رفتار سازه در طول آتش سوزی متحرک عمودی

This study considers a multi-storey composite frame subject to a fire which travels vertically between three floors. Previous work has analysed the behaviour of this structure when subject to simultaneous fires on three floors. It highlighted the importance of the cooling regime adopted and the relative axial stiffness of the steel beams to the overall behaviour of the structure. This paper extends that work by investigating the more realistic case of a vertically travelling fire. Various inter-floor time delays are considered as well as two floor beam sizes. It is found that the inter-floor time delay affects the global behaviour substantially. The behaviour is also in part dependent on the stiffness of the floor beams. Axial forces caused by thermal expansion in individual floors may induce cyclic loading on the column which is not normally considered in structural fire design but may be important in determining structural behaviour. Identifying a worst-case rate of vertical fire spread is not possible due to the range of structural responses, so it is recommended that designers consider several rates of spread and ensure structural integrity for each.

A performance-based approach to structural design specifies how buildings should perform rather than construction details such as material properties or member sizes, as in prescriptive design. It has been adopted in many areas of structural design in recent decades but has only recently been introduced in the field of structural fire design. It is particularly relevant to the fire safety design of high-rise structures as such buildings tend to combine large open-plan offices, long evacuation times and unusual construction methods, all of which fall outside the scope of applicability of most prescriptive-based fire design codes. However, the design of high-rise buildings to resist fire using a performance-based approach requires an understanding of their structural behaviour when heated. Several recent high-rise building fires 1., 2. and 3. have shown that very large, multiple-floor fires are possible in such buildings and that the structural response will involve a similarly large portion of the structure. Consequently, attempting to analyse and understand the behaviour of high-rise structures on an element by element basis, as is common for ambient temperature design, is unlikely to meet the needs of designers using a performance-based approach. Fire in high-rise structures also tends to be difficult or impossible for fire-fighters to tackle, so much so that full burn-out may occur on one or more floors while evacuation or fire-fighting on other floors is still taking place. In such situations, ensuring the stability of the overall structure has clear life safety implications. This leads to a requirement to understand the behaviour of structures subject to fires that travel between floors, perhaps with some floors cooling after burning out while other floors are in the early stages of heating. This paper aims to build on the very limited work that has so far been devoted to understanding the global response of high-rise structures subject to multiple-floor fires. Usmani et al. [4] and Flint [5] examined the collapse behaviour of tall buildings similar to the (undamaged) WTC towers 1 and 2 when subject to simultaneous multiple-floor fires. Depending on the relative stiffness of the floors and columns, two distinct collapse mechanisms were found for such structures [6]. More recently, it has been shown by Usmani et al. [7] that these collapse mechanisms can be reproduced in simplified, two-dimensional (2-d) models made up of column and beam sections, thus removing the need to model the complex three-dimensional (3-d) truss systems used in the WTC design. Usmani et al.’s work was only strictly applicable to structures using the unusual truss flooring systems found in the WTC towers; however, it has subsequently been shown that similar collapse mechanisms can occur in more generic tall buildings (e.g. Fig. 1) with beam spans of as little as 10 m [8].

It has been shown in this paper that a time delay between floor fires will affect the global response of high-rise structures. It has also been shown that in general neither simultaneous nor vertically travelling fire can be considered a worst-case scenario as they result in different structural responses, either of which may be the most serious. For short inter-floor time delays the structural behaviour was found to be very similar to fires occurring simultaneously on the same number of floors. However, a key difference observed was the cyclic movement induced in columns at each floor level as the fire progressed upwards. This cyclic deflection pattern has not previously been considered when designing against fire. It will be of significance for fire design, particularly for connections which will already have severe demands made on their ductility capacity under fire loading. With larger inter-floor time delays the global structural behaviour changed. This is particularly clear for the strong beam structure. Whereas, with small inter-floor delays, floors expand against the relatively weak column restraint and do not deflect substantially, in the slow travelling fire the cold surrounding columns provide more restraint, and large deflections do occur. This has an effect on the overall floor forces in the non-fire floors as well. Cyclic movement of the columns also occurs here, albeit with a larger time interval.