2.1.7(ish) - Calculating Truss Forces

An example of solving forces on a truss. Similar to 2.1.7 - Calculating Truss Forces General Steps: 1. Check "2J = M + R" to make sure it is solvable 2. Replace pins and rollers with reactionary forces a. Usually draw arrows into the joint - if it ends up being negative, reverse the arrow 3. Choose the pivot point (usually the pin) 4. Solve reactionary forces a. List ccw and cw moments b. ∑Moments = 0 (solve reaction forces) c. Update diagram d. ∑Fy = 0 i. List down & up forces ii. Put all downs on one side of =, and ups on the other iii. Update diagram e. ∑Fx = 0 i. List left & right forces ii. Put all lefts on one side of =, and rights on the other iii. Update diagram 5. Solve members a. Redraw the diagram with only forces. Assume all members in tension (arrows point away). b. Isolate a joint (choose one with the least unknowns) c. Draw the free body diagram of the joint (resolve angled forces into Fx and Fy) i. Use "θ = tan-1 (Opp/Adj)" (with distance) to solve for θ if necessary d. List ALL X & Y forces (with direction) in a T-chart i. Use trig to set x and y variables in terms of the member name (e.g. "ABx" might become "AB(Sin θ)") e. Choose X or Y (depending on which has the least unknowns) and solve for the unknown f. Sum 1 of the forces = 0 (similar to step 4d and 4e) g. Sum of the other forces = 0 h. Update the drawing Isolate the next logical joint, and repeat steps