# Thin Plate Bearing on Concrete

@Kootk My latest project has a CFS L6x6x6” long, made of 10ga material (t=0.14”). Centered on the 6x6 flat is a welded 7/8” dia threaded rod with 1/4” fillet weld all around. The metal plate bears on concrete and a 20K service load is applied to the threaded rod.

The 7/8" threaded rod is trying to push through a 6"x6"x0.14” sheet into concrete.

This seems light. Does anyone know of an easy method, other than using FEM to determine the capacity of the thin plate bearing on concrete with the 20K service load? I haven’t been able to find anything in Roark that addresses the condition.

Dik

You know the CISC method for thin base plates? The cold formed literature has something similar for track wherein you get to take a multiple of the track thickness around the stud as your effective base plate. I’d look that up and try to apply it here. Including the weld, your effective diameter resisting would be 7/8" + 2 x 1/4" + 2 x 0.14" x multiplier. Seems unlikely that you’ll get to 20k this way.

What is this thing?

Thanks… I was using that approach with confined concrete bearing and ‘punching shear’ through the steel plate. Looking at the total bearing area in concrete to see what amount of steel plate is engaged and seeing if the moment capacity of the plate would resist the bearing value. The connection is for the top of an auger pile supporting concrete grade beams.

Dik

Does anyone have a quick formula for the bending moment at Mf in the attached sketch? assuming the free edge of the cantilever is at Db?

Thanks,

Dik

Roarks will have something for point load on circular plate which would be conservative, perhaps too much so.

Alternately, you could pretty easily sum the moment over a wedge of the annulus outside of the weld.

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What is the strength of concrete?
I don’t think you can rely on any significant contribution of the plate based on bending, but D(effective) = 0.875 + 0.5 + 0.28 = 1.655"

So bearing area = 2.15 in^2
qc = 20,000/2.15 = 9,300 psi

Seems a little high, but it is confined, so may be okay.

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For 25MPa concrete, the limit load confined bearing stress is about 4ksi. Combined bearing and steel shear is about 28K ultimate vs. 30K factored load, using 3/16" plate and the area of concrete involvement beyond the pin is about 4" diameter. The 0.14" thick plate does’t come close. I have to look at the 4" dia moment to see if I can transfer the balance of the bearing load. Working on that now… I thought it would be more simple. I have to determine the moment per unit length of the circle at the weld, determine the load on 1/4 of the circle and centroid of same and divide it by the length of segment. I was thinking there would be a simple formula for a circular footing with a point load in the middle… but, haven’t found it.

Thanks… Dik

Dik:
The bearing stress qb will not be uniform, but rather more triangular in shape, to zero at the max. radius. In fact, the bearing stresses will be a very sharp transitioning bell curve beyond the fillet weld toe, but this will help the bending stress in the thin pl., I guess. Why not provide a .75” thick pl. (doughnut?), 3-4” in dia. fillet welded to the end of the round bar, and then to the thinner pl.? This gives you the bearing area on the conc. which you need.

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Thanks… cannot get it to work in flexure, I’m going to rely on bearing only. Thanks, I was curious about what type of bearing stress would be present. Almost works with 3/16" material.

Dik

what about the sum of the two cases ? Bares en pag 252.pdf (1.1 MB)

Here the source
Bares en index.pdf (513.2 KB)

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Thanks very much pisani… The service load resistance is about 8K and with 20MPa concrete, the service load resistance is about 13K… I’ll post my SMath program later today.

Dik

Flexure doesn’t work worth a ???. Just relying on ‘punching shear’ through the plate and confined bearing with concrete. Thanks…

Dik