Can someone give an example of when you would use each?
I was under the impression that you would use Plate Mat if you have created a mesh for your mat. However, I looked at the "Modeling base plate with anchors in STAAD.Pro" and it uses Elastic Mat, but I would have thought this would use Plate Mat.
Thanks!
PLATE MAT was introduced in STAAD as a way to overcome some of the limitations of the ELASTIC MAT feature. Thus historically, ELASTIC MAT was introduced in the program before PLATE MAT.
ELASTIC MAT uses the layout of the nodes of the foundation to determine the shape of the foundation and the influence area of each support. Hence, it can be used regardless of the type of entity (beams, plates or solids) used to model the foundation.
PLATE MAT uses the plan area of the individual plate elements to determine the influence area of each support. Thus, it can be used only in cases where the foundation is modeled using plate elements.
Some of its limitations of ELASTIC MAT are:
Conveying information regarding the presence of openings (holes) in the mat is a tedious task with this method. As there is no direct way to inform the program where the opening is located, or its shape and dimensions, the mat has to be subdivided into regions in such a way that the nodes located on each side of the opening are specified through separate elastic mat commands. Thus, a rectangular mat with one rectangular opening has to be defined through four ELASTIC MAT commands. For polygonal or circular shaped openings, the task is tedious to the point where the elastic mat is not a viable option.
The contour formed by the nodes that constitute the list for the ELASTIC MAT command must form a convex polygon as explained in Section TR.27.3 of the STAAD.Pro Technical Reference Manual. The internal angle formed by 2 adjacent segments connecting 3 consecutive nodes in the list should be less than 180 degrees. Boundary nodes where this condition is not met are called re-entrant corners. A simple way to understand this concept is that if a rubber-band is tied around the perimeter, corners and sides of the mat that are not in contact with the rubber band will form a re-entrant corner.
If the mat is at an incline to the horizontal plane, the influence area calculation may fail.
All of these limitations can be avoided by using PLATE MAT. But if none of these conditions exist, both methods should produce the same results.
PLATE MAT cannot be used if the foundation is modeled using something besides plate elements, such as solids. In such cases, you could add a layer of fictitious plates (small E, small thickness) and then apply the command to those plates.
Answer Verified By: RFreund
This was very helpful, thank you.
Quick question - I'm trying to imagine a situation where you would use beams. Could you provide an example of this?
The methods used for ELASTIC MAT and PLATE MAT calculate the spring constant based on the assumption that soil is behaving as an area spring, which is why the influence area is computed and multiplied by a subgrade modulus.
I can't think of a scenario where beams would be used to model the foundation where the soil is treated as area springs.