Wood Gable End 1

[GalileoG]

Hello fellow eng-tippers,

I am working on the design of a 75’x75’ single-storey wood building with a gabled roof. 4 walls will be constructed to eave height followed by the 2 gable end walls (triangular walls).

I have a few questions about the gable end detail. I have attached a detail to this post for your comments.

-How can I maintain continuity of the 2 top plates (my diaphragm chord) at the ridge where the top plates will simply butt each other?
-Architectural requires a 2’ overhang – would I need to detail this or is that typically just stick built by the contractor? If so, any ideas as to how the overhang can be framed?

Thanks for your help.

 

[msquared48]

Depending on your local snow load, the overhang is normally accomplished with 2X4 flat material at 2 to 4 foot spacings.

I would drop the double top plate of the gable end wall by 1.5" to allow the 2X4 flat material to cantilever and have a back span to the first truss beyond the gable end. Then block between the 2X4 material. This is pretty normal construction. Sometimes the first truss is dropped 1.5" too to allow a greater back span.

Mike McCann
MMC Engineering

 

[DaveAtkins]

Make sure the contractor is OK with stick building the gable--usually a gable end truss is used.

Regarding the diaphragm chord, I would use the double top plate BELOW the gable for that purpose. The sheathed gable end truss must act as a shear wall to transfer the diaphragm force down to the double top plate.

DaveAtkins

 

[GalileoG]

Thank you for the responses.

DaveAtkins,

I went with the gabled wall because a gable end truss would have to transfer quite a bit of drag force. Also, the architect prefers the gable end wall.

I am having a hard time imagining how the wall plates BELOW the gable end wall can be engaged as the chords for the roof diaphragm/sheathing. Do the chords not have to be fastened directly to the diaphragm sheathing? The section as it is currently detailed now, can what you have suggested work, if so, how?

 

[msquared48]

There should be no problem with nailing the roof sheathing to the blocking and the blocking to the double top chord of the gable end wall. Just specify the nailing required to develop the shear.

Mike McCann
MMC Engineering

 

[AELLC]

I don't understand - a gable end roof truss has web diagonals to transfer any magnitude of drag force commonly specified. There are "inset" pieces of 2x4 verticals to take the place of studs so that you have something to nail the sheathing to.

When the structure is finished, you cannot tell the difference, so why is the Archi opposed?

 

[msquared48]

The gable end trusses normally only have verticals, and top and bottom plates to which the plywood sheathing is nailed to transfer the in plane shear from the roof diaphragm. The double top plate of the gable end truss acts as a shear collector, and the bottom plate as a shear distributor to the double top plate of the wall below.

The only diagonals in the plane of the truss are usually at the interior trusses, not the gable end truss. The diagonals shown in the section are the diagonal struts from the bottom of the gable end truss to the roof diaphragm to laterally brace knuckle joint at the connection of the bottom chord of the gable end truss to the wall below.

Mike McCann
MMC Engineering

 

[AELLC]

msquared,

Maybe it is different in your area. GET's here definitely have diagonals, but they are fewer in number (panel points are spaced out longer)

 

[AELLC]

Here is a PDF of some trusses. AO1GE and AO2GE are the GET's.

 

[DaveAtkins]

I am saying your gable end wall can act like a shear wall (or two shear walls butted together, if you like). The sheathing and nailing must be able to transfer the shear from the diaphragm down to the double top plate below.

DaveAtkins

 

[PrescottEngineer]

I have attached a PDF showing some of our gable end details. All the details show a gable truss. The first two details are for a smaller overhang and show the "outlookers" flat. The second two details are for a larger overhang and show the "outlookers" on edge. We use the bracing details when the depth of the truss exceeds 6' and the bridging details below 6'.

I agree with AELLC that I can't see the advantage of a gable wall over a gable truss from an architectural standpoint.

As stated in some of the posts above, I agree that by sheathing the gable end truss you have addressed the drag force issue. I am in Arizona and have seen gable end trusses both with and without the diagonals depending on which truss manufacturer provides the truss.

As far as connecting the top plates at the ridge why not just use a Simpson strap applied to the top face of the top plate? I added a typical detail for this condition to the attachment. This is not really an issue if you have a gable truss.

As far as connecting the chord, which is the top plate in this example, directly to the sheathing, I would say no, you do not have to connect the chord (top plate) directly to the sheathing. At the eave walls, I assume you are going to have blocking between the trusses and you will nail the sheathing to the blocking and attach the blocking to the top plate with a Simpson A35 or RBC or toe nails. I think the top plate will still act as a chord member even though it is not directly connected to, or maybe we should say "touching", the plywood.

 

[AELLC]

The trouble with Structural Engineering is you have to spend so much time at University figuring out how to derive differential equations, there is no time left to look at practical examples of engineering, and then when you start working, nobody has the time to mentor you because everyone is working 80 hrs and being paid only for 40 hrs because the Archi's only pay 50% of the invoices.

So inexperienced engrs go thru all this mental anguish about chords and diaphragms and such.

Strictly speaking, yes (!) the chord need to be in contact and in the same plane as the diaphragm. However, in reality, an indeterminate width of roof sheathing ends up being the chord, seeing how the sheathing is nailed to the gable end truss.

So the G.E.T. detail is fine and everything works, without any way to really quantify all this. It is much easier to construct than a gabled endwall.

 

[GalileoG]

Thanks again everyone. You've all been very helpful!

I have one more question that I would like to pose:

If I were to make the switch to a gable end truss, I will need to rely on the truss to transfer lateral wind forces. Can the truss supplier design a gable end truss that can take 35psf of lateral wind load when the truss is 15ft deep at its peak? Seems a bit much.

 

[focuseng]

They might be able to if you tell them to. Diagram the truss with its loading and call a few up to find out. Always good to have a few truss mfg you are friendly with to ask these questions. The software they use can input all kinds of loading but they only do special loading if directed by someone. I keep 4 sales guys in my contacts list for just these kind of questions and usually ask at least two to make sure I am not getting a line of bull.

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MAP