Pinjointed

Pinjointed refers to a structural system in which the connections between members are modeled as pins or hinges, allowing rotation at the joints but transferring only forces along the lengths of the members. In this idealization, joints do not resist moments, so bending in the members is not carried by the joints themselves. Each member is treated as a two-force member, with the force acting along its axis and equal and opposite at each end.

This concept is central to planar and spatial truss analysis. Because members are assumed to carry only axial forces, the internal forces can be found by applying equilibrium to the joints (method of joints) or by cutting through the structure (method of sections). A pinjointed planar truss is statically determinate when the number of members m and joints j satisfy m = 2j − 3 (for standard supports); otherwise, the structure is statically indeterminate or unstable. Real systems approximate this idealization, with joints designed to minimize moment transfer and allow rotation, while some rocking or friction may occur.

Advantages of pinjointed structures include material efficiency where loads are predominantly axial, simple fabrication, and straightforward analysis. Limitations arise because bending moments and shear cannot be carried by the members, joints must permit rotation, and real connections introduce stiffness, friction, or play that can affect behavior. Pinjointed frameworks are common in truss roofs, bridges, cranes, lattice towers, and educational models, where their simplicity aids both construction and analysis.

In engineering practice, pinjointed models serve as useful approximations for certain lightweight, rigid frameworks, while more rigid joint assumptions are used when moments play a significant role.