If you’ve every tried to apply a tapered surface load to plates in RISA-3D, you’ll soon realize that each plate element must have a uniform load/pressure applied. If you use the area load tools, you’ll notice that they only work for beams. RISA News has published an article illustrating a common method for applying tapered surface loads to your model:
It’s easy to apply tapered surface loads to plates in RISA-3D by stepping up the loads from one level to the next.Using the selection tools on the left-hand toolbar allows you to apply the surface loads to a group of plates at the same time. Just select the lower row of plates by unselecting the entire model and selecting the plates using the Draw a Box selection tool .Apply the first magnitude of Surface loads to the selected plates, making sure to select Apply Loads to Selected Plates.Unselect the entire model, and use the Draw a Box selection tool to select the next row of plates, and apply the loads next magnitude of loads to this level. You can continue this method all the way up the model.
Users should note that the load should be the average load that is present on the plate.
Loads for tanks:
Another useful tool present in RISA-3D is the rectangular tank generator. This gives the option of loading the tank with an equivalent fluid pressure just based on the density of the contained fluid.
Of course, if you have something on the inside of an underground tank, you can use a generic density (say 1pcf) for your contents. You can then copy this basic load case into other basic load cases for your soil or other tapered loads that are present.
Once in separate basic load cases, you can multiply the applied loads by their respective factors to represent the equivalent fluid pressure of the soil or contents. Your soil multiplier will have the opposite sign from the multiplier for the contents on the inside of the tank.
From there, you can apply all of your load factors for each load combination. You may likely need a case for an empty tank and one for a full tank with no backfill (leak test scenario). You will have even more combinations if your tank is partitioned and each can be full or empty at different times.
Tips for tapered loads for walls:
A fast way to apply a tapered load to a wall is to create a rectangular tank with the auto-generation tool, and then delete the other three sides.