Structural design for ponding of rainwater on roof structures

Ponding of rainwater is a special load case that can lead to roof collapse. In Dutch building practice the most frequently occurring damage cases are failures of flat roof structures caused by ponding of rainwater. In the Dutch code for loadings and deformations NEN6702 [1] and the Dutch guidelines for practice regarding ponding NPR 6703 [2], principles and guidelines for the determination of rainwater loads are given. The Dutch code [1] prescribes a complex iterative procedure for ponding of rainwater. Today, there are a number of computer software programs available to support the structural designer in this iteration method. However, to keep insight in the process of rainwater ponding, a simple design method for ponding of slightly sloping flat (steel) roof structures was developed. The method is described in the first part of this article. In the second part a sensitivity analysis for design and construction inaccuracies is presented. It is shown that roofs, that are seemingly stiff enough to withstand ponding, need partial safety factors substantially greater than normally used to account for construction inaccuracies. A proposal for the partial safety factor related to roof stiffness and construction inaccuracies is given.