Cantilevered Diaphragm vs. Steel Columns

[JungleJoe]

Hi all,

Having a bit of a disagreement with a co-worker on this pool house that I'm working on. I have attached elevation views. The covered porch extends out 18' from the main structure. In my mind this extends out too far to be designed as a cantilevered diaphragm. Also note how the covered roof comes into the main roof at a different height than the top plate height. I am thinking about designing the straight columns as cantilevered steel posts to handle the shear forces (Seismic Zone D). My co-worker thinks we should do standard 6x6 wood columns and design the covered porch as cantilevered. Wanted to see what you all thought, and if a cantilever design is appropriate I don't see a great way to bring the shear back into the main structure's roof diaphragm.

Thanks in advance!

Pool House elevation views

 

[JoshPlumSE]

The cantilevered roof diaphragm doesn't line up with the diaphragm from the main building. If so, how do you define the back-span to resist the cantilever moment?

 

[CN-EIT]

What type of diaphragm are you using here; WSP or metal deck? If it is a WSP I believe that the NDS SDPWS has aspect ratio requirements that must be satisfied for it to qualify as a cantilevered diaphragm (see section 4.2). In either case, it seems to me that I would use the exterior posts in the LFRS (maybe not necessary if your aspect ratios check out) and then detail a load path for the diaphragm shear where the roof diaphragm in question does not meet your double top plate. Other more experienced engineers may have better insight here.

 

[JungleJoe]

Yeah so my thoughts are to design the columns as cantilever steel. Then where the covered porch roof comes back to the main building there will be a short, slanted pony wall that bears down onto the top plates of the wall. There will be sheathing all the way up the face of the wall and pony wall. There are also beams from the steel columns back to the main building that bear on studs inside the wall.

My co-worker's idea is to tie a double rafter onto the pony wall that runs parallel to the direction of the roof rafters for the covered porch. A portion of the slanted roof overbuilds on top of the main structure's roof so the that's where he's thinking the diaphragms tie together.

 

[JungleJoe]

What type of diaphragm are you using here

Plywood sheathing. Everything on this project is wood framed with the possible exception of the columns that I think should be steel.

 

[JoshPlumSE]

I personally prefer the concept of cantilevered steel columns at the front. You probably still need the pony wall on the back side though.... As you have to drag any shear from the diaphragm down into shear wall or strong walls or such.

Could you make a cantilever diaphragm work, maybe? I just think the detailing of that would get a lot more complicated. I'm not sure how your co-worker's concept transfers the moment from one diaphragm to the other.

 

[dhengr]

JungleJoe:
I would be inclined to use two identical stl. moment frames. One out at the two vert. cols. and with a sloped beam member btwn., and over, the two vert. cols. You have a pretty weak exterior wall as a shear wall back at the main bldg., what with the O.H. door and other openings, so another similar moment frame would be within that wall. Then, there would still be the two sloped rafter beams tying the two moment frames together. Archs./designers do all kinds of crazy things, seldom improving the structuring of their mess. Watch those two stone bases under the outer cols., they tend to attract car bumpers using the O.H. door, and can wipe out a stl. col. quickly.

 

[KootK]

I believe that the applicable limit here is that your cantilevered diaphragm can't project more than 2/3 of the depth of the diaphragm backspan. Here, your cantilever looks to be longer than your back span. So, all else aside, it seems at though a cantilevered diaphragm solution would not be code compliant.

 

[JungleJoe]

Thanks for the thoughts everyone.

with a sloped beam member btwn.

Hadn't considered that. The thinking is that the shear won't transfer into the steel columns unless the rafters tie into steel beams that bear on the columns? We have wood beams running left to right so we'd probably need to change those to steel as well if we end up going with the steel column design. Seems like a ton of steel for this pool house. I agree that's probably how it should be done, but you know contractors...

cantilevered diaphragm can't project more than 2/3 of the depth of the diaphragm backspan

I'm not familiar with that criteria. Can you tell me where I might find more information on that? I figured that would be in my Breyer book, but I can't find it.

 

[Enable]

I'm not familiar with that criteria. Can you tell me where I might find more information on that? I figured that would be in my Breyer book, but I can't find it.

Not sure if this is the provision KootK is thinking of, as the AWC talks about limits in terms of the width (not backspan), but none the less this also applies:

https://www.awc.org/pdf/codes-stand...ws/AWC-SDPWS2008-Commentary-ViewOnly-0906.pdf

Cantilevered Diaphragms: Rigid wood-frame diaphragms shall be permitted to cantilever past the outermost supporting shear wall (or other vertical resisting element) a distance, Lc, of not more than 25' or 2/3 of the diaphragm width, w, whichever is smaller.

However, in Seismic Behavior of Base-Level Diaphragm Anchorage of Hillside Woodframe Buildings from the Consortium of Universities for Research in Earthquake Engineering we find a provision for 1/2 of back span

Existing cantilevered wood diaphragms are acceptable provided they do not cantilever more than one half of the diaphragm back span (anchor span).

 

[KootK]

Enable's provision was the one that I was referring to with width = backspan in this context.

 

[JungleJoe]

Thanks, Enable.

I'm looking at the 2015 SDPWS and they have modified the maximum cantilever length to be 35' compared to 25' in the 2008 version.

Also, very interestingly, in the 2008 SDPWS the cantilever cannot exceed 2/3 of the diaphragm width, but in the 2015 version The L/W ratio can now be a maximum of 1.5/1. The 2/3 ratio now only holds for torsionally irregular structures over one story in height.

So the length of my cantilever to the end of the roof sheathing is 22'. Does W refer to the width of the cantilevered diaphragm or the width of the nearest shear wall line? In either case, by inspection the 22' would not exceed 1.5:1