56 thoughts on “Homework 23.B”

    1. Bridget, you have enough information to calculate the angle formed by AMC. Once you have that angle, you can find the perpendicular distance from L to MJ. Hope this solves your question.

    1. I did not assume that MK is perpendicular to LJ, but I did get that LJ is a zero force since there are only 3 forces at L, and ML and LK are colinear, which means that LJ is 0.

    2. Hi Kate. Point L actually forms a Y joint which acts the same as a T joint, just instead of the 0 force member being perpendicular it is at an angle. Because we know the angle is not 0, the member LJ must carry no load so that the forces balance. Hope this helps!

    3. Hi Kate, you do not need to determine whether LJ is perpendicular to segment MK because LJ is the non collinear member of a Y-joint. This means that LJ will be a zero force member. Hopefully this is helpful.

    1. I draw the forces pointing away from the point of interest (tension) and then plug in all the calculated values, without changing the sign. This should keep all the signs in order.

    2. There are multiple ways to go about approaching this problem, however, I would probably draw the forces pointing in one direction, in order to keep the signs consistent throughout the remainder of the problem.

    1. You need angle IED if you want to find the horizontal and vertical components of the load in EI. This is the angle made by the member with the horizontal line, and hence components would be dependent on this angle and not angle IEH. Hope this answers your question.

    2. My guess here is that by finding angle IED, you can then use geometry more effectively to find angles such as angle IEH, and angle EIH. I believe this also allows you to find and equate angles DEC, HIE, JKI, and so on, going up the structure.

    1. You do not need to solve for the reactions and you will only need to use the loadings at the top at point V to be able to solve for the members we are looking for

    1. If you take the upper part of the truss it might be easier as you don't have to find the reaction forces. However, if you take the lower part then you will have to find it.

    2. I was able to solve the problem without solving for the reaction forces. I suggest making a cut through JH, HK, HI, and IE. This will expose 4 members, some of which are zero force members. So after you find which ones are 0, you can just solve for the rest using moment equations or sum of forces equations. Hope that helps!

    3. I solved it without the need for reactions, if you take the FBD cut to not include the points of reaction, you should not need the forces. I have found this is usually the easiest way to solve the problem.

    1. As stated by others, I do believe that it is your choice to include the (+/-) signs or to just list the magnitudes and state the nature of the forces. Personally, I just list the magnitudes of the forces and state whether they are in tension or compression.

  1. When solving these sorts of problems does it make a difference if the problem asks for the load at IE or EI? I know in other contexts, the order of the letters signifies the direction of the force so therefor IE=-EI but is that the case here as well?

    1. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

    2. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

    3. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

    4. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

    5. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

    6. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

    7. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

    8. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

    9. The order doesn't matter in this question in my opinion. Naturally, you find all the zero-force members before cutting through the leftover forces you have to find. What matters is the direction you point those forces in when you attempt to find them. If you point them away from the FBD and the solution is positive, it is a tension force and vice versa.

  2. For our work and calculations for the problem, are we required to show calculations for the zero-force members? Or can we just state that we visually "performed" the method of joints and found which members were zero-force members that way?

    1. I didn't show the work out for the zero-force members but I did give the reason why the members were 0. For example, member LJ is only connected to two other links, so it is a zero-force member.

  3. By using trig you can solve for the top angle. And then by similar triangles you can solve for the length of HI. This then allows you to find the moment easily.

  4. In this case, would we need to consider the reaction forces at A and C? I could obtain my answers without them; however, I am unsure. Can someone please explain this?

    1. Hi Dominic. I think that is a safe assumption to make that we would need to find at least a couple of the reaction forces in order to solve this problem. For my solution, I ended up not needing them in the end, it really just depends on where you place your cut when solving with the method of sections.

      1. From my professor, it seems that you can almost always (in this course) find a way to go about solving without needing to worry about the reaction forces. For me and others that I have talked to, they weren't needed.

  5. How do we use trig to find angle HIE? I've seen suggestions to solve the top angle so for that do we awesome like KJ is a length of 2d? I think maybe I am just overthinking this.

    1. Hey Hannah! If you want to solve for HIE, try solving for HIK first. Since all of the lines inside of the entire system (aka a giant right triangle) are parallel, you can assume that HIK is equal to ACE. Using the given vertical and horizontal lengths of the entire triangle, you can solve for ACE which ultimately gives you HIK. From there, you should be able to find HIE. However, when I solved this problem I didn't need HIE - only HIK.

    2. Alternatively, if you do not want to get involved with angles in your solution process, you can use similar triangle proportions to find the length of moment arms. This will allow you to find the value of HJ using moments. Afterwards, you can solve for the x and y values of EI using the sum of forces in the x and y directions, then you can calculate the magnitude of EI using the Pythagorean theorem.

  6. For this problem I just want to help people out by saying that this whole truss system is a giant right triangle. So you can find the angle of that giant triangle, and the angle will carry over to smaller triangles within the system. Use can use this hint to help solve for the y component of Force IE.

    1. Hi Andrew. I suggest you show the force at each joint and then also include a x y diagram showing their directions. That's what I did if they were obvious. If not, maybe show the sum of forces in x or/and y. Hope this clarifies things.

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