Safety Assessment Of Small And Medium Span Bridges Considering The Randomness Of Transverse Position Of Vehicle Load

At present,the multi-girder bridge safety assessment based on the reliability theory is mainly focused on the primary component s of the structure.The statistical parameters of the structural resistance in the specification are based on single beam s.Therefore,the calculation of the vehicle load effect shou ld take the load distribution factor(LDF)into consideration.Especially for the multi-girder bridge safety assessment,it is necessary to obtain the reliable lateral load distribution factor of the main girder to accurately calculate the maximum load eff ect of each girder.In this paper,small and medium span multi-girder bridges are selected for investigation.The statistical parameters of the LDF are analyzed by considering the randomness of the lateral distribution of the vehicle load.Then,the reliab ility of the bridge is evaluated by using the statistical parameters of the LDF.The optimal transverse position for overweight trucks to cross the simply-supported multi-girder bridges is investigated.In addition,the influence of the vehicle lateral loading position on the safety of bridge operation is also analyzed.Based on the results of this dissertation the following conclusions can be drawn:(1)The statistic properties of the live load distribution factors were obtained by considering the random vehicle transverse positions and their corresponding frequency of occurrence.A group of prestressed concrete girder bridges designed following the AASHTO LRFD code was investigated in this paper.The bending moment LDFs for each girder under different truck loading conditions were obtained from finite element analysis.Then the probability distribution of the LDFs was obtained based on the LDFs calculated at each transverse loading position and the corresponding frequency of the truck transverse position.It was found that the distribution type of the LDFs has a strong correlation with the girder spacing.The span length and the girder spacing both have a significant impact on the coefficient of variation(COV)of LDFs.(2)By treating the LDF as a random variable with the obtained statistic properties,the reliability indexes of bridge girders were evaluated and compared to those predicted using the traditional method which treats the LDF as a constant.Based on the results of this study,it was found that by adop ting the single maximum LDF value the traditional bridge evaluation methods may be too conservative and may lead to underestimated bridge reliability indexes under certain circumstances.This suggests that the actual statistical properties of the LDF should be considered when a refined analysis is desirable in case unsatisfactory assessment result is obtained by the traditional evaluation methods.By doing this,unnecessary bridge rehabilitation or demolishment may be avoided.(3)The optimal transverse position for overweight trucks to cross simplysupported multi-girder bridges was investigated and the suggestions for the management of overweight vehicles were provided.The optimal transverse truck position was defined as the loading position where the maximum bridge bending stress of the girders is minimized.Finite element analysis was performed for a group of prefabricated concrete bridges under the action of overweight trucks.The effects of superstructure configuration,length of bridge spans,number of gi rders,type of girder connection,and support conditions were all investigated.The findings revealed that the transverse loading position of overweight trucks has a significant impact on the responses of multi-girder bridges.For example,for the 20-m simply-supported T-beam bridges with five girders,the maximum bending stress at the midspan of the girders when the overweight vehicle moves cross the bridge along its centerline is 15.1% to 37.2% larger,depending on the girder connection condition,than th e maximum bending stress when the overweight vehicle moves following the optimal transverse position.This finding also reveals the fact that the condition of girder connections has a considerable effect on the bridge response under vehicle loading.(4)The influence of different lateral loading position on the safety of bridge operation is analyzed.The first-crack strength and the ultimate strength under different truck loading positions were also checked.It was found that for all the bridges analyzed,when the truck followed the optimal transverse loading position the first-crack strength of the bridge was always greater than that obtained when the truck followed the bridge centerline(except for the bridges that the centerline of the bridge coincides with t he optimal transverse position).Take the 20-m T-beam bridge with five girders for example,the truck load that initiate the first crack was 142.4 ton with the truck following the optimal transverse loading position,which is 5.9% greater than that(134.4 ton)obtained when the truck followed the bridge centerline.In addition,the ultimate capacity under the former condition was 423.5 ton,which is 8.9% greater than the ultimate capacity under the latter condition(388.9 ton).These results indicate that the optimal transverse loading position obtained from capacity consideration also stands from the perspective of durability and serviceability which are usually more of a concern than the capacity for the bridges considered.