Double every third joist....

[KevinChez]

I recently came across a plan where the architect calls for every third floor joist to be doubled. I don't understand why.

Anyone have an idea?

Thanks.

 

[Shotzie]

There is some good discussions here on the topic: Link

 

[KevinChez]

Thanks...

 

[dik]

Seen it done numerous time on schools and wood framed early structures... Only seen it with joist spacings of 12" or 16", not 24".

It's used to increase the load carrying capacity x 4/3 for joist structures. The rational, I understand, is that it's part of an assembly where members are increased in strength by 'load sharing' due to the proximity of the framing members... If the added members fall within that spacing, by definition, it also qualifies as load sharing. It becomes part of the load sharing system.

Dik

 

[jdgengineer]

While I'm sure it is done frequently and has a good history of use, I don't personally like it. I'm not convinced that the sheathing / bridging / blocking is stiff enough to engage all of the joists to allow this much load sharing. If joists are 16" oc, this means the doubled joist occurs only at 48" oc. I suppose, if the sheathing had a span rating of 48" oc (say 1 1/8" plywood) I could be convinced a bit easier, but with typical 3/4" floor sheathing I don't like it. Just my 2 cents.

 

[msquared48]

I have to echo jd's sentiments here.

I would typically double at every other joist, and have in the past, but not every third joist.

Mike McCann, PE, SE (WA)

 

[JohnnnyBoy]

If this is being used to increase the capacity of the floor system and rely on bridging of plywood, why wouldn't the spacing of the joists just be decreased to get the required capacity? If spacing is at 16" c/c and every third joist is doubled why not space at 12" c/c, or likewise if was spaced at 12" why not space at 8". Spacing would still need to be considered to ensure fastening of sheathing but I don't see why spacing wouldn't be reduced and not have to worry at all.

 

[JohnnnyBoy]

Apologies for the above question as it was answered in the referenced thread.

 

[Ron]

It is based on a "theory" of stiffness absorption....which only works if the "membrane" (i.e., sheathing) is rigid enough to carry the load for full transfer. Thin sheathing is not sufficient to do this. Agree with JohnnyBoy...decrease the spacing.

Same concept as when you have a metal stud wall and you add studs around windows or other openings but don't change the mid-wall studs....deflections are not compatible and serviceability issues result, particularly with rigid facades like stucco.

 

[BAretired]

If a 12" spacing is needed, it might be preferable to provide double joists at 24" centers in order to provide mechanical clearance.

BA

 

[Triangled]

C_r, repetitive member bending stress increase, makes sense to me.....but this system of dbl, sgl, sgl, dbl, feels odd, and perhaps even a little uneven. If I got may way to justifying some kind of bending stress increase, which I may do after working through the 'is it 3 joists or 4 joists in the system', i'd be bothered by the varying stiffness. that is, every 4' or so is a dbl joist with effectively double the stiffness of its nearby joists. i'm imagining ripples in an older floor

 

[KevinChez]

thank you for all the info

 

[JoshPlumSE]

I could see doing something like this to help with vibration issues.

You essentially have double the stiffness locally in that region. So, the mode shapes will be a little different and can't carry through the structure so easily.