The ponding problem on flat steel roof grids

Due to several reasons, a flat steel roof grid of industrial or commercial single-story buildings may be very flexible in the vertical direction. Due to its flexibility, a ponding effect can take place. The rainwater fills the space created by vertical deflection of the roof grid and this space increases under the gradually accumulated rainwater until equilibrium is reached.Ponding may be full or partial, and it is a self exciting phenomenon that can lead to grid instability. The problem is strongly non-linear, since the rainwater distribution on the roof grid in not known a priori.A closed form response to particular ponding cases and a simple grid geometry is achieved. Moreover, a non-linear finite element technique is presented in order to study the small/large deflection solution to the ponding problem of a complex flat steel roof grid.Finally, several steel roof grids are examined in order to illustrate the effect of precambering, grid dimensions and vertical flexibility on the ponding effect. The practical significance of a large deflection analysis for practical situations is also elucidated.