According to the code, amplified Accidental Torsion is applied if

• SDC = C, D, E, or F with Type 1a or Type 1b horizontal structural irregularity

Initially it is unknown if torsional irregularity Type 1a or 1b should be applied, but the only way to determine this is to actually apply the irregularity and look at the calculation results.

As part of the calculations the following are determined at each relevant level from the deflections of the seismic lateral and torsion loadcases (in each permutation of positive and negative directions):

• δ_max = maximum horizontal displacement for all column/wall nodes on that level

• δ_min = minimum horizontal displacement for all column/wall nodes on that level*

• δ_avg = (δ_max + δ_min)/2

• A_x = min( 3.0, max(A_x, (δ_max / (1.2 ×δ_avg))2 )) where A_x is the value for this level for the torsional loadcase

At the end of these calculations, for each relevant level with A_x > 1.0 the accidental torsion loads on each level are multiplied by A_x only when the ratio δ_max / δ_avg > 1.2 or 1.4.

The analysis results are automatically updated if the accidental torsion loads are amplified.

When the accidental torsion loads have been amplified, additional tables are output in the Seismic Design Report which give the displacement and calculated A_x values for each direction and level.

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, or if torsional irregularity 1a or 1b exists - consequently 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 section 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). This will also mandate RSA.

7. You can then run Analyze All (RSA) to see if the torsion amplification occurs - to do this refer to the values of A_x in the seismic accidental torsion tables. If A_x for both building directions <= 1.0 then there is a note under the table to that effect which also states “A_x,used = 1.0 in all cases - no amplification applied”.

   

8. If the above note is displayed - there is no torsional amplification, 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 torsional amplification - 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 δ_max/δ_avg for each level. If > 1.4 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.

13. Check the design and 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).

According to the code, amplified Accidental Torsion is only applied if

• SDC = C, D, E, or F with Type 1 horizontal structural irregularity

As part of the calculations the following are determined at each relevant level from the deflections of the seismic lateral and torsion loadcases (in each permutation of positive and negative directions):

• Δ_max = maximum story drift in a floor at the building edge with a rigid or semi rigid diaphragm

• Δ_avg = average story drift at two opposing edges in a floor with a rigid or semi rigid diaphragm

• TIR = Δ_max / Δ_avg

Initially it is unknown if torsional irregularity should be applied, so the only way to determine this is to actually apply it 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, or if torsional irregularity exists - consequently 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 ‘Plan irreg 1 - torsion’ 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 section 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 ‘Plan irreg 1 - torsion’ which will also mandate RSA.

7. You can then run Analyze All (RSA) to perform the calculations, after which you should open the Seismic Design report to review the results.

   For SDC = B, the TIR is contained in the Structure Plan Irregularities list as shown below.

   

   For SDC = C, D, E, or F, to see if the torsion amplification occurs, refer to the values of A_x in the seismic accidental torsion tables. If A_x for both building directions <= 1.0 then there is a note under the table to that effect which also states “A_x,used = 1.0 in all cases - no amplification applied”.

   

8. If the above note is displayed - there is no torsional amplification, 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 torsional amplification - 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. 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).