IMPACT AND FATIGUE IN OPEN DECK RAILWAY TRUSS BRIDGES

The purpose of this study was to investigate various factors affecting the impact values and fatigue lives of various members in open deck railway truss bridges. The vehicle model used in the study has three degrees of freedom: bounce, pitch and roll.; Lumped mass models were used for the truss bridge. A 175 ft riveted truss bridge was selected in the study. All joint connections of the bridge were considered to be either hinged or rigid, whereas connections for bracing members were assumed to be hinged. The connections for the floor beams were considered as either rigid or semirigid. For a bridge with hinged joints, the floor beams were considered to be rigid and simply supported. For partial bridge models, the stringers were considered to be semirigid connected, with lumped massed at the mid-span points.; In the analysis, only the vertical degrees of freedom for the bridge were considered. The stiffness equation for the connections and dynamic equations of motion for the bridge and vehicles are given in Chapter II. A train with three four-axle locomotives moving at speed of 60 mph on the bridge with rigid joints was studied in Chapter III. The spring constant was taken as 7 kips/in. per wheel and vehicle damping was omitted. The bridge was studied with and without two percents of critical damping. The vehicles in the train were considered with no initial displacement and roll and with the initial displacements and rolls of 0.5 in. and 0.04 radians, respectively. In Chapter IV, the same bridge and the same train with the same initial displacements and rolls were used. The train was moving at 60 mph on the complete bridge model with hinged, rigid or semirigid joints. In order to reduce the computing costs, the partial bridge models were developed to study the impact factors, force and moment ranges in the fatigue critical members (i.e., the hangers, floor beams and stringers). The partial bridge model was also used to study the effect of: (a) stringer stiffness, (b) floor beams not connected at the truss joints, (c) stringer vibrations, (d) vehicle damping, (e) train speed, and (f) long vehicles in train consist.; In Chapter V, fatigue lives and impact factors were investigated using the partial bridge model. Trains with three freight cars were used with initial displacements and rolls as specified previously. Spring constant per wheel was taken to be 15 kips/in. Three different freight trains were studied: (a) one hundred 70-ton cars, (b) seventy 100-ton cars, (c) a train of 70-ton and 100-ton cars, mixed in various proportions. For the first two cases, various train speeds were considered, but for the third case only 50 mph speed was investigated. Both bridge and vehicle damping were neglected, since their effects tend to cancel each other.; In Chapter VI, a study was made of the impact factors in bridge members due to flat wheels and track irregularities. The maximum wheel flat height was taken as 0.2 in., maximum peir settlement as (+OR-) 1/4 in., and maximum camber error as (+OR-) 1/2 in. The track roughness spectra for class 6 track was used in the analysis, and only selected members of the 175 ft bridge, as previously mentioned, were studied. The train of three 70-ton freight cars with spring constants of 11.2 kips/in. per wheel were used. The train was moving at various speeds and with initial displacements of 0.25 in. and rolls of 0.02 radians. Damping of the bridge and vehicle were neglected.; In Chapter VII, a summary of the conclusions from Chapter III through VI is presented.