Workflow to check for torsional irregularity and amplification

Tekla Structural Designer
Tekla Structural Designer

Workflow to check for torsional irregularity and amplification

When using the seismic code ASCE 7, torsional irregularity and amplification must always be checked for. Initially it is unknown if torsional irregularity 1a or 1b should be applied, but the only way to determine this is to actually apply them and look at the results. A suggested workflow for proceeding is outlined below:

  1. Initially the SFRS is not sized/ designed so you actually have little idea how laterally stiff the structure is hence if 1a or 1b exist, and there is little point checking for this and amplifying torsion at this stage.
  2. Hence we recommend you conduct an initial full lateral and seismic design using ELF with 1a/1b OFF (assuming other irregularities don't exist that mandate RSA).
  3. SFRS members will be auto designed for the seismic forces to regular code requirements and checked to special seismic.
  4. You may then need to manually increase sections sizes to pass special seismic provisions AND seismic drift.
  5. This process may require some iteration since the lateral stiffness may increase the seismic loads, since these are a function of this.
  6. ONLY when the SFRS/drift checks are all passing at this stage, you can run the seismic wizard again and activate 1a (not 1b) which will also mandate RSA.
  7. You can then run a check-design (or just run Analyze All (RSA)) and see if the torsion amplification occurs - the additional amplified torsion result tables will be included in the Seismic design report if it does.
  8. If there is no torsion amplification - i.e. no extra tables for this in the seismic design report - the design is complete at this stage. You have checked for torsional irregularity and there is none. You could therefore revert to using ELF at this stage for final output.
  9. If there is, size any members of the SFRS that now fail with amplified torsion if this is the case. This will increase the stiffness which may affect seismic loads and drift/torsion checks.
  10. Run Analyze All (RSA) again when this has been done.
  11. From the Seismic Design Report torsion tables calculate the ratio dmax/davg for each level to check if the irregularity is 1b (extreme).
  12. If the irregularity is 1b (extreme) then run the seismic wizard again and enable 1b. The Wizard will then check if RSA analysis is still allowed for the SDC. Complete the Wizard and check design if so.
  13. Iterate as necessary.

Alternatively to the above process, more experienced users might want to go straight to using the RSA method for the pre-design stage (though we would still recommend doing a full gravity and lateral design for regular drift/ wind before attempting seismic design). Note that RSA is generally acknowledged to be less conservative than ELF. So doing an ELF pre-design might actually result in section sizes that are over-conservative (in relation to what RSA might achieve).

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