of the reinforcement requirements. This does not mean that you can use 15-25% less reinforcement, it means it has under designed by 15-25%.
It does not mean that FEA is wrong. FEA is reporting Mx, My and Mxy. It means that the design application tacked onto the end of the FEM is wrong because the developer does not understand design using FEM, or they are trying to cheat (hopefully it is the first but either is worrying).
This will become obvious below but we must distinguish between FEM analysis and software that produces reinforcement/prestress requirements based on FEM analysis. All FEM produces is a set of stresses which, when converted to moments, describe the moments on the floor system that have to be designed for.”
This is why I don’t trust FEA programs in general, While teh moment they give can be correct if you get the model correct (this would take to long to go through all the problems i have seen with people modeling in FEA programs), and just to show that i don't trust other 2D programs wither, I also have problems with 2D programs due to the fact that it they normally ignore torsion thus makes it very hard to do anaylsis with either compatibly or equilibrium torsions.
Sorry again for the lng post and I will get off my soap box now. now to eat some fish with my chips of my sholder.
When in doubt, just take the next small step.