Steel joist bottom chord extension and stabilizer plate

[EBF]

My understanding is that the bottom chord extension for OWSJ at columns is required for to provide stability during construction. Does that mean that it's ok to cut back the bottom chord after construction is complete? Has anyone done this?

 

[JAE]

There's also the concept that the extension is used to laterally stabilize the bottom of the supporting beam as it runs over the top of the column.

If the beam has adequate vertical stiffener plates over the column and the fear of the column top buckling sideways isn't significant then the extensions could be removed, yes.

Their presence always bothered me as they formed a continuity that I never believed the joist manufacturer takes into account.

This is from an SJI - K Joist specification:
[blue]
5.1 USAGE
This specification shall apply to any type of structure where floors and roofs are to be supported directly by steel joists
installed as hereinafter specified. Where joists are used other than on simple spans under uniformly distributed loading
as prescribed in Section 4.1, they shall be investigated and modified when necessary to limit the required stresses to
those listed in Section 4.2.

When a rigid connection of the bottom chord is to be made to a column or other structural support, the joist is then no
longer simply supported, and the system shall be investigated for continuous frame action by the specifying
professional. The magnitude and location of all loads and forces shall be provided on the structural drawings. The
specifying professional shall design the supporting structure, including the design of columns, connections, and
moment plates*. This design shall account for the stresses caused by lateral forces and the stresses due to connecting
the bottom chord to the column or other structural support.

The designed detail of a rigid type connection and moment plates shall be shown on the structural drawings by the
specifying professional. The moment plates shall be furnished by other than the joist manufacturer.

*For further reference, refer to Steel Joist Institute Technical Digest 11, “Design of Lateral Load Resisting
Frames Using Steel Joists and Joist Girders." [/blue]

 

[lexpatrie]

Well, here comes another essay....

The joist stabilizer plate is required at "column joists", per OSHA. It's probably there to prevent the joist from rolling over back when steelworkers would "walk" on the joist, the joist has extremely limited capacity for this kind of load when the full span is unbraced, similar to walking on a 2x12 that's 20 feet long without any wood deck on it. I specifically remember some footage from the old Basic Design for Stability seminar series showing Ted Galambos sitting on a joist with a suspension harness and a hardhat in the lab and the joist swinging back and forth.

I've found mention of a 2002 policy but nothing farther back. But I have not done a detailed or thorough search.

I have a circa 1994 building locally that doesn't have them.

Enforcement Policy on Column Joists, OSHA, 7/8/2004. (this mentions a 2002 policy that's rescinded/superseded) [Edited to add: Here's the policy, cpl-02-01-034, sort of. It seems like it's just referring to another publication in the federal register. Federal Register Volume 66, Issue 12 (January 18, 2001), This is the January Federal Register, publication, go to page 630.

The second Federal Register mentioned in the 2002 policy doesn't seem to have "stabilizer" mentioned anywhere in it, but here's the link to the PDF, Federal Register, Volume 66, Issue 137, July 17, 2001]

As to the stabilizer plate, the SJI specifically says don't weld it. Or perhaps more precisely, a former Canam Steel Joist design engineering manager shows a detail that says "do not weld", and mentions it is TYPICAL. (Yeah, it's showing a girder truss, but if you look, there's a plate coming at us for the joist that's on top of the girder truss coming in perpendicularly). I don't want these welded on anything I design (not that I have, lately, say 2017).

Take a Few Moments to Do it Right, Lucas, Structure Magazine, November 2006

Yes, I think you can remove them, but why bother? Removing them also makes your steelwork look "older" because it's missing something that should be there (an erection aid/OSHA compliance item) that will tend to mislead any future engineer as to when the structure was built. I suppose there's always the satellite imagery on Google Earth and the building department/assessor, and there's maybe going to be weld remnants you could see (from the ground?), but still.

The rest of this may or not pertain to your particular design situation......​

Secondly, there's a tendency in some structural communities to use these as braces for the beam (for lateral torsional, probably in the downward/gravity direction), hurricane wind zones and lighter roofs might get lateral torsional in the opposite direction.

They aren't plausible braces.

They aren't designed for a continuity force (induced compression when the joist deflects downward and the seat rotates), either, and if there's no bridging at the first bottom chord panel point, the unbraced length for that bottom chord seems to be the full length of the joist, or at best the bridging spacing which you really DON'T have in design, because the Joist Manufacturer designs and locates them, "bridging per SJI", meaning you can estimate where they are, but don't actually know until you get those shop drawings. The bottom chord buckles with very little compression and it is not an effective brace.

If you have bridging/bracing at the bottom of the first panel point, it's there to brace the joists for uplift, NOT to provide bracing for a steel beam/column. It could perhaps be done for that as well, or it's a separate (uplift versus download) case and you're not adding load to the bridging, but bridging is pretty spindly, and to me the connection and the stiffness of the bridging is questionable in terms of being an effective structural steel beam brace.

I'm kind of going through this with Ask AISC at the moment, as one of their older design documents (Cantilever Roof Framing Using Rolled Beams, AISC, circa/post 1994) shows "no stiffener required" at the beam on top of the column and the sort-of source document and/or inspiration for their publication, Design Guidelines for Continuous Beams Supporting Roof Structures, Rongoe, NASCC 1996) (after AISC's rolled beams document?) [AISC gives "unknown" for th date of the document, by the way) and the prior CISC's Roof Framing with Cantilever (Gerber) Girders and Open Web Steel Joists, first printing in 1989, shows a check for a web crippling or stiffener design of the beam.

I'm not fully convinced the web crippling check is all that needs to be done here, because with cantilever framing, there's an overhang and the beam at the column can be in compression on the bottom flange, CISC gets into this discussion where the effective length is 7.5x the span in some situations, even at a 5' cantilever, that's 35'+ of effective length.....

Current AISC guidance and manuals has a lot of discussion about stabilizing the beam web / column, the most straightforward is a stiffener at the beam bearing location. I don't think they show a "fitted" stiffener, but that's perhaps typical.

(Source: Steel Tube Institute)
This is "reasonably close" to the stiffener, Their post isn't really on this topic, it's just a decent visual depiction of the situation. The text does mention a vital need to stabilize the column out of plane....)

You may find this article interesting (I've read it):
Cantilever Roof Structural Failure Caused by Excessive Construction Loads, Flawed Design, and Poor Maintenance, Hal Cain, ASCE Forensic Engineering, 2009

This looks interesting (Don't have, haven't read):
Beam-over-Column Bracing Requirements in Continuous Steel Frame Buildings, 2012, Hauck and Moe, ASCE Structures Congress, 2010

 

[JAE]

Well I didn't think that the original question had anything to do with joist girders...only joists at column centerlines.

But lots of good info - thanks lexpatrie

 

[lexpatrie]

Well, you'd either have to have a steel beam or a joist girder at the top of the column, I originally had that second part under a spoiler but it didn't work correctly, stuff bled through for some reason. So I took the spoiler tag off it.

If anybody is interested in the background (further), May 2001 article from Modern Steel Construction.

New OSHA Erection Rules, How they Affect Engineers, Fabricators, Contractors, Barger, West, Modern Steel Construction, May 2001.

(images are a bit blurry, and it looks like they have a weld indicated between the truss bottom chord and the stabilizer plate). Maybe not, if I can find a copy of it in my garage I'll update. (Update, I don't think I have it, the main location issues start at June 2001), but if you read the text of the article it says you don't need to weld the bottom chord extension to the stabilizer plate.