What is the best approach in designing a multi-storey building with mat foundation support is it to include the mat-soil interaction in staad by using plate mat support and analyze the building together at the same time or assumed fixed support without modeling the mat foundation together and just pass it on to staad foundation to design the mat?
We have to acknowledge that by assuming fixed support without mat the forces in the superstructure frames would be different compared to the analysis wherein mat is modeled together with the super structure. I have asked this question because I've watched an online presentation from bentley about modeling and designing multi-storey concrete building wherein the support is just assumed to be fixed then exported to staad foundation for mat analysis and design.
In the analysis point of view what would be the recommended approach is it to include the soil-mat interaction or just assumed fixed support without modeling the mat?
Reply from the support team is very much appreciated thanks.
I would not include the mat in the analytical model of the structure unless it significantly simplifies the design of the mat foundation. Modeling the mat with shells and compression only springs complicates the analytical model and typically is not required by code. For example, ASCE 7-05 section 12.7.1 specifically permits the base of the structure to be fixed. For seismic, flexible supports softens the structure leading to an increased period and a reduced base shear. Hence, the seismic forces are larger if the base is fixed. I recommend separate models for the superstructure and the foundation.
Thanks a lot for the clarification Helmueller.