Embedding pipe in concrete pier

[BigBridges]

Hello,

I am a Civil PE designing a cantilevered column for a freestanding solar array. The posts will be steel pipe and the foundations will be a drilled concrete pier. The client would prefer to embed the pipe steel into the concrete rather than using a base plate. I used Enercalc's embedded pole calc for determining the depth of the piers and I have heard it is possible to use this feature to determing the required depth of embeddment for the pipe into the pier.

My question is, how can I determine the allowable passive pressure and maximum passive pressure (of concrete) for the input card? I am using f'c = 3000 psi concrete and the drilled shafts are 18" and 24" depending on the height of solar panels and size of pipe steel.

I appreciate any help offered!

-Ryan

 

[msquared48]

You can either use the lateral soil pressure values listed in the IBC - in fact twice the tabulated values if you read the footnotes, or get a geotechnical report.

Mike McCann
McCann Engineering

 

[msquared48]

What I was addressing above was the soil pressures. I read yourpost again and have the feeling now that you are talking about the steel bearing on the concrete.

The ties in the concrete pile should take care of this concern, and I would fill the pipe with concrete to control the pipe sidewall buckling issue.

Mike McCann
McCann Engineering

 

[BigBridges]

Thanks for the quick response. Yes, I already have a soils report and was able to determine the depth of piers. I am asking how to find the required depth of the steel posts into the piers. To do this using Enercalc I need to change the allowable passive pressure to reflect that of concrete, if that makes sense.

I can go conservative and embed the pipes the full length of the piers (6-7 ft), minus some cover, but I would like to use less pipe if possible.

Thanks again!

 

[csd72]

just use bd^2/6 and the allowable concrete bearing pressure to calculate the required embedment depth.

 

[BigBridges]

csd72, thanks for the tip. I will use that in my analysis.

 

[miecz]

Carrying the bending moment this way will cause tension in the concrete at the sides of the steel post, in the direction of the load. Be sure you have enough developed hoop steel to take the tension.

 

[spats]

To expand on the previous response, bd^2/6 is the section modulus ("S") of a rectangular section, where "b" is the OD of the pipe, and "d" is the depth of embedment (both in inches). Since the pipe is pushing on the concrete on one side top, and one side bottom, this rectangular section can be considered to resist the moment at the bottom of the pole. The moment arm should be measured to mid-depth of the embedment.

Simply calculate M/S, and compare it to a maximum allowable concrete stress of 0.35 x concrete compressive strength, or 1,050 psi. Of course, this will be a trial-and-error process.

You need to confine the concrete well with ties within the embedment zone... use at lease 2 or 3 ties fairly closely spaced at the top of the pier. For a little added conservatism you might want to lower the allowable concrete stress, and with 6'-7' piers 18" to 24" diameter required, I certainly wouldn't go any less than 12" emebedment. How about "embedment = pier diameter" sounds pretty good!

I don't see how you can use the enercalc program... "allowable passive" is resistance per foot of depth, and would be meaningless for what you are trying to do.

 

[BigBridges]

I thought it may be possible to manipulate the enercalc program to arrive at a solution. My thinking was I could set the maximum allowable passive pressure to the allowable concrete stress. I then set the allowable passive pressure higher than the max, so that the program automatically uses the max value. This would eliminate the passive pressure per foot of depth issue. I might try it later just to compare. I'll stick with the M/S = 0.35*f'c though.

I am detaling 3 full winds of spiral at the top of the pier, which should be more than enough developement to resist the tensile stresses, etc.