Method of Joints Query: Shedding Light On Truss Calculations

In summary, the conversation discusses the topic of method of joints and using the Skyciv truss calculator for analysis. The speaker encounters a problem when adding additional members to the truss, which results in internal static indeterminacy. This means that the truss cannot be solved using joint equilibrium equations alone and may require computer aid. The issue is further explained by the arrangement of the additional members and their impact on load distribution.
  • #1
tina-duncan
13
0
Hello all,

I have recently been attempting to refresh my memory in regards to method of joints and have been making some reasonable progress, however I have ran into a stumbling block that has left me quite puzzled.

I have been using the Skyciv truss calculator to verify my work and for the 1st analysis shown below I am fine and have no problems working out all the forces. However, in the 2nd analysis when I add further members, I cannot understand how the force values in each member are the values stated. Supports are at A & B, with an external force applied to D at 45 degrees.

Could someone possibly shed some light on how this works please?

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  • #2
Looks like you've introduced internal static indeterminacy by adding in those members, such that you cannot solve the truss using the joint equilibrium equations alone. Often these extra members carry no load when added outside the original load/support 'envelope', but apparently in this case because of the member arrangement there are some small forces introduced based on joint and member displacements ( if FC was not there, the other additional members would not carry load). Difficult to solve indeterminate trusses without computer aid.
 

1. What is the method of joints and how is it used in truss calculations?

The method of joints is a technique used to analyze the internal forces in a truss structure. It involves breaking down the truss into its individual joints and analyzing the forces acting on each joint. This method is used to determine the internal forces, such as tension and compression, in each member of the truss.

2. How do I identify the reactions at the supports when using the method of joints?

To identify the reactions at the supports, you must first draw a free-body diagram of the entire truss structure. Then, use the method of joints to analyze the forces at each joint. The reactions at the supports can be determined by applying the equations of equilibrium to the entire truss structure.

3. What are the key assumptions made when using the method of joints?

The method of joints assumes that the truss structure is in equilibrium, meaning that all forces acting on the truss must balance out. It also assumes that the joints are connected by frictionless pins and that the members of the truss are connected by rigid joints.

4. Can the method of joints be used to analyze any type of truss structure?

Yes, the method of joints can be used to analyze any type of truss structure, as long as it is in equilibrium and the key assumptions are met. This method is particularly useful for analyzing simple truss structures with a small number of joints and members.

5. What are the limitations of the method of joints?

The method of joints is limited to analyzing truss structures that are in equilibrium and have a small number of joints and members. It also does not take into account the weight of the truss structure itself, which can affect the internal forces. Additionally, the method of joints may not be suitable for truss structures with complex geometry or variable loading conditions.

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