Base shear values

[tigrouis]

Robot gives the base shear values in the X and Y directions aproximatletly as being 3 to 1

staad on the other hand gives for the same data and structure the base shears to be as expected aproximately the same.

As you apreciate this difference has a signifficand effect on bending moments on the lower columns.

Thank you for your help

[Pawel.Pulak]

Have you compared the results of dynamic modal analysis? Are the vibration frequencies and mass participations similar in both software?

If the answer to above questions is "yes" then:

Are you using in Robot the seismic code which enables scaling of base shear? If yest have you activated it? - see the screen capture below:

Regards,

[tony.ridley]

Pawel can you elaborate on this answer please. 

If I set base shear to "inactive", do I then need to manually do it through "direction definition" and add a ratio for x, y, or z?

If I set to automatic, how does Robot figure out the predicted base shear it's strying to hit, and how can I verify this number?

Tony

[Pawel.Pulak]

In case of "inactive" base shear Robot is giving directly the value from response spectra analysis. If modal analysis was run with insufficient number of modes and for some direction mass participation was significantly smaller than 90% it may result in significant reduction of base shear for such direction.

In case of "automatic" base shear Robot is internally calculating base shear according to formulas from static force procedures and scaling base shear from response spectra analysis to these values.

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Regards,