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
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Accidental Torsion in Seismic Analysis
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: Accidental Torsion in Seismic Analysis
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
RE: Accidental Torsion in Seismic Analysis
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