huangyhg

appropriate use of fy
if a structure is designed with a36 and someone made a very big "woops" (not me), and the steel is tested and comes back with fy > 50 ksi i believe it is ok to use a572 or a992 for a second look (provided the other requirements of the standards are met). is it appropriate to use fy = say 61 ksi if that's what the test shows or are you limited to a max of 50?
any opinions?
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reevaluation and initial design can follow two different rules possibly. initial design is limited to the yield of the grade specified, regardless of what any test reports might show. the challenge with using something other than speced yield in any situation is that while the piece tested may in fact show fy=61, how do you guarantee that every other piece on the job has an equivalent yield.
that said, most a36 produced today will in fact have a yield reasonably consistent with a572 grade 50. you can't call them the same thing because one is a carbon steel and the other is a high strength low alloy steel.
i would limit yourself to no more than fy-50.
what was the mistake? was the steel tagged as 36ksi yield strength from the mill, and tested to be 61ksi. or was it tagged as 50ksi steel and tested to be 61ksi? i don't think that i would do anything more than justify a localized over stressed area utilizing the 61ksi. i would not account for it thru out the structure.
i would treat this as an existing building. asce's guideline for stuctural condition assessment of existing buildings has provisions for testing materials of existing buildings. i can't find that asce specifically says what to do with the tests, but it seems to imply that one should use the test results in a structural evaluation, with no reductions from the tested values.
is there a problem that the material is almost 50% stronger than the minumum spec ?
i guess your code makes assumptions of ftu based on fty ... could you show that ftu of the material exceeds ftu assumed in the code?
i recently heard at an aisc seminar that recently most of the a36 angles are almost 50 ksi because they are being made from recycled 50 ksi material.
the problem is an existing structure that i am checking was designed with a36, but is significantly overstressed (on the order of 200%, even using the newer spec). the material was tested and a majority of the coupons came back with a yield stress around 60-65 ksi and ftu around 90 ksi. armed with that information, i'm not sure what values are appropriate to use when checking the capacity as is.
so you're questioning the material the structure was built from ? would hardness testing help you out ?
i'm asking if it is ok to use 65 ksi in a design check since that's what the test came back as. i don't know if there are other requirements (such as fy/ftu ratios, chemical compositions, elongation %'s, etc...).
eit:
if i'm reading correctly, it sounds like you're checking multiple members. if that's the case i would use the actual fy for the member that was tested and for the others use fy somewhere between 50 and 60. you could probably justify 60 if there's a reasonable certainty that the steel came from the same mill. i know it makes for tedious work but you'll sleep easier at night. this is not uncommon in bridge work; sometimes we're faced with rehabbing an old structure with no available plans.
if you're not dealing with any fracture critical members elongation shouldn't be a problem. if you need to retrofit
well, technically you would gather all your material tests, establish a mean, a coeff. of variation, etc. and then set as fy the 90% confidence value of the scattered plots of the tests....a sort of statistical method i guess.
there might also be an ibc section (chapter 17) or astm out there that gives direction to this.

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