STAAD Pro outputs plate corner forces in the units lb-in/in or kip-ft/ft, basically moment per unit length in the local coordinate system of the plate elements. So, do I need to multiply the Mx and My at the corners by the plate dimensions parallel to local y and local x axis to obtain the total Mx and My at the corner of the plate element?
A follow up question is how to obtain total moment at a plate corner if that corner is being shared with another plate or plates? Can I algebraically add moments at a corner from all plates sharing that corner/node?
Thank you.
The Staad reports plate moment in the format "force-length/length" which is the moment intensity per unit width of the plate . This moment intensity may vary from value determined at the other corner node of that width .
At corner , you can’t get the moment . There you can only get the moment intensity . The moment will be for overall edge which is the total of the contribution of the moment intensity varying across the entire width .
So by merely multiplying any of the corner moment intensity with the width , one can’t call them "total Mx and My at the corner of the plate element " .
If you average out the two moment intensity of the two end corners and then multiply with the width then it will give you the idea of the total edge moment of that element . But yet these moments are the approximate one .
The reason being , for plate elements, STAAD determines the values of stress exactly at the center of the elements based on the four gauss quadrature points.
Knowing the strain-displacement and stress-strain relationships, the stresses at the corner nodes of the elements are then calculated by extrapolation. And hence the stress values at those points are approximate values.
The continuity of stresses across that node from one element to the next is not guaranteed. This means, the stress that is calculated there from one element may not match the stresses for that same node calculated from each of the other elements. Typically, this limitation is addressed by using the average stress at that node from all the elements connected to that node (AVERAGE OF ALL THE CORNER STRESS OF THE ELEMENTS MEETING AT THAT NODE ).
So one may refer to the center stress which is the value actually determined by Staad . Multiply the moment intensity determined at the plate center with the edge width (for rectangular element) to get the total moment developed at that plate element .
However, by generating finer meshing , the result would be closer to the realistic value . You would notice that in the finely meshed element , the corner stress and the center stress values are almost same.
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Thanks for your detailed reply. I tried various refinement levels in meshing and obtained results consistent with your explanation above. The corner stress at a shared node from adjacent plates gradually came closer with increased mesh refinement. Does STAAD Pro uses 4 Gaussian Points with quadratic shape functions for bending?