CORE WALL analysis 1

[Rick89]

I have been doing a bit of designs that involve steel and concrete frames as the main resisting systems.
Now, we got a project six-storey reinforced concrete medium commercial building that requires an elevator, which our design team want to include a core-wall (C-shaped) to take care of the 'total' seismic shear(Vt). This way we assume the frames to only carry dead and live load.
A friend in our team will use SAP2000 for analysis but we need to crosscheck the output by approximate manual calculation.
So please help on the following questions:
- Is it safe or a common practice to let the core-wall take all the lateral load while the frames only carry DL+LL?
- can we divide the C-section and analyse as if it were three planar walls where torsional strength is ignored?
- what are the approximate formulas/equations used to roughly determine V(shear), M(moment) and T(torsion)?

 

[Rick89]

I have read some papers discuss the torsional resistance of C-section (T-section) to lateral torsional effects but they seem to all use finite element (software) when composite action is considered in the analysis.
The books mentioned above are really helpful plus the IBC seismic design manual. By reading through these documents, I would say we are still on the safe side even the composite action is neglected. So the walls (3 walls) would be assumed to resist lateral forces independently, given that detailing follows properly.

 

[Trenno]

Does your elevator core have concrete between the top of the door opening and slab soffit? In other words, will your C shaped core have a coupling beam to 'close' the core section.

I looked at a very similar problem a few years ago.

Link

 

[Mjkkb2]

Replying to the OPs original questions

- Yes, it is as long as the detailing and allowance for movement follows and is compatible with your assumptions
- Yes you can, as that's how many of the buildings were designed when it had to be done "by hand". Neglecting the composite effect of the "flanges" of the C is ok. It is done in my practice when we do things by hand.
- Others cited good sources on this one.

And just to add, I wouldn't expect that you get similar results with the SAP (or any other software). AS some say 'there are many ways to skin the cat'. Only because the software got you X results it doesn't mean that you made a mistake if you get Y results doing it on paper with a pencil. AS long as you make reasonable assumptions and have a direct load path with all applicable loads accounted for there's nothing wrong with your hand calc being off.

good luck