How Short is Too Short for a Steel Joist of a Particular Depth 3

[KootK]

Just like the title says. Say I have a 16" joist. How short of a span could I go to before it's not feasible to produce such a thing? Additional info:

- Yeah, I'll call somebody if I don't get what I need here.
- Not concerned with economy. It's a client/mech decision.
- Tables seem to stop around h x 12 but I've certainly gotten shorter than that in the past.




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.

 

[XR250]

I guess at some point you just use a stick of 2 1/2" tube steel

 

[EngineeringEric]

Use Joist Substitutes for spans that are short

 

[JD2]

About 10 years ago I asked Vulcraft basically the same questions and got the following:

Joist depth/Minimum Base Length

10"/5'-2" 12"/5'-6" 14"/5'-10" 16"/7'-0" 18" rod/7'-4" 18" crimp/9'-4"
20" rod/7'-8" 20" crimp/9'-10" 22"/10'-6" 24"/11'-0" 26"/11'-0"
28"/11'-6" 30"/12'-0"

Those are only Vulcrafts numbers and it's 10 years old so take it with a grain of salt.

 

[dik]

They used to fabricate 8" 'Corridor Specials'... These were used for spans to approx 10'; I don't know what the minimum span would be. If Vulcraft is correct, then I would guess about 5'.

Dik

 

[TLHS]

I have no idea what they /would/ produce, but from a mechanics standpoint there's really no point unless you have an intermediate node point for the truss. That implies that the minimum buildable span for a joists with 45 degree web members would be approximately 2 x depth, with some vertical members in there to give you load paths from the top/bottom of truss. At 60 degrees to the horizontal you could go to 2*Depth/sqrt(3). So I'm throwing my hat in the ring at a span of something in the range of 18.5" to 32".

This would be dumb. I bet someone would make it, though, if you really pushed.

More realistically, I think that it's probably twice the end panel length plus one standard interior panel. That'd be \/\/. So ballparking, it'd be somewhere in the 4*Depth range, maybe lower with a more aggressive web angle.

 

[dhengr]

KootK:
You probably need at least one panel point at mid-length of the top chord or the top chord member would almost make the span, by itself, as a simple beam. Then, you need the two end diags. from the bot. chord up to the end bearings, plus the end bearing lengths. Then, the issue becomes how tightly you can bend the diag. bar stock without cracking it, and still be able to get enough welding in place. In the end you are moving from a truss with triangular diags., almost to a vierendeel truss.

 

[gravityandinertia]

I worked for a joist manufacturer briefly and never ran up against this. As you've said Hx12 is the general rule of thumb. For something significantly shorter than that, I think you would be dominated by shear, and so a simple member with enough shear capacity would be adequate, with less production costs.

Is my thinking off here? Am I missing something?

 

[JD2]

Just some additional information that did not seem pertinent at the time I replied the first time - Vulcraft also provided the min. panel size for the particular truss depth

joist depth/ panel size

10"/1'-7" 12"/1'-7" 14"/1'-7" 16"/2'-0" 18" Rod/2'-0" 18" Crimp/4'-0" 20" Rod/2'-0" 20" Crimp/4'-0"

All of the remainder of the depths to 30" had a panel size of 4'-0"

 

[KootK]

Just the information that I needed. Thank you gentlemen. Hopefully I can switch to joist substitutes right about the time things get too short for the desired depths.

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.