Influence of a PV insulation layer on the point load distribution of corrugated roofs

Abstract

This research aims to investigate the influence of Polyisocyanurate (PIR) insulation on small surface load distribution on corrugated steel roofing sheets. PIR foams are used for thermal insulation in industrial buildings. The distribution of surface loads induced by solar panels support structure might be altered by PIR foams. This is completely ignored in the design calculations according to current standard which might results in overconservative designs. In this study, the following main question is going to be investigated using experimental and numerical methods: ‘’What is the influence of PIR insulation on the transverse distribution of small surface loads on corrugated steel roofing sheets?’’.

The methodology involves conducting bending experiments to obtain strains and deflection data, which then are validated using numerical models. The experiments have been conducted on corrugated steel sheets supported by rollers. The PIR insulation is connected to the steel sheet by bolts, and a loading cylinder applies vertical force at the PIR insulation panel side. Two loading configurations are selected to simulate the representative loading conditions in real-life structures. Vertical deflections and strains in the bottom flanges of the corrugated sheet are measured during the experiments. The experimental data are used for the validation of the numerical models that are created by using ABAQUS FEA.

First, a linear F.E Model was developed and validated with experimental results up to the initiation of nonlinearities. The results of the linear models shows clearly that the PIR layer influence considerably the stress distribution in the adjacent ribs to the one of load application. A nonlinear analysis is conducted to determine the maximum load that the roof structure can resist. To capture geometrical nonlinearities more accurately, eigen buckling analysis was conducted to implement initial geometrical imperfections. Considering the imperfect initial geometry of the steel sheet and its plastic properties, maximum strength of the roof system could be estimated. As a note, the damage in the PIR insulation material was not considered as it is beyond the scope of this study, the material was assumed to be linear elastic.

The experiments demonstrate that the behaviour of the roof structure is primarily governed by the rupture of the PIR insulation, occurring prior to any yielding in the steel sheet. The linear numerical model predicts the behaviour of the roof structure within the elastic phase. The linear model can be used to study the influence of the several influencing parameters such as the dimension, PIR material properties and the loading conditions. Expanding the span from 2 to 6 meters with a single mid-span surface load increases the adjacent rib contribution by 20%, meaning that their normalized strain values relative to the load location's maximum strain grow by 20% in the elastic phase. The nonlinear model is employed to establish the force vs. displacement curve and identify the strength of the roof. The non-linear numerical models show that the ultimate load is found to be 8000 N when a single surface load is applied directly on the steel sheet. The experiments show yielding of the steel at 7300 N after the loading jack ruptures both the PIR surface and layer, then the jack reaches the profiled steel sheeting. For design purposes, it is recommended to exclude the influence of the PIR insulation beyond its elastic phase. Compression tests conducted on the specimen revealed damage within the PIR material at a load of 6250 N, which is considered the upper limit for the PIR's elastic phase.