Hello,
I have an 185ft tall building. The model was set to use the first floor level at 23ft for the "Ground Level" We then switched to using the base level. The moments and shears at the base of the columns jumped 10x. For example, at one column, the bottom reaction went from M = 500k-ft to 5,000k-ft and V = 50k to 500k. This was similar at all lateral columns. A jump in values is expected but a jump of 10x seems disproportionate. Any ideas on what may be causing this.
Thank you,
Brian
The ground level diaphragm is fully restrained (see: communities.bentley.com/.../ram-frame---criteria---ground-level) so you have prying action and huge shears in the base are expected (you can perform a sum of moments about the foundation as a rough way to hand check this). There is also the possibility that the applied loads have changed due to a change of period/frequency as well as mass/exposure.
Looks like the prying action and huge shears are expected when you make an upper level the ground level? This is the opposite. I was going from an upper level to the base level. Applied loads changed very slightly to account for the extra mass/exposure. Any tips for the hand check? Take the sum of the diaphragm load x height at each level in a certain direction and see if that equals the sum of my moments in that direction at the base?