Vibration of the structure due to human footfalls can be modeled in ETABS through a time history analysis. I have put together a simple beam example to demonstrate how you could set up the pedestrian step loading in a time history analysis. Please note that I have not used realistic parameters for this example as the purpose here is simply to show you how to set up the model, not how to determine the parameters, such as load magnitude, stride length, time between steps, etc.
The four steps required are:
1. Define as many load cases as the number of foot fall positions that you want to simulate in your model. If you have 100 foot fall load points, then you will need 100 load cases. You may want to number these so it is obvious as to the order; I have used 'Step 1', 'Step 2', etc.
2. For each load case apply a point load at the position at which the load will be applied for the respective step. I prefer a unit load at each position so that I can adjust the overall magnitude more easily elsewhere.
3. Define a single time history function that represents the impulse of the foot fall. This could be a unit impulse or if all of the foot fall positions will receive the same load, you can set the magnitude in the function definition. Note that if you have more than one foot fall impulse function, there is no reason why you can't define more than one.
4. Finally you need to define a time history analysis case. There are two analysis types that you can use, as follows:
a. Modal time history based on Eigen modes - this option uses an Eigen modal analysis to carry out a modal time history analysis. You need to ensure that you've captured enough modes for the structure under consideration.
b. Modal time history based on Ritz modes - Ritz modes are a better option for modal time history analysis, but when used in a modal time history analysis, require you to use each of the foot fall loads defined in Step 1 as a starting load vector. Moreover, you will need a mode for each of these loads. So if you have 100 load cases, you'll need at least 100 modes.
For any of these two analysis types you need to add each of the foot fall load cases to the Load Assignments section, as shown below. For each load case, specify the impulse function, a scale factor (this is optional depending on whether you included the full magnitude in the function; ultimately the overall result will be Load*Function*ScaleFactor), and an arrival time. The arrival time is the important thing here as it will define when each of the loads is applied, so you need to work out the timing between steps. Finally, make sure that the “Number of Output Time Steps” and “Output Time Step Size” are specified appropriately to cover the duration of the time history for which you want results.
