The Moving Load tool was added to the Special Loads window in version 7.0. So what does this new tool do and why would you need it? The Moving Loads tool creates additional load cases based on a specific load case and then moves either a concentrated load, pair of concentrated loads or uniform load across the chords according to how you specify the load to be moved. The tool will allow you to model several different loading situations quickly and easily. The button for the tool is located on the bottom right of the Special Loads window.
 
 
So let’s run an example to see how the tool functions. The first step is to choose the Base Load Case; this is an existing load case that will be used to place the moving load cases on.  When you select the Moving Load button the load case that is current will become the Base Load Case. For this example I will be selecting LC#1 Regular.  To do this make sure that the current load case is LC#1 and click on the Moving Load button.
 
At the top of the User Moving Load window you will find the Base Load Case to which the moving loads will be added. Now we are ready to create our moving load cases. For this example let’s assume that the customer will be placing a Water Heater between the trusses in the attic but does not know its exact location. The unit has a weight of 400 lbs, is 4’ tall and takes up a space of 2’x 2’.  So where do we go from here? Because of the height restriction on the water heater we can narrow down the area where the water heater can be placed. For this example the load will not need to be applied until 9’ away from outside of the truss. Now we need to determine how the load will be applied to the truss.
 
There are two possible scenarios so we will run through both possibilities.
 
 
 
The first is that headers could be applied between or across two trusses which would give us two 200 lb point loads with a distance of 24” between them. The second case would be that a wall (solid blocking) could transfer a uniform load to the truss; in this case the load would be 200 plf over a 24” distance. The conservative approach with the loads is to assume that a single truss could carry the full load in this particular scenario. Let’s start with the first case.
 
 
For the First Case we specified a Total Point Load of 400 lbs and then checked the Two point loads check box. This brought up the Distance between text box, in which we place 2’. Then we specified the starting and ending location for the moving loads (please note this is based on the centerline of the loads so we used a distance of 10’ to 18’). Next we need to determine the Distance to Move; this is how far we move the load for the next new load case. For this example we used the minimum (6”). As a user you can change the duration of load for the new load cases, although in this example nothing was changed. To finish the creation of the new moving load cases we specified the load as a Dead Load and applied the loads to the Bottom Chord only. After selecting OK you will be returned to Special Loads and the last new User Defined Moving Load.
 
For the First Case there were 17 new load cases created.  Below is a picture of the first User Defined Moving Load showing the two point loads on the truss.
 
 
Now let’s add the loading for the Second Case. We start by going to LC#1 and selecting the Moving Load button. The information was retained from the First Case so we will only need to change the loading information, which is 200 plf over a 2’ distance. After we select OK we end up back in Special Loads on the 34th User Defined Moving Load Case.  There were also 17 new load cases created for the Second Case.
 
 
Example of the moving load in the first load case created for the Second Case loading.
 
 
The Moving Load tool can be a powerful addition to modeling those non-typical loading situations.  You can quickly produce an efficient design that will cover many possible loading scenarios.
 
 
 

 This page last modified on 10/30/2007