by Mike-SE » Sun Sep 15, 2013 7:00 pm
the force is 12 kips.
For everybody, this could be on the test. You MUST be able to solve this problem in 30 seconds or less.
Loading is symmetrical (As will almost all truss problems on the exam)
Reaction each end is 15 kips.
Vertical component of member FD is 15 kips
Triangle CDF has ratio of V:H of 15:12. Vertical is 15, therefore horizontal is 12 kips
Look at joint F, only member EF has a horizontal force. Therefore, force in EF MUST equal 12 kips.