how to design a beam modelled as shell elements 1

[msh69]

Hi everybody,
the statement of the problem:
cut and cover metro station (3.5m below the ground level)
top and bottom slabs are 90cm thick, outer shear walls are 70cm thick, inner shear walls are 30cm thick. dimensions are approx. 26m x 23m from axis to axis (1 part of a total station) It has been modelled by using shell elements. Due to geometry, an approx. 10.5m span length has to be passed by a beam of 170cm height. Beam has also been modelled as shell element.
the designer's approach:
the designer has tabulated F11 forces, found (or assumed- not clear from the report)neutral axis, took moments of F11's with respect to neutral axis, found moments and calculated the reinforcements.
my approach:
Tabulate stresses(sigma's), plot against the height of the beam, locate the neutral axis, calculate forces, and so on. I also suggested to the designer, model the beam as frame element but the designer claimed that due to the thickness of the top slab (90cm), it could not be connected (shell to frame) properly.
The question is what is the usual or practical way of designing such beams?
Thanks in advance.

 

[14159]

If you were using ETABS, I'd say to call the desired shell a "spandrel" which will give you the forces that you need to design it as a beam. I'm not sure if this is possible in SAP.

I don't really like the stress summation idea because I've seen really weird results from this in the past. Most people are simply not FE experts enough to do this without screwing something up.

Using good modeling techniques, there's no reason it can't be a frame element. Look at it like a tee-beam and calculate A, I, etc. Then the effective concrete width must be subtraced away from I to keep from double-dipping. This is what I'd do.

DBD