Rebar weight related 1

[Vikasgkothari]

I am a chemical engr but presently looking after petrochem project construction cordination.I came across a typical problem. All the rebars used in silo structure are less than normal weight i.e. in the range of 94% to 97%. Individually they are acceptable but are the collectively OK

 

[dougantholz]

If the reinforcement is under weight, it is probably actually undersized. It could be that the actual diameter is within the tolerances but typically steel is a pretty uniform 490 lbs/ft^3.

 

[jike]

I would not at all be surprised if manufacturer's run as close as they can to just within the acceptable low end tolerance. If it is within the specified tolerance, then you have no choice but to accept it.

 

[Vikasgkothari]

Dogantholz,
Thanks. What is the standard practice for design ? Do we consider the negative tolerance as the base case ? If so, then even if all the rebars are within negative tolerance limit, the structure will be safe enough. Could you please advice based on standard practice if its acceptable ? What would be implication?

 

[Zambo]

I don't know what the tolerance is for the weight of rebar. 94%, i.e a tolerance of +/-6/% seems reasonable.

What is more interesting is the tensile strength, again I don't have the code in front of me but I assume the tensile strength is based on the theoretical area of the bar, not calculated from the unit weight. If all your rebar complies with the tensile strength requirements then I recommend that you should also rest at ease regarding the unit weight.

 

[msquared48]

Take some representative samples to a local testing lab and have them do a tensile test to failure. Pull the samples from the material delivered to the jobsite, not what the supplier gives you...

Mike McCann
MMC Engineering

 

[hokie66]

It would be of interest to know where you are and whose mill the bars came from. How did you discover the underweight problem?

 

[concretemasonry]

Since you had the bars checked for weight, what procedure did you use. There should be an ASTM standard for the material and testing procedure. I don't have my books at home right now. Often, ASTM has separate requirements for the + or - of individual and another for the average of a set of samples. This may be the case with your standard.

You should also be able to get a mill report for the steel for the purposes of composition. Where are you and what is the appropriate standard referred to in the project specifications?


Dick

 

[Vikasgkothari]

Dick,
The weights are not measured but taken from mill report only. However the mill uses ASTM standard to report(I suppose)

 

[BigH]

Let's look at ASTM A 615/A 615M-96a for Standard Specification for Deformed and Plain Billet Steel Bars for Concrete Reinforcement. Table 1 gives the nominal weight for deformed bars (e.g., Bar Size 6 (19mm) is 1.502 lb/ft or 2.235 kg/m). Section 11 gives a discussion on Permissible Variation on Weight (Mass). It states that the variation from the nominal weight shall not be more than 6% for deformed bars. Overweight shall not be a cause for rejection. The specification also gives very detailed comments on deformations as to shape, size, frequency, etc. Of course, a variation on the deformation arrangement can lead to a variation in the weight. I don't understand why there are suggestions as to going for tensile tests, etc. just because the weight varies from the nominal. Check your requirements vs the specification - if your weight (mass) is more than 6% lower than the nominal, you can reject. The contractor is required to supply conforming steel. If the weight is within the tolerance and the other requirements of the specification (and or special conditions if any) are met, you can't reject unless you are willing to pay. Normal QA, in my part of the woods, is to take samples from time to time on the delivery (contractors typically do it and if they don't we do) and have the mill test results verified by an independent testing company.

 

[hokie66]

I also would not be testing for tensile strength to resolve this issue. Maybe if for concerns that the wrong grade is being supplied, but here you are getting shortchanged on the amount of material supplied. If ALL the bars are undersize, I would raise a stink regardless of what the standard says. We have a right to base our designs on published bar areas without having another "capacity reduction factor" imposed by a supplier deliberately competing unfairly.

To answer your question directly about what area is used in the design, it is the nominal bar area, not with the tolerance deducted.

 

[concretemasonry]

The use of a mill test report for compliance is usually not acceptable unless it is identified as being the only product used in the project/e;ement.

A mill reprot in most industries is just a report of sampling from a run of materials produced and not the materialls supplied to a specific project or even where the materials were used in the elements of a project.

