Deck ledger connection

[CEmonkee]

I have a deck ledger question. The IRC supplement (R502.2.2.1.1) has the following:

"Placement of lag screws or bolts in deck ledgers. The lag screws or bolts shall be placed 2 inches in from the bottom or top of the deck ledgers and between 2 and 5 inches in from the ends. The lag screws or bolts shall be staggered from the top to the bottom along the horizontal run of the deck ledger."

In the plans for a deck design I did, I specified 1/2" lag screws at 16" o.c. staggered, with a 2" edge spacing. However, the contractor installed (2) 1/2" lag screws every 16" o.c. There is 2" between the edge and the top lag screw, and 1-1/2" between the edge and the bottom lag screw (see attached photo).

I think this connection detail meets the spacing requirements in the NDS (unless I'm not interpreting the requirements correctly). Table 11.5.1A (Edge Distance Requirements) for loading perpendicular to the grain, lists a minimum edge distance of 4D for the loaded edge (2" for a 1/2" lag screw) and 1.5D for the unloaded edge (3/4" for a 1/2" lag screw). I also ran a calc for the loading on the screws and I can show them good.

Am I missing something? If my calcs are ok and the contractor provides them to the building inspector, will he still have a problem with the connection since it is not per the IRC?

 

[JStructsteel]

reading the first paragraph, you need one at the top, then one at the bottom, and the spacing between the tops would be 32", and the bottom 32" but 16" between top and bottom.
Thats the way I understand the paragraph to read.

I just think you have too many lags. Better have them take them out

 

[Ron]

The reason for staggering the lag bolts is to reduce the potential for an interconnecting crack between the holes. Depending on the width of your ledger, this can be an issue.

The crack can develop from drying shrinkage or creep, though ledgers are not usually heavily loaded so creep would likely be less of an issue.

The edge distance given by the code is the minimum in the absence of engineering that can show otherwise. You have apparently done that and the building official can review and approved based on submitted supplementary engineering information, particularly when you can back it up with NDS compliance.

 

[mark1234]

If the question arises certainly calcs to back it up would satisfy the building department.

I always like using Simpson SDS 1/4" lags. Great shear value, less impact on both the ledger and studs, no pre-drilling required, more likely to connect to the stud in the center with adequate edge distance.



MDJ

http://www.windspeedbyzip.com

http://www.groundsnowbyzip.com

 

[CEmonkee]

Thanks everyone for your input. I was doing my calcs and I'm having trouble showing it good. So I thought I would check my calculation procedure by verifying the spacing requirements in IRC Table R502.2.2.1. I must be doing something wrong, because I cannot show good what they have in the table.

Say for example I have a Southern Pine ledger and a Spruce-Pine-Fir band joist (as described in the table heading). Also the deck LL = 40 psf and the DL = 10 psf. The joist span is 10', so the loading per foot along the ledger would be:

w = (10 ft/2) * (10 psf + 40 psf) = 250 plf

For 1/2" lag screws, the recommended spacing in the IRC table is 18" o.c.

If the screws are 18" o.c., that would mean the load each screw carries would be:

P = 250 plf * (18"/12 in/ft) = 375 lb

For the allowable load per lag screw (prior to applying adjustment factors), I'm calculating Z = 180 lb (considerably less than the required value of 375 lb).

I have attached a copy of the IRC table along with a screenshot of my spreadsheet calcs. I have checked the formulas in the spreadsheet by running some examples from Breyer (Design of Wood Structures, 6th edition) - the results match, so I don't think I have any errors in the spreadsheet equations.

Does anyone see an error in my calculation procedure (either for the loading on the screws or the allowable screw load)?

Thanks for your help!

 

[CEmonkee]

The attachment for my last post didn't get attached - hopefully this will work.

 

[concretemasonry]

Residential or commercial construction?

Many local codes over-rule the national codes because of experience and do not allow lag bolts and permit only though bolts if you are attaching to a rim joist or similar. - That plus the detailing, construction moisture/mold problems are the reasons so many frees standing decks are built. Many situations do not permit access to provide trough bolting. They just set the posts 2' beyond the wall to avoid the footings and the deck is cantilevered back 2' to the structure.

You cam still make one or two selective connections (limited moisture exposure) to the existing structure to provide some lateral resistance. A deck surface with properly attached deck boards on a diagonal will also help with lateral stability. - All of this is beyond computers, and a "sit back" observation always helps.

Dick

 

[CEmonkee]

Dick - lag bolts are allowed in my area.

My problem (described in my last post) is that I can't show the spacing specified in the IRC table as good (I must be making a basic mistake, but I'm not sure what it is). If someone could take a quick look at my calcs and let me know if I'm doing something incorrectly, I would really appreciate it.

 

[lsmfse]

Don't you have 2 bolts? I couldn't see it in your spreadsheet.

