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RE: NEW STRUCTURAL STEEL

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Title: RE: NEW STRUCTURAL STEEL

>I was taught in Grad school that I should always consider A36 to have an Fy=50ksi
>since mills today produce mostly dual spec steel. This recommendation was brought
>up in an EBF lecture. Does anyone think it is appropriate to always use A36 w/ Fy=50ksi
>in all NEW construction for any application (even angles and channels)?
If you specify A36, it is my opinion that you should calculate the strength of a member based upon Fy = 36 ksi, because that is all the producer is obligated to provide per ASTM A36. It is likely that the actual yield strength is higher than that however. Wide-flange shapes should be specified as A992 (Fy = 50) and doing so will let you design with Fy = 50 ksi. M, S, and HP-shapes and channels are commonly specified as A36, A529 grade 50, and A572 grade 50 with industry trending towards the two 50 ksi specs (but not 100 percent there yet...check with your local fabricator). Angles and plates are usually A36, although there may soon be a trend toward moving these along to a 50 ksi base steel too.

Now for high-seismic design where you are not calculating the strength of amember but rather the demand that will be induced on a connection, column or other member due to inelastic deformation of another element in the structure, your assumption of Fy should be different. For example, in a moment frame, you expect to hinge the beam, so the connection to the column and the column has to be good enough to do it. That means you have to use a more reslistic estimate of Fy. In the AISC Seismic Provisions, we use an Ry factor. For A36 w-shapes, Ry = 1.5. For A572 grade 50 w-shapes, Ry = 1.1. The end result is that the expected Fy is essentially the same.

Hope this helps.
Charlie