Risk Analysis and Plastic Duct Design

First Order Second Moment (FOSM) analysis techniques, employed in the development of limit state design codes, allow designers to account for the variations and random nature of the material properties and load effects inherent in the design of engineered structures. A generated non-linear equation is used in conjunction with an existing design formula to model the load-deformation behaviour to failure of a flexible plastic duct in free-air compression. The duct was modelled as if it were subjected to the BS EN 50086  : 19941 parallel plate compression test load of 450 N at which the internal vertical diametral strain (VDS) must not exceed 5%. The duct was modelled with a predetermined internal diameter and wall thickness. The statistics of those random variables considered were modelled using normal and lognormal distributions. Duct reliability was evaluated by imposing a deformation due to an applied vertical load and calculating the reliability under assumed random variations in the pipe diameter, pipe wall thickness and material modulus of elasticity. The FOSM analysis provided a more rational description of duct behaviour.