Field Measured Dynamic Effects and Load Distribution in a Prestressed Concrete Light Rail Bridge

Live load performance of a recently constructed, prestressed concrete bridge was investigated to determine the Impact Factors (IMs), the Girder Distribution Factors (GDFs). The bridge was subjected to controlled static and dynamic loading using two Denver Transit Partners (DTP) electric-multiple-unit rail cars prior to a commuter rail line being placed into service. The rail cars travelled along northbound and southbound tracks of the four span, horizontally curved, bridge at various speeds. A total 24 tests were conducted and response was measured using a combination of strain sensors and accelerometers installed at critical sections on the supporting girders. Results for IMs and GDFs obtained from the field tests are presented. These results are also compared against relevant code provisions and these preliminary results indicated that IMs are insensitive to train speed and are smaller than investigated code provisions and that measured GDFs were different than those provided by these code provisions.

Live-Load Girder Distribution Factors for Bridges Subjected to Wide Trucks

An important problem facing engineers and officials in the United States is the constraint imposed on transportation due to limitations of bridges. These limitations typically constrain vehicles to minimum heights and widths, to minimum and maximum lengths, and to a maximum allowable weight. However, with current demands of society and industry, there are times when a truck must carry a load that exceeds the size and weight of the legal limit. In this situation, the trucking company requests from the state departments of transportation an overload permit. For a truck with a wheel gauge larger than 1.8 m (6 ft), the process of issuing a permit for an overload truck requires a tremendous amount of engineering efforts. This is because the wheel load girder distribution factors (GDFs) in the design specifications cannot be used to estimate the live-load effect in the girders. In some cases, an expensive and time-consuming finite element analysis may be needed to check the safety of the structure. In this study, the finite element method is used to develop a modification factor for the GDF in AASHTO’s LRFD Bridge Design Specifications to account for oversized trucks with a wheel gauge larger than 1.8 m. To develop this factor, nine bridges were considered with various numbers of girders, span lengths, girder spacings, and deck slab thicknesses. The results indicated that use of the proposed modification factor with the GDF in the design specifications can help increase the allowable load on slab-on-girder bridges.