Activity Overview
In this simple activity, students get a chance to feel the forces in a circular tank or silo. They learn why many vertical tanks and silos have added circumferential reinforcing along their lower sections.
Learning Objectives
By the end of the activity, students should be able to:
- Describe in general terms how the lateral forces in a vertical tank vary with depth
- Explain why the lateral forces exerted when the tank diameter is allowed to increase are smaller than the forces needed to push the tank wall back together
- Report where additional circumferential reinforcing is often found in real tanks or silos and why it is at that location
Tips for Using the Activity
Students should be encouraged to enjoy the Activity, and especially to feel the forces that are produced in the model.
They should not be rushed or under pressure to obtain the “correct answer” or a high grade.
Geological students should be reminded that soil behaves much like the marbles used in this Activity
Equipment Design
We considered many designs for this Activity. As usual, we wanted the students to be able to feel the loads. Three circumferential segments of 4” ABS drain pipe were found to work well as a tank, and to produce adequate forces. Alignment of the tank wall segments was initially problematic, but addition of a base disk solved this problem.
Loading was provided by marbles, but one could presumably use pea gravel (preferably non-crushed so that individual stones have a strong convex shape) or grain instead of marbles. We tried using water in a bag, hoping that its reversible deformation might provide a contrast to the irreversible motions of the marbles (due to friction), but it was messy when the water invariably spilled, the bag extruded between the wall segments and got trapped there, the loads were smaller and the reversible deflection that a fluid theoretically provides was difficult to detect experimentally.
Notice that much of the same equipment is used for the Tanks, Culverts, Dams and Retaining Wall Activities.
Additional Information
See the Downloadables section for more information on this activity.