huangyhg

rebar weight related
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
find a job or post a job opening
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.

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

dick,
the weights are not measured but taken from mill report only. however the mill uses astm standard to report(i suppose)
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.
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.

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