Old Steel Roof Joist Analysis

[jpw2913]

I am having to analyze some old steel open web joists. I don't have the joist designation, unfortunately. From my research, I found out the building was constructed in 1946. I don't have the "80 Years of Open Web Steel Joist Construction" by SJI and I'm not sure the joists in question would be in there b/c they are a little odd. The web members are a mix of angles and flat steel (all diagonal web members are flat steel except the two outermost, while the vertical web members are angles). I've attached a rough sketch of the joist. Before I by the "80 years" book from SJI, I was wondering if anybody had any info on a joist like this or would know where to look for load tables, etc.
Thanks for any help, see attached sketch.

 

[strucguy]

For a joist that old, you are probably better of doing a quick analysis using basic structural analysis software. I am guessing you already have the basic member info. with that in had, modeling a joist shouldn't take that long. I would go ahead and assume 36 ksi steel.

If you don't have any means to identify the joist...I wouldn't count on any SJI table. What is the guarantee that the joist was originally designed per SJI specifications.

 

[msquared48]

No standard SJI documented Steel Joist that I have ever seen and I have that catalogue. You will not find it there.

More likely than not, these are ones that were designed and fabricated for that particular building. If so, you will have to do a computer analysis, but that should be simple enough.

There have been other manufacturers over the years like Macomber and Great West that are not documented in the SJI catalog. I could be wrong and this may be one of them, but de3finitely not GW or Macomber.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

I would analyze the joist by hand. It is likely that the material is ASTM A7 steel with minimum yield of 33 ksi. A36 was not available until much later.

BA

 

[hokie66]

It is just a Pratt truss, and is determinate. Probably designed by method of sections or joints, or maybe graphically. You really only need to check the critical members, which can be selected by inspection.

 

[efsinc]

Watch out for for stress reversals in the flat webs under your new loading conditions.

 

[jpw2913]

Thanks for the help guys!

 

[strucguy]

Don't forget to check deflections. Evaluating member forces can be done by hand as suggested by others. But, if you are even interested in knowing the deflection, having an analysis software is very helpful. Good Luck!

 

[BAretired]

Deflections can be determined accurately enough by using the moment of inertia of the chords and increasing the resulting deflection by 10% to account for shear deformation.
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Alternatively, accurate deflection can easily by computed by hand as well.

BA