Steel post in concrete, minimum cover

[MooCow]

Is there some standard that cites the rules for steel posts in concrete?

A 3" diameter steel post is 12' long. 6' of it is above ground. The 6' below ground is encased in 18" diameter concrete. The post is not placed in the center of the concrete. The section where it is closest to the edge of the concrete is 5". So, if you draw a line through the center of the circle, it would be 5" of concrete + 3" post + 10" concrete = 18" diameter concrete. (A perfectly cented post would be 7.5" + 3" + 7.5".) The post has a constant vertical load of 300 lbs. Lateral (side load) is 0 to 300 lbs. I feel this thing is already over-engineered. I think 3' in the ground, 12" diameter concrete would have been sufficient, but it's what the customer wanted. I know when you set a rebar cage in a hole in the ground, there needs to be at least 3" of concrete cover between dirt and rebar. I imagine the same would apply to a steel post. We installed 15 posts as described above. 3 of them aren't perfectly concentric with the concrete. The example above is the worst one. My argument is it won't affect structural integrity.

 

[JAE]

I know of two:
1. "Engineering principles".... i.e. the load and calculation of how the concrete would behave under vertical and lateral loading.
2. "AISC Specification section I2 is for Encased Composite Members

For item 1, we usually try to keep steel shapes at LEAST 4" clear just to have some good concrete "meat" and minimize the chance that the outer, thin concrete would crack and spall away from the body of the main concrete.

For item 2, AISC I2 states that encased composite members requires that an encased shape also have longitudinal bars and lateral ties around the embedded shape. This would require the min. ACI cover on the rebar.
 
You didn't describe what the post is supporting, or the loads on it so I can't speak to whether 5" is adequate or not. If there's a significant lateral load on the pipe, and it is in the direction toward the 5" narrow section of concrete, then perhaps cracking of the top 5" edge of concrete could occur. Then it would definitely not be "over-engineered".

To answer your question - no, I don't believe there is a "standard" that dictates cover unless you equivalate it to rebar cover requirements.

 

[MooCow]

Thanks for the quick reply. Lateral load won't be much. The posts are part of an "H" frame to hold telecom equipment. Each H frame consists of 3 posts, 5ft apart. 2 pieces of steel strut are attached horizontally across all 3 posts using U-bolts. The several pieces of telecom equipment with a total weight of 600 lbs. will be attached to the struts. The maximum offset of the equipment from the centerline of the posts will be 6 inches. (Imagine a a cube 12" x 12 x 12, weighing 200lb attached to the side of a pole.) The 5" part of the concrete is actually more to the side of the load, not front or back.

 

[BridgeSmith]

Yes, the 3" of cover is essentially so you get a minimum of 2" cover, accounting for any unevenness or voids, which is to prevent rusting of the reinforcing. It would be the same for the pipe. If you have a rebar cage around it, there should be no concerns of it breaking out of the concrete, especially with only 1800 ft-lbs of moment. I'm not sure how you'd go about proving that numerically, though.

 

[TLHS]

If I were using it for corrosion control, then I might want three inches, but honestly this isn't going to act like cover on rebar because you don't have the same extent of bond to hold together cracking. What concrete is usually doing in this situation is two things:

1. Fitup to make your post tight to the hole and give you good lateral bearing contact
2. Increasing bearing width for lateral resistance horizontally or increasing base bearing area

It's not normally a function of 'add three inches to the post' or whatever, because that's unlikely to govern.

Uncentered is not ideal and likely affects some theoretical failure mechanisms, but not at the practical concrete demand level that you'd be looking at for something like this. Soil would heavily govern.

What you're describing could honestly end up with some not completely negligible wind loads on it if the equipment is reasonably dense. Your internal post is grabbing 5ft x 6ft of potential equipment area. I'd be running a quick post foundation check using the provisions in the IBC for post foundations. It's a half page capacity calculation that can be compared against wind load. I also generally don't go less than 4' depth for anything I care about, and the difference between 12" and 18" diameter in things of this scale is basically negligible. The concrete volumes are small and the labour is pretty similar unless they're hand auguring.

 

[MooCow]

TLHS, you're right. You can't compare 3" cover for rebar because that's just for corrosion. The reason for the diameter and depth is because of the engineer's load calculations. So, I can't pin down some kind of measurement to say "all you really need is this much concrete". The customer will refer back to the engineer for his opinion and it's unlikely the engineer will accept, partly because he'll be conceding all that extra concrete was unnecessary.

JAE, you mentioned something useful "...cracking of the top 5" edge of concrete..." If the concrete fails, it's likely to happen at the top because during lateral force, the top of the concrete will see the most movement and stress.

The customer installed strut, equipment and cables to the poles before they notified us of their dissatisfaction. So, having to remove the concrete and poles to redo them would be a huge operation. The poles are in the right place. The holes in the ground weren't lined up. Maybe I can ask them if we can leave everything up, break the concrete 24 inches down and replace with new concrete with the Sonotube forms in the correct positions. Then, fill and compact soil where any the void is left. This will address the concerns of weakness at the top edge, as JAE stated. I know the rest of it isn't ideal and we're not adding a cold joint in the concrete cylinder, but hopefully this will be sufficient.

Thanks everyone for your input.

 

[BridgeSmith]

The reason for the diameter and depth is because of the engineer's load calculations. So, I can't pin down some kind of measurement to say "all you really need is this much concrete". The customer will refer back to the engineer for his opinion and it's unlikely the engineer will accept, partly because he'll be conceding all that extra concrete was unnecessary.

The size of the concrete is most often not required for the confinement of the post, but to provide adequate bearing area to transfer the loads to the soil. As long as the post is inside of rebar cage (AKA, within the core), and the reinforcing has enough cover to prevent spalling of the concrete, the post is restrained by the reinforcing.

 

[Tomfh]

There is no clear rule.

The min 3” cover in your case will most likely be fine. The soil will yield long before the post tears out of the concrete, so it’s not a strength issue having “only” 3 inch of cover.

If someone is unhappy you can dig out some soil and add more concrete at the top.

 

[1503-44]

You couldn't find any telephone poles?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."