Lawyers have a saying that you should never ask a question that you don't know the answer to. You are now in the ridiculous situation that if your design is challenged you will have to either demonstrate that you took the most conservative path, or else demonstrate that one or other of your analysis methods was wrong.

Cheers

Greg Locock


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Either method is generally considered valid for design purposes so long as all load delivered to the system get resisted somewhere. Concrete is usually a flexurally ductile material and quite capable of redistributing load. If your FEM and tributary area results differ by more than, say 20%, I'd recommend seeking an explanation for that. Either your tributary area technique isn't accurate enough or your FEM model has gone awry. Either one is a good check on the other. Done properly, I'd say that FEM would be moe accurate. But then, if you don't check it against tributary area, how do you know that you've modelled it properly?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

It depends how u calculated the tributary area. Generally if u ignore the slab and consider that all the load is carried by the beam it won't converg with the FEM since the slab has out of plane stiffness and it can carry some of the load. If u a really want to compare then u must assign in ur FEM model modifiers to the out of plane stiffness (or assign ur slab as a membrane).

The main problem with the FEM result is that it does not consider
- staging of construction - It basically models the building assuming it was fully supported until it is all constructed and then all of the temporary supports are removed.
- it does not consider inelastic effects, cracking etc and redistribution resulting from it.
- it does not consider creep and shrinkage effects.

The problem with tributary areas is getting them right and allowing for the correct areas and any redistribution that happens due to axial shortening of supports and other effects.

I think the answer is normally somewhere between the two, but conservatively I would always use the worst of the 2.

Transfer beams are the most important member in the structure, it is logical to give them a higher factor of safety anyway. If you consider Robustness in your design, normally the only way to justify a transfer beam is to increase its factor of safety even further!

Thank you guys for your valuable response. Those replies gave me some more insight into the matter. I am a recently graduated civil engineer so that i have lot to know. I got a thought that the supports assigned to the corners also absorb some of the loads in FEM model.

Quote (rapt)

I think the answer is normally somewhere between the two, but conservatively I would always use the worst of the 2.

I agree to the Rapt's point. But I'm curious about very complex structures where the manual calculations are very tedious or difficult to carry out, how we are going to compare the results like that.?

Do the calculations by hand like we have always had to previously! Maybe you could put your analytical knowledge you learnt in university to good use to come up with a way of automating it.

If you were able to set up your FEM model to not allow axial shortening in the supporting members and not allow deflection in your transfer beams, then I would think that you should get an even more accurate version of the tributary area approach. P.S. this method is trademarked by me so anyone who has never thought of it before and wants to use it, please send donations!

We run three anaylsis methods.

1. Fea
2. Rapt (trib method with excel load take down)
3. Rules of thumb

If any of these fail to meet a +/- 15% rule we spend a few hours figuring out why.

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