Modeling of insulation in 19th century metal framed structures

In 19th century metal framed construction, load-bearing girders were encased in “jack arch” fire-proofing. Since the masonry arches were filled with “early concrete” or other filling materials (which provided thermal insulation to the metal girders), their thermal properties are necessary in determining the temperature profiles of cast iron/steel beams during fire exposure. This paper will present the results of a detailed sensitivity study to investigate the effects of changing the thermal properties of “early concrete” and masonry within the bounds of variability on the fire resistance of the flooring system. Four sets of 2D thermal finite element analyses will be conducted for a typical geometry of the “jack-arch” flooring system. Results will be presented in terms of time-temperature evolution curves at different representative locations of the metal beam (upper flange, mid-height of the web and lower flange). In each analysis set, the thermal properties (i.e. thermal conductivity and specific heat) of one insulating material (masonry or “early concrete”) will be kept constant and those of the other vary. The temperature-dependent thermal properties are selected according to the appropriate upper and lower bound curves derived from relevant experimental data, either found in open literature or in current design codes. In all the analysis cases, the thermal properties of the metals (cast iron, wrought iron and mild steel) are the same as those for modern steel due to close similarities in the chemical compositions of these materials. The results of the sensitivity study demonstrate that the temperatures in the metal beams are not different from those when assuming the insulation has properties similar to those of concrete. It will be concluded that a model of insulation adopting the EN 1992 part 1 - 2 thermal properties of concrete can provide an accurate estimate of the temperature evolution in the steel/cast iron girders.