I am getting excessively high displacements when I analyze my mat in STAAD Foundation Advanced. Any ideas what could be the reason ?
STAAD Foundation Advanced assigns the compression only attribute to the spring supports underneath the mat. So when you analyze the mat, please ensure
All load cases that are included as part of the job are real life cases like DL+LL, DL+WL, DL+LL+/-WL, DL+LL+/-EL etc. Refrain from having cases like Wind load ( WL ) acting by itself or earthquake load ( EL ) as part of the job because if you do, the overturning moment due to the lateral loads acting alone, may cause the mat to lose contact with the soil. When that happens, the compression only springs would start getting into tension and would start getting deactivated. The iterative solution may cause more and more springs to get deactivated and subsequently it may cause the mat to just take off leading to absurdly high displacements.
Selfweight of the mat is included as part of all the load cases considered for analysis which would help in counteracting any uplift/overturning. To ensure this, go to the Selfweight and Deadweight factor table and make sure that the selfweight factor for the various load cases is non-zero.
The vertical loads are all applied with proper signs ( for example gravity loads are applied with –ve signs )
To ensure that only the real cases get included as part of the job, you should go to Job Setup > Edit Current Job and check that only the appropriate cases appear within the Selected Load Cases window on the right hand side. If you find the component cases like the LL, WL, EL as part of the selected load cases, you should take those out and click on the Edit Current Job to save the change.