A prestressed roof is threatened with failure caused by pending of water on its surface. As this the fault of the design or of the construction of the prestressed members? The trouble began with plugged drains but it is getting worse.
This kind of trouble is uncommon. It occurs most often in southern states where live loads for roofs are relatively low since no provision is made for heavy snows. When pending occurs it creates deflection which may in turn lead to deeper ponding and progressively to failure. Obviously it is important to make sure that the service-load prestress force is actually achieved in prestressed members. It is equally important that the design provides for the long-term volume changes anticipated. This design calculation requires allowing for the creep and shrinkage of the concrete used, the relaxation losses for the prestressing steel actually used and any anchorage slip or frictional loss due to holddown devices. For each of these allowances the characteristics of the particular materials used must be taken into account. The calculation also requires knowledge of the modulus of elasticity of the concrete at the time of stress release. Finally, dimensions of forms for the prestressed members must meet the design dimensions within permissible tolerances. The engineer should check with the manufacturer of prestressed members to be sure that the assumptions used in the design of longspan roof structures adequately represent the actual materials to be used. The permissible margin of error is obviously narrower for members with high span-depth ratios (especially where low live loads are specified) than with low span-depth ratios. When the roof itself is constructed care should be taken to locate drains at all possible points of maximum deflection and see that no drains are misplaced in a high position. An alternative is to provide adequate slope in the roof. If architectural constraints make such measures impossible the design capacity should be reviewed to ensure that the flexural strength is adequate to prevent progressive deflection and possible failure from ponding.