Reinforced concrete encased duct bank 5

[BSPE90]

I am a structural engineer and am working on a switchgear enclosure foundation. The electrical engineer is also requiring us to provide a design for a concrete encased duct bank for four 5" schedule 40 conduits in a 2 x 2 configuration and is requiring that it be reinforced. I generally had thought these were generally in the scope of the electrical engineer and didn't even know they reinforced the concrete encasement.

Is there a standard for the design of the reinforced concrete encasement of conduits? And how is it determined whether a concrete encased duct bank needs to be reinforced or not?

 

[vmirat80]

DoD does this all the time, but we don't reinforce the concrete. Sometimes we just use flowable fill instead of full-strength concrete. The encasement is just to prevent a backhoe from damaging the conduits. I think the electrical engineer is over-designing, unless there are poor soil conditions over which the duct bank must run. Find out why they want it reinforced and re-post.

 

[dauwerda]

My past experience with a large utility also did not use reinforcement. They were generally encased with lean concrete (flowable fill) except where they crossed under designated drive aisles, these areas were required to be 4000psi concrete, still no reinforcement though.

I am not aware of any typical designs/details/procedures that use reinforced duct banks (but they might be out there).

 

[JoelTXCive]

I have done one with reinforcement before.

If you ask the electrical engineer for a loading you will probably get a blank stare.

In theory the duct bank is uniformly supported by the ground and your your span is zero.

To me that means you just need temperature and shrinkage reinforcing around the perimeter of the duct bank. I also threw a few stirrups in just so that the longitudinal bars can be tied to something and the assembly can be formed up.

If you really want to reinforce it, then my suggestion would be to take a box culvert standard and mirror the reinforcement setup.

 

[Rabbit12]

Our electricals here have a detail on their sheets showing reinforcing in there duct banks. It's not necessary unless like vmirat80 says you have extremely poor soils.....even then I'd have to question it.

JoelTXCive I'm not even sure you need T&S steel. Why do we care if it develops some shrinkage cracks?

 

[JoelTXCive]

Rabbit - I agree. I don't think any reinforcement is necessary, but if the electrical engineer wants some reinforcement, then T & S is what I would provide. (unless they can give me a certain loading or scenario that justifies more)

 

[JoelTXCive]

BSPE90 -

You might already be aware of this, but red colored concrete is sometimes required for these duct banks. I have no idea what the specifications are for the colorant and there are usually zero design budget dollars available to figure that stuff out, so..... I put a generic callout on my sheet that says: "Reference Electrical Sheets and Specifications for any required concrete dyes & colorants."

 

[dauwerda]

I would just treat like a prescriptive strip foundation out of the IRC and throw two longitudinal #4 bars at the bottom and call it a day unless the sparky had a more descriptive case of what he is trying to design for.

 

[dold]

I did a DOE project recently for what was effectively a barn. Nevertheless, it was wired up with all the latest tech/security stuff/etc. The ductbanks were: reinforced, red pigmented (extremely expensive apparently), had conductive traces, and warning ribbon buried every couple feet above it. I bet the ductbank cost more than the rest of the concrete work due to the pigment alone.

 

[Rabbit12]

dold, the pigment is expensive? I never would have thought it would be. The colored concrete I've been around is made by throwing a bag of colored powder in the concrete truck.

 

[r13]

Check traffic load, if it is under road way. There is usually a standard detail sheet to indicate conduit layout, and reinforcement at where I worked. The reinforcement is meant to provide minimum protection for under ground structures, and to bridge over weak soil spots. You shall check traffic load, if it is under/cross road way.

 

[dik]

Unless roads, just flowable fill... done lots of these... filled trench, reinforced, with coloured concrete over and ribbons and signage (168Kv stuff)... any frost penetration issues? poor soil, etc.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dold]

Rabbit, I was told by the contractor that the last project they did with this spec, they were able to cut like $30k off the budget just by eliminating the red pigment from ductbank mixes. To be fair, these are fairly long runs - probably around 1000ft. It's a very big, remote site. I don't know if it was some sort of special pigment with other additives or something, but the contractor was pissed when they required it on this one.

 

[dik]

Depending on what it is... sometimes red concrete is good... the last project where there was red concrete... there were 4 conductors (one was a spare) roughly 4 km @ $75 per foot for each conductor... likely would have really 'light up' a backhoe operator...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[STrctPono]

Concrete jacketed duct banks are not reinforced in my neck of the wood either. For 99% of applications there is probably no need. The jacket is there to protect the lines in case someone tries to dig them up, it's not typically a structural concern. The only time I have reinforced them is when we are tying the duct bank to the footing of a retaining wall or maybe we will reinforce the last 20ft of it as it approaches and is supported on the rear side of a bridge abutment backwall. Even under a roadway, if in competent (I hope) soil there should be no need to reinforce it. I have, however, been a part of projects where we supported utility duct banks on deep foundations.

 

[JedClampett]

We reinforce electrical duct banks and require them to be red. There's a lot of reasons that don't hold up to close examination, but bottom line is, that if you're in a water or wastewater treatment facility, you're likely to be digging a trench through it or a hole into it. There's a lot of casual concrete (waste concrete, thrust blocks, foundation extensions, etc.), but if the backhoe operator sees red, he's been warned. Keep digging and you might die.
They're reinforced with #4's around the perimeter at 12 inch spacing and #4 ties at 24 inch centers. There's no real design, but I have used the reinforcing to span the duct bank over a temporary trench.
Side note: Whenever we have a VE (Value Engineering) phase on a project, the first thing the contractor wants to eliminate is duct bank reinforcing. So we have them quantify the amount the owner will save, and it invariably comes out to $5000 or $10,000. On a twenty million dollar project. So we laugh, and say put it in per design. It actually makes us look good, that the contractor can only find one thing we've "overdesigned" and it doesn't amount to anything.