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Manhole covers are generally made up of two pieces of triangular plates to form a square cover. One may wonder why two rectangular halves are used for a rectangular cover. To understand this, one should note that a planar surface is usually in contact with three support points.
For a triangular plate, the centre of gravity (CG) normally lies within the zone bounded by three support points even when loaded, thus creating a stable support system for the manhole cover. For a rectangular plate, the CG of a loaded manhole cover may lie outside the three support points, causing it to rock. Hence, the potential problem of rocking produced by vehicular traffic by rectangular traffic could be eliminated by using two triangular halves.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.