TO ALL ETABS USERS 1

[hsn199]

I have posted this on the ETABS forum but it seems it didn't get any attention. The drift in my model accounting for combination "EQUAKEX" is exceeding 0.02, which is not allowed by UBC 97.

The natural period in my building is about 5.89 s which is too much in my belief. I've checked the elasticity of concrete materials, and stiffness modifiers were only applied for slabs and connecting bridges (.35 for flexure). Everything seems to be right.

I've provided an attachment of my model (connectivity at bridges ends is Rigid-Hinge). Any help given in relation to this matter is really appreciated. Thx.

PS: When running the analysis without application of stiffness modifiers, the period is somewhere near 4.45 s. Application of stiffness modifiers for SLABS ONLY, made the period 5.89. I believe if something was done wrong, it was done wrong here.

 

[SethGuthrie]

I don't know anything about ETABS, but in other finite element analysis programs, if the slab is modeled as shells and the masses are schmeared, then there is the potential of finding a mode shape that represents the slab waving, which could have a long period, but isn't the principal mode you are after. Assuming the program has some graphical way to view the mode shapes, you can confirm if this is happening or not. If it is, then I suppose you would need to cycle through the modes until you find the 'real" first mode and somehow override that T in the seismic load.