Question

A gate in a salt water canal is hinged at A and is held closed by the pin-connected assembly CDF. (SEE FIGURE)

Answer 1

First, the hydrostatic force can be reduced to a single equivalent force by integrating hydrostatic pressure over the depth of the salt water. The magnitude is Fh and should be applied at a point between A and B such that it gives the same moment as the hydrostatic pressure, kind of finding the centroid of the hydrostatic pressure. Call this depth d.

Answer 2

For the two arms CD and DE, they are two force members, therefore forces are axial.
By symmetry, each will have an axial force of Fs (s for strut, since they are both in compression).
Consider member AB, hinged at A.
Fs and Fh create opposing moments about hinge A, such that the sum of moments =0, or
Fs*((1.1+0.4)cos(20)=Fh*d