Failure of holding down bolt assembly

A completed car park canopy collapsed shortly after its completion even though there was no live load, wind load or accidental load. The structural system of the car park canopy was a 'T' steel structure of unequal cantilevers connected to the ground at the base with eight 25-mm holding down bolts. The collapse was a progressive one starting with the steel structure located at one end of the roof. An investigation of the causes of collapse was carried out. Before the collapse, the maximum tension acting on a holding down bolt was not more than 10% of the tensile strength of the bolt. This investigation shows that the use of incompatible bolts and nuts such as in the case reported leads to the total loss of the tension capacity of holding down bolts in 3 d.