Truss Analysis: Method of Joints

• JordanIV
In summary, the conversation discusses a truss analysis by method of joints, where the forces at joints "A" and "G" have been determined and the focus has now shifted to determining the loads at each joint. The angle at joint "A" has been calculated to be 63.34° for a 10' member and 53.13° for a 15' member using inverse tangent. The calculations for joint "L" are believed to be correct, but there is uncertainty about the symmetry of the truss and whether the forces at "L" and "A" are equal and opposite. The expert confirms that the forces in the truss are symmetrical and that the angle at "L" is the same
JordanIV
I'm doing this truss analysis by method of joints and I have determined the forces at "A" and "G" and have moved on to determine the loads at each joint. I have determined the angle to be 63.34 for the 10' and 53.13 for the 15'(I took the inverse tangent of 20/10=63.34 and I took the inverse tangent of 20/15=53.13). As you can see I have calculated the forces at joint "A" and i then moved on the joint "L". For Joint "L", I feel that I have made a mistake somewhere and this may have made my other calculations wrong. I want to know if my calculations made at "L" are correct. Also, I believe that "AL" would actually be symmetrical to "KL", but am not sure of that. The truss is also in symmetry so the other side will just match up with the other side.

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Your calculations for joint "L" appear to be correct. The forces in the truss are indeed symmetrical, so that AL = KL. That means that the forces at "L" will be equal in magnitude but opposite in direction. Thus, the force at "A" will be equal and opposite to the force at "K". The angle at "L" is the same as the angle at "A" (63.34°) since they are connecting the same two members.

1. How do you determine the reactions at the supports in a truss?

To determine the reactions at the supports in a truss using the method of joints, you must first isolate each joint and draw a free body diagram. Then, apply the equations of static equilibrium to solve for the unknown forces at each joint. The sum of the forces in the x and y directions must equal zero, and the sum of the moments about any point must also equal zero.

2. What is the difference between a zero force member and a redundant member in a truss?

A zero force member is a member in a truss that does not experience any internal forces, meaning it is not necessary for the stability of the truss. A redundant member, on the other hand, is a member that can be removed without affecting the stability of the truss. Redundant members often have zero forces, but not all zero force members are redundant.

3. How do you determine the forces in the members of a truss using the method of joints?

To determine the forces in the members of a truss using the method of joints, you must first draw a free body diagram of each joint and solve for the unknown forces using the equations of static equilibrium. Start by assuming the direction of each unknown force, and then check your answer by making sure the sum of the forces at each joint equals zero.

4. Can the method of joints be used on any truss?

Yes, the method of joints can be used on any truss as long as the truss is stable and determinate. A stable truss is one where all the joints are connected by at least three members, and a determinate truss is one where the reactions and forces in all the members can be determined using the equations of static equilibrium.

5. Are there any limitations to using the method of joints for truss analysis?

One limitation of the method of joints is that it can only be used for trusses that are in equilibrium. If the truss is not in equilibrium, additional methods such as the method of sections or the method of joints and sections may need to be used. Additionally, the method of joints may become more complicated and time-consuming for larger and more complex trusses.

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