Plywood Gusset Plates on Lumber Rafters
Plywood Gusset Plates on Lumber Rafters
(OP)
Hello all
I realize there are similar question asked previously but I think this one may be a bit different. I have been asked by a manufactured housing company to design a plywood gusset plate for a rafter (not a truss). The rafter is 12/12 and spans 7' (its a small module). When designing a gusset plate for a truss you don't usually take moments into account as the members of a truss are analyzed for tension/compression forces only.
Can this assumption be used for a rafter as well, my feeling is no. How would moments be accounted for in the gusset plate design?
Also, different from trusses where there is no outward thrust, will there be an outward thrust from this rafter? In this case the rafter is hung from an LVL ridge beam.
Any help would be appreciated.
I realize there are similar question asked previously but I think this one may be a bit different. I have been asked by a manufactured housing company to design a plywood gusset plate for a rafter (not a truss). The rafter is 12/12 and spans 7' (its a small module). When designing a gusset plate for a truss you don't usually take moments into account as the members of a truss are analyzed for tension/compression forces only.
Can this assumption be used for a rafter as well, my feeling is no. How would moments be accounted for in the gusset plate design?
Also, different from trusses where there is no outward thrust, will there be an outward thrust from this rafter? In this case the rafter is hung from an LVL ridge beam.
Any help would be appreciated.





RE: Plywood Gusset Plates on Lumber Rafters
RE: Plywood Gusset Plates on Lumber Rafters
RE: Plywood Gusset Plates on Lumber Rafters
Regarding the second question, technically yes, but this can be averted by notching the rafters at the head and tail for flat bearing at the ridge beam hanger, and at the wall top plate. You can also avert the force by installing a metal strap across the top of the ridge beam holding the opposing rafters together.
Mike McCann
McCann Engineering
RE: Plywood Gusset Plates on Lumber Rafters
Thanks again
RE: Plywood Gusset Plates on Lumber Rafters
Still confused in Michigan....
RE: Plywood Gusset Plates on Lumber Rafters
Anyway, the cut at 24" is measured along the length of the rafter stating from the bearing end and not the ridgeline.
Thanks again
RE: Plywood Gusset Plates on Lumber Rafters
RE: Plywood Gusset Plates on Lumber Rafters
Thanks again everyone, I appreciate the interest so far.
RE: Plywood Gusset Plates on Lumber Rafters
Is there a ceiling joist or is it a vaulted ceiling space?
Can you post a picture or sketch?
Also, not sure how/why this "modular" or panelization works. How is the roof "panel" configured?
Again, pics or sketches would be ideal.
RE: Plywood Gusset Plates on Lumber Rafters
RE: Plywood Gusset Plates on Lumber Rafters
I also wonder about the stability (bracing) of the 4 play trusses. It doesn't look like the rafter 'extensions' would do much to brace the compression chord of these trusses. Perhaps it will be a simple matter to improve this but...don't overlook it.
If the 4 ply trusses provide sufficient vertical capacity (and they are braced adequately) I would think the bending in the plywood will be maneagable but there will be bending and I would guees that the plywood plates will need to run up on the rafters much more than you show so you can develop bending wiht the connection to the rafter..
RE: Plywood Gusset Plates on Lumber Rafters
Thanks as always
RE: Plywood Gusset Plates on Lumber Rafters
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RE: Plywood Gusset Plates on Lumber Rafters
This company uses hinged plates for all their trusses. But, this is a rafter and the hinge plates won't work. They actually use hinge plates in this case only to hold the folded roof together...once it is on site the roof is rotated into place and then the splice would have to be reinforced with a plywood gusset.
RE: Plywood Gusset Plates on Lumber Rafters
I would look at the bending in the rafter at the point of the splice.
The plywood gusset will need to develop that bending moment on the fastener group on EACH SIDE of the splice.
For bending in plywood I have considered material thickness of one helf the plywood thickness and calculated section properties for that. You might need two plays or something similar. Often when there is a decent amount of bending, I find it better to use a scab piece with greater length (over which to design the moment in the faster group.
RE: Plywood Gusset Plates on Lumber Rafters
You have at least 2 major problems to deal with:
1. The rafters will have a shear, moment and compressive load at the splice. The forces on the nail or bolt couple in this connection will probably be very high.I would never try to make a connection like this with a wood member.
2. It doesn't look like there is ridge beam on this building. Therefore,the structure itself is unstable when the "temporary ceiling framing" member is removed. You will have a hinge at the top of both walls and at the ridge. There is nothing to keep the walls from spreading out when snow or wind loads are applied to the rafters.
RE: Plywood Gusset Plates on Lumber Rafters
In general, the whole idea of this splice is terrible and will result in a horribly constructed house with virutally no craftsmanship involved whatsoever.
You can "engineer" just about anything you want. I could run an FEA on a shoe box to determine it's strength, but we all KNOW a two year old girl can step on it and crush it.
This is not practical. Any carpenter in the field constructing this house would scoff at this detail. It is fundamentally unsound and any "design" of this splice connection would be based on horible construction techniques.
Garbage in -> Garbage out.
RE: Plywood Gusset Plates on Lumber Rafters
I do not agree that it is a GI/GO scenario (necessarily). Also, even with a suitable structural ridge, there will be bending and shear at the splice loation.
I think that ultimately the simplest solution might be to add a "sister" to the rafters in order to achieve sufficient lap so that a connection with moment capacity can be made as well as to provide a member with sufficient and knowable bending capacity at the splice location. That wasn't the original question though, so.....
RE: Plywood Gusset Plates on Lumber Rafters
Truss repair doesn't directly address bending in the plywood gusset in alot of cases. Min guesset sizes and min nailing spacing in truss chords based on testing is how the bending is embedded into the design