Objectives
- Improve upon the Isotherm Method, the Isotherm Method is a simplified approach that gives relatively similar results to design codes. However it does not properly model the behaviour of the slab at elevated temperatures, as the material degradation is either classified as having 100% or 0% strength based on whether the concrete is cooler or hotter than 500°C. Although this method has an advantage of being simple to implement, it may not fully represent behaviour in punching shear. Therefore a more complete analysis will be conducted and compared to the work done by Thornsteinsson and design methods in Eurocode 2.
- It was also noted that the slab would bow when heated from beneath, however when the end of the slab was restrained there were significant stresses around the column-slab boundary, these stresses would reduce the punching shear strength of the slab. The effect of the additional thermal stresses need to be included in the thermo-mechanical model and suitably analysed.
- When under compressive stress, concrete behaves differently during heating than it does when no stress is applied. The thermal expansion reduces and can even result in shrinkage, a full thermo-mechanical model would need to include Load Induced Thermal Strain (LITS) behaviour of the concrete. To adequately analyse the effects of LITS, a number of different loads will need to be applied to the slab. The results can then be compared to assess the effect it can play for flat slabs where the loads may not be sufficiently large enough to cause significant changes in strain.
- Account for the moment redistribution that would occur in a full structure, a reduced bending moment due to moment redistribution can improve the punching shear strength at a column.
- Conduct an analytical analysis of the cross section of the reinforced concrete slab under heating. This should take into account the change in strength of the concrete as the temperature varies through the depth of the slab.