Foundation Design

Question

I have a few questions: 
 1. For a 90'H, 28'W and 84'L fully-enclosed building with a mat foundation, is it better to use plate mat or elastic mat? 
 2. What is the difference between using "compression only" and "none"? 
 3. What is the difference between using "Y only" and "Y"? 
 I was directed to a write up within this forum on the subject, but it didn't cover all my queries, hence the reposting of this. 
 Thanks in advance for all your help!

Modestas · Accepted Answer

1) If the mat is modeled using plates, PLATE MAT option would be a better choice.

2) Compression-only springs are capable of carrying compressive forces only. Thus, they are automatically inactivated for load cases that create tension in them. If "none" option is selected, then both tension and compression forces are resisted by the support.

3) If X or Y or Z is selected then a spring support is generated in that direction plus 3 other directions receive a fixed support, e.g., if Y is selected, then FY is supported by a spring; FX and FZ and MY are fixed supports; and MX and MZ are free. If XONLY, YONLY, or ZONLY are selected then only a spring support in that direction is generated.

Sye · Answer

As you mentioned, the soil cannot be considered to offer any resistance to uplift. So you should define the plate mat with the compression only attribute. For the cases where you are getting high displacements, you may want to ensure that these cases include the selfweight and other gravity loads that would resist the overturning. For example do not have a lateral load acting by itself and analyze it with a compression only spring. Ensure that you have the gravity loads as part of that load case too. If there are component cases like live load/wind load or seismic load acting alone, you may consider specifying the supports twice. For the component cases do not use compression only. At the end of all the component cases put a PERFORM ANALYSIS and CHANGE. Following that you redefine the PLATE MAT and this time with the compression only attribute. Subsequently you specify your real cases ( using REPEAT LOADs) and follow that up with another PERFORM ANALYSIS at the end.

Sye · Answer

Whenever the mat is modeled using plates, please use PLATE MAT. 
 The following wiki offers more detail on the options 
 http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/29301.difference-between-elastic-mat-and-plate-mat

Modestas · Answer

Yes, if resisting loads are lower than uplift loads then instabilities will be reported.