×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Accidental Torsion in Seismic Analysis

Accidental Torsion in Seismic Analysis

Accidental Torsion in Seismic Analysis

(OP)
I am performing a seismic analysis on an apparatus per ASCE-4. I am using an equivalent static load per the code and applying it to a full nonlinear finite element model. The model is fully detailed (distributed mass, no stick and lumped mass idealizations, etc). I'm now being told I need to account for accidental torsion. My interpretation after reading the code and the commentary is that a detailed finite element model accounts for this (3.1.1.(d) The model shall represent the actual locations of the centers of masses and centers of rigidity, thus accounting for the torsional effects caused by the eccentricity) and that this is only required when an idealized model is used that is "apparently symmetric." Do I really need to account for accidental torsion?

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Accidental Torsion in Seismic Analysis

I believe that you do still need to account for accidental torsion.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Accidental Torsion in Seismic Analysis

(OP)
OK, but why? I'm trying to understand the physical significance of this.

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Accidental Torsion in Seismic Analysis

Torsion can have some pretty deleterious consequences if not accounted for. For conventional structures, it's rather difficult to pin down either the centre of rigidity or the centre of mass with much accuracy. If your structure is symmetric, the accidental torsion is a must. If it's heavily asymmetric, than the accidental torsion probably won't be much of a penalty anyhow.

What exactly is an "apparatus" anyhow?

There is code committee talk afoot that may lead to elimination of accidental torsion requirements being waived for structures analysed with non-linear algorithms (RSA/THA).

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Accidental Torsion in Seismic Analysis

As I understood it, accidental torsion is due to whatever is being built not being build as ideally as we model it. Mass isn't as evenly distributed in reality as in our models, so the 5% eccentricity is to account for that, along with stuff that may not have been accounted for in the model 100% accurately.

RE: Accidental Torsion in Seismic Analysis

(OP)
KootK. Its a thick wall steel box on some tubular legs. The box is slabs bolted together. The legs are weldments that are bolted to the box and the floor. It is not symmetric. The FE model is made directly from the CAD assembly file.

structSU10: I got the impression that the problem was the use of idealized models that may use evenly distributed mass to model something that can actually vary, like a poured concrete floor, or contents of a building that are somewhat random and can get moved around. Or, lumped masses in a non-FE model. The commentary to the code says, at C3.1.1 (a)., "The amount of detail used to represent a structure in a mathematical depends on the structural configuration and the use of the model. Finite element mathematical models are used to represent complex structures..... Specific considerations and requirements pertinent to idealizing complex structures are provided here." Then at (d), "For Lumped-mass stick models...", and at (e), In an apparently symmetric system...", i.e. a lumped mass stick model. Also, in "Evaluation of Code Accidental-Torsion Provisions from Building Records" by Chopra, et al, "The additional+/-0.05b, known as accidental eccentricity, is introduced to account for building torsion arising from discrepancies between the mass stiffness and stress distributions used in the analysis and true distributions at the time of an earthquake." My finite element model is complete and detailed and unless i'm getting serious variations in the density of a rolled steel plate, I dont believe I have any such discrepancies.

I'm not trying to be argumentative here. If I gotta do, I'll do it, I just want to understand it.

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Accidental Torsion in Seismic Analysis

one might be able to get the center of mass with a good degree of accuracy but the center of rigidity may be less reliable...no such thing as a truly pinned or fixed connection..as-built condition can vary from assumed design condition, etc

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources