We know that whenever force is applied on a cross section or beam along its axis, it either tries to compress it or elongate it. If applied force tries to compress the member force is known as compressive force as shown in fig (a). If force applied on member tries to elongate it, force is known as tensile force shown in fig(b)
If a compressive force is applied on the member as in fig.(a), the member will always try to resist this force & a force equal in magnitude but opposite to direction of applied force will be induced in it as shown in fig(c), Similarly induced force in member shown in fig(b) will be as shown in fig.(d)
From above we can conclude that if induced force in a member of loaded truss is like the fig(c) we will say nature of applied force on the member is compressive. If Nature of induced force in a member of truss like shown in fig(d) then we can say that Nature of force applied on the member is tensile.
