OpenBridge Designer can perform SSI – Soil-Structure Interaction with the complete modeling of the structure, including superstructure, mesostructure, infrastructure with foundations’ details, and soil modeling springs. The soil-spring constants can be extracted from gINT reports or from PLAXIS and within RM Bridge the structural model can be created and analyzed for seismic forces.
The usual bridge structure numerical model uses individual springs to model the ground as a rigid base with the assumption that the soil has a very stiff behavior. In these general cases, we assume that we have a model of a soft structure on a relatively much more stiff soil. The flexible damped structure is much less rigid than its base for tall, slender, narrow bridges that have a long period and quite low natural frequencies. A stiff soil will be those dense materials that have a very high average shear strength.
Thought, we all know that despite this assumption can be a correct simplification for most of the time, the soil does not behave as a rigid boundary; and, in some cases, replacing a multilayer nonlinear elastic soil with a single-digit might not be leading the analysis to the most economical analysis. Therefore, earthquake engineers should determine when this is not a valid assumption.
The Soil-Structure Interaction considers additional DOF (Degrees Of Freedom), elastic behavior, deformation, and damping of the soil. SSI is a function of:
- Stiffness of the structure relative to the stiffness of the soil.
- The slenderness of the structure relative to the footing width.
- Mass of the structure relative to the mass of the soil support.
So, when does SSI is going to be important? SSI can become beneficial for the analysis of stiff structures on more soft soils. In these cases, it could be advantageous for the model structural response analysis to include more DOF, with additional modeling properties that will add modes of motion, consuming more seismic energy in the system. This will increase the natural period and the damping in the modeled system, and then decrease the predicted damage to the structure.
For simplification, a Stiffness or a Flexibility Matrix can be introduced to substitute all elements; but, when justified, modeling complete infrastructure models requires ground information in the form of equivalent soil springs that will simulate the soil around the structure.
Additional information can be found at the link below:
communities.bentley.com/.../ssi---soil-structure-interaction-in-rm-bridge