Cantilever deck stringer - fix for sagging 2

[taz112358]

I'm looking at a typical wood framed elevated deck with (4) 2x12 stringers spanning 7' across supporting 13' of deck between stringers. The issue is twofold, one, these stringers are cantilevered approximately 4' from center of column to outside ledger. They've attempted to support this with a knee brace, matching the columns @ 10x10 but really only spanning about 1'-9" center to center @45deg. A little over 2.5' is still hanging out there and even though it theoretically works in RISA with a 40 PSF LL and 10 PSF DL it's still sagging on the outside and the knee brace isn't something I really like to rely on.

Do you guys have experience with knee braces? Wouldn't they just shrink over time and end up not doing any bracing at all?

The second issue is that the two columns are resting on different types of foundations. One is either a driven or dug pile supported by the ground at some depth. The other is resting on a huge concrete foundation for the pool, 42" up above grade. I'm assuming some differential settlement is causing the column closest to the cantilever to be lower than the other, contributing to the sagging observed on the outside. It's about 1" lower on the outside than the inside.

Anyway, I'm thinking about adding a couple of steel channels to the stringers to beef them up, but connection details are getting a little tricky. I want to jack up the outside of this deck to level first of course, then add the steel. I guess the knee brace can stay even though I don't want to rely on it doing anything to support the outside of the cantilever.

Any help would be much appreciated, going to get some ideas from a few resources now but I thought I'd post here because there is always so many good ideas being shared and I didn't find anything similar in the search.

Cheers!

 

[XR250]

I would not expect the "stringer" (I call it a beam), to sag over that distance. Put a straight edge on it to confirm.
Sounds more like the settlement you mentioned.

 

[Tomfh]

Agree the "stringer" should be stiff enough not to cause noticeable sagging due to bending.

Check the settlements as noted, including slipping of connections. It's a shortback span and it wouldn't take much slip to result in noticeable sag of the cantilever.


As for the knee brace, is the column even stiff enough to resist it?

 

[taz112358]

Good idea on the strait edge, I'm also sure I'm going to find it's not actually bending/deflecting but rather the whole length just isn't level.

The column is also a 10x10 FS so it's got enough strength. I'm not sure how I'd measure the differential settlement but all the connections seem to be just fine. I don't think they've repainted in 5 years or so and there's not any evidence of movement with cracking or anything. Looks like it's all in the same spot as when they built it.

Thanks for the responses! Sorry I'm down in Texas we call em stringers.

Going to sketch up a fix with two channels on either side of the beam and setting new bolts to the boss man. I don't think he wants to involve steel but I'm not sure how to make it right and stiffen it up without it.

 

[diyfarmer]

I think I'd avoid steel too just for aesthetic reasons.

If there is in fact bending, any reason you can't use a longer knee brace at a steeper angle and support the beams closer to their ends?

 

[MIStructE_IRE]

Is this actually stable? If columns are pinned top and bottom my guess is the whole thing is leaning.

 

[dhengr]

JTBuTexas:
That looks kinda like a cobbled mess, and someone who knows something about structures should take a good hard look at it, rather than just adding stl. channels to the 2x12s, whatever they are called. For the most part, the bolted joint movement should have taken place and settled down after some (a few) significant canti. loadings. You have some slop in all the oversized holes, and then some crushing, in bolt bearing, as the bolts really do come into play. Then, at some point it got painted and joints haven’t moved much since. All the bolts should probably be tightened, and with washers too. That small knee brace doesn’t do much, it’s too short and the slack or slip in its joints just makes it decorative. With the 4’ canti., and the 3’ backspan, the shorter/right post is actually almost always a hold-down, not really taking much gravity loading. Thus, its t&b connections will move accordingly, and should be designed to take that max. uplift, approx. .76kips upward. The longer/left post actually takes more gravity loading than your sketch might suggest, something on the order of (40 + 10lbs./sq.ft.)(7’ x 13’)(7’/2)/3’ = 5.3kips., and this is a soil bearing pressure of about 9kips/sq.ft., on a bare 10x10 post. You may want to find out what the actual footing conditions on that post are and correct that. With the connections on the shorter/right post tending to move up, and the longer/left post connections compressing and the whole post settling, it isn’t hard to imagine a 1” settlement out at the canti. tip, that’s just a trig. problem. Then, also, check the whole deck structure for stability. By the way, the left most deck joist is just called a joist or a rim joist, not a ledger.

 

[taz112358]

Thanks for the helpful responses. Yes the boss man agrees with you farmer we aught to keep the repair done with wood. We are going to pull the bolts, jack up the deck on the outside and add (2) 2x10 to those knee braces and redo the bolts. Edit: also increasing the size of all the thru bolts from 3/4" to 1" to reuse the holes without having to drill them out bigger.

Yes I ran this in RISA and got loads even higher than what you've posted dh with the self-weight added in. Defiantly settlement in the column driven into the ground once I went back out there and put a string line on the stringers. The hold down connection at the other column is a heavy duty Simpson column base so no issue there.

Cheers!