A PI-controller for hybrid fire testing in a non-linear environment

Hybrid Fire Testing (HFT) is an innovative testing methodology: it improves the existing experimental method by updating continuously the boundary conditions at the ends of a tested specimen. However, HFT methodologies are still in their infancy and, despite several previous applications, a rigorous framework for formulating the general problem is still lacking. To address this need, this paper proposes a new framework based on linear control system theory. Adoption of this robust theory to HFT allows deriving the general state equations and stability conditions of the problem. It is shown that the use of a P-controller leads to a stable process and that the conditions of stability can be formally expressed. However, the P-controller presents a major drawback, namely an inability to adapt to changes of stiffness in the system. A PI-controller is developed to overcome these limitations, and the corresponding state equations are established. A virtual HFT (using FEM) is performed on a 2D steel frame to compare the two different controllers. The results show that a PI-controller is more efficient in reproducing the global behaviour of the frame. The proposed methodology is versatile and can be used when the surrounding structure is non-linear, including when it is also subjected to fire.