Garage Door Headers 1

[medeek]

Usually when determining the size of a header for garage door I typically only consider the gravity live and dead loads acting on the beam about its strong axis and then plug them into the beam calculator here:

Medeek Beam Calculator

Most residential garage doors attach to the vertical jambs of the door so any wind loading will not be seen by the header but by the jambs which is picked up by the king studs at the sides of the door. However this is assuming that the pony wall above the header is non-existent or relatively short. What I am wondering is what about a garage door at the gable end of a building where the possibility of biaxial loading might become more significant. A properly built structure should have some form of lateral bracing about every 10 ft o/c for trusses but I have seen plenty of construction out there that seems to neglect this important detail. As such the wind loads (C&C) on the gable end will be felt by the garage door header and sizing of the header may be affected by this increased biaxial loading situation D + (.6W) or D + 0.75(0.6W) + 0.75(S).

I am looking for any white papers or other resources on this topic as well as any comments or experience dealing specifically with wind loading on garage door headers.

Another possible loading configuration is when the double top plate is missing and the beam serves as a collector. In this case we have bending and axial loading, however this situation would not seem to be as common.

 

[jayrod12]

Correct Koot, However IF the floor beam or header are not there, the plates or angles are sized to take the forces. Therefore I think it is reasonable to ignore the axial load unless there is no plate or angle present.

 

[KootK]

I do precisely what you've described Jayrod. It's built into the dogma of our trade. However, it is not rational. Imagine a deep LVL header with a couple of properly designed top plates running above. I would expect reality to play out like this:

1) LVL absorbs whatever gravity loads are tributary to it. Check.
2) When chord forces develop, they are shared between LVL and top plates in proportion to stiffness and fastening. Check.
3) LVL, which is now a beam column, fails due to overstress and crushes innocent family in front of fire place. Doh.
4) Properly designed top plates are still adequately designed for any chord forces that may develop in future. Check.

Does this ever happen? No. Do things like load casing and composite behavior help? Sure. These things aren't the point however. The point is that beams can't simply be told not to accept undesirable axial loads. They, like me, are rather poor listeners.




The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

Has anyone ever tried putting an entire stud gable wall into STAAD or RISA and applying some loads to it. I'm not talking about shear loads, just the gravity loads and the out of plane perpendicular load to the wall. I'm wondering how accurate this would be compared with manual methods and what it would show with regards to KootK observations about axial loads in the header.

I think as engineers we make alot of broad brush assumptions, some of which probably work just fine, but what is really going on with the structure. Obviously for practical reasons such FEA simulations are prohibitively expensive and time consuming for most residential work.

For instance if you have a large gable wall full of windows (beach house design) with a rafter roof. An FEA approach might be appropriate given the complexity of the wall and the high wind loads. In this sort of situation the biaxial bending of the king studs and any beams spanning the windows would be significant in my opinion.

 

[jayrod12]

Koot,

What about if you specified no nails between the header and the top plate? (I'm totally just playing devil's advocate here and suggesting a situation that is highly unlikely)

Would you still assume the header takes the axial load?

 

[KootK]

@Medeek. I haven't FEM modelled any wood. And I wouldn't admit to it I I had. I'm fairly certain, however, that a header beneath a gable truss effectively becomes the bottom chord for that gable truss. And then, clearly, axial loads ensue.

@Jayrod: I would not assume axial load in that scenario. Although some amount of axial transfer would still come from... wait for it... SHEAR FRICTION! This is your fault -- you baited me into this.

I've been working on something related in my head for some time. I think of it in steel but it can be adapted to this. Assume that:

1) I've got a header that can handle all of the gravity loads that it will be exposed to.
2) I've got a top plate assembly that can handle all of the axial loads that it will be exposed to.
3) Horizontal shear transfer between header and plate assembly is a given. 100% composite behvior.

Is there any rigorous way to say that the composite section is acceptable for the combined loading without doing a beam column check on the composite section? My gut tells me that everything is indeed fine. I can't prove that though.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.