Fire Resistance of Reinforced Concrete Filled Rectangular Hollow Sections

Composite structural elements are widely used due to their efficient use of each material's strength, enabling each of them to work in the most favorable conditions. In the case of steel-concrete composite structures, this means allowing concrete to work mostly in compression while it is steel that carries the tension stresses. More specifically, when talking about columns composed of steel tubes filled with reinforced concrete, these kinds of sections present a wide array of advantages born out of their composite behavior, such as reduced cross sections requirements (and more available space for architectural design), fire resistance provided by concrete (as opposed to steel profile alone), reduced construction time and no need for formwork, controlled and reduced creep and shrinkage, among many others. However, despite all the advantages provided by these columns, their behavior in case of an accidental fire situation is not easy to simulate and often requires complex software analysis. With this in mind, this study has focused on analyzing and understanding the behavior of reinforced concrete-filled rectangular hollow steel columns when exposed to fire conditions (accidental simulation as modeled by ISO834 fire curve), and the development of a simplified tool that allows for the estimation of the fire resistance of these. Additionally, the method has also been used in a study project with the purpose of verifying its applicability and usefulness as a simplified predesign tool. This study is deivided into 5 chapters. The first one is the introduction, where the basic information regarding the materials contemplated and composite sections can be observed. Additionally, the objectives of this study can be found in this section. The second chapter is a study of the state of the art on fire resistance calculation for this type of composite column. This has been achieved by means of a thorough analysis of past and current methods used for this purpose, as well as those provided by the current European standards The third chapter delves into the methodology and steps followed in the development of the simplified mechanical analysis tool developed during this study. The thermal response has been obtained through the use of CDM DOLMEN's module IS.FUOCO, and starting from the heat map obtained, the mechanical response tool has been developed. This consists of a simplified 4-point approach that is able to simulate the flexo-compressive behavior in this situation. The fourth chapter is comprised of a project currently overseen at Essebi Ingegneria. This project is presently being designed with reinforced concrete columns, but an alternative design with reinforced concrete-filled rectangular hollow steel columns, with the fire resistance being obtained through the tool developed and explained in the previous chapter. The fifth and final chapter is characterized by the conclusions, where an analysis of the results found is developed as well as a comparison between RCFRHS columns - RC columns and the design methodologies utilized for fire design.