Investigation on the Analytical Behavior of Built-Up Steel Section Encased in Concrete Under Fire Loads
"Due to the increasing number of fire accidents that are able to leave behind great losses and complete collapse of structures, structural fire safety has become a major consideration in the design of high rise buildings. In this research, the numerical model of concrete encased dual I-shaped steel columns during fire loads is investigated and discussed. The interest behind such type of sections lies in its wide usage in tall buildings especially when the case requires carrying huge axial and lateral loads in biaxial directions. This research proves that the sections studied provide an excellent fire resistance rate which increases the time needed for evacuation and saving property.
The aim of this research is to evaluate how certain factors can influence the critical time of the column as well as its buckling behavior under fire loads. This can be considered as a guide for structural designers who seek for designing columns that satisfy certain fire resistance rate. The parameters that were considered are:(1) the applied load level, (2) stiffness of surrounding structure to column, (3)section dimensions,(4) concrete cover, and (5) axial distance from concrete surface to longitudinal bars. Another aim of this research is to check the ability of an approximate analysis method in providing reliable response of these columns and thus acting as alternative of Heat Transfer Method. This approximate method is based on dividing the section into layers at the location of experimentally recorded temperature-time histories and then linking load amplitude to its corresponding layer.
In order to achieve the posted aim, numerical investigation using ABAQUS software was used in order to overcome the drawbacks of performance-based tests including the shortage of resources and time. In order to check the accuracy of such finite element software, two FE models were analyzed under fixed and pinned boundary conditions and the results were compared to experimental results available in the literature.
After being satisfied with the approximate method and simulation software, twelve models of double HEA section totally encased in concrete were developed in ABAQUS software. These twelve models belong to three types of sections subjected to high and low load levels, as well as, high and low surrounding stiffness. The high and low stiffness of the surrounding structure was provided experimentally in  by the usage of restraining frame with variable beam spans resulting in variation of stiffness. As an approximate approach in the numerical model, the high stiffened surrounding was defined as fixed boundary conditions and the low one as pinned connection.
Finally, it was found that decreasing the load level and increasing the concrete cover have a big influence in increasing the critical time (fire resistance rate) of the column. Although increasing the stiffness of the surrounding plays a great role in reducing the displacement at mid height by around 28.2%, its effect on reducing the critical time is insignificant and can be eliminated by designers. Also, it was noticed that as the slenderness of a section decreases, the displacement values are reduced and the critical time is increased. However, the effect of slenderness (section dimensions) on the restraining axial force was not clearly understood because increasing the section dimensions (reducing slenderness) under low loads and fixed connection reduces this force and under high and pin connection increases it.
Noura Ahmad Mokhtar Wehbi
Dr. Adnan Masri