Horizontal Irregularity 1a (ASCE7-05)

[rrmiv]

Does anyone know of an exception based on the magnitude of the story drift? I have a two-story building with special concentric braced frames relatively symmetrical, seismic design category D. When I calculate the Torsional Irregularity, I am outside of the 1.2 limit, but the seismic drifts are very small, around H/600.

For example: Assume 18 feet from the second floor to the roof. One corner has a story drift of 0.36", the other corner drifts 0.23". The average drift x 1.2 = 0.354". This would be defined as Torsionally Irregular even though the maximum story drift is less than 3/8". That seems ridiculous. Am I missing something? Seems like there should be an exception for when the magnitude of the story drift is very small.

 

[rrmiv]

Anyone have thoughts on this?

 

[JoshPlumSE]

I don't understand how a relatively symmetric building (assuming relative symmetric mass distribution) can be outside the 1.2 limit.

I take it you've applied the 5% accidental eccentricity and that is the cause of your torsion. If so, then I think you've got a structure that's relatively sensitive to torsion. It's borderline, but it's there.... How much additional eccentricity (beyond the 5%) is required for such a borderline case?

 

[rrmiv]

JoshPlum, thanks for the reply. It isn't perfectly symmetrical, but close. And yes, the 5% accidental eccentricity is applied.

My point is that when the magnitude of the drift is very small, there does not have to be much difference from one corner to the other to exceed the 1.2 limit. As shown in the previous example, just over 1/8" difference from one corner to the other. For a building of any size, that's a tiny torsional movement. Seems like there should be an exception based on the actual magnitude of the drift.

Sorry, I don't understand your question "How much additional eccentricity (beyond the 5%) is required for such a borderline case?"

 

[JoshPlumSE]

My question just took your calculation 1 step further. From Section 12.8.4.3 of ASCE:

Ax = multiplier for accidental torsion = (delta_max/1.2*delta_avg)^2 = 1.034

Therefore, your 5% accidental eccentricity becomes a 5.17% eccentricity with the additional amplification. That's a very minor ammplification of the accidental torsion. It's pratically within the roundoff error of the calculations.

 

[rrmiv]

I see your point that it doesn't amplify much in Section 12.8.4.3. But the other reference sections of Table 12.3-1 come into play also, such as the Increase in Forces and for higher Occupancy Categories whether Equivalent Lateral Force is allowed (Table 12.6-1).