DOI: 10.25413/sun.13295720.v2
title: Modelling of 3D structures in fire as skeletal frames using the Fire Beam Element (FBE) for simplified analysis and design

description: n recent years performance-based design has become more popular in structural fire engineering as it often leads to more cost effective and safe designs. Real incidents, such as the Broadgate fire in 1990,and large-scale experiments, such as the Cardington tests, have shown that composite structures have significantly higher resistance in fire than what is typically predicted by prescriptive designs based on isolated members. However, performance-based design tools require significant expertise and computational effort. Simplified tools, such as the Fire Beam Element (FBE) methodology and the Slab Panel Method (SPM), can individually be used to analyse the skeletal frame of a structure in fire or composite floor panels respectively. However, few tools exist for consulting engineers to be able to comprehensively, but efficiently, consider global structural analyses for fire.<br>The FBE methodology is based on a finite element that has a movable Neutral Axis (NA) and can consider material and geometric nonlinearity. In this research the FBE methodology is extended to include a three-dimensional (3D) beam element and is implemented in a finite element software, OpenSees, so that it could be applied to 3D skeletal composite structures. The SPM is a design tool for composite slabs that considers the concrete deck and unprotected secondary beams as a whole. In the SPM, the ultimate load-carrying capacity is calculated for the slab panel taking into account the Tensile Membrane Action (TMA) mechanism that develops due to large deformations of the slab panel. In this thesis, a design methodology is proposed linking the FBE methodology with the SPM. In the proposed design methodology, the composite slab and the supporting skeletal structure are considered as two separate systems which are able to interact with each other.<br>The3D FBE design methodology is validated with three-case studies, obtained in literature, which are used to validate the behaviour of the supporting skeletal structure. The case-studies show that the 3D FBE is able to capture the behaviour of the structure where the Bernoulli-Euler assumption holds, and typically shows good correlations between FBE and literature supplied deformations. The second case study investigated indicates that continuity should be taken into account when determining the yield line pattern and load distribution to the support beams. The third case study highlights a limitation of the FBE analysis that is applicable when the primary support beams lose their strength, and the loads are carried through Membrane Action. Lastly, the FBE and SPM design methodology are applied to a ten-storey office building which provides a proof of concept of the overall design methodology. The results obtained from the FBE analysis demonstrated the behaviour of the supporting skeletal structure and the impact that the rest of the structure has on the support beams. The ultimate load-carrying capacity of the slab panel are calculated with the updated edge deflection determined with the FBE analysis It is highlighted how SPM predictions can be updated based on improved information regarding perimeter supporting beam deflections.