Existing wood Hip upgrade 2

[sumbumb12]

So I have an existing wood framed structure and we are adding equipment to the roof.

The problem is that the hip is overspanned and can't be sistered because the rafters are framed into it on both sides.

So I was thinking about using some SDS (or similar) wood screws to bolt a unistrut (or similar) to the bottom face of the hip, inside the attic. I haven't gotten to calculating what sizes would be required, but I figure it would just be a composite section and I would have to check the screws for shear at the wood-metal interface.

This seems pretty unconventional, but I can't think of a good reason not to do it.

So I'm putting it out there to you guys - can you think of a good reason not to do this?

 

[jdgengineer]

That would be very unconventional

Can you just post down to a ceiling beam?

 

[msquared48]

I agree with jdg.

Mike McCann
MMC Engineering

 

[XR250]

You could also hang another hip underneath the existing that is the same width and fasten with a 2x4 or 2x6 vertical on each face between rafters.

 

[sumbumb12]

These are all good ideas/points, but I still don't have a good reason not to.

I'm not sure that "a reason not to" should be the standard, but I feel like if we are going to discount a method, we should have a good reason.

 

[dhengr]

Go for it..., even if it is a bit unorthodox, you’re the one who has to do the calcs. and prove that it works. Let us know how the connection calcs. are going; what screws, what spacing and end and edge distance, what slip at each screw before they all come into play, etc. etc. Then let us know how much more the hip has to deflect before your combined/composite section really starts acting like a composite section. What is the force, stress and strain in the unistrut, at the various stages?

You said...., “I'm not sure that "a reason not to" should be the standard, but I feel like if we are going to discount a method, we should have a good reason.” Experience as a Structural Engineer helps with this determination. Do all the calcs. and show them to us. Almost nothing is impossible if you have enough money, time and effort to spend on it. When you get done with your hip beam problem, list all the reasons the square wheel never caught on, I’d be interested in seeing that too.

 

[charliealphabravo]

As you have discovered these free span hip configurations don't often work on paper. I suspect that their origins are in prescriptive methods and regional construction practice rather than any rigorous rational method. In fact most of these hips appear in jurisdictions where an engineer is not required for home design anyway. In my experience these roofs rely on an unclear/ unconventional and highly redundant load path through arching and buttressing action in the sheathing and roof joists. Also the roofs are rarely symmetrical with forethought into opposing rafter reaction.

I had a point here now what was it?

...oh yes. Even tho you say the hip rafter is overspanned, I doubt that the hip/ridge rafter is the limiting factor in the design or that reinforcement of the rafter alone will make the system work on paper or in the field. The loads are carried mostly through the sheathing and the reaction forces in the opposing joists. Also these hip rafters act a bit like ridge plates on steep roofs. You will need to ensure that the supporting walls and floors can provide the additional reaction. The connections in the load path are also highly stressed I believe and would need to be looked at.

 

[XR250]

@Charliealphabravo

Well said.
I don't see a whole lot of problems with them in the field - except older ones that are lower slope.
Shows what we know!
FWIW, I always design the hips/valleys as beams and support them at the peak.
Wreaks havoc on homeowners when they want to finish the attic. I think most of them just covertly remove the posts.

 

[msquared48]

Sumbumb12:

The reason is KISS.

Mike McCann
MMC Engineering

 

[sumbumb12]

msquared48:

You know, that's a pretty good reason not to.