Calculation of reliability for time-varying loads and resistances

Abstract There are many elements of structural loads and resistances that vary with time. For bridges, the live loads generally increase; that is, the mean of the live load density function increases with time. The resistances may increase, as in the case of concrete curing or timber seasoning; they may also decrease, as in the case of prestressing losses, material deterioration, or timber moisture gain. In calculating reliabilities, the time-varying nature of the above-mentioned elements is often ignored or represented only by a single averaged coefficient. A method is presented for including such important time-dependent elements in the reliability equation. The equation is derived and applied to the reliability analysis of a prestressed concrete bridge girder. The analysis considers the reliability against flexural cracking due to live loads over a 50-year life. Both the concrete compressive strength and the live loads are assumed to increase with time. For the assumed distributions and time relationships the time-reliability equation indicates higher reliabilities than those found by conventional level-III reliability analyses.