Phenomenon of rainwater ponding on light-weight flat roofs (slopes £ 5%) is one of the most recurrent causes of structural failures. This phenomenon generally affects roofs with small slopes in the case of thunderstorm showers or blocked rainwater gullies. Due to the load of water, the girders deflect which in turn increases the load of water and so on. For "flexible" roofs a structural failure can occur if the evacuation of rainwater is less rapid than its retention. The importance of the phenomenon is recognised and its effects emphasised but poorly - or not at all - treated in French rules and Eurocodes.

In order to help a practitioner in comprehension of this phenomenon and in taking part of it in the design, CTICM puts for public use a "ACUDO" Excel application (only in French at the present time), downloadable. This application allows for taking into account the effect of rainwater ponding in a roof valley by determination of the final state of the studied system in an iterative calculation.

This application is an evolution of one developed for a study published in Revue Construction Métallique N°1-2010: "Effet de l'accumulation d'eau dans les noues horizontales de toitures à versants symétriques de faible pente (Effect of rainwaterwater ponding in horizontal roof valleys of roofs with symmetrical small sloped sides)" (T.M. Nguyen, Y. Galéa, M. Lukic).

The studied structural system is composed of two slopes of a roof next to the valley between two adjacent portal frame rafters. The slopes are truncated at the third upward purlin. The system is composed of beams modelling the rafters, the valley beam, the purlins and the corrugated sheeting (waves parallel to the rafters). The data for the application - in addition to geometric and material proprieties of the beams - allow for the introduction of a valley slope (by adjusting the roof slopes introducing the difference in levels of the rafters), various support conditions of the purlins and of the valley beam, flexibility of the rafter supports due to the rest of the portal frames,.

The system is subject to a loading resulting from a combination of actions (generally at Ultimate Limit States). This loading is characterised by an uniformly distributed surface load on the entire slope of the roof (permanent and snow loads,.), a distributed load on a given rectangular surface (drifted snow and maintenance loads,.) as well as an initial water load defined by the maximal rainwater depth above the supposedly non-deformed roof (height of emergency overflow or of roof upstand sills, stationary water depth taking into account the capacity of rainwater discharge system,.).

The deflection of the roof (valley beam, purlins, rafters and sheeting-equivalent beams) under the abovementioned loads results in accumulation of water with a depth exceeding the defined maximal rainwater depth. The deflection and the water depth are determined at each node of the system. Water load is updated at each iteration of the iterative calculation before reaching a sufficient convergence on displacements.

ACUDO application provides:

·         Local deflections at mid-spans of the valley beam, of the 1^st purlin and of the sheeting in the lower part of the roof slope

·         Curves depicting the depths of water retention volume along the slopes of the roof

·         Deflection of the roof in a cross section at mid-distance between the rafters (with its initial level and water depth)

·         Various results concerning the depth and volume of the water, deflection and maximal bending moment in the valley beam, the 1^st and 2^nd purlins, the sheeting in the lower part of the roof slope.

·         Amplification factor for each of the abovementioned results in comparison to their initial values (water) or to their "linear-elastic" values (deflections, moments) determined at the 1^st iteration (without water ponding) of the calculation.

Installation

ACUDO is an EXCEL application developed in the frame of Microsoft Office 2003. It is standalone and does not need any specific installation. It as been successfully tested with Office 2007 under Windows XP, Vista and Seven.