A-Frame Shearwalls 1

[MightyPirate]

My recent project is designing a 3-story A-Frame structure in a high wind area (lake front). In addition the front and rear transverse walls are packed with windows.

So, my question is there literature regarding the design of A-Frame buildings? Mainly I'm looking for information on the transverse shearwalls where they are triangular shaped and where they're 3 story stacked.

Also, would it be feasible to instead of designing the shearwalls as stacked shearwalls to design the whole wall (33' high x 32' tall) as a perforated shearwall with the roof rafters as the chords?

-MightyPirate, EIT

 

[MiketheEngineer]

Need a bit more info - but to meet IBC2006 - you might have to resort to steel moment frame. I have had to do that in two/three story atrium walls that are not in line with the main wall system.

 

[msquared48]

Thinking a little out of the box here...

Being a trianglularly shaped structure, this is the most stable structural configuration if properly connected at the joints.

Could you use TJI's continuous from the bottom floor to the roof apex and make the triangle work to resist the lateral forces. I think that if you loaded each of the TJI members to resist the wind force, you might be able to get it to work without any end shear walls. Computer model it with the floor joists included.

However, you will still need a window wall system to take wind forces transverse to the endwall.

Mike McCann
MMC Engineering

 

[MightyPirate]

Thanks msquared48, I'll check that. I was talking around the office and that seems to be the consensus as well. The only problem is that the roof rafters are 38' long and are not continuous but I think the chord forces will be small enough that I can get a splice to work for those.

No literature though, huh? Searched APA's site and the local university library and couldn't come up with anything on A-Frame construction.

Also, I assume that as the roof diaphragm shear will be transferred from the roof sheathing to the roof rafter/chord (mentioned above with the splice) above the end wall and from that rafter/chord to the foundation that I don't really need sheathing in that wall except to take gable end wind forces.

 

[MightyPirate]

msquared48:

Just an FYI, after modeling the structure in STAAD I came up that a single 2x8 A-Frame at the gable ends can take the tension force of 5016 lb (at the base) and shear force of 1773 lb (at the peak) from the wind loads on the roof diaphragm.

So, anchor the base using a beefy simpson hold-down. Not sure yet what I'll do at the top, a shear ring or bolts, 1773 lb seems a bit much for screws or what not.

Thanks for the input.

MightyPirate, EIT

 

[HouseBoy]

I think that wood I joists may be limited in that they are not "permitted" to be used if sloped more than 12:12.

Just a thought that you might want to check into before too long.

 

[MightyPirate]

Never seen anything on it but we're going with conventional dimension lumber.

 

[HouseBoy]

I recall in the past, the I joist manufacturer did not allow too steep of installation as the joists would begin to act lile a column and the allowable stresses would not apply.
Perhaps this has changed in recent years. I've not seen anything about it lately. I just thought of it while reading the suggestion about TJI's full height.
I see that you wrote you have discontinuous framing members so I suppose long pieces (such as you could get with MWJ) are not needed. Like I said, it just came to mind.
While I'm thinking about it, I'm not sure the roof "diaphram" will even act like a diaphram. Will it? The wall on the leeward side is fastened at the bottom edge to the foundation isn't it? Of course, it is on the windward side too. I wonder if there might be a need for some sort of diaphram on the inside of the building (maybe at the level where you describe the rafters are discontinuous).
Just wondering. I'm sure you have been thinking about this longer than I have. I'm just trying to visualize how either one of the sides of the A frame will actually be engaged like a diaphram. I don't think the thing is going to be stable without some interior shearwalls and (probably) a horizontal diaphram in there too if the rafters are not capable of spanning bottom to top and anchored properly to the foundation at bottom and to the opposite side rafters at the top.
Think about it....

 

[MightyPirate]

Houseguy:

After thinking about it a bit more I think I agree with you and I think I'll redesign each rafter frame to take it's own tributary width (16") rather then assuming that a roof diaphragm will take the shear to the gable end walls.

The STAAD model I used has a hinge in the rafter where the discontinuity occurs and the structure is still stable (mainly due to fact that the rafters take mostly axial loads). I've located the hinge at the same point as the 3rd floor joist.

 

[csd72]

I cant see how you can get mainly axial forces in your rafters when they are supporting the cladding.