Vaulted Hip Roof - Screened Porch 1

[XR250]

This is a roof design that one of the engineers in my area did. The contractor sent this to me as he was concerned about the rafter spread. It seems sketchy and very sensitive to shrinkage, cut quality and nail slip. It also provides no resistance to uplift moments. I checked the strap tension and it is only about 1,500 lbs. The strap capacity is about 2,000 lbs. total for both -so that is not unreasonable. It is also uses the house for bracing but it has minimal overbuild and only about a 12ft. connection length. What do y'all think?

 

[jdgengineer]

I don't like it at all. I am of the camp that wood moment connections don't really exist (or at least likely won't perform well). Any shrinkage will lead to deflection. Those straps are impossible to pre-tension so they will slip a little bit before the load is engaged.

 

[KootK]

What do y'all think?

1) I think that I'm glad that I don't have to compete for engineering work in your market.

2) I think that you may have inadvertently carved out a neat niche for yourself being: the guy who doesn't screw up the lateral spread thing.

3) I think that, in addition to the other concerns mentioned, which I share, this sets up a problematic, tension perpendicular to grain issue in the rafter ends. Test one of these to design load in the lab and I bet they'd tear apart very prematurely.

4) I think that, with a well installed diaphragm in place, many of these setups work for a good long while. But, then, I know that you know that sometimes it goes off of the rails with respect to the lateral spread business.

5) I think that I dislike this even more at a three ply girder than I do at the common frame condition.

 

[XR250]

Thanks for the feedback.

1) I think that I'm glad that I don't have to compete for engineering work in your market.

I'm glad you don't have to either!

2) I think that you may have inadvertently carved out a neat niche for yourself being: the guy who doesn't screw up the lateral spread thing.

It is one of my pet peeves

4) I think that, with a well installed diaphragm in place, many of these setups work for a good long while. But, then, I know that you know that sometimes it goes off of the rails with respect to the lateral spread business.

I agree. I have seen plenty of these hold up fine and have also seen plenty of 'em spread.

5) I think that I dislike this even more at a three ply girder than I do at the common frame condition.

Definitely.

 

[jayrod12]

Although I share the same concerns, I also feel that in this instance, with appropriate details to cuts and nailing, the hip style roof condition improves the chances of this working. The connection to the house will stop the left side from spreading, and the hip roof sheathing on the right, with an adequate connection to the two ply hips will allow the main sheathing to span between them all deep beam style.

I wouldn't put this out as a new construction detail, but if I was checking an existing structure, I'd bet that this would be performing adequately and I wouldn't be pushing for upgrades.

 

[XR250]

Geez, just noticed this note on the drawings. The Engineer certainly has a lot of faith in these strap connections.
I can't imagine getting a whole lot of stiffness out of this.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1590765836/tips/STRAP_xj18gc.pdf[/url]...

 

[jayrod12]

Oh that's ugly. That would not fly with me one bit.

 

[phamENG]

Oh...it'll fly alright...