Improving the robustness of steel frame structures under localised fire conditions

PurposeThe robustness of building structures in a fire has recently drawn wide attention. This study presents the progressive collapse analysis of steel frame building structures under localised fire. The main objective of this study is to propose methods to enhance the structural collapse resistance of such structures in fire.Design/methodology/approachA modelling method was developed and validated against both experimental and analytical studies. Then, a series of robustness analyses were performed to investigate the interaction among the members and the pattern of load distribution within the structures. These analyses show that lateral resistance and load redistribution have a vital role in the robustness of the building. Thus, two approaches have been adopted to enhance the robustness of the focused steel frame during a fire.FindingsIt is found that increased size of floor beams and vertical bracing systems are effective measures in preventing whole structure collapse. The larger beam section is able to prevent catenary action so that the load in the failed columns can safely transfer to the adjacent columns without buckling. On the other hand, the bracing system improves the lateral resistance that can accommodate the lateral force when catenary action occurs in the beam.Originality/valuePrevious studies have focused on the collapse mechanism of steel frame structures. However, the parameters affecting the structural robustness in a fire have not yet been explored. To address this gap, this study adopted numerical modelling to undertake parametric studies to identify effective methods to improve the robustness of such structures under fire conditions.

MECHANICAL BEHAVIOR AND CATENARY ACTION OF RESTRAINED STEEL BEAM UNDER FIRE

To investigate the mechanical behavior and catenary action of restrained steel beam under fire, experiments were performed on five H-section restrained steel beams exposed to ISO-834 standard fire. At first, mechanical property tensile tests were performed on 3 room-temperature specimens and 8 high-temperature specimens, and variation laws of the material properties of steel materials with temperature rising were investigated by the high-temperature steady-state tests. Through the fire experiments, the temperature data, mid-span deflections and failure modes of all specimens were obtained. The experimental results show that: (1) a restrained steel beam is prone to in-plane buckling failure under fire; (2) the loading ratio n and axial restraint stiffness Kx have great influences on the catenary action of restrained steel beam under fire; (3) when the loading ratio n is constant, the greater the axial restraint stiffness Kx, the later the catenary action occurs; when the axial restraint stiffness Kx is constant, the greater the loading ratio n, the earlier the catenary action occurs.

Effects of Corrosion on Compressive Arch Action and Catenary Action of RC Frames to Resist Progressive Collapse Based on Numerical Analysis

Many negative factors can influence the progressive collapse resistance of reinforced concrete (RC) frame structures. One of the most important factors is the corrosion of rebar within the structure. With increasing severity of corrosion, the duration, robustness, and mechanical performance can be greatly impaired. One specific side effect of rebar corrosion is the significant loss of protection against progressive collapse. In order to quantify the effects of rebar corrosion on load-resisting mechanisms (compressive arch action (CAA) and tensile catenary action (TCA)) of RC frames, a series of numerical investigations were carried out in this paper. The previous experimental results reported in the literature provide a benchmark for progressive collapse behavior as a sound condition and validate the proposed numerical model. Furthermore, based on the verified numerical model, the CAA and TCA with increasing corrosion and an elapsed time from 0 to 70 years are investigated. Comparing with the conventional empirical model, the proposed numerical model has shown the ability and feasibility in predicting the collapse resistance capacity in structures with corroded rebar. Therefore, this numerical modeling strategy provides comprehensive insights into the change of load-resisting mechanisms in these structures, which can be beneficial for optimizing the design.

Influence of the Bolt Material Properties on the Ultimate Capacity of End Plate Bolted Joint Subjected to Column Loss

This paper investigates the performance of extended stiffened end-plate bolted beam-to-column joints subjected a column loss scenario by means of finite element simulations. An advanced numerical model was developed, and its effectiveness was validated against the experimental results. The influence of the bolt strengthening on the column loss action was investigated changing the grade of bolts. The results showed that the joint performance under column loss scenario are deeply related to the development of the catenary action that depends from the connection ductility; therefore increasing the bolt material strength will provide beneficial effects on the joint capacity under the column loss.

Strengthening of Precast RC Frame to Mitigate Progressive Collapse by Externally Anchored Carbon Fiber Ropes

The robustness of precast reinforced concrete (RC) frames is relatively poor, while the precast RC frames are strengthened to mitigate progressive collapse, avoiding “strong beams and weak columns” and the anchorage failure of strengthening materials under large deformation condition are the key problems. Aiming to discuss these problems, this paper carried out an experimental research of strengthening on three half-scale assembled monolithic frame subassemblages to mitigate progressive collapse. One specimen was strengthened by implanting carbon fiber rope (CFR) with polymer into concrete, one specimen was strengthened by binding CFR with special knot, and the last one was not strengthened. The failure mode, collapse failure mechanism and strengthening effect of subassemblages were discussed. Analytical models of load capacity increment contributed by CFR and construction suggestions of precast RC frame to mitigate progressive collapse were proposed. The results indicated that none of the strengthened specimens had anchorage failure. The two strengthening methods significantly increased the load capacity of the subassemblages in the catenary action (CA) stage with little effect on the flexural action (FA) stage and compressive arch action (CAA) stage.

Numerical investigation on load transfer mechanism of bonded post-tensioned concrete beam-column substructures against progressive collapse

This paper presents the high-fidelity finite-element-based numerical models for modeling the behavior of prestressed concrete (PC) beam-column substructures to resist progressive collapse under column removal scenario. After careful calibration against data, the validated numerical models are further employed to shed light on the influence of bonded post-tensioned tendons (BPT) with a parabolic profile on the load transfer mechanisms of PC frames against progressive collapse. The effects of parameters, including initial effective prestress, profile of tendon and lateral constraint stiffness at the beam ends, are also investigated. The study shows that, due to the presence of prestressed tendons, the mobilization of compressive arch action in the beam at small deflections demands stronger lateral constraints, and the ultimate load resistance of PC beam-column substructures depends on combined catenary action from non-prestressed reinforcement and BPT at large deflections. For a given constraint stiffness, the initial effective prestress of BPT has less significant effect on the overall structural behavior. For prestressed tendon, a straight profile usually employed in structural strengthening can improve the initial structural stiffness and yield strength, but is less effective in enhancing the ultimate resistance against progressive collapse than the parabolic profile.

Study on the Component-Based Model of an All-Welded Beam-Column Connection for Progressive Collapse Analysis

The mechanical behavior of all-welded beam-column connections of steel frames during progressive collapse was numerically studied using finite element simulations. The validation of the numerical model was based on a previous test model. The analysis results indicated that the stiffness of the all-welded beam-column connection in the elastic-plastic stage was mainly provided by the shear stiffness of the panel zone, and the axial compression on the column had a substantial impact on the capacity and ductility of the all-welded beam-column connection. An improved component-based model of the all-welded beam-column connection was proposed. To verify the accuracy of the proposed model, a beam-column assembly with an all-welded connection was established and the influence of the catenary action, column axial compression, beam-column stiffness ratio, and dynamic performance was parametrically analyzed. The validation results showed that the proposed model was able to simulate the behavior of all-welded beam-column connections at large structural deformation.