Effect of Elastic Soil Structure Interaction on Modulus of Subgrade Reaction

Abstract

Many structural designers represent footings as hinged or fixed supports in the structural models. Sometimes they represent the footing as a group of springs with a spring's coefficient K (kN/m) equal to the area of modulus of subgrade reaction k sub s (kN/m²/m). Designers assumed this modulus in different ways that do not consider soil structure interaction, such as the American Concrete Institute (ACI) way of calculation. ACI calculates k sub s by dividing the allowable bearing capacity by 1.0 cm assumed settlement or calculates 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 sub s. However, k sub 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 in 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 sub s. There is a 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.