Design of rc-beams, modelling slabs as shell elements and supporting beams as frame elements

[cyrosmith85]

Hi,

This is my first post.
First of all I would like to thank you all for the help you have provided, by sharing opinions and giving advice on many structural design issues that I have come across more than once. Thank you!

Second, concerning the subject; I have a pretty clear idea of how shell/membrane elements work when used to model slabs. Now, about the design of the rc-beam supporting that slab, I recently found a comment in an eng-tips forum which suggested that if the slab is being modelled as a shell, one should not design the beam based on the information of such an analysis, because the reinforcement reported is much less than what is truly required.

I have always designed with slab-membrane elements and supporting beam-frame elements. Just recently, I learned about SAFE, and when comparing the bending moments of supporting beams of a membrane-on-frame model (from ETABS) with those of a shell-on-frame model (from SAFE) there is a significative difference, generally lowering the flexural demand of the shell-on-frame model (from SAFE). With these recent founding, and taking advantage of such a tool as SAFE (FEM), what I would do is to design all beams that are part of the lateral-force-resisting-system (LFRS) with a model having all slabs modelled as membrane elements. That way I would be neglecting the slab stiffness in the seismic analysis/design and only rely on the frames and/or walls to resist wind and or earthquake forces.. but those beams that carry gravity loads only I would design using SAFE (the shell-on-frame model) thus benefiting from considering the slab working part of the load out before delivering it to the beam, resulting in an more economic beam design $.

Now, I would do the above mentioned as long as it's safe to proceed that way. That's what I'd like to know your opinion on. I understand it would not be a traditional conservative design (like the membrane-on-frame model), but provided conservative loads and good detailing are a given, do you think it´s a good design approach? or is it not safe at all to design the only-gravity beams for the bending forces that SAFE reports using the shell-on-frame model?.. If so, why is it so?

As I understand, finite element analysis (if correctly used) reports a more realistic scenario in terms of what is going on with stresses and forces; so why would these results be unsafe to use?

Thanks, and greetings from Mexico.

 

[mushtpk]

I am much more interested in detailing of beams meant for only gravity loads only and these are being affected by lateral forces, as these beams are part of the whole structure.
In case ur object is carry out analyses and design on gravity loads, it's OK.
U can employ lateral loading to the SAFE model for practically true design.
Since SAFE has logarithm to work out the actual/ cracked section, it is used for deflection assessment and rebar design.

 

[cyrosmith85]

Hi, just read your reply, tankyou mushtpk!

Yes that´s what I think, since it was a basement floor and it´s way far from where the significant seismic induced rotations take place, I ended up designing it only with what SAFE reported.

I am interested in the effect of lateral displacements on gravitational elements too, you should take a look at Jack Moehle´s book "Seismic Design of Reinforced Concrete Buildings", it has very interesting content on the subject.

Thanks again, and best regards!