torsion irregularity in RSA

[chekre]

hi,

Does anyone know the procedure to verify, while using the results of response spectrum case, if the building is irregular versus torsion so we can amplify the initial eccentricity? i think we should create some sort of combination using the resulting mode !! any ideas ?

i would be grateful.

thank you.

 

[asixth]

I thought there is an eccentricity input when you create your spectrum cases. My code requires a minimum 10%.

 

[chekre]

hello asixth,

thanks for your reply. the code suggests an initial eccentricity of 5%. if your building is irregular versus torsion, then an amplification factor should be calculated and multiplied by the initial 5%. for static earthquake, it is quite simple. u need to verify the average displacement versus the maximum displacement using the load case of the static earthquake.
however, it is not as simple for the response spectrum case. u should create some sort of modal combination. i have no idea how to do it.
thats why i am asking for the procedure.

regards,

 

[Agent666]

Usually the codes I deal with (New Zealand) say for the torsional irregularity check you are referring to that the equivalent static loads are used when assessing the max vs average displacement check. These are the same as the first mode deflections by definition for a given direction of loading.

ETABs does have an output within the tables outputs that reports this ratio, but not sure what it reports for a modal case (one would like to assume it's based on the first/fundamental mode for a given direction). You could confirm by just looking at a particular mode deflections and comparing with a hand check.