Effect of Elastic Soil Structure Interaction on Modulus of Subgrade Reaction

Many structural designers represent footings as hinged or fixed supports in the structural models. Sometimes they represent the footing asagroup of springs withaspring's coefficient K (kN/m) equal to the area of modulus of subgrade reaction k_s (kN/m²/m). Designers assumed this modulus in different ways that do not consider soil structure interaction, such as the ACI way of calculation. ACI calculates k_s by dividing the allowable bearing capacity by 1.0 cm assumed settlement or calculated settlement without soil structure interaction. This paper shows that the soil structure interaction has a significant effect on the modulus of subgrade reaction. Usually designers assume K for edge as half of the inner one and the corner as quarter, which is based on the area that each spring represents assuming uniform k_s. However, k_s is not uniformly distributed under the footings. Considering the soil structure, interaction results change relatively in the normal forces in the columns. For the paper model, this change leads to increase in the outer columns and decrease the inner columns. For the complex structural systems, the distribution cannot be estimated. The change in the normal forces changes the resulting contact stress and settlement, which changes the resulting k_s. We are in need for more research to this effect to simplify an accurate calculation. Currently, use of a geotechnical finite element (FE) program that can consider soil, foundations, and superstructure effect is highly recommended.