Fire performance of timber-concrete composite floors

Due to the burning characteristics of the timber and theinsulative properties of the char layer, the effects ofreduced timber material properties due to elevatedtemperature in both shear and compression parallel tothe grain had a very small impact on the overallperformance of the floor units. This was shown by thevery small residual section left behind in the full scaletesting and the mode of failure, in that if reduced sectionproperties were significant the floors would have failedat an earlier stage by shear or crushing at theconnections in the timber beams.It is important to note that in these small scale tests theentire section was heated through while simulatingbeam behaviour, and in the case of a real fire thetemperature gradient across the section would be vastlydifferent and most definitely not uniform. Thisemphasises the fact that reduced section properties areimportant to consider for smaller, more slendermembers than large heavy timber members. Thebehaviour of wood in fires is such that only a smallheated layer of approximately 35 mm is affected by a firebelow the char layer [17], and beyond this thetemperature of the wood is at ambient conditions. Thiseffectively nullifies the importance of reduced sectionproperties of heat affected timber for these compositefloors until the member section is so small that collapseis imminent due to the reduced cross sectional area ofthe timber members alone. This was found to be thecase in the full scale testing, however it must be notedthat the notch connection spacings were designedbeforehand with these types of failure in mind. Acomposite beam design with a small number ofconnectors spaced very closely together willundoubtedly reduce the shear capacity of the timberbetween the connections in the beam, and increase theeffect reduced section properties have on the collapsebehaviour of the floor, most likely making the reductionsin mechanical properties of the timber shown abovemuch more significant.