For the plane truss shown in the figure, the number of zero force members for the given loading is

A three hinged parabolic arch $$ABC$$ has a span of $$20$$ $$m$$ and a central rise of $$4$$ $$m.$$ The arch has hinges at the ends and at the center. A train of two point loads of $$20$$ $$kN$$ and $$10$$ $$kN$$, $$5$$ $$m$$ apart, crosses this arch from left to right, with $$20$$ $$kN$$ load leading. The maximum thrust induced at the supports is

The unit load method used in Structural analysis is

For a linear elastic structural system, minimization of potential energy yields