How Do You Determine Forces in a Truss Structure?

[rikka22]

Homework Statement

Determine the forces in each member of the truss shown. Also find the values for any reactant forces

http://img201.imageshack.us/img201/6853/trussnu9.jpg

Homework Equations

F_x= 0
F_y= 0

The Attempt at a Solution

I set θ in the corner of J₁
tan θ=12/16
θ = tan^-1 12/16
θ = 36.87

I set Φ in the corner of J₃
tan Φ = 12/9
Φ = tan^-1 12/9
Φ = 53.13

This is where I began to falter. I could not figure out how to diagram everything out. If someone could just at least give me a diagram of where things are supposed to go, I can sum up the x and y forces by myself. This is pretty urgent, and I was absent for this class so my only help is messy notes from another student. This isn't in my textbook. Any help will be appreciated.

 

[chaoseverlasting]

You want to resolve the forces. If a force $$F$$ makes an angle $$\theta$$ with the horizontal, the horizontal force is $$F_x=Fcos\theta$$ and the vertical force is $$F_y=Fsin\theta$$.

Give us something to work on and we'll lead you on.

 

[thomate1]

I would first start by identifying the forces acting on the truss. These include the external forces (such as the applied load and any support reactions) and the internal forces within each member of the truss.

Next, I would draw a free body diagram of the entire truss, showing all the forces acting on it. This will help me visualize the problem and determine which equations to use.

To find the forces in each member, I would use the method of joints or method of sections. The method of joints involves analyzing the forces at each joint of the truss, while the method of sections involves cutting the truss into smaller sections and analyzing the forces in each section.

For the method of joints, I would start by analyzing the forces at the joints J1, J2, and J3. I would draw a free body diagram of each joint, showing all the external and internal forces acting on it. Then, I would use the equations Fx=0 and Fy=0 to solve for the unknown forces.

For the method of sections, I would first choose a section of the truss to cut, such as the section between J1 and J2. Then, I would draw a free body diagram of the section, showing all the forces acting on it. Again, I would use the equations Fx=0 and Fy=0 to solve for the unknown forces.

In both methods, it may be helpful to label each member with a letter (such as AB, BC, CD, etc.) and assign a direction for each member. This will help keep track of which forces are positive and which are negative.

Once all the forces have been determined, I would check for equilibrium by making sure that the sum of all forces in the x-direction and y-direction are equal to zero.

In terms of reactant forces, these would be the support reactions at the joints J1, J2, and J3. These can also be found using the equations of equilibrium and the method of joints or method of sections.

Overall, the key to solving this problem is to carefully analyze the forces and use the appropriate equations and methods to solve for the unknown forces.