Tapered cantilevered PSL/LVL beam

[skimboard20]

Hi all,

I am working with an architect that would like to cantilever floor joists from the interior of the house to the exterior in order to form an 8 ft cantilevered deck. I am aware of the thermal bridging and waterproofing issues associated with this, but right now I am trying to convince myself that the joists are strongth enough and satisfy deflection requirements.

The joists are 26 ft long (8 ft cantilever, 18 ft backspan). The house is located in an area with a relatively large snow load (65 psf).

Right now I have settled on PSL 3-1/2 x 11-7/8 @ 16"oc for the floor framing. This conservatively satisfies deflection requirements. However, instead of furring up the rest of the floor to allow for a sloped deck, the architect is wondering if we can taper the PSL joists at the cantilever. Assuming 1/4 in. per 1 ft slope, we lose about 2 in. of joist depth at the end of the cantilever (11-7/8" deep over support, 9-7/8" at cantilever tip).

My first question is if anyone thinks tapering PSL material is a bad idea to begin with. My second concern is the additional tip deflection that will arise from the continuously reduced cross section (if anyone has any resources on how to calculate deflection for a member with a linearly variable cross section, I'm all ears).

Mainly I'm just looking for some fellow structural engineer insight, which I so happily accept on this forum.

Thanks!

 

[driftLimiter]

You could conservatively estimate the deflection by making the cantilever segment depth equal to the minimum depth of the taper. To get more accurate you can integrate d2x/dy2 = M(x)/[ E I(x)] slope deflection equations

Just be careful with those PSLs they are very absorbent and degrade rapidly with moisture. Even if the deck system is wrapped and water proofed, you will want to take special measure to ensure even the smallest amount of moisture doesn't get trapped near the surface of the beam.

You can refer to the last page of TJ-7102

Link

 

[MotorCity]

Why do you need to slope the deck? Most decks I've seen have small gaps between the deck boards that allow water to drain off between the cracks. I would also suggest not having the beams start inside the house and extend out to also serve as deck supports. When the deck needs to be replaced (it will need to be replaced) are you going to remove the entire beam and tear up the inside of the house?

 

[XR250]

If you have a 2d beam program you can analyze it with multiple, varying depth sections. If you don't have one, you need to get one for a variety of reasons. You might want to try to use multiple treated 2x12's and sister them to main span. That way you can also have a step down at the deck - which will improve the chances of your waterproofing working. Might be a stretch though for 8 ft. cant @ 65 psf LL.

 

[RontheRedneck]

I have not designed a lot of Glulams. But what I recall is that the manufacturer said the plies of lumber in the top and bottom were higher grades than in the middle.

PSL and LVL are likely different. But in any case, my suggestion would be to contact the manufacturer. Get an answer straight from the horse's mouth.

And as others have said, the moisture problem is very real.