Preventing progressive collapse through strengthening beam-to-column connection, Part 1: Theoretical analysis

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Abstract

This is part one of a two-part paper reporting the results of an investigation into the characteristics of catenary action and their effect on structures. In this paper, it is shown that when catenary action occurs, the bending moment will decrease significantly in the beams. It is also demonstrated that when a column is destroyed by a blast, the failure strain will be reached at the simple beam-to-column joint. To enhance the survival capability of the steel framed structures subjected to a terrorist blast, novel retrofitting schemes are proposed in this paper for strengthening the simple joints of existing tall steel framed structures by changing the partial-strength shear-resisting joints to the full-strength moment-resisting joints, ensuring the full development of catenary action. To support the above mentioned viewpoints, the catenary behavior of a truss model is evaluated and exact results are given. In the subsequent companion paper, these views are also corroborated by means of finite element analysis.

Introduction

Unlike military structures, ordinary civilian buildings have traditionally not been designed to resist blast loads in the past. However, recent terrorist threats to important buildings of high public or commercial relevance have demonstrated the need to evaluate the vulnerability of these buildings. Few skills are required to manufacture a crude bomb that can cause serious damage, and as a result, bombing has long been the most common terrorist tactic. It is widely believed that during explosions most casualties and injuries sustained are not caused by the pressure, heat, or fragments resulting from a bomb detonation, but by the local failure of a building and its disproportional progressive collapse later [1]. Therefore, ensuring resistance to a blast without producing deadly local failure and mitigation of progressive collapse is a crucial aim.

In order to avoid damage caused by blasts, it is very important to prevent terrorists from reaching a potential target. The most cost-effective means of protection is to maintain a stand off distance between any possible explosion and important buildings, because the blast effects of a bomb diminish dramatically with an increasing range. In addition to the above mentioned common-sense way of keeping dangerous matters away from buildings, there are traditionally three ways to improve the ability of the structure to survive blast loads: enhancing local resistance, increasing redundancy and adopting ductile structures 2., 3..

Many structural design standards 4., 5., 6., 7., 8., 9. have acknowledged the threat posed by abnormal loads and progressive collapse since shortly after the Ronan Point collapse 10., 11.. After the terrorist attacks against the Alfred P. Murrah Building in Oklahoma City in 1995 12., 13., 14., 15. and the World Trade Center in New York in 2001 16., 17., much more research has been conducted. In general, there are two approaches to reducing the susceptibility of structures to progressive collapse: local resistance to the abnormal load and redundancy. The latter approach requires that when any one component fails, alternative paths are available for the load in that component and a general collapse does not occur. With this methodology, structural continuity is crucial. When key elements carrying gravity loads are destroyed, it is important that the remaining structure can redistribute the loads without collapse 18., 19., 20., 21., 22., 23., 24..

When a load-bearing steel column is removed by a blast, the steel beams connected to this column are required to transfer the load borne by the column and to bridge over the damaged area. As a result, catenary action 24., 25. will be involved. If the catenary action force is adequate within the beams, the local damage to the column will not cause catastrophic consequences. Catenary action in steel beams also received attention by researchers 26., 27. in structural fire engineering. Yin and Wang 28., 29. developed a simplified hand calculation method of catenary action in the axially restrained steel beams at elevated temperature.

In this paper, the characteristics of catenary action due to the removal of a column are investigated. The need for strengthening the joints of a simple construction is illustrated. Two retrofitting schemes are proposed for strengthening the fin plate beam-to-column connection of existing buildings. Through linking beam flanges together, the original partial-strength simple joint is changed to the full-strength moment joints. In the companion paper, the viewpoints mentioned in this paper are corroborated for realistic structures through modeling by the nonlinear finite element method.

Section snippets

Analysis of catenary action

Normally, beams are mainly employed to resist bending moment only, and are seldom called up to resist an axial force. However, when the column supporting the beams is destroyed by a severe blast, the beams will sag significantly, as shown in Fig. 1. Because of the diaphragm effect of the floor slab, which is almost rigid within the floor plane, the movement in horizontal direction is greatly restrained. The sagging of the beam generates a rather large axial tensile force, creating the catenary

Retrofitting scheme

As mentioned before, ductility is very useful for preventing blast-induced damage. It is true that structural steel is one of the most ductile engineering materials. However, material ductility alone is not a guarantee of structural ductile behaviour because steel components can fail in a premature manner, in which only a small part of the structures is damaged and the stress level in the rest of the structure is still very low. In particular, these premature failures raised important concerns

Exact solution of catenary action of truss model

The susceptibility of different beam connections to the effect of catenary action will be examined here, for the purpose of illustrating the need for reinforcing the joints of simple steel construction. When the tension is extremely large, the contribution of the bending moment approaches zero and the structure changes from a beam to a tensile truss.

In addition, because of involving both material and geometric nonlinearities, the truss is the only structural type for which exact results can be

Summary and conclusion

This two-part paper reports part of results of an investigation devoted to the analysis of catenary action. In this paper, there are three contents. Firstly, a theoretical analysis of catenary action is conducted. It is shown that the deformation and the catenary action can reduce the bending moment significantly through axially restraining the beam.

Secondly, the necessity to strengthen the simple beam-to-column connections in an existing steel building structure is discussed. For fully

Acknowledgements

This work was supported by the EPSRC Dorothy Hodgkin Postgraduate Award (DHPA) scheme and the National Science Foundation of China (NSFC) under Grant No. 50478119, and these supports are gratefully acknowledged. The quality of the paper was enhanced significantly by the valuable comments of two reviewers, much help was given by Dr. Antonis Zervos, Prof. Stuart S. Moy, Dr. Christine J. Winter and Dr. George D. Chellapa of University of Southampton, and they are greatly appreciated.

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