Calculating truss forces
A truss is composed of slender members joined together at their end points. A simple truss will be composed of triangles.
Steps to calculate
Is it statically determinate?
To find out if a truss is statically determined, use the equation 2J=M+R where R represents the number of reaction forces, J represents the number of joints, and M represents the number of sides/members. A roller joint has has one reaction force and a pin joint has two. The truss in the picture has 5 joints, 7 members, and 3 reaction forces. If you plug these numbers into the equation, you will find that the truss is statically determined. If it wasn't statically determined, you wouldn't be able to continue solving any farther.
Find the angles
All of the triangles in this truss are the same size, so finding the angles wont be hard. The triangles are right triangles so we can use SOACAHTOA to find the remaining angles. If we chose to find angle E, we would know all of the other angles in the truss. To find angle E, you will need to find the tangent. The tangent is equal to opposite over ajacent or 6/5. The angle with be the number you get when you find the inverse tangent of 6/5. Angle E will be 50.2 degrees. This makes the remaining angle 39.8 degrees.
Find The sum of the moments
After you draw your new truss with labeled angles and reaction forces, you will sum the reaction forces. Pick a point (perferably a pinned point) and use the equation EM=O (the sum of the moments are equal to zero). Remember that force*distance equals a moment. A clockwise rotation is a negative moment and a counterclockwise rotation is a positive moment. By doing all of this, we can find that the force of FAY is equal to -250 lbs. In order to find the remaining reaction forces, you will need to find the sum of the forces in both the x and y direction. The forces in the y direction on our truss are -500, -250(FAY), and Fey. If you add these together and set them equal to zero, Fey will be 750 lbs. FEX is the only force in the x direction so FEX equals 0lbs.
Finding member forces
Draw a free body diagram for the truss and label all known forces and angles. Assume tension for all members. Pick a joint to work with, preferably one you know a lot about. You will need your EFx=0 and EFy=0 equations again. You will solve these members in the same way you would solve force vectors. You will need to know your angles and use SOACAHTOA to find each member's force. If you complete all of these steps, you will be able to calculate the entire truss.