Dynamic Load Allowance for Reinforced Concrete Bridges

Abstract eng:
A dynamic load allowance (DLA) is a quantitative measure of dynamic effects exerted in addition to static loads by moving vehicles on highway bridges. The American Association of State Highway and Transportation Officials (AASHTO) recommends using 33% as a maximum value of DLA [1]. The paper presents results of extensive investigation of DLA, which was carried out for the Florida Department of Transportation (FDOT) for a broad class of prestressed reinforced concrete bridges. FDOT manages over 6400 such bridges and precise information about their performance, due to heavy (so called: permit) vehicles, is critical for its Maintenance Office. Two parallel tracks: experimental and analytical, were designed and implemented as integral parts of this research. Each track resulted in data which was used to calculate DLA. Comparison of these results showed a good correlation between experiments and analyses, and was used to verify and validate finite element (FE) models developed. All experimental tests were carried out on a selected, typical, prestressed, reinforced concrete bridge #500133 on US 90 in north eastern Florida. It was a newer bridge, built in 1999 with three simply supported spans and AASHTO type III girders. A selected span of the bridge was instrumented with: (a) two linear variable displacement transformers (LVDT), (b) 38 strain gauges and (c) 14 accelerometers installed at the top of the bridge deck. 35 static and dynamic tests were carried out with the speed of 48 km/hr and 80 km/hr. Three representative permit vehicles were used for the tests. Data acquisition from experimental testing allowed for calculation of DLA. The second research track consisted of FE model development and computational analysis using LS-DYNA computer code. In addition to the bridge #500133, two other local, prestressed, reinforced concrete bridges were selected for analytical studies of DLA. Their superstructure consisted of AASHTO type II and IV prestressed reinforced concrete girders. The FE model of the bridge #500133 was validated and verified using experimental data. DLA factors were calculated for all bridges as functions of vehicle types and their velocities. They were determined based on maximum displacements and strains. Several factors were identified as triggering significant DLA. They included: surface imperfections (as joint abutments and bridge approach depression), loosely attached cargo producing so called hammering effect, and characteristics of the vehicle suspension. This paper presents the latest results of the project, for which some earlier data was reported in [2] through [4].

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