Analysis of AB bar: AB is balanced by two forces, hinge A and hinge B. According to the balance condition, these two forces must be equal in magnitude and opposite in direction, so the force of hinge B on AB must be along the direction of BA, and the force of AB bar on BC bar should be along the direction of AB.

With C as the fulcrum, it is assumed that the force of AB rod to BC rod is F', which is obtained by moment balance.

F 12BC？ sinC = F′？ ACsinA

SinA=sinC again

Get f' = bc2aacf =12f =18n.

For BC rod, there are three forces: the force F' of F and AB rod and the force of hinge C. According to the equilibrium condition, the force of hinge C on BC is equal to the resultant force F and F' and opposite in direction. If f increases, it can be seen from the above formula that the force F' of AB on BC also increases, and the increase times are the same. According to the parallelogram rule, it is known that the direction of the resultant force of f and F' is unchanged, so the hinge C.

So the answer is: 18, unchanged.