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Equivalent Lateral Force procedure with Torsional IrregularityHelpful Member! 

Helpful Member!  Delchi (Structural) (OP)
1 Apr 10 0:42
I'm trying to understand how to reconcile the following two provisions regarding torsional irregularity in chapter 12 of ASCE 7-05:

First, Section 12.3.3.4 requires the 25% amplification of design collector forces determined from Sec 12.8.1, which is the Equiv Lateral Force Method, when you have certain irregularities including a Torsional Irregularity. It seems like this assumes that you can use the Equiv Lat Force method for these structures.

But then, Section 12.7.3, second paragraph, requires that structures with a Torsional Irregularity be analyzed using a 3-D model. It appears from this that the Equiv Lat Force Method is NOT allowed for structures with Tor Irregularity.

So, Can you use the Eq Lat Force Method for type 1A irregularities, or is it not allowed? I am stumped.
WillisV (Structural)
1 Apr 10 8:15
See Table 12.6-1 for which analysis methods are or are not permitted.  The ELF procedure is certainly allowed for some structures with torsional irregularities.  The required use of a "3-D" model does not mean you can't use the ELF method for loading on the 3-D model - they are mutually exclusive requirements.   
slickdeals (Structural)
1 Apr 10 8:30
The results of a dynamic 3-D analysis will have to be scaled to match 85% of the base shear from a static analysis based on ELF.

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