There is no substitute for sampling of materials on site if the is a real question of compliance.

As an example, in the cement industry, a mill report is routinely supplied to users/dealers. This is for an average of the product produced and posibly shipped immediately, unless the shipment was made from a general inventory.

Once it(the product)gets to the first distributor/dealer/manufacturer it may be put into a mingled inventory.

Once the product is at the construction site, there may or may not be a link to the specific material referenced in the mill report. The hope of identifying the specific material in an element is nearly impossible unless it has a tag on it. This is when it can be definitely identified and tested.

If you are dealing with a material that has no provision for oveage quatities/qualities the importance of minimal underages is not really important, unless there is a breach of the basic specificarions backed up by actual tests.

Dick

 

[BigH]

Not my take on mill test certificates. For each batch of steel rebar, there are tests done. The heat number is noted and the rebars are tagged with the heat number so you can compare the mill test certificate heat number to what is delivered to site. For cement, it is correct in that the manufacturer tests each batch (or day's production) but as it all goes into large silos, they would present an average for the silo. On deliveries to our sites (in India, Laos and Indonesia) our contractors always ran a few of the tests to show compliance (strength and vicat setting).

If there is no link of the produce delivered to site and the mill certificate, the product is removed from site or segregated until it can be proven to be in compliance. That's the best way to handle it. The contractor is charged (he stipulated in his contract) that he would use compliant materials. If he can't "prove" it, then he is in non-compliance. (I realize that for many jobs this line of reasoning may seem overkill - or not followed - I have typically worked on large jobs (several hundred million so contrator's requirements are quite clear).

 

[dougantholz]

BigH has identified the right procedure (star for him). If the bars meet the ASTM specified (assumed to be A615) then you have no cause for rejection as the bars meet the requirements.

 

[frv]

Given that the length of rebar is not in question, the area of the steel is therefore slightly below what is assumed for design.

Take into account that the actual yield strength of the bars is going to be well above the 60ksi minimum assumed for design. Probably well in excess of 6%.

F(assumed)=As(assumed)*Fy(assumed)

F(actual)=0.94(As(assumed))*1.06(Fy(assumed))=0.9964F(assumed)

You really have nothing to worry about.

 

[hokie66]

I may be in the minority here, but I still think the OP is entitled to bars with the nominal area or close thereto. Especially in things like silo design, area of bars is the controlling thing for controlling ring tension, not the yield capacity.

 

[BigH]

There is no question that if the steel rebar is out of compliance with the required specification, then it should be rejected. hokie66 is correct that the project is entitled to compliant bars. What one must do is determine compliance - if in, then okay as it has been taken care of and has been stipulated as such. If outside the spec, then the contractor is to remove and bring in compliant rebar - or the designer can review his design and if accepted the contractor can take a reduced payment.

 

[concretemasonry]

If you are worried, have the bars sampled and tested by a independant testing lab to determine compliance with the project specifications. So far, you have not said where the project is or what product/material specification is required.

Mill reports are usually not a substitute for prudent enforcement for a project under construction. Just because ASTM standards may have been used for the testing procedures, that does not have mean the rebars meet the project specification for the material in question. Testing procedures are different product/material specifications.

ASTM standards and/or procedures have to power until adopted by a legal entity or applicable code. Until that happens, they usually considered a "state of the art" document.

Based om your reading of the mill reports you are concerned, so it is your call. Like most ASTM product standards, there is a provision on payment. If additional testing beyond the contract is done, usually the person requesting the pays if the materials meet specifications. If the material fails there usually is procedure for retesting or possible rejection with costs born by the contractor or supplier, depending on contract documents.

Once you have complete information it can be turned over to the structural engineer (in this case) to determine if he feels the products are adequate for the design.

Dick

 

[dougantholz]

The bars are fine - don't do a thing. If they fall within the ASTM then the bars are compliant and sufficient. The ACI code is written around tolerances and assuming your design was sufficient in the first place there is no need to do anything.

 

[frv]

hokie66-

I am totally unfamiliar with silo design (and tension rings, for that matter). How is it that Fy is not important in the strength of these structures?