Mat Foundation (1m) w/ Pedestals. Deep Beam Effects?

[edomingo]

I have a mat foundation that supports building columns (~1m x ~1m; around edges of mat footing) and very large equipment pedestals (imagine 2mx4m pedestals; in the middle of the mat footing). The Mat Foundation is roughly 13m x 13m with Rock Anchors at the corner to help with overturning. The foundation is seated on rock so the subgrade modulus is large (~110MPa / m).

I've created a STAAD model with all the design loads on the pedestals applied to the plate mat footing and I am getting various bending (Mx,My,Mxy using Wood-Armer) in the plate bending which I am designing the reinforcement to. My question is do I have to consider "Deep Beam Effects" on footing? Or can I design the footing as a regular footing? And if this isn't the right away of designing this any advice would be appreciated!

 

[TRAK.Structural]

I've designed mat foundations in STAAD in the way that you are describing. Not sure what codes you are using, but this doesn't sound like a situation where I would use deep beam provisions. My experience with models for mat foundations is that the support conditions are the driver for what results the analysis comes up with; as such it's critical your model represents closely the actual conditions. A few tips/suggestions from my experience with this are listed below:

• Consider checking the analysis with different subgrade modulus values. I'm not a geotech but generally I think the subgrade stiffness is an input that can vary widely when it comes to actual field conditions. An upper/lower bound check could be useful
• Make sure you are using compression only springs/plate supports for the subgrade reaction
• Check local effects like punching shear at concentrated load locations and your rock anchors
• Investigate the results such as bearing pressure with the program generated stress contours to verify it matches the expected behavior
• Check vertical deflections to make sure they are within the right magnitude