[Rio]

We're doing a 2nd story addition using 2x12 Doug fir @ 16" on center spanning about 16' to the bearing wall. This is well within the tables.

There is one part, about 5' wide, in the middle of the 2nd floor where the span bumps out to 19'-8". This is still within the tables but barely, 19'-10" is the max for 16" o.c..

We're thinking of tightening up the spacing in this area to 12" o.c. in the thought that it should make it stiffer and more like the rest of the framing but I'm not sure if this is a good idea or not. I think it should help.

Does anyone have any experience with this? TIA for any advice, pro or con.

 

[Golden view]

I think it's a good idea. Will it interfere with mechanicals or anything? You could also double the joists at that area, which will make both areas have roughly the same deflection. Better than 12" and closer in strength.

 

[ScipioAfricanus]

I think it is a good idea.

The spans you mentioned are good for residential sleeping areas only though.
Other living areas require 40 lbs. live load though.

Andy.

 

[Rio]

I think it's a good idea. Will it interfere with mechanicals or anything? You could also double the joists at that area, which will make both areas have roughly the same deflection. Better than 12" and closer in strength.

I didn't think of doubling them, the 12" o.c. won't interfere with mechanicals I think but it is tighter than doubled 2x12's at 16" o.c. albeit only 2-1/2"
. So you think the deflection will be closer than if it's a single at 12" o.c. I'm not sure how to calculate that as it's the whole system that needs to be calculated, not just the single unit.

Also thanks Scipio for the heads up on the 30 psf for sleeping areas vs. the 40psf for living areas.

 

[hdavis]

Use any of the beam deflection calculators, if you're looking to match deflection. You don't have to consider the floor as a single system unless you're doing something like vibration analysis.

 

[hdavis]

I get about ~8 5/8 inch spacing to match deflections, so double up on 16" centers.

 

[CarpenterSFO]

Maybe throw in an extra row of blocking to tie the dissimilar areas together, no matter what you do with the joist spacing.

 

[ScipioAfricanus]

According to the CBC your deflection delta is still at 360.
Those are not terribly accurate though.

Andy.

 

[CarpenterSFO]

According to the CBC your deflection delta is still at 360.
Those are not terribly accurate though.

Andy.

Can one bump from #2 to SS in these situations, or does that cause some other problem?

 

[ScipioAfricanus]

Sure, he go up to SS, That would stiffen the floor and make it stronger. But his situation is rather difficult to determine since he is obviously in a SDC of D0 D1 or D2 his project being on the 2nd floor is required to be engineered.


Andy.

 

[Rio]

Lots of good suggestions here. Thanks to all!
Rio

 

[makethecut]

Anything over 12' I just go with wood I beams. Lighter. Easier. And the same price as 2x12 if not a little cheaper depending on where u are at.

 

[tyb525]

And collapse faster in a fire.

 

[Rio]

And harder to work with and detail out.

 

[JT Wood]

Consider 8" spacing so you won't get burned on any layout


(Unless there is mechanicals to be put there)

 

[Roofcheck]

Throwing the fire part was a great point, but if my place burns I want it all gone to start over not live in a charred anything.

 

[hdavis]

Throwing the fire part was a great point, but if my place burns I want it all gone to start over not live in a charred anything.

He was referring to an unpredictable collapse - the kind that kill firefighters.