16' cantilever deck rotted!!

[bdruehl]

possible for bid job..

16 foot cantilever deck with main beams 13 foot spacing (13 ft trib to the cantilevered 7x20 glulam beams which extend into the residence say 15 feet)...100 lb live load required!!! haha!

the glulams are rotted. NO POSTS allowed ofcourse by request by owner. idea is to cut the beam at the exterior of house then the fun begins with the attachment to the non cantilvered portion of the glulams (which are exposed) with some steel member scabs. i have my doubts to the ability of exiting glulams (and their attachments too) which probably were designed to half or less of the 100 lb balcony live load required now.

thoughts, or even better, anyone do this so i dont recreate the wheel and make a square?

 

[bdruehl]

just now instead of my original kneejerk thought of using a steel member butted against the cut glulam, and the details of that attachment (a but-plate with tension straps and compression straps back into the beam, a bunch of welds, etc??), i suppose 2 C channels on each side of the glulam is much more intelligent?? thinkin out load here.

 

[DaveAtkins]

I am with you--it may be difficult to make this work.

First of all--are you sure the live load requirement is 100 psf, and not 40 psf? I don't know--I haven't done residential work in a few years.

Second of all--I am almost certain you won't be able to just butt an extension against the end of each existing glu-lam. You will need to run the new members (I like the idea of a steel channel each side of each glu-lam) back into the floor of the house. The contractor will need to access the floor from underneath and install a lot of through bolts or lag screws.

DaveAtkins

 

[woodman88]

The owner may prefer steel tube beams in place of steel channels at each side of the GLB for the look. If you can, use a steel post to the cantilever bearing for transfer of the reaction. Placing a steel u-plate over the beam and welding (if possible) or bolting (better shear bolt values steel to steel instead of steel to wood) will also work.

Garth Dreger PE
AZ Phoenix area

 

[bdruehl]

due to fire requirements the beams will need to be wrapped anyway... and the load requirment, pretty sure that its 100 psf for residenital (cantilivered) balconies over 100sf in size. i havent seen an exception yet. PLEEEaase let me know if you know of one.

i dont like the "butt" into design either, but why would it be that it wouldnt work altogher? im thinkin the plate (welded to the extension beam and to the tension compression bolting "straps" 6 feet or so into the house beam) might be real thick at the butt, because the wood cant take the compression at the bottom end therefore needs to be strong like bull, but the idea wouldnt be flawed? why would that produce more of a force than the channels on the side?

discussion apprecieated.

 

[DaveAtkins]

I didn't read your post closely enough. Yes, you could put the channels on both sides of the glu-lam under the floor, and field weld the cantilever beam to those, using a plate of some sort.

But is that easier?

DaveAtkins

 

[bdruehl]

definetely not easier, but ive been doin some more woodshedding, and it seems i need alot more steel for an "MC" channel then i would for a single butting into I beam. all in the shape i guess... i suppose stopping 8 feet out or so and then bolting a smaller shape for the rest of the cantilever is also intelligent (and as requested by client, a replacement of the tapered ornignal glulam).

any more discussion on any un-obvious potholes for this type of connection with large moment? mega bolting required...

 

[bdruehl]

see scribble drawing tif
well, interesting... the glulam is failing under the preferred c channels (option 2each side of GL) configuration stressed to the minimum moment loads for the cantilever...
however, it wouldnt be wrong would it, to count the channels that are imparting the load to the beam thru the bolts also as composite to the beam for added strength- then the beam wouldnt fail!?... (will have to think about that one, should work cuz the beam cant deflect due to the channels, hmmm)that would be good because i dont like the option #1 - must check the VMD diagrams to see failure points - (the channels are designed to extend about 6 feet into the house for right now)_


the other configuration (option#1) seems to sidestep beam strength problem, with parrallel shear values for tension and compression bolts , but the detailing is more cumbersome.. the strength of the old glulam is also troubling- is the glue still good!?
also the steel channels (option2)weigh more than an I beam would cuz of the nonefficient shape. (option one is the I beam butted against the cut glulam)it is weird to me that option 1 turns out to be best option. this configuration seems to be a less desirable solution altogether when i look at it, even from a potential failure standpoint, but ive been wrong before!

(bolt hole reductions ignored for now)

and i know my drawing is horrid... and still just thinkin out loud - no "dont be a dummy" finger pointing! haha

seems it is not an entirely boring problem for the forum?

 

[efsinc]

regarding design loads....
balconies are notorious for surprise loads, and 13' strikes me as a pretty big residential cantilever.

In Malibu everyone crowds to the rail at the to see the sunset....and the balcony collapses.

In Chicago everyone crowds to the rail to watch the parade....and the balcony collapses.

It sounds like a fun job, but I recommend real care in load development, attachment to the structure and balcony rail design.

if the beams need to be wrapped for fire, what is the cause of the existing 'rotting'? be careful that the remainder of the existing structure is 100% sound that you rely upon to support your newly designed reinforcements.

good luck!