Rebar Estimations 2

[YIGO]

Hi everybody,

I am an EE and I am involved in duct banks estimations. I need to know how to assess rebars quantity for a given concrete defined area.
Thanks for your consideration.

Yigo

 

[Focht3]

Your question is too general. Please provide a specific problem - with lots of specifics. That is, if you want useful assistance...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[Qshake]

You are:

1. Going to specify or estimate the amount of reinforcing area to place in the duct bank?

2. Taking off quantities for the reinforcing someone else put in the duct bank?

3. Estimating the weight and ultimately the cost of the reinforcing someone else put in the duct bank?

back to:

1. A EE shouldn't estimate the amount of reinforcing to place in a duct bank. However, in a pinch if the bank is sitting on the bottom of a trench that is to be backfilled such that it is a square of concrete surrounded by soil - ACI will give the minimum steel required for temperature and shrinkage.

2. If you are taking off quantities that a CE in your firm gave you, they need to give you a final weight. Its simply easier for them. Else you'll have to find the weights per length which are commonly found in tables in concrete textbooks.

3. See the answer to number two and use $0.65/pound for black steel and $0.90/pound for epoxy coated steel.

 

[dik]

As a WAG, if the duct bank is lightly reinforced, you might consider 250 pounds of reinforcing steel per cubic yard. If it's heavier, then you might consider 400 pounds per cubic yard. If it's even heavier, it's best to do the 'sums'...

I'd use the above for budget 'guessing' only and if the matter is critical, then I'd do the actual calculations. If you need it, I can fax or eMail you a sheet with the bar sizes and weights per foot, either Metric (eg, M15) or Imperial (eg, #7)... post your fax or eMail address if you want it...

 

[YIGO]

Thanks All,

Dik, the information you provided is what I'm looking for.
This is my email:yigosire@yahoo.com. Please provide futher information.

Yigo

 

[cvg]

just a stupid question from a civil, but why reinforce a duct bank? I assume this is in a trench? I have seen duct banks designed before where the ducts are encased in concrete, but primarily to protect the ducts from damage (ie: the accidental backhoe operator who gets electrocuted when hitting the duct bank. I have never seen one with rebar.

If not in a trench, usually in a casing or raceway, no concrete or rebar needed.

 

[Bbird]

Cvg,

A duct bank can be very large say in a matrix of 12 by 10 and needs both strength and protection from concrete. Cable ducts must remain separated from each other throughout the service life and electric cables are expensive. The ducts are spaced very close to each other and need steel reinforcement to provide adequate strength.

Duct banks are seldom designed indivually but follow standard details proved over years of application.

Yigo and Dik

With all due respect I think the rebar quantity quoted by Dik is too high.

Chas Reynolds in "Reinforced concrete designer's handbook" 1957 and Charles E Reynolds & JC Steedman in 1974 " quoted the following:

The amount of steel in an ordinary reinforced concrete structure varies from 1 ton in 5 cu.yd. of concrete to 1 ton to 30 cu.yd. [265 to 44 kg/m3, 1 ton=1.016tonnes, 1 yd=0.9144m]

Warehouses, girder bridges, rectangular tanks, water tower,silo,circular bunkers,pits & pit head gears: 10 to 15 [132 to 89 kg/m3]

Factorties,residential buildings, retaining walls, culverts, swimming baths and open circular tanks: 15 to 20 [89 to 66 kg/m3]

Arch bridge, rectangular bunkers, pyramidal tanksin the ground and elevated conical tanks: 8 to 10 [166 to 132 kg/m3]

The 250 to 400 lb/cu yd is equivalent to 148 to 261 kg/m3 in metric term. I use 100kg/m3 as the starting threhold of congestion in rebar. I kept a reocrd of 4 power stations where 365000, 185000, 110000 and 6332 m3 of concrete were poured with 33600, 18000, 7000 and 482 tonnes rebar consumed giving an overall rebar content of 92, 97, 64 and 76 kg/m3 respectively. I have allowed people designing gas turbine foundations getting away with 75 kg/m3 rebar content.

I would be surprised if a duct bank needs more than 75kg in a m3 of concrete. My figure would be to go for 60kg/m3 or 100 lb/cu.yd, which is good enough for normal pad foundations, in the estimate.

 

[dik]

The rebar quoted at the low end are budget costings for typical beam reinforcing. I have no idea of how he was going to reinforce the duct. It's conservative for most applications and the added cost for rebar would likely have little impact on the overall cost. With laps and multiple ties being used between the ducts, it is likely on the low side. In comparison, for a light wall I would use about 100 lbs/yd.

The high end was for typical column reinforcing. If the individual ducts require ties around them and multiple longitudinal bars are used, then it is likely conservative but may be in the ball park.

As noted... it was a WAG, not knowing the specifics. 250 lbs/yd, is equivalent to about 150 kg/m. Thanks for the additional information; if there is a fair amount of mass concrete or large elements, the values you note may be on the light side for something smaller...

