Ledger - Stud Wall Connection w/ Lags: Shear or Bearing?

[cmhski]

Hi Everybody,

I am designing a deck w/ a 4000 lbf jacuzzi & 210 psf snow load. I end up having that each joist (@16" o.c.) bears on the 4x ledger (w/ hanger) with a total load of 1500 lbf. I want to use lags to connect the ledger to the 2x6 studs in the wall. Using the NDS ASD Table 11J (& applicable factors), I get that I need 3- 3/4" lags @ 16" o.c. That just seems ridiculous... Also, I am concerned that the large lags will cause the wood to split.

Can it be argued that it is bearing (Table 11.3.2), rather than shear that controls? Or do I need to specify a closer lag spacing w/ blocking between studs?

Thanks,
cmhski

 

[hokie66]

cmhski,

Is the 210 psf a typo? That would be about 20 to 30 feet of snow! Are you connecting your joists to the face of an existing wall? What is the cladding? Better connection would be to place studs under joists or provide beam and columns adjacent to wall similar to what you must have on the outer side. When you say 4x ledger, what do you mean? In summary, not enough information.

 

[COEngineeer]

yeah, like hokie said, check your snow load. What is the reason you use such a thick ledger? I think there is a top flange hanger that will support 1500 lb, or you can use face mount hanger. Also, instead of using lag bolt try to use ledgerloks. Go to

http://www.fastenmaster.com

for more info.
It is a lot better than lag bolt. I will look at my simpson catalog when I am at work.

 

[civilperson]

That is correct snow load for Tahoe (Nevada or California).

 

[dik]

For those loads, I'd consider using through bolts with rings or modify the detail so that the forces can be transferred by bearing... weld a plate on the top of the ledger and maybe notch the ledger into the studs.

 

[structuralaggie]

I think your concerns with the lag bolts are well founded. I know Frank Woeste at Virgina Tech has done some extensive research on deck and balcony design, which included the attacment of ledgers. I think I downloaded it before, but that was a few years back. I couldn't find it just know, but I didn't try that hard. It would be worth looking at. Sorry I can't give you a link.

 

[structuralaggie]

It was an article in the Journal of Light construction. I remember now. I'm not sure if it is available online or not.

 

[RockEngineer]

When your loads get that high you probably don't want to count on connecting to the rim board and wall studs to support all your vertical load. You would be better off adding a girder and posts about 2 ft off from the wall and then your other girder and posts and the end of the deck. I've designed decks for over 250 psf snow load and hot tubs here in Washington and you don't want them pulling away from the structure. Three 3/4" lags into every stud is not a good idea. The same is true with the Ledger Lok screws. They have a slightly higher capacity than regular lag screws but still not a good idea.

If you provide the extra girder for the verticzal loads all your connection to the wall studs will have to resist is the lateral loads for wind and seismic.

 

[COEngineeer]

really? 210 psf in Truckee? The worst one ive seen in colorado is 110.

Well, I guess you can always add girder right next to the wall or frame the joist the other way and rest the girder in the wall.

 

[RARWOOD]

I would follow RockEngineer's advice. If you are able to, get a copy of "Wood Design Focus" Volume 16, Number 2 Summer 2006.
It has several articles co-authored by Frank Woeste, P.E. regarding testing that has been done on deck ledger connections and guardrail connections.

I think a review of those articles would discourage you from using lags or through bolts to attach your ledger to the wall studs.

 

[structuralaggie]

The article RARSWC is refering to was what I was talking about. It was a good artilce and worth reading.

 

[RockEngineer]

Just for info I have a link to a white paper on snow loads in Washington State. Go to the appendix in the back to see some of the design ground snow loads around the state. Quite a few over 200 psf. The largest is 760 psf at Sunrise on Mount Rainier. We get lots of really heavy wet snow in the Cascade Mountains (not like the dry fluffy stuff in Utah and Colorado). It can be really fun and challenging designing in some of the ski areas where I engineer a lot of log homes.

http://seaw.associationsonline.com/documents/WABO-SEAW Snow Load White Paper.pdf

 

[cvg]

Sierra Sludge was our nickname for the stuff as compared to Rocky Mountain Champagne powder. If you have ever skied, the sludge is much heavier, thus requiring much higher design loads.

 

[COEngineeer]

wow... thanks.. I guess I cant complain anymore then hahah.. I hate it when I have to put (2)-1 3/4x11 7/8 LVL for certain rafters (they refuse to use trusses). I guess I cant complain too much because there are a lot worse snow load.

 

[cmhski]

Hi Everybody,

Thanks for the all the replies! And yes, we have "Sierra Concrete" here in Mammoth Lakes, CA. Actually, the 210 psf is if I assume that the snow is showeled when there is only 6 ft of it covering the deck. Last year we got 668 inches of snow over the season! And most homes are owned by 2nd homeowners, i.e., they are not necessarily around to showel their decks.

In general, the owners don't like to have a "forest" of posts underneath their decks as it obstructs the view from the downstairs windows. That is why I'm trying to avoid specifying girder/posts close to the wall.

The weird thing is, that the typical existing decks here have survived just fine with face mount joist hangers nailed onto 2x ledgers, which in turn are connected to the studs with 1- 5/8" lag @ 16" o.c. Generally, the failures occur when the soil builds up around the bottom of the outside posts (over decades of poor landscaping management), leading to dry rot.

To COEngineer: the 4x ledger gives a higher capacity per lag than a regular 2x according to NDS ASD Table 11J.

Thanks,
cmhski

 

[ctcray]

Could the ledger be attached to the rim board of the house and have the joists frame to the ledger with hangers? Seems that would be a better connection than dropping the ledger below the joists and trying to hang off the studs.

 

[cmhski]

ctcray,

Yes, the joist hangers are connected to the ledger. That part is easy... The tricky part is connecting the ledger to the rest of the house... In order to get the highest possible capacity according to NDS ASD Table 11J, the main members have to be loaded parallel to grain, i.e., the lags have to be going into the studs, rather than the rim board.

cmhski

 

[RockEngineer]

I hear builders all the time tell me thay have been doing something the same way for 50 years and it hasn't failed yet so why should they change. As engineers the design load isn't the failure load. The design snow load should be the 50 year snow storm which you may never see and then the structure has a lot of residual strength beyond that design load. Not failing under the first heavy snow or even 20 years of heavy snows is not the criteria for design.

I like the answer some of the instructors in some log home building classes give to students who think they have a better way of doing things that have not been well thought out "You could do it that way, but I don't recommend it". You have choices and as an engineer it is your responsibilty to make the hard decisions. Putting that many lag screws into studs may work fine but I don't personally recommend it. I'm sure I do a lot of things other engineers would question that I feel comfortable with.

When a deck does fail it can have disasterous consequences and failing decks are becoming more and more prominant as builders and engineers stretch the materials and designs to the limit. Some people want to live in a glass house with nothing obstructing their view of the outside. If they want no obstruction go outside. oooouch!

Good Luck.

 

[ctcray]

I would add additional blocking behind the existing rim board to have a 3" main member than use ledgerlok screws (8 or 9 per bay) to attach your ledger at the rim. They're a lot easier to install than 3/4" lags. I agree that the 3/4" lags will likely split the studs.

The ICC evaluation report with design values for LedgerLok screws:

http://www.icc-es.org/reports/pdf_files/ICC-ES/1078.pdf