Gable Wall with Stairway 3

[medeek]

I've come across this a number of times with my own designs and now this particular design:

A stairway next to a gable wall or exterior wall. My first thought is to call out a balloon framed wall up to the 2nd story top plate(s) but then that makes the rim board and double top plate of the first story discontinuous. Any thoughts on the best way to frame this small section of wall next to the stairs?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

If the plies are separated doesn't that give the whole assembly more section modulus? Solid blocking between the plies 24" or 32" o/c should help to further stiffen things up. I will post a detail shortly.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Two separate, independent plies would be twice as stiff as a single ply. However, two plies snug and composite would be eight times as stiff as a single ply. Two plies separated by a gap but made to act compositely would be better still. The trick is making the separated plies behave compositely. Blocking would be one way. Another would be using the plates above and below to stitch the plies together into a box beam. You'd probably be stuck with toe nailing on the bottom though. Nobody loves toe nailing.

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.

 

[KootK]

Upon further inspection, I see that you've already thought of the plates as the webs of the box beam.

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.

 

[medeek]

Not much really going on here, but here is the detail:

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

A solid bead or two of LN/LNP-902 (Liquid Nails Subfloor) probably wouldn't be a bad idea either to make sure the LVL-plate-RIM assembly works as a unit.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

WFCM Fig 2.1k Floor Diaphragm Opening Limits:
Exterior walls adjacent to the opening shall be framed using full height studs where the opening is less than 2' from the exterior wall.

Then if you turn to the prescriptive section is shows a stairway next to a platform framed wall...

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

The big question is can we make the boxed beam work as a single unit, does that assumption hold true? Or is the safe bet a "wind beam" at this location?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I vote yes, we can make it act as a unit with some attention to detail. Not sure what the difference is between this and a wind beam though.

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.

 

[medeek]

When I hear "wind beam" I assume you replace the void and LVL with a larger glulam or PSL beam. Disregard any composite action with the plates and just run the numbers on the beam taking the full out-of-plane load. This would be the safe bet, especially if the length of the opening gets larger. The 12'/50% rule in the WFCM should place a limit of 12' on the max. length this "wind beam" should have to span.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[AELLC2]

Why is the opening in the second floor same as the projected length of the stairway? Why not minimize the length to that required for headroom?

 

[medeek]

Good question AELLC2.

The ceiling height in the model is shown as 8'. In reality it will be 8'-1". Subtract about 7.5" for the first step gives me a headroom of about 89" give or take an inch or two. This is assuming there are 13 treads. The framer may opt for 14 treads instead which would give me about 81" of headroom. I always like to err on the side of a little extra headroom for a stairway, for reasons of meeting code and also for functionality for the occupant.

In this particular instance the client made a point that they wanted more headroom and width so moving large pieces of furniture into the upper floor space would be less problematic. I've damaged a lot of drywall in my lifetime moving stuff up and down tight stairways.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jdgengineer]

I think there is great conversation going on here and I don't mean for this to come off badly, but do you think this conversation is tredding into "penny wise, pound foolish" territory?

In my area I wouldnt get much grief for balloon framing 15' of area. Deflection I don't think is a concern as you have an adjacent low roof that can brace the wall.

I think where I would run into flack would be when I tell the contractor they have to use:

1) Continuous rim with interior LVL beam.
2) put beads of liquid nails to help bind them together.
3) nail them together with nonconventional nailing.
4) but small strips of blocking between the two.
5) insulate the space prior to closing (likely out of sequence)

If it were me and we werent using balloon framing, I'd 100% go with full width 4x10, 6x10 or PSL rim to span in weak axis across the opening. Sure the material may be more expensive but less labor and less likelihood of missinstallation.

 

[medeek]

The porch roof will help brace the wall, I wasn't going to consider that but it probably adds a significant amount of stiffness to the entire assembly when you consider an 8' deep diaphragm.

I agree that the safe bet is just to put a "solid" beam into the slot behind the rim joist and eliminate most of the special detailing required to form a composite beam.

Regardless of using a "wind beam" there still needs to be appropriate connections between the wall plates and the beam so that the loads are properly transferred into the beam from the the wall. What would be a more conventional way to nail the plates to the beam? I still think you are toe nailing into the first floor top plate and nailing down through the second floor plate as shown in the detail above, I'm not sure how else to do this.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

A few options:

1.) 1-1/4" Rim Joist on exterior with a 3-1/2" x 9-1/2" PSL beam behind to span the 10'-10" of the stairwell.
2.) Remove the Rim Joist in this section and install a 5-1/4" x 9-1/2" PSL beam that spans the full 10'-10" of the stairwell.
3.) Remove the standard Rim Joist (1-1/4") along the entire length of the wall (24') and install either a 3-1/2" or 5-1/4" continuous PSL beam the entire length of the wall.

Or perhaps some other option I've overlooked.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

With option 2, you would probably want to install straps between ends of PSL and Rim Joists.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

The porch roof actually is pretty close to the rim joist, see image below:

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I agree that the porch diaphragm gives you something but I'd be pretty hesitant to rely on it.

1) Normally, I'd assume the porch to be loading the wall laterally, not bracing it.

2) To utilize the diaphragm, by the book, you'd probably need to connect a bunch of things that aren't, by default, typically connected.

3) Owners generally feel at liberty to modify things like porches as they see fit. Most lay people -- and builders for that matter -- are unlikely to recognize the role played by the porch diaphragm in stabilizing the wall joint. And you're unlikely to saddle the building with some kind of plaque to that effect (porch removal = death).

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.

 

[medeek]

I probably should have checked the numbers first. Given the high wind loads in our area (155 mph ult.) a 3-1/2 x 9-1/2 PSL loaded in the weak axis will deflect almost a full inch with a clear span of 10'-10" and a trib. length of 9'. However, a 5-1/4" beam will pass with flying colors, so I've updated my detail to:

The eccentricity is small enough that I think it makes sense to just sandwich them together and make sure they act as one unit. I still like the idea of disallowing any splices in the members in this region of the wall and will keep my notes specifying that.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jdgengineer]

I like your new detail a lot better.

I'd personally let them splice the wall plywood at the rim but up to you. It's where they want to splice it and anywhere else would require blocking (assuming you have blocked shearwall).

 

[medeek]

The second detail should be pretty stout. The top and bottom plates will also contribute as well as the sheathing when the wall is sucked outward.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com