Reinforcement requirements for short pile foundation

[rteja]

I have a short pile foundation of Dia 600mm and length 1000mm, to support a circular steel post anchored to the foundation.
From my calculations, tensile stress in concrete due to bending moment is less than allowable tensile stress.
My question is do i still need to consider a minimum reinforcement for the pile? If yes, what is the recommended minimum reinforcement?

I have considered 3 layers of Dia 12mm bars at the top, tack welded to the anchor bolts for concrete breakout strength (attached image for reference)

 

[jayrod12]

How much moment are we talking about? Is this supporting a light pole or something similar?

Where does the moment go at the bottom of the "pile"?

What kind of soil/rock are you drilling into?

Honestly, where I practice, we would never weld the rebar to the anchors. In a pile that is 600 diameter we would have 6-15m verts and 10m rings at some spacing. Also around here, we would never put a 1000mm pile, unless it happened to be socketed into exposed bedrock.

 

[BridgeSmith]

If you don't need to reinforce the concrete, why do it? Put a nut and a washer at the bottom of the anchor bolts (or use J-bolts) and call it a day.

I also agree with jayrod that welding to the anchor bolts is a bad idea. You obviously don't have much load, but a typical poor field weld can lead to fatigue fracture under very little stress.

 

[rteja]

Thanks Jayrod12. I'm having a moment of 6.2kNm and shear of 2.25kNm at the top of foundation. It is to support a flag type directional sign. The soil is slightly silty with medium gravel and sand.

 

[jayrod12]

No freezing chances? I still feel that 1000mm long is a bit shallow for me... those loads are not overly significant however and I don't really see there being a need for much reinforcing.

 

[BridgeSmith]

Taking a closer look at the detail, I'd be more concerned about corrosion, due to the anchor bolts being recessed below the surface elevation, than the small moment and shear. Likely, you could do without reinforcing, but if it was me, I would develop the anchor bolts, provide a washer and nut at the bottom, and consider providing a minimal reinforcing cage around the anchors down to the bottom. Alternately, extending the anchor bolts to the bottom and omitting the reinforcement, would also mitigate virtually all the tension in the concrete.

If it's in a location subject to frost penetration, as jayrod mentioned, the shallow depth would concern me as well. Here in Wyoming, we neglect the top 5 feet of soil for lateral resistance on our drilled shafts due to the loosening effect of frost heave.

 

[swearingen]

As a side note, the ASTM A325 designation shown is for a headed bolt, which is not what you have. You should be specifying ASTM F1554 Anchor Rods.


-5^2 = -25 ;-)

http://www.eng-tips.com/supportus.cfm

 

[BridgeSmith]

A side note on swearingen's side note: ASTM Standard A325 has been superseded by F3125, which now includes A325, A325M, A490, F1582, and F2280, as bolt grades under the consolidated standard. For threaded rods, F1554 would be the correct spec.

 

[skeletron]

Clause 15.9.2.1 (A23.3-14) allows for the minimum reinforcement of 0.005 times the gross area of the supported member. In Clause 22.8 (Plain concrete design for drilled piles) there is some guidance on minimum eccentricity for applied loads, reduction factors, and comp/tens stress limits. Despite this being shallow and lightly loaded, I would still be curious about the soil-pile interaction.