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Evaluation of ponding analysis methods using 3-D finite element modelling

A. Schouten, J. Locht, J.P.B.N. Derks
Schouten Engineering Consultancy B.V. Breda, the Netherlands

Water accumulation is studied by application of the developed in-house software WACSIM, able to model the ponding behaviour of one-dimensional, two-dimensional and three-dimensional structures. All analyses are done under the regime of the Dutch building code NEN 6702, the latest version of which requires, in a water accumulation analysis, a reduction of the structural stiffness data by a model factor gM , intended as an instrument to guarantee larger safety. The first part of the article reports statistics of a large number of cases, showing that a prescribed fixed value of the model factor yields a big scatter in the real safety. The second part of the article is a comparison of the computational results with the outcome of the analytical results as published in this Heron edition. The third part of the article is a demonstration of the importance of a three-dimensional analysis in which the complete real roof structure is adequately modelled. Simplified computational schemes yield different failure modes than those occurring in a full three-dimensional model.

Key words: Ponding, flat roofs, accumulation of rainwater, steel, statistical evaluation, finite element analysis, comparison, case study