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Re: Existing steel joist manufacturer?

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There is no way to read the tag and definitively know anything of value
about the joist.  If the joists were fabricated by a member of the Steel
Joist institute then the name of the manufacturer would be embossed on
the tag.  If these joists are 24" deep then you know that the first
number probably corresponded to the depth.  If they are some other depth
then it doesn't.  There never was any standard for tag information.

But even if you have that, you would need to know when that particular
manufacturer switched from two different grades of steel (36 and 50) to
only one before you'd know for sure what you had.  Several manufacturers
used 50 ksi steel in J series joists, and just gave away the extra
strength.  Conversely, some tested each heat, and used the test value
for Fy, thus robbing you of some of your reserve.  It is unlikely a
different grade of steel was used in the top chord from the bottom
chord.  It is also unlikely that web members would be fabricated from
different grades of steel.  However, the chances that the chord steel is
different from the web steel are fairly high, particularly with cold
formed chords as you describe.

The top chord is usually larger than the bottom.  Thickness and/or strip
width will change from one to the other.  It is also fairly common, but
not universal in that era, for joist webs to decrease diameter from the
end towards the center.  Not continuously, but expect one or two
changes.  So be very careful when you measure, and don't assume that a
standard gage or diameter was used - the joist manufacturers have often
had large enough orders (or owned their own mills) to get custom sizes.

Good luck, have fun with your micrometer, and get the SJI's 60 year
digest.  If you need any additional help, write me.

Phil Hodge