# Deck pier question



## Pcinspector1 (Mar 22, 2016)

I recently had a conversation with a deck builder in regards to the diameter of the post pier and at what point the pier should meet the required diameter.

The deck required a 20" round diameter pier and depth of 36". The deck builder indicated that as long as the pier at the base is 20" in diameter the cylinder could be a lessor diameter. The deck builder cones the bottom to aid in preventing uplift. Would I be wrong in requiring the complete cylinder pier to be 20"

Would the coned portion of the pier need to be below the established frost line of 36"?

Your thoughts?


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## fatboy (Mar 22, 2016)

It will be interesting to see the opinions on this. IMHO, the size of the pier would be base on the load bearing capacity of the soil. I would not have an issue with an inverted cone, as long as the base in contact with the soil met the required diameter. Maybe I am over-simplifying it.........


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## mtlogcabin (Mar 22, 2016)

Same principle when using a Bigfoot    http://www.bigfootsystems.com/


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## Glenn (Mar 22, 2016)

The builder is correct.  For a 36" deck pier (that is not generating resistance from skin friction), the minimum diameter is for the bearing area in contact with the soil.  The base of the pier would follow the rules for a footing and footing projection.  The bearing area (min. dia) must be at least 6" thick.  The projection (where the lower portion extends horizontally beyond the upper narrower section) cannot be larger than the thickness of the footing.


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## fatboy (Mar 22, 2016)

Long time, no hear from Glenn!  Are you coming ti the Chapter meeting April 1?


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## Keystone (Mar 22, 2016)

Pcinspector1, we as in our office, base those dimension from DCA6. The minimum base thickness appears to be 7" and gets greater depending upon the beam span.

The problem I run into, when contractors attempt to do this method. The auger the pier, typically a 14", then take a digging bar to hog out the base. 9 times out of 10 then minimum base height dimension is never meet cause how can one effectively create a 6/7" base height?  Can it be done yes, but as I said 9/10 do not or have a difficult time attempting it and they end up with a 28" wide base by average 10" high base. No time or cost savings there.


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## ADAguy (Mar 22, 2016)

A basic methods and means? Good Old boy vs what do the drawings say, you do have drawings don't you? If so ask the engineer who stamped them.


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## Keystone (Mar 22, 2016)

ADAguy, PCinspector said"conversation" not inspection or drawing.


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## tmurray (Mar 23, 2016)

This is all we see here. I didn't realize that some people build the full width the whole way.


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## Pcinspector1 (Mar 23, 2016)

ADAguy,

Were still are allowed to do residential and Two-family structures without engineered plans here. Same with decks no engineered plans would be required on the subject matter I posted. It was an interesting question and mtlogcabin's post shows a Bigfoot which is the same concept. I typically see the auger bit used on most piers here, but some contractors use a standard size auger and when the pier needs to be larger they clean out the sides but this contractor asked why when in his opinion it was not necessary. This is why I posted the question. Obviously the contractor would save $$ on concrete cost but has to be more precise in the post bracket post connection with


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## Glenn (Mar 23, 2016)

Go to the very bottom of this link to view the 2015 IRC.  What you are asking is shown in the new figure.  This is clearly allowed and intended by the IRC.  http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%205.html

I also go over the subject of deck foundations in the "foundations" session of my free, ICC-approved online course "Down the Load Path" at www.buildingcodecollege.com

Fatboy, Yeah, I've been too busy to enjoy the forum lately...or to attend chapter meetings...  Hope to again someday soon.


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## steveray (Mar 23, 2016)

> The projection (where the lower portion extends horizontally beyond the upper narrower section) cannot be larger than the thickness of the footing.


Then how does this apply to basement lally columns in the IRC?


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## Glenn (Mar 24, 2016)

> Then how does this apply to basement lally columns in the IRC?


It does apply.  Its about the 45 degree bearing plane.  Unless engineered with steel to resist punching shear.


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