Failures of roof structures caused by wind uplift forces

Abstract

The weakness of the tying down systems between the structural elements has been identified as the main cause of the failure of light roof structures due to uplifting pressure from the wind. These weaknesses have caused part of the roof structure being blown off during thunderstorms. This project report presents the results of a study carried out to evaluate the load carrying capacities of joints for zinc/asbestos sheeting to purlin, purlin to rafter and rafter to roof beam in the frame structure for light roof construction. The capacities of these joints were determined from laboratory tests on respective samples and also from previous related tests. Comparisons were made with the calculated pressures from the wind and hence the pulling out forces exerted on each type of joint on modeled buildings representing the low and high rise houses located areas similar to those which experienced failure. Details of the structural configuration and types of joints adopted were within the range found in actual construction. The result show that the number of tying down points between members within a specified area of roof and the depth of penetration of the nails at three points currently adopted are inadequate. They are only sufficient to sustain forces exerted by the wind speeds of up to wind speeds of up to 10m/s and 50m/s in low and high rises building respectively. It was also proposed that the pitches between purlin and also between rafters in the present construction be reduced in order to achieve more connection points.