Reliability analysis of PSC-I girder railway bridge based on measured train axle loads

Current Korean railway bridge design standard adopts the strength design method for concrete structures and the allowable stress design method for steel structures. It is required to keep the Korean railway bridge design standard in pace with the global trend by introducing the reliability-based limit state design (LSD) method superseding the traditional design methods. Reliability indexes of existing railway bridges should be first assessed before the settlement of a target reliability index in the LSD. In this point of view, the reliability index of an existing representative PSC-I girder railway bridge is estimated using the measured values of railway vehicle axle load and the statistical information of material properties presented by preceding researchers. The statistical load data are measured over a month on a commercial railway line of the Joongang corridor where heavy freight trains are frequently operated in Korea.

The target reliability index and the failure probability should be settled to introduce the reliability-based LSD method to the railway bridge design. These values should not be far away from secured values of existing structures. Therefore, it is the first task to evaluate the reliability levels of existing railway bridges, making it possible to provide the new design standard with the reliability levels similar to the current design method. The reliability index denotes the distance from the mean to the failure probability in the limit state equation.

The statistical distribution of train axle loads is evaluated in this paper. Train axle loads are measured in a freight/passenger sharing corridor. The failure probability and the reliability index are estimated from those load effect for a typical PSC-I girder bridge which is one of the most widely used railway bridges in Korea. The bending moment caused by train axle load is used as load effect on the girder. Statistical properties (mean, coefficient of variation (CoV), statistical distribution type) for resists are hired from precedent research results.

Table 1 shows statistical properties of parameters and results by the reliability analysis. The reliability analysis is conducted by use of the statistical parameters that express accurately the lower and the upper tails of the statistical distributions for the sectional flexural strength and the live load moment, respectively. The reliability index of the PCI-I girder is estimated as 6.6703 (failure probability of 1.2764 × 10⁻¹¹%). Subsequently, the sensitivities of each design parameter for the reliability index are analyzed. Those results show that the concrete strength has a biggest effect on the reliability index. Table 1 Statistical information used for reliability analysis and reliability index for the girder.

Item

Mean

CoV

Flexural strength

18,184 kN-m

18,184 kN-m

Moment caused by D

3,506 kN-m

11.806%

Moment caused by L

1,750 kN-m

15.860%

Reliability index

β ₀ = 6,6703

Failure probability

1.2764 × 10⁻¹¹%

Next plan is to estimate reliability indices for railway bridges of other types reflecting domestic rail vehicle properties, and to assess the reliability level provided by existing design standard. Finally, these reliability indices will be used to establish the target reliability index required in reliability-based railway bridge design standard in Korea.