Fundamentals of Soil Structure Interaction Analysis for Integral Bridges

March 4, 2021


Soil-Structure interaction (often referred to as SSI) is required when structural and ground displacements are depended of each other.  It is usually required when the relation between soil and structure stiffness are comparable. On those cases a coupled structure-soil model is required. 


This 1-hour case study covers the theoretical background of soil interaction analysis with application methods according to PD 6694-1:2011. Attendees can get many useful knowledge on integral bridge design.


Key Points


1. Background of Soil-Structure Interaction

From this session, 3 analysis method, Limit Equilibrium Analysis, Spring Analogy, and Continuum Media will be presented with its characteristics. Practical design method and important notes for flexible foundation, laterally loaded piles, and retaining structures under laterally loaded pressure.

2. Design of Integral Bridges

Methods of analysis according to PD 6694-1 and example for integral bridge on piled abutments will be explained and demonstrated.



Approaches of Soil-Structure interaction


In order to consider soil-structure interaction, 3 approaches can be applied: Linear equilibrium method, spring analogy, and continuum media.


Limit equilibrium analysisSpring Analogy

Continuum media


Flexible Foundations and Laterally Loaded Piles


For flexible foundations and laterally loaded piles, spring analogy can be applied with Winkler spring or p-y curve. Both can be simulated in midas Civil by “Surface Spring Support” and “Point Spring Support” feature.


Spring analogy: Uncoupled springs - Winkler model

P-y curves

P-y curves 2

Retaining Structures


For retaining structures, lateral load pressure can be calculated and applied. For this, different formulations can be applied such as Rankine theory, Eurocode7, etc. In midas Civil, this approach can be applied by applying external loads, and adding nonlinear springs.


Retaining structures 1

Retaining structures 2

Retaining structures 3

Retaining structures 5

Retaining structures 6

Integral Bridges


Finally, for integral bridges, both linear equilibrium and soil-structure interaction analysis can be used depending on the structure type. When applying linear equilibrium in midas Civil, external pressure load can be applied to the abutment. 


Methods of analysis

Soil-structure interaction

Integral Bridges

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About the Author
Pere Alfaras | Principal Bridge Engineer | Arcadis UK

Alfaras is a Principal Bridge Engineer at Arcadis UK with 10+ years of experience in bridge and structural design. He has been a lecturer for Finite Element Method of Civil Structures at the UPC/Barcelona Tech for 5 years. Some of the projects Alfaras has been involved in include the Lower Thames Crossing in UK and the "Eix Diagonal" Motorway in Spain. 

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