Point thermal transmittance of rib intersections in concrete sandwich wall panels

Concrete sandwich panels are widely used building elements. They are made by two reinforced concrete wythes separated by a layer of lightweight material: the central layer is inhomogeneous due to the presence of concrete ribs which tie the external wythes and act as thermal bridges. International Standards allow to evaluate the average thermal transmittance of concrete sandwich panels as a linear combination of the transmittance of the solid concrete ribs and of the lightened parts - calculated as if the temperature field were 1D - and linear and point thermal transmittances associated with thermal bridges. In a recent work we have addressed the problem of finding an accurate correlation for prediction of linear thermal transmittance values. The goal was reached upon use of a fast and accurate Spectral Element Method. In this work we complete our study investigating the point thermal bridges and determining the associated point thermal transmittance. Point thermal transmittances in sandwich panels are associated with the concrete rib intersections, like in the four panel corners, and require 3D numerical simulations for their evaluation: the computational effort required to approximate the point transmittance is much larger than that needed to estimate the linear one. For this reason we present and discuss a solution strategy based on the use of low-order polynomials (p = 4) on three grids of increasing refinement, starting from a very coarse one: results have been improved through an iterated application of Richardson extrapolation. This procedure assures a good trade-off between accuracy, as required by Standards, and computational cost. A dataset of 1080 point transmittance values is obtained varying systematically six geometrical and thermophysical parameters. A simple power law correlation in terms of a single variable depending on linear transmittance of the intersecting ribs is introduced and its accuracy assessed.