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enter image description hereI know if a rod is hinged at its two ends and if there is no load act upon it along its length then the forces exist only along its length but i don't see how this characteristic comes into play ?

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  • $\begingroup$ A picture would help $\endgroup$
    – Fred
    Jan 20 '18 at 2:14
  • $\begingroup$ There are different interpretations of hinges. Carpenters interpret it quite differently from engineers. :) $\endgroup$
    – Jem Eripol
    Jan 20 '18 at 7:45
  • $\begingroup$ @Fred I added one $\endgroup$ Jan 20 '18 at 8:27
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    $\begingroup$ Probably because those rods are operating in tension (not voltage...) $\endgroup$
    – Solar Mike
    Jan 20 '18 at 9:06
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I think this question is about additional cross bracing.

With a rectangular frame in a vertical plane the ability to carry compressive loads is excellent, but a comparatively small lateral force can deform the rectangle into a parallelogram which will collapse under the same loading. Since the cross bracing will operate to restrict primarily tensile and (to a lesser extent) compressive forces, these struts can be hinged.

If there's a possibility of the main cross bracing struts bending, compressive forces could cause buckling. Additional cross bracing can be applied to the main cross bracing to keep the strut in the line of the force it is designed to restrict. These will also act in tension and compression (mainly tension), and can be hinged.

In both cases the idea of the hinge is to reduce the effect of lateral forces that might bend the strut.

Power transmission towers are pure structure, and have several levels of cross bracing - as can be seen in the picture. Though each level of bracing adds mass, it adds less mass (and less resistance to air flow) than using a thicker beam or a reinforcing plate would to achieve the same rigidity.

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If you imagine a square or rectangular shape made from rods joined with points at the corners. If those pins act as hinges then the whole shape can easily deform, effectively acting as a 4 bar link.

On the other hand if the shape is a triangle then even if the corners are hinged then it can't be deformed without stretching one of the bars. Also, whichever way you try to deform it one of the bars will always be in tension. So the stiffness of the structure doesn't depend on the rigidity of the joints.

This is the fundamental reason why breaking up a structure into triangular sections tends to be an efficient way to make a stiff, lightweight structure.

This approach also tends to be fairly straightforward to design and analyse.

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It could be just to meet a section of a code, or facilitate construction. Or it could be part of the design that is meant to work with configuration and tension of power cables.

Basically in long truss sections they use L shaped metal profiles and a gusset plate and a set of bolts, but use of hinges is not prohibited.

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