Gable End Shear Wall / slopped diaphragm
Gable End Shear Wall / slopped diaphragm
(OP)
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.
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





RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
EIT
RE: Gable End Shear Wall / slopped diaphragm
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Gable End Shear Wall / slopped diaphragm
Here it is again.
EIT
RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Gable End Shear Wall / slopped diaphragm
EIT
RE: Gable End Shear Wall / slopped diaphragm
EIT
RE: Gable End Shear Wall / slopped diaphragm
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
RE: Gable End Shear Wall / slopped diaphragm
EIT
RE: Gable End Shear Wall / slopped diaphragm
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Gable End Shear Wall / slopped diaphragm
Or do you need the triangular section to have shear capacity.
EIT
RE: Gable End Shear Wall / slopped diaphragm
No. There is the area of wall betweeen the two long vertical wind column s that can be developed into a shear wall, probably perforated, in this area.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Gable End Shear Wall / slopped diaphragm
I see now, so there has to be a shear wall in the triangular portion of the wall. This may be a problem because I realized my sketch does not have the windows laid out exactly as the architect has them. In his scheme the triangular portion is basically all windows (the middle window I have shown is larger). They were hoping to use wood but I think they will need to change the window layout because currently I have no space for a shear wall in the triangular portion. Unless there is another solution? My boss recommended to use a steel frame (which is most likely what we will end up doing) but it would be great to be able to propose another solution.
Thanks again.
EIT