Polygon Window Header

[AMB DEC]

Hello,

I am working on a project in which a homeowner would like to add a new polygon shaped window in the gable wall of the home. They will also be replacing the existing windows in kind. The roof is framed with scissor trusses @ 16" o.c. and the eave on the gable wall is 16". I am assuming 25 psf roof dead load and the roof snow load is 135 psf (320 plf total load to the header). I would like to present my concept for the design to the group to see if anyone has any feedback or advice with my approach (drawing attached). I plan to frame the polygon window with an arch style header and splice plate. I plan to use engineered lumber for the header to minimize any wood shrinkage and with screwing the plate to the header hope to prevent any slippage at the connection. The header will be supported at the ends with joist hangers into king studs. I am a little unclear with this splice plate design if there will be thrust at the ends of the header, but I have provided blocking/nailing in the shear wall to transfer this force from the header to the shear walls. My biggest unknown with this design is if the header will have any tendency to deflect and crack the window. Any input from the group is greatly appreciated and if I can provide further information to help address my question please let me know.

Thank you,
Austin

 

[jayrod12]

Regarding the splice plate, I don't see any huge red flags there. Lots of small diameter fasteners as you've shown are better than a couple large ones. How much deflection would you expect a flat beam of that length to deflect under those loads?

Does your three ply 7 1/4" lvl header over the rectangular windows have enough out of plane capacity to span to the king studs? That one jumped out at me as more concerning than the upper one. I'd almost feel you'd be better with a 3 ply 2x6 on flat in that instance, since that header is seeing negligible gravity loads.

 

[dhengr]

AMB DEC:
Make that gable end truss a double truss ganged together, kinda like a girder truss. The outside truss is as you show in your drwg. and the inner truss is one of the common scissor trusses, and they are screwed together, side by side. Brace the bot. chord diagonally, back up into the interior trusses and rood diaphragm. Discuss the king studs bearing points 5' from the ridge, plus the regular end bearings with the truss designer, just to make sure this doesn’t cause any problems with this truss arrangement. And, set the whole girder truss on the double top pl. Then, below the double top pl., btwn. your rough opening framing and the window frame, allow for some deflection, in your detailing, probably within the window frame (window head) itself, to protect the glass. This shouldn’t cost as much as the steel pl. ridge connection and the PLS beams.

 

[AMB DEC]

Hi jayrod12,

Thank you for reviewing my question/design. The total deflection for the header as a flat beam would only be 0.088". Yes, I did check the three ply lvl header for out of plane wind loads and it had adequate capacity and deflection. However, you raise a good point and I might as well orient that header in the direction which sees the larger loads, and for this case is going to be out of plane.

Thank you

 

[AMB DEC]

Hi dhengr,

Thank you for the response, I appreciate it. Since this is an existing home and I do not have any info on the existing trusses or manufacturer I am going to shy away from modifying the trusses and try to accomplish my work below the trusses.

Thank you

 

[XR250]

Why not put the steel in the middle of a 2-ply wood member - like a normal flitch plate? Other than that, it seems like a reasonable plan. There will be a small amount of thrust but that can be mitigated by limiting your vertical deflection.

 

[AMB DEC]

Hi XR250,

I am thinking that I would prefer to use screws rather than bolts for the flitch plate to minimize any slippage at the connection down the road when the snow load is applied to the roof. But maybe with this arch style header & flitch plate this is not as much of a concern? Thank you for the feedback on the design and the insight on the thrust component.

 

[XR250]

Have the holes drilled the same size as the bolts to minimize slippage

 

[AMB DEC]

Ok thank you.