CSiBridge – 08 Dynamic Vehicle Loading: Watch & Learn

CSiBridge – 08 Dynamic Vehicle Loading: Watch & Learn


this tutorial will illustrate how a moving vehicle can dynamically excite a bridge we’ll run an HS20-44 truck at three different speeds to see the different responses our composite bridge has two spans each with a length of 100 feet it is 36 feet wide constructed with steel girders cross brace diaphragms and a concrete deck it has two 12-foot lanes but we will only load lane 1 we start by going to the loads tab to define our truck when using a vehicle load for a dynamic analysis it is important to use a truck load and not a lane load we next add our vehicle to a vehicle class next we will define our bridge load pattern we will call the pattern truck with a type of bridge live clicking on the modify bridge live load button allows us to specify how the trucks will run over the bridge here we can specify the vehicle the lane the start time and the speed for the first truck the start time will be zero and the speed will be 75 feet per second for the second truck the start time will be six seconds and the speed will double to 150 feet per second for the third and last truck the start time will be 11 seconds with a speed of 300 feet per second we are using speeds considerably higher than typical to cause a significant excitation of the bridge we will use a 15 second duration for the load and a step size of 0.05 seconds the last step is to modify the truckload case from a static moving load case to a time history case in order to capture the dynamic effects moving to the analysis tab we select the truck case and click the modify button note that the case for the truck load pattern was created with a type of multi-step static we will change this to a time history analysis to include dynamic behavior we will leave it as a linear analysis but will change to direct integration the program automatically creates a default load application using a ramp function to apply the truck loads which we will not change we will ask for 320 time steps again at a step size of 0.05 seconds we are now ready to run the analysis note that the truck load case is now a dynamic case we will shut off the dead and modal cases as we do not need these with the analysis complete we can display the deformed shape in this case we will show the shape at one-half second into the analysis however the dynamic effects of the vehicle loading can best be seen by creating a movie here we will create a movie for the truck load case we will increase the magnification to make it easier to see and make sure that the show vehicles option is checked when we play the movie we see that the first truck has a limited dynamic effect on the bridge and the second truck slightly more however the third truck causes significant excitation of the first mode of the bridge once again the first truck at 75 feet per second the second truck at 150 feet per second and the third at 300 feet per second in conclusion bridge structures may be sensitive to the dynamic effects imparted by moving vehicles and CSiBridge offers a powerful and easy way to include and view these behaviors this ends this tutorial on dynamic vehicle loads