Bearing load on concrete curb

[jdgengineer]

In residential applications we often have bearing reactions from posts on relatively slim curbs (say 6" wide). The curbs are not very tall (say 18"). When calculating the capacity of the curb, is this just a bearing problem as the height is small enough that buckling is unlikely or are there other checks (side face blowout) or similar that should be checked?

 

[Guastavino]

I'm assuming the curb is then sitting on a wall footing?

Also, how big a load we talking?

And any potential for uplift?

With those questions in mind, my initial guess would be a non-issue unless the loads are really high and/or there is potential uplift.

 

[KootK]

Usually it is a non-issue but there is a stability issue that warrants some consideration, at least in passing. The curb is a cantilevered column of sorts with a pin ended column above it. Lateral sway at curb/post joint is the governing stability issue even if the curb itself is essentially rigid due to its proportions. On occasion, I'll assume the joint to be displaced laterally an inch or so and then use the resulting moment demand to design the curb vertical bars in within a reasonable distance of the post. It's not real rigorous but it's something.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

Thanks guys. Usually the loads are small, but hypothetically let's us assume it's a HSS5x5 column with a 6"x12" base plate with 25 kips. Say the base plate is 3/4" thick.

Would you assume relatively uniform bearing pressure below? This would equate to 350psi which would be well below concrete bearing stress.

I can check with some assumed lateral displacement to calc vertical bars at KootK mentioned. But would you also check a shear failure from a side bursting?

I've seen a 20° failure slope assumed for bearing ledges that then have horizontal bars designed for shear friction. Doesn't quite apply to a curb condition. Would you assume a failure plane of something similar and check the shear along that slope?

 

[KootK]

Would you assume relatively uniform bearing pressure below?

Yeah. It's mostly baloney but that's what is assumed and it seems to work out alright.

But would you also check a shear failure from a side bursting?

My understanding is the conventional bearing stress is the stress that you can use without concerning yourself with side bursting. Not like there's a whole lot that you could do about it in a six inch curb anyhow.

I've seen a 20° failure slope assumed for bearing ledges that then have horizontal bars designed for shear friction. Doesn't quite apply to a curb condition. Would you assume a failure plane of something similar and check the shear along that slope?

I don't know and have wondered about this myself. I've seen that same check and have never understood under what conditions the regular bearing is insufficient. Maybe it has to do with situations where you've got that lateral "N" force on the connection like with corbels.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

Thanks KootK.
I'll go with just bearing and take some comfort that at least 6" diameterx 12" long is used frequently for cylinder testing and those things can take some serious loads before fracturing.