Wood Diaphragms - Cantilevered & Open Front Structures

[Charred]

I am working on a design where the front portion of the building will not be able to resist lateral loads due to multiple openings, aspect ratio limitations, and shape irregularities. I decided to look at designing this an open front structure with three shear walls. However, the roof diaphragm will cantilever ~25'. I know there are two different configurations listed in SDPWS in sections 4.2.5 (Open Front Structure) & 4.2.6 (Cantilevered Building). Is it applicable to design these two systems in conjunction with each other? It seems to me that the wood diaphragm would not have the stiffness or rigidity to transfer loads at the cantilevered portion, but I may be wrong. Has anyone ever came across a situation like this?

 

[mike20793]

You can definitely do both, but the detailing will be a handful. Your going to need transfer diaphragms all over the place to get it to work out. I would start by sketching an idealization of the plan for us to review and look at the 2015 SDPWS. They are a little more specific on the rigid diaphragm assumption and how to handle it and they allow the cantilever length to increase.

 

[msquared48]

Man, I would surely strongly consider a steel frame before going there...

Mike McCann, PE, SE (WA)

 

[mike20793]

Yes, I agree with the steel frame before attempting that, especially if it's a relatively smaller building.

 

[JoshPlumSE]

Remember the lessons Northridge taught us about this type of structure. The gravity systems at the open side are subject to large deflections. Therefore, these gravity systems MUST be capable of withstanding these large deflections and sustaining their gravity loads.

In my mind this means, the gravity system should be design for P-Delta effects probably of the anticipated inelastic deflections. But, also their connections should be capable of withstanding the large deformations as well.

It's an interesting theoretical exercise to get an open front / cantilever diaphragm like this to work. But, it is not generally a good thing to do in practice.

 

[Charred]

A steel frame is likely to be a worst case scenario. As for gravity loads, there is plenty of wall, columns, beams, etc. at the "open" end of the building to resist gravity loads. The entire construction is wood framing with open web wood roof trusses to account for the long spans.

 

[JoshPlumSE]

Resistance of gravity loads in Northridge wasn't in itself the problem. It was resisting them when also subject to extreme lateral displacements. The type of open front / cantilever diaphragm system you describe really invites these extreme lateral displacements.

 

[Guastavino]

If you don't already, get Terry Malone's book too. Has some great complex examples. Also, he is available to contact through woodworks. They give free advice and he is a leading expert. He has helped me numerous times.

 

[manstrom]

We are going to see a lot more of these conditions. The 2015 SDWPS restrictions on narrow shear walls are the nails in the coffin for exterior shear walls. Windows are getting bigger too. These things don't look like big wood houses any more, they look like concrete/steel midrises.

You can get away with a 35' cantilever if you check the deflection and rotation. Read up in the 2015 sdpws. It is a free download if you don't have it.

That steel frame is going to get VE'd out right away. That and it will cause settlement issues. Also, the delfection on a frame is going to have to be very tight in order to make it play well with interior shear walls.



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Charred]

I do not have very much capacity, if any at all, for an internal shear wall. This is a restaurant with a large dining area that has 45'+ spans of open web wood roof trusses. The front of the building has two large "tower" parapets but my roof structure will be at the same level and this is where the 25' cantilevered diaphragm is giving me a problem because the walls are 26'6" tall with many arched windows and architectural features. Several times in the past, I have had to just tell the architect that the window sizes need to decrease or a different spacing needs to occur in order for my aspect ratios on the shear walls to work out. Steel is not really an option here.