Experimental and numerical investigations of fire resistance of a hybrid slab

Abstract

In Uganda, there has been a growing use of hybrid slabs which include timber steel and concrete within floor systems. On the 26th of September 2022, the government of Uganda through the minister of works and transport issued a ban on building houses with hybrid slab floor systems until enough studies have been done to deem it safe as a construction method. Therefore, there was a need to conduct an experimental and numerical investigation on the performance of the hybrid slab when subjected to a fire. The experimental and numerical model parameters and specifications considered were obtained from the baseline study carried out on buildings, designers and contractors employing the use of hybrid floor systems in Uganda. A 4x3m slab model was built on a construction site in Katta, Wakiso district. The model was constructed with a 75mm concrete topping cast on expanded metal lathe fixed, on top of 100x100mm Eucalyptus grandis beams, spaced 600mm cc which were simply supported on I-beams top flanges. The I beam were supported by SHS 75x75x5mm columns at the ends. The I-beams were spaced 2000mm perpendicular to the 4m side. Burnt clay brick wall were constructed around the perimeter of the structure to keep the heat in. Three openings were constructed on the 4m side and two openings were constructed on the 3m side. Due to the unavailability of the large laboratory furnaces to carry out the experiment, a natural fire was considered as the fire scenario to be used for experimenting following Eurocode specifications. It was observed from the experimental setup that hybrid slab could with stand a natural fire for more than one hour with the ability to maintain its load carrying capacity. Numerical simulation was done using ABAQUS-CAE a Finite Element Model programme. Natural fire curve, ECPC, ASTME119 and ISO 834 fire curves were used for numerical simulation. The numerical natural fire curve observations were consistent with the experimental results. The hybrid slab therefore meets the east African code recommendations of one hour fire resistance without collapse based on natural fire tests and therefore can be used as a construction method. The ability of the slab to withstand fire action will depend among other factors the composite action between the timber and concrete therefore an investigation of the degree of composition between the timber and concrete was be determined considering three-inch wire nails and expanded metal lathe as shear connectors. Four test samples were cast according to Eurocode 4 for each of the connectors. A Universal Testing Machine (UTM) was used to test the samples under monotonic loading, and linear variable displacement transducers (LVDTs) were used to measure slip. It was determined that both the nails and expanded metal lathe create partial composite action between the concrete and timber elements.