Look on either end of the joist.  There will be a tag.  Take down the numbers on the tag and then call the manufacturer or match numbers in the 75 year joist manual.

if you know the cross section of bottom and top chords, you can calculate the moment inertia and section modulus of the joist.

dcceecy...doesn't help...joist tables are mostly empirical.

Ron,
I've seen that said a few times here, but wish you would explain.  Since it is a long time since I worked in the US, it is a long time since I have used bar joists.  But I always thought they were designed and could be analyzed in the same way as any other truss, provided you know how.

hokie66...they can be and I believe they are now, but I haven't done anything with open web joists in quite a while either.  Early on, they were empirically designed (mostly the welding) by load testing.  The reason for that was the poor weld profiling that was necessary because of the geometry...you couldn't get a proper profile weld in place on the "wiggle bars" or the flattened angles, so they just welded wherever they could and load tested them.

As I recall, if you did the analysis as a strict truss, the numbers didn't work with the old tables.  I've analyze them as deep beams,neglecting the web and gotten closer.  Obviously parts of them have to be looked at like a truss, particularly when considering buckling of the first diagonal under load reversal.

Might be an exercise for a few beers and a boring night.

hokie66...ps..that info came from the Canadian counterpart of the Steel Joist Institute or directly from the SJI as a result of a failure investigation we were doing on the collapse of a Sears store roof in St. John, Newfoundland in the late 80's.  I doubt that I still have the file, but I'll check.

there must be some tag troll at work in my neck of the woods. i've looked at a LOT of existing buildings and never seen a tag on a joist but maybe twice.

I've seen them on almost all joists I've ever seen.

 

Same here....routinely look for and find tags when dealing with existing joists.

I have seen tags on almost every joist except one building where the Owner told me that they specifically removed them because they were a food manufacturer. We cut selected pieces from the joist and had them tested. It turned out they were "H" series but the age of the building fell in that time period where it could have been either.