Analysis of Trusses – Method Of Joints | Engineering Mechanics | ME/ CE

by / Wednesday, 15 November 2017 / Published in Technical Corner

Real Life Application
For everyday occurrence, trusses are incorporated into numerous structures all around us, though we probably rarely notice. Here are just a few examples:
1.Bridges – Arch bridges, suspension bridges, cantilevered bridges.
2. Roofing – Practically all modern domestic housing, both timber and steel, employs trusses to span ceilings and support the roof cladding materials.
3. Cranes – The cranes employed in urban construction sites almost universally employ steel truss construction. Industrial lifting cranes in hangars, warehouses and factories also often employ truss construction to bridge substantial gaps while maximizing load carrying capacity.
4. Hangars – Every large warehouse building or aircraft hangar depends upon the rigidity and length of truss roof construction to span the enormous unsupported ceiling spaces.

Explanation
Method of joint works by considering pin at joints in the truss structure. We isolate the pin at joint by drawing its FBD, and replace all truss members connected to the pin by the forces that they exert on the joint. The magnitude of the forces is then determined by applying the equations of equilibrium ([ΣF = 0] and [ΣF = 0]) on each joint.
A common confusion is to determining whether each member is in tension or compression. There are many ways to go about doing this, but the trick is to stick to only one method consistently. One simple way is to treat compression as a pushing force and tension as a pulling force.
If the force exerted by the member on the joint is pointing inwards towards the joint, it is pushing and therefore the member is in compression. The opposite, where the force is pointing outwards away from the joint, signifies that the member is in tension.

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