Statics: A truss structure is used to hold power lines as illustrated

A truss structure is used to hold power lines as illustrated. The power lines provide forces skewed to the tower. Calculate the reaction forces at the base of the tower.


Solution

↻+ (clockwise-positive)
→+ (right-positive)
↑+ (upward-positive)

→+ΣFx = 0    assume Ax is postive
Ax - 300sin30º + 400sin25º = 0
Ax = -19.05N
Ax = 19.05N (←)

↻+ΣM_A = 0
-By(4) + 400cos25º (4+2) + 400sin25º (10) -300cos30º (2) -300sin30º (10)=0
By = 461.50N


↻+ΣM_B = 0    assume Ay is positive (↑+)
Ay(4) - 300cos30º (2+4) - 300sin30º (10) + 400cos25º (2) + 400sin25º (10) = 0
Ay = 160.83N