Benefits from nonlinear constitutive laws in civil engineering

Civil engineering is certainly one of the fields where progress can be seen by the majority of the population. More and more complex structures are being designed and built to withstand more and more complex loads, in order to combine durability, serviceability and structural safety. In other words, it's the search for a certain structural performance that's targeted these days. Whether for housing, agricultural production, public utility or industrial purposes, the dimensioning of such structures is still based on the application of linear methods, resulting in a high degree of conservatism. The presence of safety margins is undeniable and fully justified.

However, demonstrating that existing structures meet the ever-increasing safety requirements calls for a structural assessment. Quantifying safety margins is a key question, and one that needs to be answered. For this, the assumption of linearity in the behavior of the constituent materials is no longer justifiable, and the latter need to be represented more faithfully.

The main objective of this article is to give an overview of the main contributions and difficulties encountered when using nonlinear behavior laws to represent material degradation. To achieve this objective, the paper will be presented in three parts.

First, an overview is given of the main non-linear phenomena exhibited by concrete and steel. Identifying these phenomena will enable us to understand the main assumptions underlying the formulation of behavior laws classically used to represent these materials. Only the main keys to understanding the frameworks used to formulate non-linear laws are presented.

Secondly, some of the benefits of using non-linear laws are presented through several examples of structural elements or structures taken from the nuclear field. The case of a load-bearing wall structure is discussed to illustrate the contribution of nonlinear behavior laws to quantifying seismic margins.

Finally, we present some of the pitfalls to be avoided when using non-linear behavior laws in structural design. A number of examples are given, clearly highlighting the difficulties faced by engineers. Recommendations for resolving them are also proposed.