Bending Equation for eccentrically loaded Base Plate 2

[SALTRAM4567777]

As we are aware of this equation being used to calculate maximum and minimum bearing pressure.
F=P/A +- 6M/bl2

When designing base plates for large eccentricity we use this equation where b is the width of base plate and l is the length of base plate. Similarly when we compute the base plate thickness we substitute t (thickness of base plate) instead of l (length of base plate).
F=6M/bt2
My question is how for the same moment we are calculating section modulus taking t as well as l.

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[JLNJ]

There are two different things going on here and you must thoroughly understand them both in order to perform this sort of design.

The moment used to determine the bearing pressure under the base plate is the global moment applied to the column. (Note that the correct distribution under the plate is a subject of some debate.)

The moment used to determine the plate stress is the local moment in the plate caused by the manner in which the bearing pressure is applied to the plate. And that's not all, of course. Another moment to check the plate for is the moment caused by the anchor rod tension forces.

Engineering is about finding and understanding the load path rather than just finding the right equation.

 

[SALTRAM4567777]

I am getting your point but I am still in doubt.
While computing the Section modulus for a rectangular section wrt to direction of applied moment we take thickness of the beam to compute or section modulus rather than the length of the beam.
Simiar situation is in this case also.

 

[SALTRAM4567777]

I agree the moment is global and local but the direction of both the moments is wrt same axis,lets say Z axis.
While computing section modulus for global moment we take length l while for local moment we take thickness t.

 

[Celt83]

While computing section modulus for global moment we take length l while for local moment we take thickness t

you may be getting confused somewhere, depending on which direction you are bending the plate about the elastic section modulus S for the plate will always have the plate thickness, t, as the squared term. Either S= B*t^2/6 or L*t^2/6.

I don't want to add confusion but do note that for the anchor tension side the critical bending section has a much smaller effective width. Review the examples in AISC Design Guide 1 and/or Design of Welded Structures by Blodgett

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[ProgrammingPE]

I think this may clear things up a bit.

The moment resolves into the same type of stress profile for the bearing pressure as well as for the plate flexural stress, but they are in two different planes (and have two different moments). The section modulus is used to determine the stress for both of them, but because of their different planes, one uses "L", while the other uses "t".


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[human909]

A picture is worth a thousand words. Nice.

 

[SALTRAM4567777]

Thanks Programming PE. The picture clears it all.