Indeterminacy is a condition of certain structures where some or all of the forces in them can't be determined. This happens to any single point when there are more than two forces of unknown magnitudes leading into it. In a structure, it's OK for some joints to be indeterminate at the beginning, just as long as there's at least one joint that you can start at. As you saw in the last section, Putting It All Together, unknown forces will become known forces as you solve more joints. In determinate structures (where you can find all the forces), all joints will eventually become determinate by solving the other joints in the structure. You only have a problem when you come to a situation where you've solved all the determinate joints and there are still indeterminate ones left.

Right now, you may be despairing that all your structures are going to be afflicted, and you'll never fix them. The second part of that statement is wrong (sorry, I can't say about the first), since indeterminacy can be cured without seriously reworking a structure. As opposed to the more technical definition given in the last paragraph, there is another, more common sense definition. Indeterminacy is caused when there are more supporting beams or forces than the structure needs to stay in equilibrium (upright, or however else you want it to stay).

A common example is a simple square, strengthened by two cross-pieces that don't touch or connect (notice the lack of a pin holding them together in the drawing at right). This structure is indeterminate, because only one cross-piece is needed to make the structure stable. The second piece is only redundant. The indeterminacy could also be shown by applying a force to it and working it out yourself or inputting it into our web programs. If you run up against indeterminacy either on your own or when using our programs, make a check for redundant beams.

Another case contributing to indeterminacy are joints with only two beams that aren't lined up (so they aren't forming a perfectly straight line) leading into them. It's ok to have these, (really!) but only as long as you have an external force acting on them (like the load or one of the supports). Otherwise, both of those two beams will end up having to have a force in them with magnitude zero. Why? Because there's no way that one of the beams could resist a force exerted by the other one. It would just move along with it, so it wouldn't be in equilibrium. Think of sticking your two index fingers tip-to-tip. Exert a force on one finger and the other one moves (you're not allowed to tighten up your knuckle, all joints have to rotate freely, remember?). The only way to keep your fingers from moving would be to have someone put their finger (an external force) pushing down from on top. You yourself can recognize this condition in a structure and correct for it, either by removing the beams altogether or pre-setting the magnitudes of the forces in them to zero.

At the other end of the spectrum is a condition where there are too few beams. Now, the structure won't be stable at all. This condition can come about when you don't build with triangles, and instead have squares (or worse!). Remember, all joints can move freely, so there's nothing holding a square from collapsing. If you're figuring out the forces in a structure by hand, you'll find that you come to a joint where there is a force acting in one direction, and no way to counteract it.

Hey, this is it, you're done. Hopefully you now have the background knowledge for building strong, long-lasting structures. We haven't even started to cover what kind of design makes a structure actually strong. Can you guess why? Because that part's up to you to find out yourself. That's why we have the web programs for you to fool around with. Get workin'! If you feel unexcited about going out there and achieving this task or would like a motivational speech inspiring you to design amazing structures using our web programs please send email to Jon (inhouse pep-talk dude)... please title the message "Pump me up Jonny Boy!"
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