Steel Column to Concrete Bond Strength? 1

[tdawgui]

I have a 1.25" sched 40 pipe embedded into the ground 6'-0" and a 12" diameter concrete collar poured around it at the surface (12" deep)...

I'm trying to figure out what the bond strength is between the steel pipe and the concrete for vertical force resistance. My concern is that the concrete bond will break and then I will be relying on skin friction and the pile action of the pipe for vertical bearing resistance instead of the concrete collar...

I know that within the concrete pour zone I could drill a hole in the pipe and shove a piece of rebar that would negate the need to worry about this bond, but I'd rather see if the bond strength will do so that I don't need to require unnecessary work...

So far, I've researched on this forum and the web... Here I found in one thread that the strength for 3000 psi concrete (I have 2500 psi or possibly less) is about 82 psi and on the web I found a bond strength of 36 psi for concrete (no strength reported)...

I'm wondering if anyone knows an equation to calculate this bond strength or can direct me toward a resource which may aid in my design.

Thanks!

 

[WillisV]

American codes for composite columns generally do not allow the use of bond strength for encased sections and require mechanical anchorage. Europeans are less conservative; however, and allow the use of about 43.5psi for encased shapes per Eurocode 4.

 

[msquared48]

Four questions with your detail:

1. Aren't you concerned with the unencased pipe rusting out below grade?

2. Can the grade sluff away over time, eventually exposing the pipe under the collar?

3. What is this pipe supporting, or is it just a fence post?

4. Any lateral concerns?

Mike McCann
McCann Engineering

 

[jike]

I would totally discount bond if the pipe is painted or it is galvanized and has not been degreased or etched.

I vaguely remember a figure of 20 psi for bond between concrete and steel piling. Don't quote me on this but double check it.

 

[tdawgui]

Four answers for your questions:

1. Aren't you concerned with the unencased pipe rusting out below grade?

How long would you anticipate before rusting would become a structural issue? I'm a rather new engineer, but I would anticipate that the duration of time would be greater than the design life of roughly 10-20 years... Don't they run steel water pipes underground?

2. Can the grade sluff away over time, eventually exposing the pipe under the collar?

Notes require that grade be vegetated and maintained to prevent this issue.

3. What is this pipe supporting, or is it just a fence post?

Supporting Mechanical Tramway: roughly #1000 vertical force.

4. Any lateral concerns?

Lateral has been designed for through pole embedment requirements.

I appreciate the concern! Any responses are welcome...

Thank you,
Tyler
Treasure Valley Engineers, Inc.

 

[JKStruct]

AISC design guide 6 gives the average ultimate bond strength (based on a limited number of push tests) as

u=0.9(.09f'c-95), psi

Then, it says not to count on it and provide a positive shear connection for encased steel sections.

Also, the pipe will rust and deteriorate below grade much quicker than you think. It needs to be galvanized, have a mastic coating, or completely encased in concrete.

 

[msquared48]

Tyler:

The time of rusting depends on the type of soil you have, plus any galvanic action present. The more acidic the soil, the faster the deterioration. You're only dealing with 1.25" pipe here. That's why galvanized pipe is used a lot in your circumstance, and most of that I have seen encased in concrete too. True about other steel pipe, particularly pile, but these are usually in regions of high water tables or poor soil conditions where the presence of oxygen to fuel the rusting process is limited.

Lastly, depending on your frost depth, I would extend the collar to at least that depth, if not further due to soil sluffing. Plants do die when not cared for.

Cheers.

Mike McCann
McCann Engineering

 

[tdawgui]

If the pipe is galvanized, would this negate any bond from forming?

FYI: I'm now planning on providing positive resistance via a threaded rod or bolt located in the concrete pour zone.

 

[tdawgui]

Thanks all for your help!