Torsional Irregularities Check in ETABS

How can torsional irregularities be calculated for response-spectrum analysis? To compute Torsional Irregularity and Extreme Torsional Irregularity, as specified in ASCE 7-05 Table 12.3-1, item 1a and 1b must be performed according to modes, and not the responsespectrum case. These irregularities in turn can trigger requirements for Torsional amplification of seismic loads. Steps are given as follows: 1) Obtain the scaling for each mode as the response-spectrum modal amplitude through Display > Show Tables > Analysis > Results > Modal Results > Response Spectrum Modal Information:

Figure 1 - Table: Response Spectrum Modal Information

2) Select Define > Load Combinations > Add New Combo. Select Modal in drop down list under Load Name and enter Mode number and Scale factor for the specific mode from Response Spectrum Modal information Table as given in item 1. Add each mode in a separate load combination, as shown in Figure 2:

Figure 2 - Load combination data

3) Select Define > Load Combinations > Add New Combo. Combine all modes using the SRSS method to determine whether Torsional Irregularity or Extreme Torsional Irregularity is applicable to the model. Each one include the related load combinations (which were created in item 2). 4) Obtain Dmax/Davg Drift for each SRSS combination per item 3 by selecting Display > Show Tables > Tables> Analysis > Results> Dmax/Davg Drift. 5) Review the story drift of the building, then compare with the maximum story drift for that specific mode i.e. Dmax/Dav<1.2 or 1.4 for each mode.

6) For each Floor where is the Dmax / Dav>1.2, Calculate the Amplification Factor Ax = (Dmax /1.2 Davg) ^2. Where Ax need not exceed value of 3. 7) Go to Define >Load case >highlight the spectrum case (example SPECx) then click modify/show .to enter the new values of eccentricity. Click the button related to Diaphragm Eccentricity. 8) The required value which will be input as following : a) Default eccentricity will be as a ratio result form 0.05 multiply of the value of amplification factor Ax (item 6). b) The value of eccentricity for the overwrite table will be as a ratio of the length of the related direction of building. It means 0.05*Ax*length of building. c) Note : you can ignore the item 8-a if you enter all floors eccentricity as item 8-b 9) Repeat the previous steps for the another direction of SPEC _y

NOTE: This procedure should be done for the default eccentricity 0.05