The Stability Analysis Of Steel-framed Columns In Fire Condition

Investigations against the causing of the collapse of the Twin-Towers of the World Trade Centre (WTC) have commonly accepted that the fire generated after the skycraft's attack is the reason for the structure's collapse. Both strength and rigid of steel deteriorate in high temperature. However such deterioration is fail to explain the structure's global collapse. The FEMA report indicates that in the fire the gas temperature is range from a high of 1100℃to a low of 400℃. In those temperatures structure is not likely to collapse because of the loss of strength and rigid of the material. Whilst in fire it is quite possible that when parts of the structure heated other parts may remain cool. Both Cardington test and studies by others have show that in reality there exist 'assistance actions' between the cool and hot structural elements that the cool elements will resist the loads which are initially resisted by the hot elements in normal temperature but can not be further resisted by the hot elements for whose loss of strength and rigid caused by high temperature. In addition, the FEMA report also indicates that the utilization factor for gravity loading is a very low 20% for exterior columns and a moderate 60% for the core columns. This multiplies with the above two factors conclude that the statement of the collapse is caused by the material's deterioration is not standable. Other reports also state that the collapse is caused by the chain progressive floor collapse. That the truss would undergo large deflections due to the effect of restrained thermal expansion which would lead to tension membrane action in the slab and the resultant tension would lead to connection failures and floor collapse, thus setting off a chain of progressive collapse. However, Cardington test have shown that connections, except snap in tension on cooling, remain under compression while the fire burn and the temperatures increase as a result of restrained thermal expansion (initially from steel and later from concrete expansion). Therefore the statement is not credible. The most reasonable explanation is given by Usmani and Chung. They use a spring to consider the laterally restraint of a single floor to an exterior column. Therefore the whole exterior column can be treated as a compressive member restrained by many springs laterally. The stability of this member is significantly determined by the stiffness of the springs. Considering the stiffness of a spring with values of 0 and∞extremely, in the view of member, the nominal length of the column in the former situation is twice of the value in the later situation. In fire situation the stiffness of the floor will reduce and the floor may also push the exterior column out, both of those might cause the column loss stability. By FEM analysis Usmani gives the following conclusion that the loss of the stability of the exterior column caused by the fire induced change of the lateral restraints is the reason for the collapse of the WTC.This paper is based on the above analysis of the collapse of WTC. Some theoretical derivations have done and the derivative results have been utilized to analyze the stability of steel framed columns in fire. Results from the general commercial software package ANSYS have been used for comparison. Good agreement obtains between the results which verify the derivative results' ability to study the aforementioned problems. In the further structural fire safety design practice especially the design of tall buildings the conclusions given in this paper should be considered.Considering the deficiency of the research on structural fire safety design in domestic and the fact that the code for fire safety is only a prescriptive code about building fire protection, section 1 introduces the structural fire safety design in other countries mainly in Europe. In the heat transfer part of section 1, the author's recent research is also introduced. Section 2 is a review about the behavior of steel framed structures in fire. Section 3 discusses the WTC collapse model and the theoretical derivations. Section 4 is about using the derivations in section 3 to study sub-frames and carrying out FEM analysis. The last section gives the conclusion and the discussion.