Joist selection

Wayne-

I was thinking of cutting a taper that did the job of the notch but
sounds like the taper at the joist end would result in a rather
shallow joist end?

But as you mention you are deflection critical , shear or bending
stress not an issue. So I think that the taper would work....check
the shear for the reduced depth. I doubt that a 12" or 16" taper
would reduce the beam stiffness much.

We're really discussing stuff that is pretty much second (or third
order) effects.

If you can't use a taper cut to do the whole job...I'd use a large
paddle bit or hole saw to define the end of the clearance relief &
just jig saw to it.

Cutting a taper out of the relief cut is really overkill.

cheers
Bob

Posted by RicodJour on February 17, 2008, 6:41 am

> > > 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?

> > 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

> Wayne-

> I was thinking of cutting a taper that did the job of the notch but

> sounds like the taper at the joist end would result in a rather

> shallow joist end?

> But as you mention you are deflection critical , shear or bending

> stress not an issue. So I think that the taper would work....check

> the shear for the reduced depth. I doubt that a 12" or 16" taper

> would reduce the beam stiffness much.

> We're really discussing stuff that is pretty much second (or third

> order) effects.

> If you can't use a taper cut to do the whole job...I'd use a large

> paddle bit or hole saw to define the end of the clearance relief &

> just jig saw to it.

> Cutting a taper out of the relief cut is really overkill.

The notch on the upper side isn't nearly as critical as a notch on the
bottom of the joist. There's also the possibility of sistering on a
3/4" plywood scab to help distribute the stress at the reduced cross
section - whether notched or tapered.

R

Posted by Wayne Whitney on February 17, 2008, 1:03 pm

> I was thinking of cutting a taper that did the job of the notch but

> sounds like the taper at the joist end would result in a rather

> shallow joist end? . . . I doubt that a 12" or 16" taper would

> reduce the beam stiffness much.

If I do the minimum taper possible over a length of about 17", I have
an end depth of 5". Does that seem sane with a 2 x 8 x 17'?

> But as you mention you are deflection critical , shear or bending

> stress not an issue. So I think that the taper would work....check

> the shear for the reduced depth.

Well, I assume the shear should be checked at the face of the support,
since going into the support the shear stresses are decreasing as the
bearing comes in. In that case, if I maintain the 5.5" section depth
at the face of the support, there is nothing to check, as I already
know a 2x6 passes bending and shear.

Thanks,
Wayne

Posted by RicodJour on February 17, 2008, 4:52 pm

> > I was thinking of cutting a taper that did the job of the notch but

> > sounds like the taper at the joist end would result in a rather

> > shallow joist end? . . . I doubt that a 12" or 16" taper would

> > reduce the beam stiffness much.

> If I do the minimum taper possible over a length of about 17", I have

> an end depth of 5". Does that seem sane with a 2 x 8 x 17'?

I'm still wondering about your sanity! ;) What sort of deflection
are you getting with a 2x8 sistered on?

> > But as you mention you are deflection critical , shear or bending

> > stress not an issue. So I think that the taper would work....check

> > the shear for the reduced depth.

> Well, I assume the shear should be checked at the face of the support,

> since going into the support the shear stresses are decreasing as the

> bearing comes in. In that case, if I maintain the 5.5" section depth

> at the face of the support, there is nothing to check, as I already

> know a 2x6 passes bending and shear.

Right. And you're doubling the cross section.

R

Posted by Wayne Whitney on February 17, 2008, 8:53 pm

> What sort of deflection are you getting with a 2x8 sistered on?

From the awc.org calculator, the span rating for a 2x8 SS DF with a
20psf live load and L/360 deflection is 17' 2" and is deflection
controlled. So ignoring any end taper, the deflection at 16' 5" will
be L/360 * (16'5/17'2)^3 = L/412, or 0.47".

I'm ignoring the existing joists when doing this calculation, since
they are 2x4s. Most of the house has 2x6 ceiling joists, so I was
quite surprised to find the 2x4s in the area I'm working on.

Cheers, Wayne

Page 2 of 9 < 1 2 3 > last >>

Similar Threads	Posted
Floor Tile Selection for Kitchen	October 25, 2006, 7:55 pm
Location of Rim Joist	March 30, 2007, 4:57 pm
joist question	September 27, 2007, 9:47 pm
Floor joist sizing	October 18, 2006, 8:36 pm
Connecting a 2x8 joist to 2x4's	January 7, 2008, 1:13 pm
Floor Joist Beam Spans	July 8, 2006, 1:36 pm
joist crack repair question	November 2, 2007, 9:30 pm
Blocking in Basement between open Truss and rim joist	November 20, 2006, 3:33 pm

XML Sitemap