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Homework Help: Statics Problem - Truss

  1. Jun 24, 2008 #1
    Im doing this problem as a part of the experiment for a Basic Roof Truss. I first started off measuring the deflection for each member of the roof truss (member dismantled from the roof structure) by applying certain amounts of weights. Next I put all the members up into a roof structure as shown in the picture with 60 degrees at each vertex. Im required to find the forces in each of the member thereotically to compare with my experimental results. Im having problems finding the forces in each member thereotically.

    The attempt at a solution
    In the second diagram,I started by using the parallelogram method and sine rule to find the magnitude of force in member A and B. As a result, Fa and Fb has the same magnitude. Then by using Fa , I used sine rule to obtain Fc as shown in the second diagram. The magnitude of forces i obtain each are about 50% error.
    The results i obtain in my experiment shows significant different amount of forces in each member.

    Could someone please show me the way to calculate the forces in the members? Thank you in advance.

    Attached Files:

  2. jcsd
  3. Jun 25, 2008 #2


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    Yes, Fa and Fb should be equal (in compression), and using the properties of a 30-60-90 triangle, Fc should be 1/2 of Fa (in tension). I don't know what numbers you're getting, and what you are comparing it to in your experimental results. You seem to mention deflection measurements under some load applied to the dismembered members? I don't understand. For theoretical load calculations, you might want to google on "truss method of joints".
  4. Jun 25, 2008 #3
    Yep that was exactly what i got. Fa and Fb as equal and Fc is half of Fa. The deflection measurements are from the dial gauges installed in each member. When they are hung vertically with a load at the bottom, the members tend to stretch and the small amount of stretch is measured as the deflection. Anyway what i go for my experiment shows that Fa and Fb is different as well as Fc. And their difference are by a lot.

    Thank you for your help. Appreciate it.
  5. Jun 26, 2008 #4
    Aren't the members of a rigid truss are not supposed to show deflection?
  6. Jun 26, 2008 #5
    This will make your problem much more difficult.

    At first you should ignore this and come up with a rough estimate. Then complete the calculation with the deflections which is tedious. The finite element method is a structured method to solve for the forces in a elegant and structured manner.
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