Large Cantileverd Pipe Design 1

[azcats]

I'm working on some billboard structures. They're mono-pole designs ranging from heights from 50' to 120'. The sign faces are as large as 14'x48'. Anyway, the columns are primarily flexural elements as the vertical loads for the size of columns are relatively small. We use round pipe for the columns and it get's as large as 48" dia x 1' thick wall. The poles are typically set in a hole that is filled with concrete and the depth is designed using non-constrained pole footings in lateral bearing. The hole size is anywhere from 12" to 18" larger in diameter than the pipe size.

I know that when using embedded pole footing designs, the moment on the pipe continues to increase below grade. One publication I read (thanks slideruleera) said that it continues to increase to about 25% of the depth of embeddment. However, all the engineers before me have calculated the maximum imposed moment on the columns at grade. I'm guessing they assumed that the concrete provided some sort of composite action and the stresses in the column began decreasing at grade. I've yet to find any justification for this assumption. There is no reinforcing in the concrete (other than the pipe) nor any type of connection between the pipe and concrete. Also, the inside of the pipe columns are not filled with concrete.

I'm looking for a bit of insight or assistance on how to analyze this situation. Am I being over-conservative to increase my moment arm down to 25% of embeddment? Is there any composite action taking place? Are there any texts or publications addressing this subject? These signs have been being built like this for 20 years and there's never been a known issue with either the concrete cracking around the pipe nor any pipes buckling at the base. This may be because the wind load used is fairly conservative. I'm not sure.

Any help or input would be greatly appreciated.

John

 

[DaveAtkins]

Since there will not be a reaction right at the surface of the ground, the moment will continue to increase until enough passive soil pressure is developed to equal the reaction to the soil (hence a trial and error problem). The 25% assumption sounds reasonably conservative, but I would verify that the statics works, using that assumption.

DaveAtkins

 

[SlideRuleEra]

azcats - Glad that "Pole Building Design" was helpful.

Another approach could be to consider the "best case"; assume the embedded pole is a laterally loaded pile - take a look at how to determine the "Depth of Fixity" in the publication "How To Determine Lateral Load Capacity Of Piles" on the AWPI page of my website.

Still another option is to check out "Design Guide: Embedment Depths for Concrete and Steel Poles" (free .pdf download) from the USDA Rural Electrification Program at this link

http://www.usda.gov/rus/electric/bulletins.htm

It is Bulletin 1724E-205 (about half way down the page)

http://www.SlideRuleEra.net

 

[miecz]

In order to consider composite action, I think you need to meet AISC Chapter I, Article I2. Sounds like you don't. In drilled shaft foundations,I've seen maximum moments 15 to 30% greater than that at the ground surface.

 

[aggman]

I don't know that I agree that you don't have any composite action. I would consider a transformed section and see what the stresses come to. It may be a significant reduction and even if it didn't become fully composite even partial composite behaivior may be more than adequate to maintain safety. I am assuming that the concrete portion is significantly bigger than the pipe, like a 24" pipe in a 48" pile. There is also consideration that the pipe likely would not fail anyway. As long as yield controls the design (no critical buckling) then it would take a decent increase in height to generate enough stress to cause yielding. I am no expert at sign structures but sometimes, especially older structures, you have to get past what the code says and use good old engineering know how to justify the design. Just something to consider.