Solving Truss Force Problems: Step-by-Step Guide and Example Calculation

[xzibition8612]

Homework Statement

See attachment.

Homework Equations

Sum of Fx, Fy, and Moments=0

The Attempt at a Solution

A and B are pins, so there is a x and y component at each of these points. Cable has same tension on either side, 1200. Take the moment at A to be zero: (-1200)(12)+(5/13)(1200)(12)+(-12/13)(1200)(7)+Gx(10).

Gx=1661 lb
1661+Ax=1107, Ax=-553 lb so Ax points to the left.

How do I find Ay and Gy? Then how would I find the force in each member? Thanks.

 

[Q_Goest]

Hi xzibition8612,
All of the points A through G are assumed to be pinned, so all of the straight elements connected to those pinned joints can only be in compression or tension. In other words, the direction of the elements is also the direction of the force, either compression or tension. There can't be any moments applied on any of the pins, just forces in the direction of the elements. So if you found the force on point G in the X direction, then you can easily calculate the force on point G in the Y direction, because the angle that the element that extends between G and F is known from the geometry they give. Just calculate the angle and you find the force on G in the Y direction. The force Gx and Gy should result in the element being in either compression or tension. Does that help?

 

[pongo38]

As it happens, you CAN solve this problem without getting the reactions first. It all depends on you isolating an object for which equilibrium statements can be made. Your choice of object will determine whether the problem is solved easily by hand. Can you identify, by inspection, any members that may be carrying a zero force?