Fig. 3.1 shows most basic triangular truss. Members are connected by pin-joints, which arrest translation but not rotation. Each member has three degrees of freedom in a plane, two translations and one rotation. Total degrees of freedom are 9. Each pin joint arrests two degree of freedom. Hence, degrees of freedom of pin-joint connected structure is 3. For keeping the structure stationary, these three degrees of freedom should be arrested. In the figure, left fixed support arrests two degrees of freedom, whereas in the right, the roller support arrests one degree of freedom. Thus the structure cannot move and the structure is called stable. This type of structure is also called rigid structure .

Fig 3.1: A 3-member truss

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L3 : Trusses
A framework composed of members joined at their ends to form a structure is called a truss. Truss is used for supporting moving or stationary load. Bridges, roof supports, derricks, and other such structures are common example of trusses. When the members of the truss lie essentially in a single plane, the truss is called a plane truss .