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Joist selection Wayne Whitney 02-15-2008
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Posted by Wayne Whitney on February 15, 2008, 7:33 pm
Hi,

I've got to sister five 17' long ceiling joists, due to removing a
bearing wall. I've done the engineering, and given the existing roof
construction, I've got two options that work:

1) Use double 2x6 select struct at 16" o.c.

2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep
top edge end notch.

The selection is deflection controlled, as 2x6 select struct at 16"
o.c. meets bending and shear.

Which option is better? The material cost for the 2x6s is about $50
more. I'm mostly working alone, so the 2x6s will be somewhat easier
to wrangle into place. Otherwise, it is notching the 2x8s versus the
extra nailing to double the 2x6s.

Thanks, Wayne

Posted by Rima Neas on February 15, 2008, 10:09 pm
> Hi,
>
> I've got to sister five 17' long ceiling joists, due to removing a
> bearing wall. I've done the engineering, and given the existing roof
> construction, I've got two options that work:
>
> 1) Use double 2x6 select struct at 16" o.c.
>
> 2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep
> top edge end notch.
>

I would go with single 2x8 sisters, less wood to buy, haul, and wrangle
with. Besides, 2x8's tend to run straighter than 2x6's. Nailing a single
sister will also involve fewer intrusions into the original rafters, not to
mention hammer strokes. Given that you are sistering existing, presumably
bone dry, rafters, I would also go with KD stuff. Then again, I cannot even
figure out my window sizes...



Posted by Bobk207 on February 16, 2008, 7:46 pm
> Hi,
>
> I've got to sister five 17' long ceiling joists, due to removing a
> bearing wall. I've done the engineering, and given the existing roof
> construction, I've got two options that work:
>
> 1) Use double 2x6 select struct at 16" o.c.
>
> 2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep
> top edge end notch.
>
> Theselectionis deflection controlled, as 2x6 select struct at 16"
> o.c. meets bending and shear.
>
> Which option is better? The material cost for the 2x6s is about $50
> more. I'm mostly working alone, so the 2x6s will be somewhat easier
> to wrangle into place. Otherwise, it is notching the 2x8s versus the
> extra nailing to double the 2x6s.
>
> Thanks,Wayne

Wayne-

Does the notch start at the end of the joist & only run 3.5"?

IMO this isn't really much of a notch ....more of a depth reduction in
the joist shear / bearing zone?

I assume it needs to be there for clearance issues?

You might consider a taper cut rather than a sharp cut out.

Timber is happier if you avoid abrupt section changes but in this
case the local stress state appears to "compress" :notch rather than
wanting to split it open.


If you don't have a copy you might consider getting one

Design of Wood Structures ASD/LRFD
Sixth Edition
by Donald E. Breyer

or a older addition (cheaper) ....always great for timber design
concepts but newer editions conform to newer codes.

& this guy is a real timber expert.


cheers
Bob


Posted by Bobk207 on February 16, 2008, 7:49 pm
>
>
>
> > Hi,
>
> > I've got to sister five 17' long ceiling joists, due to removing a
> > bearing wall. I've done the engineering, and given the existing roof
> > construction, I've got two options that work:
>
> > 1) Use double 2x6 select struct at 16" o.c.
>
> > 2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep
> > top edge end notch.
>
> > Theselectionis deflection controlled, as 2x6 select struct at 16"
> > o.c. meets bending and shear.
>
> > Which option is better? The material cost for the 2x6s is about $50
> > more. I'm mostly working alone, so the 2x6s will be somewhat easier
> > to wrangle into place. Otherwise, it is notching the 2x8s versus the
> > extra nailing to double the 2x6s.
>
> > Thanks,Wayne
>
> Wayne-
>
> Does the notch start at the end of thejoist& only run 3.5"?
>
> IMO this isn't really much of a notch ....more of a depth reduction in
> thejoistshear / bearing zone?
>
> I assume it needs to be there for clearance issues?
>
> You might consider a taper cut rather than a sharp cut out.
>
> Timber is happier if you avoid abrupt section changes but in this
> case the local stress state appears to "compress" :notch rather than
> wanting to split it open.
>
> If you don't have a copy you might consider getting one
>
> Design of Wood Structures ASD/LRFD
> Sixth Edition
> by Donald E. Breyer
>
> or a older addition (cheaper) ....always great for timber design
> concepts but newer editions conform to newer codes.
>
> & this guy is a real timber expert.
>
> cheers
> Bob

Wayne-

I must have missed this thread plus I didn't specifically answer your
question....

I'd go with the 2x8's and taper cut them.

cheers
Bob

Posted by Wayne Whitney on February 16, 2008, 8:53 pm

> Does the notch start at the end of the joist & only run 3.5"? IMO
> this isn't really much of a notch ....more of a depth reduction in
> the joist shear / bearing zone?

Yes, everything you say is right, it is a depth reduction needed for
clearance. The reduction from 7.25" to 5.5" is just under 25%, the
code maximum.

An immediate taper cut is not possible. The roof is constructed with
a 1x4 bearing on top of the 2x6 ceiling joists, and the 2x4 rafters
bear on the 1x4. Crazy 1908 construction. So I have to actually
reduce the depth to 5.5" where it is under the 1x4, hence the 3.5"
long end notch. I don't want to modify the roof construction, as that
would open a whole other can of worms.

> Timber is happier if you avoid abrupt section changes

So after the notch clears the 1x4 obstruction, I should do a taper to
increase the depth from 5.5" to 7.25" more gradually? Is a 45 degree
taper OK or should it be shallower?

Thanks,
Wayne

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