Answer to Question #234588 in Civil and Environmental Engineering for Zainnm

Question

28. In modeling a nonrigid mat foundation by using elastic springs, should a uniform modulus of subgrade reaction be used along the whole base of mat?

Answer 1

By using a bed of springs to simulate the flexible behaviour of mat subject to loads, care should be taken in selection of the modulus of subgrade reaction. In fact, the modulus of subgrade reaction depends on many factors like the width of the mat, the shape of the mat, the depth of founding level of the mat etc. In particular, the modulus of subgrade reaction is smaller at the center while it is larger near the mat’s edges. If a constant modulus of subgrade reaction is adopted throughout the width of the mat, then a more or less uniform settlement will result when subject to a uniform load. However, the actual behaviour is that settlement in the center is higher than that at side edges. Consequently, it leads to an underestimation of bending moment by 18% to 25% as suggested by Donald P. Coduto (1994).

In general, a constant value of modulus of subgrade reaction is normally applied for structure with a rigid superstructure and the rigid foundation. However, a variable modulus of subgrade reaction is adopted instead for non-rigid superstructure and non-dominance of foundation rigidity to account for the effect of pressure bulbs.