Question
In modeling a nonrigid mat foundation by using elastic springs, have a uniform modulus of subgrade reaction be used along whole base of mat?
Answer
A bed of springs to simulate flexible behaviour of mat subject to loads, care must be engaged in collection of modulus of subgrade reaction. In fact, modulus of subgrade reaction depends on factors like width of the mat, shape of the mat, depth of founding level of mat etc. In particular, modulus of subgrade reaction is smaller at center even as it is larger near mat's edges. If constant modulus of subgrade reaction is adopted throughout width of mat, then a more or less uniform settlement will effect while subject to a uniform load. Yet, actual behaviour is that settlement in the center is higher than that at side edges. So, it directs to an underestimation of bending moment by 18 to 25 percent.
In broad, a constant value of modulus of subgrade reaction is in general applied for arrangement with a rigid superstructure and rigid foundation. Yet, a variable modulus of subgrade reaction is adopted in its place for non-rigid superstructure and non-dominance of foundation rigidity to explanation for effect of force bulbs.