SECONDARY DEAD LOAD AND THE LIMIT STATE APPROACH

The article explores one aspect of cp110 - the treatment of variations in secondary dead load. The limit state code instead of using a single safety factor adopts a partial factor format dividing the safety factor into a series of sub-factors which are applied separately to loads and strengths. It is hoped that better information on loads and strengths will lead to greater discrimination making the range of safety factors wider resulting in savings of materials. Probability analysis enables code drafters to derive different factors rationally by gathering statistics for the different variables in a structure. It is suggested that separate safety factors could be derived by fitting standard distribution curves to the results. Secondary dead load, which covers various items like roofing, fencing, services etc is singled out for detailed consideration. It is suggested that probabilistic treatment by itself is not satisfactory in dealing with this. This is because of alterations or repairs to parts of structures using different materials which can result in different loading. The designer is therefore advised to make sure that the structure has an overall reserve strength and where secondary loading is critical to exercise especial care in specification and proper supervision and maintenance. (TRRL)