رفتار سازه در محفظه آتش تحت رژیم های حرارت مختلف - قسمت 2: (دماهای متوسط دال)

The effect of varying the thermal regime in a highly restrained composite beam in a steel frame structure is studied using a finite element model. The variation of through depth thermal gradients in both directions of the orthotropic slab was studied in the first part of this paper. In this part the effect of varying the mean temperature increase in the slab is investigated using the same model of the British Steel restrained beam test.

In this second part of the paper we concentrate on interpreting the results obtained from the finite element grillage model [1] of the British Steel restrained beam test from varying the mean temperature rise in the slab in both directions (parallel and perpendicular to the composite beam) while maintaining the thermal gradients to the reference values. The joist maximum temperature is also kept unchanged as in the reference case (represented by the test measurement) and is equal to 850°C. The information about the model and all other relevant background information is given in Part 1. The focus of this study is the effect of an average temperature rise in the slab, this is studied at first in the context of a simple restrained beam to establish the main principles of behaviour. The results of applying varying levels of mean temperature on the model of the British steel restrained beam test will then be discussed on the basis of these principles.

As concluded in Part 1 of this paper, the structural responses observed in the models studied here are also consistent with the expected patterns of behaviour. The general conclusion for the effects of varying mean temperatures also produces the same patterns of behaviour as identified in the reference calculation [3]. Also the structural response is manifested more significantly in the variation of internal forces as in Part 1, however there are considerable greater changes is deflections than in Part 1. The larger deflection variations seen here are because mean temperature changes cause changes in length which have a much stronger effect on deflections relative to the changes in through depth gradients (which impose curvatures which can more easily be restrained by a stiff structure). From the studies in this paper it is clearly seen that mean temperature increases result in large increases in deflection (through thermal expansion) and large increases in internal forces (P–δ forces) because of the high lateral restraint in the structure modelled. The equivalent mean temperature variation also has a significant effect on the timing of the development of the redistribution mechanism of tensile membrane action in the transverse slab direction, which occurs between the reference temperatures of 400–600°C depending upon the temperature regime applied. From the investigations carried out in this paper (both parts) it can be concluded that the response of composite frame structures subjected to local fires in internal compartments is governed to a large extent by the interaction of the following key phenomena