It perform mainly two functions - -
(i) It carries the load of brick wall at ground storey
(ii) It provides moment resisting beam monolithic to columns against lateral forces
As for (i) is concerned the beam width generally min 300 plus 50mm pcc below
it making total width width 400mm act as a footing to transfer the weight of masonry
to underlying strata.
The strata is filled strata in most cases but compacted and will have sbc of 5 tonne/m2.
Considering a wall height of 4.0m upto GL(little on higher side), the load of
BK wall is = 2.0 Tonne/ running m
The base pressure with plinth beam = 2.0/(0.4*1.0)=5 tonne /sqm which is safe
Hence theoretically there is no need for design of plinth beam for this loading.
But practically Plinth beam are designed for wL^2/30 as minimum requirement
at mid span
(ii) the moments from lateral analysis needs to be fully carried by Plinth beams
(support moments) without any concessions
