finding steel joist size from dimensions

[jay156]

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.

 

[jayrod12]

Try the steel joist institute manual. They've got information dating back a long way.

 

[KootK]

Link

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[RWW0002]

I assume you have already looked for a joist tag attached to the end of the joist. I have had some luck obtaining information from the joist suppliers from tag information in the past. If you are looking for a preliminary design estimate (while waiting on results from SJI per KootK's link), there are some old tables available from SJI for common chord sizes for the various chord numbers. It is in the 50 and 75 year joist digest from SJI, but I might be able to dig up a copy for you if you need it. You can also do some basic analysis using the joist specification section and the data gathered from your field measurements and get pretty close to the actual capacity.

 

[hawkaz]

The steel joist institute has a form on their website (See KootKs link above) where you can fill in the measurements and they can tell you what they think the joist is.

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.

 

[JAE]

Probably better to use KootK's link but the table below was something I picked up many years ago from Vulcraft - they quit publishing it because I think they found that variations in the actual chord sizes was in conflict with the table. The table was primarily used to offer designers a feel for the width of the joists in terms of clear distance for openings between joists.

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[RWW0002]

JAE's chart is the same one I was referring to.

 

[SteelPE]

JAEs chart is found in my 75 year joist manual too. The table is not applicable to k series joist though

 

[jike]

Best way to know for sure is to remove the joist tag and read it. Sometimes, there may be multiple tags on some joists.

 

[JAE]

SteelPE - my recollection is that the table was for H joists.

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[Once20036]

If you have the chord sizes and depth you can calculate the allowable tension/compression.
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.

 

[msquared48]

Actually, MY SJI 60 YEAR MANUAL SHOWS THE SAME TABLE, AND ALTHOUGH NOT APPLICABLE TO k JOISTS, IS VALID AS A possible chord size only FOR sj, "s", s, j OR h JOISTS AS STATED AT THE TOP OF PAGE 11..

I am not screaming. I just do not want to type all of that again...

Mike McCann, PE, SE (WA)

 

[L2L]

Once20036:
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!

 

[Once20036]

I was reading this thread thinking to myself... this seems so familiar...

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.