Response spectrum analysis

[bkal]

Hi,

we are doing a response spectrum analysis of a concrete structure modelled with shells (plates) and beams, fixed base. The fundamental horizontal natural frequencies are calcualted in the range where the spectral acceleration is at peak (80% of the mass). However, when we do a response spectrum analysis and check the base shear (calculated by summing up nodal reactions) it appears that the acceleration at which the strucutre respoonds is much lower (reaction divided by mass), almost as low as the zero period acceleration. This does not look righ.

Has anybody else come across a similar problem?

 

[SethGuthrie]

Make sure your spectrum covers the full range of mode shapes in the model. If there is a long period mode for the structure that's off the end of your chart it might cause something like that. I would also suggest CQC combination Method.