14 thoughts on “HOMEWORK 22 – SP 25”

  1. I found that this problem is very similar to the example and quiz problem we did in class. My approach to the problem was to cut the truss vertically and use the right side since it contains fewer external forces.

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    1. I solved this problem the same way. It was very helpful to use the method of sections and use the right side. This greatly simplified the equations needed to solve for the requested forces.

    2. Yes, this is the same method I took to solving the problem as well. I first used moment equations and xy force equilibrium equations to solve for the reaction forces, and then cut the truss vertically to focus on the right side of the truss.

  2. I’m not sure what I’m doing wrong, but when I try to solve for the reaction forces, they don’t end up cancelling correctly and I cannot solve them; when I substitute the reaction forces into each other, I keep ending up with the same equations.

    1. It could have to do with the place that you cut the truss into sections. I found that the only place that was good to cut was directly along the number of what I am trying to find as in the 1,2,3. What I did wrong at first was label the distances wrong and flipped the horizontal and vertical distances for the lines that could be the problem as well.

    2. When solving for the support reaction forces, try to create force and moment equations that allow you to express supports in terms of P. By doing this you can create relations between the support forces and the load across members 1, 2, and 3 when creating the section force and moment reactions.

  3. I recommend using the sum of the moment equation and sum of the forces equation in a strategic way in which you don’t have to solve coupled equations using substitution, which may be causing you this error. For example, you could sum the forces in the y direction to isolate member two from the other two unknown members. You can use this similar strategy on the other members in order to minimize error chance.

  4. I’m not sure if I’m doing it right or wrong, but when I solved for the load members 1, 2 and 3, I got zero load for all three. I got the reaction forces and then I took the moment from point B, H and K and solved for the loads.

    1. I got nonzero values for the loads carried by 1, 2, 3. I would check that you solved correctly for Ay, Kx, and Ky. In addition, make sure you account for the P force acting downward on the truss (assuming you made a section by splitting the truss at 1, 2, and 3 and using the right side). You should have Ay and the force P at B in some of your moment equations.

  5. For this problem, I found that a vertical cut through 1, 2, and 3 and taking the force and moment equations. This minimizes the external forces overall simplifying the problem

  6. When looking at problems such as these, I have found a helpful approach to be keeping track of unknowns through the problem. If you can find an equation that allows those unknowns to be solved for you have a good idea that you are on the right track to solve the problem.

  7. For this problem, I first started taking the sum of all forces in X and Y then took the moment about A to find the reaction force Ay. Afterwards, I made a vertical cut just a little to the right of ED. With that section, I took the sum of forces in X direction then took the moment about B before finally doing the sum of forces in the Y direction to find the load carried in 1,2, and 3.

  8. For this problem, I use a moment equation and both SUM Fx=0 and SUM Fy=0 on the entire system first to solve for all reaction forces. Putting the moment equation on a support point is best so that reaction forces can be ignored during the moment calulation. I chose to start joint analysis at Joint A because it is the closest joint to the three members and I went from A to adjacent joints finding loads at all members touching each joint. I only moved to a joint when I had 1 unknown remaining in the y component on that joint and the other y components I knew, or the same thought process for a single unknown x component.

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