Question:

I am using STAAD.Pro to analyze a mat foundation that supports a steel frame. I have used the PLATE MAT command and set the springs to COMPRESSION only. I encounter 2 problems :- a) Instability warnings b) Some springs still go into tension

How do I go about correcting these problems?

Answer:

A detailed discussion on this topic is available in the article posted at the following link

http://communities.bentley.com/products/structural/structural_analysis___design/m/structural_analysis_and_design_gallery/253408.aspx

Question:

What is the difference between choosing a foundation support of the type Y-ONLY instead of Y?

Answer:

The answer to this can be found in FAQ # 6 of the following link:

Y-Only support

The YONLY type of support is useful for modeling a friction-free  base. However, stability in lateral directions has to be provided by alternative means like additional restraints along X and Z.