Development of robust connection models for steel and composite structures in fire.

Abstract

Structural engineers and architects have a responsibility for incorporating fire safety into their building designs in order to minimize loss of life and property. To meet this requirement, extensive research has been carried out, aimed at obtaining better understanding of the performance of steel and composite structures under fire conditions. Recent research indicates that the robustness of steel connections is vitally important to the fire resistance of steel-framed composite buildings. The development of effective connection models is a key issue in this research field. This PhD research is focused on the development of robust connection elements, for modelling steel connections at elevated temperatures. In this work, a robust simplified two-node connection element has been developed, for modelling the behaviour of the bolted end-plate connections between steel beam and column at elevated temperatures. The proposed numerical procedure is based on the model proposed by Huang (2011), incorporating additional developments to more precisely determine the tension, compression, and bending moment capacities of end-plate connections in fire. The proper failure criteria are proposed to calculate the tension capacity for each individual bolt row. In this new model, the connection failure due to bending, axial tension, compression and shear are considered. The influence of the axial force of the connected beam on the connection is also taken into account. This new model has the advantages of both the simple and component-based models. A total of 22 tests are used to validate the model. From these validations, it is evident that this new connection model has ability to accurately predict the behaviour of the end-plate connection at elevated temperatures, and can be used to represent the end-plate connections in supporting performance-based fire resistance design of steel-framed composite buildings. For modelling the behaviour of partial end-plate connections between steel beams and columns under fire conditions, a simplified robust 2-node connection element has also been developed. The rotational response of a partial end-plate connection at elevated temperatures comprises of two stages. These stages are due to the shift of the compression centre of the connection from the end of end-plate, to the centre of the beam bottom flange at large rotation. The model proposed in this research accounts for these two stage behaviours, representing the partial end-plate iv connection as a 2-node non-linear spring element. Characteristics of the spring, such as stiffness, tension, compression, shear strengths and bending moment resistance, are determined based on a component-based approach. This model therefore retains the advantages of both the simple and component-based models. Compared to normal component-based models, this simplified model has very good numerical stability under static solver condition, and is computationally efficient. Fourteen tests are used to validate the model, showing that the model is capable of accurately predicting the behaviour of partial end-plate connections under fire conditions. A series of numerical studies has been conducted on a 2D steel frame, subjected to ISO834 Fire and Natural Fire, in order to investigate the influences of the connections on the behaviour of steel structures. It is clear that the model can be used to represent the partial end-plate connections in performance-based fire resistance design of steel-framed composite buildings. According to full-scale fire tests, tensile membrane action within the concrete floor slabs plays an important role in affecting the fire resistance of composite buildings. It is well known that the development of tensile membrane actions relies on the vertical support along the edges of the slab panel. However, there is at present a lack of research into how vertical supports influence the tensile membrane actions of the slab. In this thesis, the performance of a generic three dimensional 45m x 45m composite floor subjected to ISO834 Fire and Natural Fire are investigated. Different vertical support conditions and three steel meshes are applied, in order to assess the impact of vertical supports on tensile membrane action of floor slabs. Unlike other existing large scale modelling which assumed that the connections behave as pinned or rigid for simplicity, the two robust 2-node connection element models described above are used to model the semi-rigid behaviour of end-plate and partial end-plate connections within the fire compartment. The impact of connections on the 3D behaviour of composite floors is taken into consideration. The load-transfer mechanisms of a composite floor, when connections fail due to axial tension, vertical shear and bending are investigated. Based on the results obtained, some design recommendations are proposed for enhancing the fire resistance of composite buildings.