Estimating the amount of re-bar??? 1

[mark416]

Hey all.

How would you approximate the ton of reinforcing in say a footing? I am doing a quick estimate of material quantities (cubic yard of concrete, ton of re-bars, etc.) for a foundation. I have not done a design of the footing and I don't know the loads applied to it, but I did guess the size of the footing. Would there be a way of estimating the ton of re-bar if I don't know the size, number or spacing of re-bar being used?

If I have a 4ft x 4ft x 1.5ft thick footing, could I just use the volume of concrete (24 cu.ft.) and multiply the density of steel (489.6 lbs/cu.ft) to get the total weight of steel required. (24*489.6 = 11,750 lbs of steel required.)

Am I out to lunch on this?

Thanks,
Mark.

 

[haynewp]

R.S. Means catalogs can give you estimates based in the realm of what you are looking for.

 

[mrpid]

The calculation you provided would only apply if your foundation was solid steel! Since it is an estimate you could consider minimum reinforcing per ACI 10.5.1 or simply 0.00333 x bw x d therefore if you have a 4' square footing x 18" your min reinf would be 48" x 18" x 0.00333 = 2.877 in^2 x 48" length (discounting cover because it is an estimate) x 2 (for each direction) = 276 in^3 / 1728 in^3/ft^3 = 0.160 ft^3 x 490 lbs/ft^3 = 80 lbs of steel per footing.

Now, I am going out to lunch!

 

[Ussuri]

Estimates are just that. And always wrong. Be careful what you do with an estimate based on concrete volume alone. Even if you include for a contigency, I will still bet its wrong.

I have worked on projects (Design and Build) where our client (the contractor) has pushed for an early heads up on reinforcement tonnage based on volumes alone. You apply estimates as best you can by looking at published guides and comparing it to previous jobs but it will still being wrong. In once case we even ended up defending a PI claim as the contractor claimed the estimate tonnage didnt match the final as constructed tonnage and we were therefore negligent. Albiet I expect they saw our PI as a way of recouping their losses.

 

[Ussuri]

Say maybe 150lb per footing.

 

[cvg]

Sometimes for rough estimates, I just use a ball park estimate per cubic yard of concrete. For light structures up to 50 lbs steel per cubic yard. for heavier structures up to 100 lbs steel per cubic yard.

 

[Teguci]

I would recommend using the CRSI Handbook to get a rough estimate for a similar sized footing.

If you need a more acurate number, there are some old DOS programs by CRSI (or even the way too expensive new programs) that will calculate how much steel you need given specific footing properties.

18" thick footing for a 4x4 - you must have some hefty soil bearing capacity.

 

[crossframe]

I've successfully used this as a basis for bridge projects. Your mileage may vary due to regional or local differences, preferences, detailing practices, etc.

 

[JStephen]

Just use a number for "reinforced concrete per yard" and you don't need to break out the rebar. Of course, the $/yard varies tremendously depending on what it is.

 

[SlideRuleEra]

As a bridge construction contractor (years ago) I was always surprised at the consistency of the ratio of rebar (lb.) to concrete (cubic yards). The numbers that crossframe has presented are in line with my recollection.

To get a better estimate for a different type structure, perhaps you can look at records from completed projects similar to yours.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[civilperson]

Try 530 pounds of steel per cubic yard.

 

[civilperson]

OOPs, factor of ten too much.

 

[mark416]

Thanks,

All great information.

Mark.

 

[UcfSE]

State DOT's may provide this for bridges for estimating purposes. You could go back into your finished projects and compile a database of rebar used per cubic yard of footing concrete, if nothing easier comes up.

 

[pba]

Good document from Crossframe!

Withour better information I have tended to use 150kg/m3. (I think that is 430 lbs/yd3).

Once I have more idea of the actual members then I fine tune the quantities.

mrpid's approach will underestimate the actual steel weight as it makes no allowance for laps, bobs or the other bits of rebar that are always needed. However this is a simple pad footing so the estimate would not be far wrong in this instance.