Question for Colorado Engineers: Deck

[COEngineeer]

Most of our clients are up in the mountain and usually we have a new construction or large remodel. We just now have more and more clients in the Denver metro area. When I look at the building department of vairous counties, they all have 8" min concrete pier with 36" into the soil foundation without any isolated footing(they call it caisson for some reason). Usually on our projects we always use 2x2 footing underneath the sonatube. Shall I start designing the decks in denver metro area to bear on the area of the pier only? Usually soil report suggests to use minimum 2x2 isolated footing. Is it possible to do decks without soil report? Do you do this 3ft sonatube without footing?

Never, but never question engineer's judgement

 

[COEngineeer]

Here is an example of building guide provided by Jefferson County, CO

http://co.jefferson.co.us/jeffco/building_uploads/uncovered_decks_j_logo.pdf

Never, but never question engineer's judgement

 

[emmgjld]

The detail of a small drilled pier is for the 'normal expansive soils' found in great abundance along the Front Range.

In the Grand Junction Area and much of the mountains, I have found extreme conditions. Either:

Expansive clays/Stiff or Firm Soils or Rock. Allowable bearing is fairly high, minimum bearing may or may not be required and only a minimum foundation is required. The 36" length is for frost protection and to provide resistance to tipping.

Very soft, compressible soils which require often requires a pad and column, as you have described. Actually, a small pier can be quite effective for the (normally) small loads associated with decks even when soft soils are encountered. The danger can be with wind uplift. If expansive soils are not the issue, the 2 foundation types may be interchangible.

 

[COEngineeer]

I thought the term drilled pier usually go all the way to bed rock and it is usually 20ft long? So you only design the bearing if it is 3 ft long? No skin friction etc.?

Never, but never question engineer's judgement

 

[emmgjld]

You are stuck on what is common in the Denver area.
The 20 ft long pier is a common minimum length in the Denver area, when dealing with expansive clays typical in the area. The various theories define the upper 'zone of seasonal moisture change' usually 8 to 12 ft and a minimum length below this 'zone' to resist uplift forces, 6 to 10 ft, possibly assisted with shear rings. So 20 ft is a nice round number.

If your look at the real end bearing capacity of the subject soils, considering the depth of embedment, possibly entering in the side friction (if you think the soils will not shrink away from the pier) then these small piers can be quite effective in many soils.