Problem statements
Solution video - H23.A
Solution video - H23.B
DISCUSSION THREAD
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HOMEWORK 23 - SP 25
12 thoughts on “HOMEWORK 23 - SP 25”
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Is the length of JK also 4d? Is there any way to solve for that or is it not even needed
I don't think you need JK length since you only have to find zero forces and forces in segment 1, 2, and 3.
How do we solve for Iy and Qy? I think we need them to find HQ and onward. Whenever I try to use moments and sum of y forces I can't get an answer.
The method I used was using the moment equations to find their general relation, and then using equations from joints Q and I, I solved by substitution. This allows you to get equations in terms of one unknown and then solve them. With this information, I moved from joint to joint until I had solved for all the relevant information, making sure to choose the less complex joints by accounting for zero-force members.
I found the moment about N and drew a FBD for Fx and Fy because that point gives us the most relevant forces to find members 1, 2, and 3. I also found that finding the zero-force members first helped me get rid of the members I didn't need to use. The HNO triangle is a perfect triangle so that helped me to find the angle theta.
You actually don't need to solve for them if you use method of sections from the left hand side.
I solved using the method of sections from the left hand side and that seemed to work. Just to confirm, member 1 would be in tension, member 2 would be in tension, and member 3 would be in compression, correct?
I approached this problem by first determining the zero force members, then making a diagonal cut through members 1, 2, and 3. By employing the method of sections and viewing the left side of the truss, there is no need to solve for the reaction forces, and the forces in members 1, 2, and 3 can be found by summing the forces in the x and y and then summing the moments about either N or H.
I agree with the method of using method of sections from the left, this greatly simplified the system. In regards to finding where the zero load components were, I looked for all of the T and Y joints. Once you identify that some components are zero load it leads to other ones becoming zero load as well. Also, be sure to account for elbow joints.
It occurred to me while solving this problem that if you choose your sections wisely, the internal reaction forces do not matter. To solve for 1-3, you should create a section in which those three forces act and the given forces act.
Does the method of sections apply to this as well or is there an easier way to do this
Yes you can definitely use the method of sections here, and I personally used it too. Which "section" you use is completely up to you, but I used a section where I only needed to short FBDs to solve for 1, 2, and 3 fairly quickly. I'd also recommend choosing a section that's near the middle or the right.