Soil Structure Interaction (SSI) in Bridge Analysis & Design
B.V. Nagesh - Principal Consultant, Nagesh Consultants, BangaloreThe process in which the response of soil influences the motion of the structure and motion of the structure influences the response of soil is termed as SSI.
Soil structure interaction (SSI) consists of interaction between soil and the structure built on it. It is primarily an exchange of mutual stresses. Most of the civil engineering structures involve some type of structural element with direct contact with soil. When external forces act on these systems, neither structural displacements nor soil displacements are independent of each other.
In most of the cases, designers do not consider the effect of SSI and they consider foundation to structure with a simple boundary condition (Fixed or Hinged). As a result, such analysis does not lead to a more realistic modelling of the structure and thus either leads an over estimation or under estimation of structural response demands.
Basically, there are 2 approaches to modelling interaction between structure, its foundation and soil that supports it, including the flexibility and damping of soil.
Substructure approach: The soil is represented with springs. The springs are typically vertical to capture foundation rotations and horizontal springs are used to capture ability of foundation to displace horizontally.
Direct analysis approach: The soil and structure are both modelled using finite elements. The soil modelling extends sufficiently around and beneath the buildings to account for site properties and seismic waves are imparted at the boundary of soil and excite the soil elements which in turn affect the behavior of the soil.
Case Study
6 x 35m Span integral PSC Girder, Integral Bridge with Sub Structure Approach
In such bridges, apart from vertical loads such as, dead load, super imposed dead load and vehicular live load. Loads due to secondary effects such as shrinkage and temperature shall be accounted due to restraint in longitudinal direction. Also, behaviour of the structure in longitudinal direction due to longitudinal forces will be different from that of simply supported spans. Two models are studied for the above loads: Model with fixed support and SSI model with sub structure approach.
From the above, it is noted that the moment on pier with SSI model is much lesser compared to model with fixed support. Subsequently, forces on piles will also be less with SSI model. SSI captures the realistic boundary condition and provides realistic forces for design resulting in optimum design of the structure.
