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Re: Torsion moment in R.C girders due to lateral loads

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Check your end conditions. If your section properties suggest that the transmitted force is eccentric with respect to the [shear center, moment center, etc.] you'll get an induced moment/torsion.

The question you must ask is: "Is this the real world effect?" IF it is, then design for it. The key is your joint modelling. I don't know how much control you have over your modelling (I learned on NASTRAN - dangerously exact is what I'd call it) but first take a look at your end conditions - pinned or fixed? single or multipoint (joist to girder)?

Is the joist steel continuous and developed into the girder? If not, and you expect service cracking then try modelling the joist as single point pinned. If the joist is anchored, then your buying joist efficiency with girder waste - does the joist benefit justify the cost. If you do decide to model the structure as pinned, you may need to add joist offsets to account for the small connection eccentricities which actually do exist.

Of course, if you're using RAM, you might consider using a dart board as a check on the model veracity. I've never used their concrete modules, but I have used RAMSteel - and the analysis is totally black box. Very efficient (from a design time standpoint), but difficult to verify the exact internal modelling.


At 10:56 PM 6/2/2004 +0430, you wrote:
Hi,

I want to ask a question about 3d models in computer analysis.

Suppose we have modeled a 3d R.C frame, which has concrete slabs and joists
as vertical load bearing system. But, when we analyze the whole system, in
lateral load cases like wind or seismic loads, some bending moments are
induced in the joists and as a result, very large torsion moments are
resulted in girders that support joists. My question is that is it true to
design joist and girders for these bending and torsion moments? Or it is
necessary to make the model such that these joists don't contribute in
lateral load bearing system of building?

Regards,
Hosein Mardanlu



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Jordan Truesdell, PE, Principal

Truesdell Engineering, Inc.
Structural Consulting and Design
434 Bluegrass Trail Studio #23
Newport, VA 24128-0342
www.truesdellengineering.com



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