Soil-structure interaction (footings and pile caps)

INDEX

• Introduction
• Module description
• Characteristics that have been considered for soil-structure

Introduction

Soil-structure interaction (footings and pile caps) is a CYPECAD module that allows users to consider the settlements of the foundations and their interaction with the rest of the structure, with regards to forces and deformation, by defining elastic stiffness constants for the foundation elements.

For footings, the stiffness is introduced as subgrade modulus. Similarly, for pile caps, the data that must be defined is the axial stiffness for each pile cap, which is determined by the ratio of its load to the settlement that is produced.

Bear in mind that the usual process is to design the foundations previously, by considering the fixities at the base of the supports, and then consider the interaction between the soil and the structure.

 

Module description

"Soil-structure interaction (footings and pile caps)" allows users to consider the settlements of the foundations and their interaction with the rest of the structure. This interaction will result in variations in the forces and deformations of the structure, causing small modifications, if any, in the result of the design of the foundations.

To assign the soil-structure interaction, from the "Foundations" menu select "Soil-structure interaction" and the program will automatically detect the existing foundation type.

 

In case of foundations using footings the assignment of the interaction is carried out by a subgrade modulus in the footing.

 

In the case of pile caps, the axial stiffness of each pile must be defined, which is determined by the ratio of its load to the settlement that is produced.

In the case of footings, the logical procedure is the previous design of the foundations by considering the fixities, in the usual way. Once the foundations have been obtained, the subgrade modulus must be assigned to each footing in order to consider the soil-structure interaction.

It should be noted that the same value of the subgrade modulus does not have the same effect on two footings of a different size, and that, in addition, the interaction between the soil and the structure will cause settlements, rotations, and therefore changes in the forces and deformations of the structural elements. As a consequence, after the second analysis and design of the foundations, with the consideration of the interaction, the program could show a warning to indicate that there is a difference between the size of the footings considered and that of the footings obtained.

This message indicates that the user should perform a new calculation and design the foundations, readjusting if necessary the subgrade modulus.

If the difference in size between the footings considered and those obtained is sufficiently small this warning will no longer appear.

As for pile caps, the equivalent to the size of the footings is the number of piles and the distance of each pile from the centre of gravity of the pile caps, which governs the axial and rotational stiffness as well as the surface and moment of inertia of the footing surface.

 

Characteristics that have been considered for the soil-structure interaction

To generate the report, from the "Foundations" menu users must select "Characteristics that have been considered for the soil-structure interaction".

The program provides a report with the usual export and print options, in which the current data can be consulted along with the data with which the last calculation was performed.

The current data can also be listed by activating "Soil-structure interaction (footings and pile caps)" in the job data report.