EFFECTS OF INCREASED SUPERSTRUCTURE FLEXIBILITY ON DYNAMIC RESPONSE OF CONCRETE BRIDGES

A study of the effects of design changes in the 1994 Load and Resistance Factor Design (LRFD) version of the American Association of State Highway and Transportation Officials (AASHTO) design code for prestressed concrete bridges is carried out in comparison with the earlier 1989 version of the AASHTO code. In the first part of this study, a detailed review of the design changes in the new LRFD code is outlined. An automated spreadsheet program is developed for the analysis of simply-supported standard AASHTO I-girder bridges and post-tensioned box girder bridges. The spreadsheet program is used to generate a set of typical bridge designs with varying spans and girder spacing to be used in the second phase of the study. Based on the study of typical designs using both versions of the code, a critical evaluation of the LRFD provisions is presented. In the second phase of the study, the dynamic characteristics of typical bridges designed by the 1989 and 1994 LRFD versions of the AASHTO code were evaluated. The response of I-girder and box-girder bridges to moving loads with and without vehicle interaction is investigated. Coupled dynamic equations for interaction between the bridge and traversing vehicles are developed to predict the dynamic responses of the bridge girder. Dynamic characteristics of the bridges are determined using a finite element computer program. A number of vehicle models are developed for simulation of heavy truck loads. A modal condensation technique is used to reduce the degrees of freedom of the bridge models. Numerical simulations are performed for cases in which different vehicle models are used. Dynamic amplification factors are calculated and compared for the cases involving different vehicle speeds and traffic patterns. Results of phase I investigation indicate that the new LRFD provisions result in generally more conservative designs. Results of phase II investigation show that there is no significant change in dynamic properties of I-girder bridges designed by either AASHTO (1989) or LRFD (1994). Box girder bridges designed with the new code do exhibit more flexibility than previous AASHTO (1989) based designs.