Balcony for upper level 1

[jimtheengineer10]

I am designing an addition for a client and he would like a balcony on the upper level. It is a 24' wide addition so the floor joists will be spanning 12' from a wall in the middle of the addition to the exterior wall. He would like to go 8' out but the cantilever distance would be greater than half of the interior joist span.

How do you analyze the joists if he were to install a diagonal member from the exterior wall to the middle of the 8' cantilever? Similar to the diagonal piece in the attached image.

 

[jayrod12]

You'd treat it as a 4 ft cantilever with a 4 ft span and then a 12ft span. The problem will be getting the exterior wall to calc out for the moment you're now putting into it with the diagonal support.

 

[JAE]

Why don't you sister other joists off the 12 ft. interior joists and extend them out?


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[jayrod12]

Still can't get a reasonable joist to work for an 8ft cantilever with only a 12ft backspan.

 

[jimtheengineer10]

JAE, are you saying basically installing a double joist but one ends on the exterior wall and one cantilevers out 8'?

 

[JAE]

If you must use 2x material (vs. engineered composite lumber) then perhaps an 8 ft. cantilever using a 16 ft. board lapping your interior 12 ft. joist.
You would have to work out the nailing between the interior and exterior to ensure an adequate load path.
You would also want to check, very carefully, the deflections. An 8 ft. cantilever seems quite long to me.

Or you could use double joists on the interior and double sistered cantilevered joists.

For a balcony cantilever, exposed to weathering, moisture fluctuations, and inherently non-redundant, I'd also consider doubling my safety factors.

Remember this collapse in Berkeley a little while ago? :

http://www.vosizneias.com/206607/20...balcony-collapse-tied-to-rotted-wooden-beams/

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[jimtheengineer10]

JAE, so double 2x joists (12' long) on the inside and then double 2x joists (20' long) with 12' sistered onto interior joists?

I was thinking of using LVL's but can they be left exposed?

 

[jayrod12]

I was thinking of using LVL's but can they be left exposed?

No. There is a parallam plus product approved for exterior use but I haven't yet come across an exterior grade LVL product.

 

[Triangled]

or try AYC all-heart glulam,
decay resistant,
20F-V13 AC/AC Fb=2000 E=1.5 (check me)
can normally get fabricated in any depth you want

 

[JAE]

I would agree with jarod12:

http://www.woodbywy.com/trus-joist/parallam-plus-psl/

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[RHTPE]

As the image that JAE shows, I would never advocate cantilevering interior floor joists for an exterior balcony. The detailing of, and actual construction of, the building envelope requires careful attention and simply invites future problems. Far too many penetrations through the envelope and it will be prone to issues a la Berkeley.

The lifespan of an exterior wood-framed deck is far shorter than the structure, even using the best P/T products. How will the deck be repaired in the future when it is overtaken by rot?

I would rather see 2 or 3 steel members penetrate the building envelope and run the deck joists parallel with the exterior wall. This way the future repair/replacement of the exterior deck will be far easier and (hopefully) will not involve any modification of the building envelope.

JMHO


Ralph
Structures Consulting
Northeast USA