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[Yogesh101]

Do we need to design plinth beam to its span? While designing a building in etabs, the r/f of plinth beam of 7m is very less even with minimum section size. So, i'm confused whether it should be designed according to its span or not.

[hishamajmalpc]

what you meant by 'design by its span'?

[Yogesh101]

What I meant was, we design the depth as span/(12-15). Do i need to do that for plinth beam?

[hishamajmalpc]

span/12 is for prliminary sizing of beam. You should check this depth for flexure,shear and deflection.

[Dr. N. Subramanian]

Dear Er Yogesh

Plinth beams are normally supporting considerable height of brickwork and hence act as composite beams. Hence they need not be designed for the normal BM. In such cases, for normal spans of 3-4 m, we assume a depth of 150 mm to 200 mm only and design for a BM of Wl^2/30. See also the IS code on piles(I think it is IS 2911). Usually 2 nos 12 mm bars are provided at top and bottom

Best wishes
Subramanian

[hishamajmalpc]

Sir,
Most of the cases there will be openings in wall (Doors/windows). Can we assume composite action in those cases?

[Yogesh101]

Thank You Sir, for your clarification. I got it.

[Ujjwal]

[someshwar ganti]

I feel that a Plinth beam cannot be designed as a Normal Continuous beam like that in a Roof beam.

1. the Plinth beam is Normally casted on a CRS masonry, which gives it support through out the length of the BEAM hence our normal BM equations will not really apply,  

   Please note for a roof beam the wall is constructed much later hence normal equations apply.


Besides the Wall load on the plinth beam, most of the sites (especially factory sheds etc) will have tractors, tippers running on the some of the bays of the plinth beams, These beams survive only because there is a CRS wall supporting the beam through out.

hence in my opinion , design it as a normal beam because the construction loads on plinth beams (atleast in some bays ) is much higher than the wall load on the beams, in such a case the CRS support below the beam will prevent the deflection etc hence the failure.

[Ujjwal]