Hi,
I've currently modelled a 22 story building and period of the structure doesn't look right (I'm getting over 3 s for the first natural period, while I would expect around 1.5 s).
Also, I can't find any mode with mass participation over 90% (all of them are less than 50%).
(file uploaded)
Please advise.
I can't find the uploaded model. Can you please upload it?
Sorry, I can't see the uploaded model or any other link from where I can download that model.
I ran the model (Oracle_VB5.rss) in v15.0 using the same options you had. As noted, the mass participation is low using 10 mode shapes. The general shape of the modes and period values seem reasonable to me, but since so much of the mass is at the bottom levels it may just require more mode shapes to get to 90% (bear in mind that since the diaphragms are semi-rigid there are literally thousands of mass points involved). I'm trying 30 modes now.
That solver option is under Criteria - General (the Ritz option is still pretty good too). The first period was right at 5 seconds and the 2nd just slightly shorter, as I recall. There are two factors about using rigid vs semi-rigid diaphragms. The first is that the diaphragm mass is lumped to a single point when the diaphragm is rigid. The second, more important, nuance is that when the diaphragm is rigid it's out of plane stiffness is neglected in the lateral load analysis. i think this also applies to the eigensolution. So I like to use semi-rigid diaphragms to all concrete two way decks. See our post on diaphragms for more: communities.bentley.com/.../ram-ss-semirigid-diaphragms