Engineering Statics

D-19


Determine the force in members EG and EF of the truss shown when P=20 kips.

A FBD of the truss is drawn in order to solve for external forces.  By 
symmetry each reaction is 50 kips.
A BBD is drawn of each side of the cut through DF, EG and Ef. 
This allows both required members to be exposed.  By taking 
moments about specific points, solving for the force in DF, 
a member cut through but not required can be avoided.
Taking moments about point F allows us to set up one equation 
with only one unknown.
Solving allows us to find EG.  Not that EG is in tension. 
In trusses usually forces in members at the bottom of the 
truss are in tension.
Using the left FBD we take moments about A.  Again we avoid 
DF and have EF as the only unknown.
EF is in compression.