Stresses that act on your structure

*Stresses that act on your structure

What are the stresses (forces) that are acting on your structure?

Compression Compression Tension Shear
Compression forces try to crush things Compression forces also try to buckle (or bend) things. Tension forces try to pull things apart. Shear forces try to break things.


Typically, columns are always in compression. The columns are put in compression as the pressure board and weights are added on top of the structure. Braces can either be in compression or tension depending on the structure design. Braces are used to resist compression in columns.

Shear is difficult to explain. The technical definition of shear is the internal stress in a beam that is a result of non-axial loading. (Yeah…..I am sure you got that). Imagine holding your hands together upright in a prayer position and then sliding one of them down. (Thanks to Ernie Change for the definition and the visual demonstration). This is shear. Shear can occur at different places in the structure depending on the design. Most teams that solved the challenge DIsigning Bridges had to deal with shear in the bridges they constructed.

Another force that acts on a structure is a rotational force (or twisting). Technically it is called torsion or torque. This rotational force is a result of forces being transferred from the vertical load (i.e. the weights being placed) to a slightly horizontal load. Many teams structures fail because they do not adequately control the tendency of the structure to twist or rotate.

Click the link on the right for a short video of a structure twisting to failure. We took this video during the Triplicity challenge. We modified our structure tester so we could test the smaller individual pieces of the three part structure. I was using a strip of paper wrapped around the columns to resist twisting. As you watch the video you will see a bend form in the paper which affected its ability to resist the twisting forces and the structure twists down. Notice that the columns did not crush. This structure failed because it was not braced sufficiently to stop the rotational forces acting on it. VIDEO
(1.4MB MPG file)

The types of stresses or forces that act on your structure (be they compression, tension, shear) fall into two general categories. “Static” forces and “dynamic or live” forces. Static forces are forces that do not move. Once a weight is placed on a structure it is a “static” force or load because it is not moving. If you were to climb on top of the pressure board and do a dance this would be a live load because you are moving. The initial placement of a weight on a structure places a dynamic load on the structure because it is in motion.

When designing a structure, the team should concentrate on carrying the static loads placed on the structure while keeping in mind that they must also deal with a certain amount of dynamic or live load during weight placement.

Most structure challenges involve testing a static load on the structure by placing weights. The only dynamic load placed on the structure is the dynamic load created by weight placement.

Years ago my brother’s team competed in a couple of structure challenges that required placing dynamic loads on the structure by: 1) applying a twist to the structure as certain increments of weight placement were reached and 2) by rolling pool balls down ramps into the sides of the structure at certain weight placement increments. I have never competed in this type challenge so have no experience in dealing with these types of forces.

* Copied from: “Diary of a Balsa Goddess”  ©2009 Heather Compton

Diary of a Balsa Goddess by Heather Compton is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.

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