subgrade need to be change as per area ?

Question

Hi 
 in my model the subgrade reaction is calculated like follows, 
 ** SBC 30T/M3 = 300 kN/m3 ** ALLOWABLE DEFLECTION IS 15mm *** SUBGRADE MODULUS = 20000 kN/m2 
 and the plate mesh i have created is 0.5mx0.5m. so in this case do i need to change the subgrade value to this 20000/(0.5*0.5) = 80000kN/m2 for internal plates 
 and for end plates 20000(0.5*0.25) = 160000 
 and for corner plates 320000 kN/m2 ???? 
 
 can you clear this doubt? thanks in advance.

Modestas · Answer

When you assign PLATE MAT support to plate elements in a model, the program automatically determines the tributary area for each plate element’s corner nodes. Then, that area is multiplied by the subgrade modulus to calculate the linear spring stiffness to be assigned at that node.

Sye · Answer

For ELASTIC MAT it works the same way as well. You just provide one value of subgrade modulus. The program will internally figure out the influence area for each node and multiply that by the subgrade modulus to generate the spring stiffness at that node. If you have modeled your mat using a mesh of plates, you should use PLATE MAT as opposed to ELASETIC MAT

Modestas · Answer

You should be using PLATE MAT option is the structure is used from plates. More details can be found here: Mat Foundation: Elastic and Plate Mat - RAM | STAAD Files - RAM | STAAD - Bentley Communities

Kris Sathia · Answer

For the ELASTIC MAT command, STAAD looks for spaces between nodes and assumes that those spaces are filled by actual physical objects like a concrete pad that can resist loads. 
 The PLATE MAT command on the other hand is going to consider only the area with the individual elements to assemble the tributary area for the nodes. 
 Your foundation is not a contiguous arrangement. It is 5 separate mats separated by gaps and connected by beams. Thus, a single ELASTIC MAT command isn't going to work for this arrangement. That is because, the tributary area calculated for the nodes at the edges of those gaps is going to include the contribution from the gaps, and thus will be different from the tributary areas in the PLATE MAT command. 
 So the remedy is, replace that single ELASTIC MAT command with 5 separate commands as shown here 
 1512 1518 1532 TO 1535 1930 TO 2097 - 2098 ELASTIC MAT DIRECT Y SUBGRADE 19998 PRINT 1513 1519 1528 TO 1531 1761 TO 1928 - 1929 ELASTIC MAT DIRECT Y SUBGRADE 19999 PRINT 1514 1515 1520 1521 1524 TO 1527 1544 TO 1759 - 1760 ELASTIC MAT DIRECT Y SUBGRADE 20000 PRINT 1516 1522 1536 TO 1539 2099 TO 2266 - 2267 ELASTIC MAT DIRECT Y SUBGRADE 20001 PRINT 1517 1523 1540 TO 1543 2268 TO 2335 - FOUNDATION Elastic-Revised.std 2336 ELASTIC MAT DIRECT Y SUBGRADE 20002 PRINT 
 This will exclude the gaps and consider just the area confined within the perimeter formed by the nodes of each command. It should be a good match with what you get with the PLATE MAT command. 
 Corrected file is attached.