Plinth/grade/ground floor beam design

@yadhukrishnanpj123 Nice to know the discussions helps you :-)

@premjit Use full information sir🤗

Plinth beams in a building has primarily 2 functions. 1) To tie the columns 2) To support the walls at ground floor level Your question is pertaining to point 2 When the beam is supported on ground, you can have 2 situations

a) The soil is hard. b) soil is soft

In Case (a) when the soil is hard,then the soil has the ability to hold the load that is coming from the wall. It is continuously supported on the ground for gravity case,and hence the beam may not bend at all.The load directly transfers to the soil with out creating any force in the beam provided you have well compacted the earth.

However note that in many cases,site does mass excavation and refill after foundation is constructued and all this decides if the soil is well compacted.

Please note that plinth beam is also a part of the frame and hence it will be resisting lateral forces and hence the size and steel in the beam wont be considerably less/nominal.

In vase (b) when soil is soft, then even for gravity case the beam may be partly or fully subjected to bending.

It is difficult to capture the bearing on the soil and you may want to go with a conservative approach

Note that ETABS also has an option to use a line spring for beams.However I believe it is not worth spending time doing all this instead look at the design wholistically and see if any reduction in wall load really changes any thing as seismic point of view the steel is any way not going to be too less.Only it reduces some mass of the wall in the design.

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