Gable End Shear Wall / slopped diaphragm 1

[RFreund]

I have a couple questions regarding gable end shear walls with a pitched roof stemming from a recent project which I believe will need to be handled with steel. There are 2 situations and they may in fact be one in the same.

1. No ceiling diaphragm (i.e. vaulted ceiling - ridge beam or scissor truss) thus requiring full height studs to underside of roof diaphragm.
2. Ceiling diaphragm (flat ceiling) with wall studs, top plate then gable studs atop the top plate.

With either of these situations I'm trying to visualize the load path.

Basically my question is: Is the entire gable wall a shear wall? In case 1 there would be one full height shear wall. In case 2 there would be an upper triangular shear wall and a lower shear wall? Are openings in the upper triangular shear wall treated the same as for a rectangular shear wall? Or could you argue that the triangular shear wall is not needed and 2 point loads at the bearing ends of the gable truss/rafter transfer the shear of your diaphragm into the lower rectangular diaphragm.

See Attached.

EIT

 

[msquared48]

What you have is what I commonly refer to as a "Window Wall".

If there is an intermediate double top plate at the low end of the scissors truss, you have a knuckle joint and a problem.

Normally for these walls the studs run from the floor to the underside of the scissors trusses, full height, with larger vertical and horizontal members as needed, wind beams if you want, to take the lateral load perpendicular to the wall.

As for shear walls, the upper wall portions are in shear as well as the lower portions, you just have to decide which ones you want to designate as shear walls and design them accordingly.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Thanks mike,
Question:
When you say you have to "decide which to designate as a shear walls", aren't both shear walls? I mean could you have all windows above in the triangular portion and solid shear wall below? How though is the shear transfered to the lower shear wall?

Thanks again.

EIT

 

[msquared48]

The horizontal beam elements of the wall should be able to be designed to act as drag struts to transfer the lateral forces from one wall to another.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Mike thanks again and again more questions:

I've attached a sketch to make sure I'm interpreting your explanation correctly.
Secondly, If this were a case were there is no horizontal beam as in the vaulted ceiling case then your shear would extend to your roof correct?

Thanks

EIT

 

[RFreund]

forgot to attach

EIT

 

[msquared48]

No sketch is posted other than your original...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

apparently we posted at the same time.

Here it is again.

EIT

 

[msquared48]

OK.

I'll have to post a couple of times for your two conditions, but here goes. Here is the scissors truss elevation with my comments:



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

And here is the second condition.

Note that you have shown a normal roof truss at the endwall condition. Usually an endwall truss is used here that has only vertical members - no diagonals. You might want to reconsider your window layout.

That being said, here are my additional comments:

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Wow. Thanks Mike I really appreciate your time.

In the first condition I understand that the wind beams will have load from wind perpendicular to the wall but are they also acting as a drag strut to transfer shear from the roof diaphragm to the shear wall? Also are the center wind column responsible for transferring any in plane forces?
Or are is this suppose to be a steel moment frame?

Thanks again.

EIT

 

[msquared48]

Yes, the wind beams act to drag the shear from and to the appropriate shearwalls.

The center wind columns do not transfer any in plane forces, only forces normal to the wall.

Depending on how deep you want the wind beams to be, it should be very doable with wood - Multiple studs, solid sticks, parallams or glulams. Steel should not be necessary.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Another question to help clarify:

If a gable wall is stick framed with either a ridge beam or ceiling joist tie does the triangular portion of the gable wall act as a shear wall or do the top plates of the gable wall act as a drag strut with is loaded by the walls perpendicular to it?

EIT

 

[msquared48]

The triangular portion of the gable wall act as a shear wall.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

For Case 1 were you saying that those windows were not possible and should be sheathed?



EIT

 

[RFreund]

Or can you have a situation where the triangular portion could be completely open and the load is transfered to the shear wall by the wind beams? Where the load would get to the wind beams at the corner top plate connection.

EIT

 

[msquared48]

I am saying that using a truss with webs at the endwall instead of an endwall truss is not the norm.

If you do use a truss with diagonals at the endwall, the windows could be installed in the truss openings as the truss would be designed to transfer the shear forces to the shearwall below, but I think it would look kinda funny.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Sorry for the confusion I was referring to the scissor truss where you have the windows hatched. Are you suggesting they should be sheathed?

EIT

 

[msquared48]

No. Just highlighting the windows. Sorry for the confusion.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Mike thanks again for staying with me on this. So with the windows in the triangular portion are you ignoring all shear resistance of this portion? Thus all shear is transfered to the wind beams at the corners where the truss is bearing?
Or do you need the triangular section to have shear capacity.

EIT