 

[Bbird]

dik,

I agree with your rebar content is a practical range for beams and columns. I audit design for a living and the rebar content is part of my sanity check. I wouldn't contemplate to upset anybody here but I have designed and supervised duct banks before. Under normal circumstance it is just a 4-inch or 100mm concrete surround lightly reinforced with 8 or 10mm diameter bars and there is little scope to reinforce it like a beam.

In cost estimate there is no harm in using the a consevative figure to cover adequately the cost.

My numbers on the as-used concrete and reinforcement quantities for power plant construction should be treated as caution if used for other applications. A power plant relies on the availability of its components and infrastructure to bring in revenue continuously and a 24-hour shut down can cost millions ( in UK the revenue loss from shutting down a 1000MW station is about £0.5 millions or US$ 0.8 to 0.9 million). Thus power plant structures are designed biased to durability. With some exceptions, like turbine supporting structures, the reinforcement is not particularly heavy but is used extensively.

 

[johnd43]

Yigo,

As a civil engineer who has the temerity to estimate electrical things from time to time, I would advise you to be much more general in your approach.

Don't kid yourself that you are getting a better answer by guessing the components and then multiplying them out. I would suggest you ask around for indicative pricing in your area for a unit volume of concrete in finished structures. Use that as your factor on the gross volume of concrete that you can envisage.

The analogy I would offer is when I am doing budgets for, say, an electrical installation in a pumping station. I count the number of drives (N) and check the dimensions of the structure. The electrical cost is then:
N x 1 cubicle each (or 2 if they are big drives, or 4 if they are Variable Speed too) x an indicative price per cubicle,
plus
the maximum length of the pumping station x N x an indicative price per lin m for cable on tray x a lot (because you guys always use more wires than you tell us first time out).
Thats the base price. Now add 100% because you guys always......
plus a percentage for method related costs,
plus a percentage (compound) for design,
plus a percentage (compound) for contractors costs, margins etc.
Now you are somewhere in the sphere of a contract price +/- risk, uncertainty and error. So allow yourself another percentage contingency and, if you are doing budgets for the owner, don't forget to add something for their establishment/management costs.

If this is a commercial bid you might by now have reached the level where you can offer the price and be be confident that you are so d****d expensive that you won't win. If the unthinkable happens and everybody else has been even more careful, then you have won the job and you are going to go bust!

Otherwise, I would go to your quantity surveyors and make them justify their exhorbitant cost by doing a proper job from the outset. The added advantage is that you have someone else to blame.

Estimating is the best fun - a real white knuckle ride.

 

[npthao121]

Hi all,
I have experienced with the matter regarding to YIGo question.
Sometime at the site, the electric guy asks me the question like this, they just want to know how much kg per one cubic meter of concrete. Probably, they use this value to deal with their sub-contractors or estimate the budget (I guessed), even cost estimation is already made before. Normally, this is about 40- 50 kg/m3 in my case. Generally, we have to carefull when pouring concrete, because the sleeves inside ductbank is very close to each other.
Cvg,
The first time when I was a fresh guy in a construction site, I also wondered why big amount of rebar has been for duct banks. Although I clearly understand that we need rebar to prevent cracks, small bending moment (even base was well-compacted, high modulus of sub grade), load impact from traffic ¡¦I still think that can reduce this rebar. When I raised the idea with the PM, he laughed at me so much and asked me ¡°do you know how much the instrument cable inside the sleeve is? And how important the ICS system is, what happen to the contractor later if something happen to the plant due to instrument cable damage?¡± the he answered ¡° we paid milions dollar for the ICS (intergrated control system), in case an instrument cable break make the ICS shut down or some equipments stop during their operation, contractor has to pay a huge amount for penalty¡± we cannot risk, anyway the cost for the item like ductbank particularly or the whole civil work in general is just small percentage of a industrial project.
The I understood the point. We may waste some rebars for absolute safe of the plant.

 

[Bbird]

npthao121,

I am glad you have independently confirmed my thought here with

(1) Rebar content of 40-50 kg/m3 in a duct bank as I have suggested 60kg/m3.

(2) There is little scope to heavily reinforce a duct bank because of the close spacing.

(3) In Power industry availability of its infrastructure is the top priority.

 

[cvg]

my experience with ductbanks has not been in an industrial setting. Mostly with existing electrical transmission line duct banks in roadways. Most were either steel or pvc, laid in the trench and backfilled with concrete. No rebar ever installed. None of these had control circuitry.

 

[Bbird]

Cvg,

The type of concrete surround to cable ducts is slightly different to the duct banks interested by the electrical engineers. Althogh I do not have the details of the cables involved in your work but steel pipe is seldom used for carrying cables in a power plant. High voltage electrical cables need robust protection even if they are outside an industrial area. Depending on the supply arrangement many owners of 11 or 33kv cables operate a permit-to-work system for any construction near their cables.

The duct banks interested by the electrical engineers have draw pits between the two ends and ducts have to be laid with radii capable to accepting the bending radius of the cables. In an industrial environment like a power plant there is an enormous amount of underground services and large tunnels have been designed just for carrying cables.