finding steel joist size from dimensions
finding steel joist size from dimensions
(OP)
I have a mezzanine with some existing steel joists that I measured up in the field. Depth, size of the angles of the top and bottom chord, diameter of the web bars...
How do I turn these dimensions into an actual joist designation so I can determine its capacity? The Vulcraft book is completely USELESS when it comes to this.
How do I turn these dimensions into an actual joist designation so I can determine its capacity? The Vulcraft book is completely USELESS when it comes to this.






RE: finding steel joist size from dimensions
RE: finding steel joist size from dimensions
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: finding steel joist size from dimensions
RE: finding steel joist size from dimensions
I am not aware of a publication where you can determine this yourself.
The joist tag is only valuable if the joist manufacturer is still in business and if they are cooperative. You will only get their job number and part number off the tag in most cases.
RE: finding steel joist size from dimensions
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RE: finding steel joist size from dimensions
RE: finding steel joist size from dimensions
RE: finding steel joist size from dimensions
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RE: finding steel joist size from dimensions
If you know the effective depth, you can get the allowable moment.
This leads to the allowable distributed load and the size of the joist.
In my experience, if you call SJI they'll tell you the weakest joist (based on the era of construction) that matches your joist depth, regardless of any other measurements you took.
I`ve never had great luck with the chart published above.
RE: finding steel joist size from dimensions
I am not screaming. I just do not want to type all of that again... :)
Mike McCann, PE, SE (WA)
RE: finding steel joist size from dimensions
Could you please guide me a bit further about how to calculate the Tension/Compression and get the allowable moment?
I went to a job site and took measurements of the existing joist (depth, angle sizes, and length)
Thank you very much for your help!
RE: finding steel joist size from dimensions
L2L - top and bottom chords are likely double angle sections. To oversimplify the process:
- Calculation the section properties for top and bottom chord
- Use AISC (in the states) to calculated allowable compression per chapter E and allowable tension per chapter D
- OR, calculate allowable compression and tension per SJI. My vulcraft joist manual has all the equations necessary. Last time I compared results, they were similar but not the same as AISC results. Unbraced length will be critical for your compression calc. Vertically the chord is braced by the web members. Horizontally it is braced by attachment to the deck. Think about 36ksi steel vs 50ksi steel
- Use the lower of the two calculated forces.
- Moment is the force * distance between the chord centroids.
Typically I`ll calculate the moment of inertia and use that to get the load that creates L/360 deflection.
Then, head into the appropriate tables (if you know the age of the joist). Looking at load & deflection its usually easy to find a pretty good match.