Wide flange steel types, properties, availability?

[koodi]

My understanding is that ASTM A36 has been phased out of production by ASTM A572 Gr. 50.

1) Does this mean using ASD, that the allowable stress should be 33 ksi (for a simply supported wide flange beam in bending)?

2) Should I expect servicability issues, such as, deflection and fatigue to be the controlling factors?

The design is in the preliminary stages. It is a 100 ft span simply supported at both ends.

 

[dicksewerrat]

When did this happen? Put both calc.s in your design so you or contractor buys the right beam.

 

[koodi]

Why should I design for 36 ksi steel if it is no longer available?

 

[JAE]

ASTM A36 was the mainstay for years - as the means of creating the steel changed (using various scrap steels, etc), the Fy of the A36 began to creep up to near 48 to 52 ksi.

This overlapped with ASTM A572, Grade 50 and for a time, a "dual grade" concept was used where a single piece of steel might meet both A36 and A572 requirements.

A new spec: ASTM A992, Grade 50 is the currently favored type and should be specified for wide flanges.

Yes...with higher Fy, the servicability checks sometimes control the design; but as always, you should be checking both anyway.

 

[koodi]

Jae,
So far as availability and economy. Apparently, according to a couple steel suppliers, the "dual grade" steel is still around. In the interest of economy, isn't it advantageous to specify either A992 or A572 Grade 50, and let the contractor choose? If it is all the same from a design standpoint, 33 ksi allowable in bending. I'm waiting till the project is "pinned down" a little better before dealing with the servicability issues.

Any more thoughts.
Thanks for the reply.

 

[JAE]

I believe that A992 is essentially the "dual grade" steel, only just now its put down in writing in an ASTM spec. So you could have a piece of steel out in your yard that meets all three (A36, A572, and A992).

I don't believe that you are then limiting anyone by simply specifying A992. There may be some differences in terms of hardness, fatigue strength, etc., but for basic buildings it shouldn't matter.

I've never let the contractor "choose" the steel, rather, I specify what I need and then if the contractor wishes to substitute a different steel he can under our contract specifications.

 

[koodi]

So far as bridge design specifications, I only found one "up to date" manual that had the AASHTO type steels with their corresponding ASTM type. The manual did not mention A992, but did mention A572 Grade 50. Thus, as long as the necesssary checks are made, it should be alright to use A992 and its higher allowable stress.

 

[haynewp]

Be careful about higher yield strength steels. A member of higher yield strength can fail before one of lower strength, and it has happened and backed up by AISC equations. Even if you design ok for 50 and it comes in at actually 60 something if can fail.
WT shapes brother.

 

[HgTX]

ASTM A 992 is not "dual grade" steel. "Dual grade" steel met requirements of both A 572 Gr. 50 and A 36--in other words, it met the minimum yield and ultimate of 50 and 65 ksi, respectively, and also the max ultimate of 80 ksi and slightly higher ductility that the A 36 spec carries. Most steel labeled either A 572 Gr. 50 or A 36 really meets both grades, regardless of whether it's specifically labeled "dual grade". (The answer to designers saying, "No, don't give me dual grade, if I say A 36 then what I want is A 36!" was simply to leave off the "A 572" notation.)

A 992 has the same minimum yield and ultimate as Gr. 50, but instead of capping the ultimate at 80 ksi they cap the yield at 65 ksi. It used to be a special version of A 572 Gr. 50 and then they made it official. So all A 992 is theoretically "dual" graded with A 572 Gr. 50, but that doesn't say much, and that's not what people are talking about when they say dual grade. If it meets the 80 ksi max ultimate and the higher elongation requirements, it'll be dual (or triple) grade.

A 992 is replacing A 36 and A 572 Gr. 50 for wide-flange beams.

Regarding whether you should bother designing for A 36...
it's not the big savings you imagine and that we were taught in school. It is no longer the case, and hasn't been for decades, that Gr. 50 is some high-strength specialty material to be brought in when A 36 just isn't enough. The cost difference fluctuates, isn't usually that much, and sometimes, depending on market forces etc., there is even a cost premium on the A 36 because the max ultimate makes it a rarer commodity than the Gr. 50. For built-up plate girders, I don't see the point of 36/50 hybrid girders; any possible cost savings would probably be offset by the effort spent tracking two kinds of material and the extra trouble to design a hybrid cross-section. (Gr. 70 is another story.)

Either way, you can't assume that the yield strength you design for is the exact yield strength you get--those specs are minimums. You can't assume that if you have 50-ksi columns and 36-ksi beams, that you're getting "weak beam strong column" ductile failure modes. A 36 and A 992 both have maximums of some sort that you can count on--but unless that maximum is less than the minimum of some other grade, you still can't guarantee that the supposedly lower-graded steel will fail first, especially because lower grades of steel tend to be more ductile.

If you want a quick side-by-side comparison of all these grades, check out ASTM A 709. A 709 Grades 36, 50, and 50S are equivalent to A 36, A 572 Gr. 50, and A 992, respectively.

Hg