Very steep roof (24/12) - vaulted

[Ben29]

I am working on a single family house ($$$$$) where they want a 20ft x 60ft building with a very steep roof pitch (24/12) and a vaulted ceiling throughout. They want the roof thickness to be shallow (12 inches).

There are no opportunities for shear walls in the short direction. I came up with this scheme of steel moment frames (my sketch below shows them at 6ft o/c, but in reality they will be spaced farther apart and at different intervals.)

I originally had the "tie beam" at the top of wall elevation, but they did not want that. They wanted it up higher - so now it is at the ~22ft elevation.

I am showing a HSS8x4 beam to run between the frames to support the top of wall and transfer the wind load to the frames. Would you agree with this approach? The HSS8x4 size is just a placeholder and I need to verify sizing.

Thank you!

 

[XR250]

Off the top of my head, I can't think of a better strategy. I would make sure the tube sizes are readily available. You also need a thru plate at the moment connections or you will bend the tube faces (this has been discussed here previously).

 

[BridgeSmith]

The roof system looks like it could design out fairly easily. However, without any internal shear walls, resolving the forces on the wall will be difficult. They seriously want a 20' x 60' house with no interior walls to break up the 60' length? That seems very odd. They do understand that a shear wall doesn't have to go the full width to be functional and save tens of thousands or hundreds of thousands in specialized framing and high strength materials, right?

 

[XR250]

However, without any internal shear walls, resolving the forces on the wall will be difficult. T

I don't see this.

They do understand that a shear wall doesn't have to go the full width to be functional and save tens of thousands or hundreds of thousands in specialized framing and high strength materials, right?

It looks like they want what they want. Money does not seem to be an issue.

 

[TRAK.Structural]

Moment frames seems reasonable to me.

What will you use for the roof purlins, and how are you going to detail the attachment to the frames?

 

[Ben29]

So this is actually just one big room that will house the kitchen / dining / living room. So it will be open concept. There are other buildings on the complex that will house bedrooms, etc. Guys, this is a vacation home.

 

[Ben29]

Moment frames seems reasonable to me.

What will you use for the roof purlins, and how are you going to detail the attachment to the frames?

I plan on having 2x12 rafters span between the wall top plate and the ridge beam. There is an HSS beam attached to the top plate of the wall. So I am counting on the ridge beam and the HSS beam to transfer the wind load to the frames.

 

[Harbringer80]

I plan on having 2x12 rafters span between the wall top plate and the ridge beam. There is an HSS beam attached to the top plate of the wall. So I am counting on the ridge beam and the HSS beam to transfer the wind load to the frames.

Run the 2x purlins from frame to frame. This gives your MF beam full length bracing. With shop weld hangers, it's way easier to erect as well. You can eliminate the steel ridge and replace the steel "top plate" with wood.

 

[EZBuilding]

How big is your lateral reaction at the base? I wonder if it would be beneficial to try and consider a floor element at that elevation as a tie.

 

[Brad805]

The moment frames seem reasonable. I would look at a 2x12 wall so I could use deeper sections and space out the frames. That will bring a WF12 into the realm of possibilities. If I were dropping this much $$, I would ask to delete the tie as well.

 

[TRAK.Structural]

I plan on having 2x12 rafters span between the wall top plate and the ridge beam. There is an HSS beam attached to the top plate of the wall. So I am counting on the ridge beam and the HSS beam to transfer the wind load to the frames.

Yea that makes sense. I was originally thinking purlins spanning the other way, but as steep as this roof is you would end up with more weak axis bending with that orientation.

 

[XR250]

If I were dropping this much $$, I would ask to delete the tie as well.

For sure