 

[lsmfse]

Also, diameter = .371?

 

[CEmonkee]

I do have two lag bolts, and I wasn't able to show them good for my loading...

So I decided to run the calcs for what is shown in IRC Table R502.2.2.1. For my example calculation I chose a Southern Pine ledger and a Spruce-Pine-Fir band joist, deck LL = 40 psf and the DL = 10 psf. The joist span is 10'. There would be one lag bolt every 18" o.c.

Regarding the diameter - a 1/2" lag bolt has a reduced body diameter of 0.371". I think that would be the diameter I would use in the calcs (at least that is what was done in an example I looked at from Breyer).

 

[OldPaperMaker]

CEmonkee,

If you use the NDS to design the ledger to rim board and you have a 2x rim (1-1/2" thick) you won't have the min. 4D penetration with a 1/2" dia. lag bolt so I don't think that you can use the Yield Limit Equations to calc. the capacity of the connection.

You might want to go to structuremag.org, May 2008 and read the article titled "Residential deck ledger connection testing & design". I think that the awc.org web site has a pdf version of DCA6 which is the precripive design found in the IRC.

Good luck

 

[CEmonkee]

Thanks for your reply OldPaperMaker.

I think the diameter I would use would be the reduced body diameter (0.371"). So the minimum penetration of 4D would be 4*(0.371) = 1.48", and the 1-1/2" thickness would be ok.

I have a copy of the DCA6 document. Table 5 on page 12 has the following footnote (#5): "Deck ledgers shall be minimum 2x8 pressure-preservative-treated No.2 grade lumber, or other approved materials as established by standard engineering practice." So the diameter that's being used for the 1/2" lag screws must be the reduced body diameter, otherwise the minimum penetration would not be met.

Anyway, if anyone could let me know if I have an error in the calculation procedure shown in one of my earlier posts, I would really appreciate it. Basically I'm trying to verify the o.c. spacing in the IRC (and DCA6) table.

 

[Cap07]

I think you would have to use the reduced body diameter for the lag screw... So you're trying to verify the spacing called out in the IRC table?

 

[CEmonkee]

Hi Cap07 - that's right. I'm running some calcs for my actual deck (and not showing the lag screws good). To check my methodology, I decided to do an example calculation using the configuration in the IRC table (please see my earlier post at 09 14:17 where I attached my calcs).

I'd appreciate it if someone could take a look at my numbers above and let me know if I'm doing something incorrectly.

Thanks!

 

[msquared48]

I see another possible problem here:

Is the ledger mounted over 1/2" sheathing? If so, then I believe that a 2" ledger dimension would give a better estimate of the capacity, which will be even lower than you show.

Also, in the photo you first posted, I see concrete below the ledger, but it is unclear to me if the ledger frames to wood framing or concrete.

In thinking further here, I bet there is some factor in the notes for the tables that has been overlooked that would equate your calcs with the other values you reference.

Mike McCann
MMC Engineering

 

[CEmonkee]

Hi Mike,

The ledger frames into the band joist, and it is not mounted over sheathing. There's a 1" plywood spacer between the ledger board and band joist. You can't see the spacers in my original photo, but they're on the other side of the screws.

I've looked over the footnotes in Table R502.2.2.1 (attached in one of my earlier posts), and I don't see any factors that I'm missing...

Possibly I'm missing something else, because when I try to verify that the spacing in the IRC table is adequate for the loads, I can't. Maybe you can see something in my analysis method that is incorrect. Just to repeat the example I was trying to verify:

Material: Southern Pine ledger and a Spruce-Pine-Fir band joist.

Loads: LL = 40 psf and the DL = 10 psf.

Joist span: 10'

Loading per foot along the ledger would be:

w = (10 ft/2) * (10 psf + 40 psf) = 250 plf

For 1/2" lag screws, the recommended spacing in the IRC table is 18" o.c.

If the screws are 18" o.c., that would mean the load each screw carries would be:

P = 250 plf * (18"/12 in/ft) = 375 lb

For the allowable load per lag screw (prior to applying adjustment factors), I calculated Z = 180 lb. So it looks like the screw spacing/joist span in the table would not work. I have to be missing something obvious, but I don't know what it is. I've attached my spreadsheet - I'm pretty sure that the equations I have for calcuating Z are correct (I've checked them against examples in Breyer, "Design of Wood Structures").

Thanks for your help!

 

[OldPaperMaker]

CEmonkee,
I think that you are looking at two different approaches to the same problem.
1. The Yield Limit Equations, from the NDS Table 11.3.1A, gives you the capacity that you have calculated in your spreadsheet.
2. The prescriptive design values found in the IRC & DCA6. According to the article in Structure Magazine the IRC values were determined by load testing at WSU (where Breyer is a professor) different connections to failure and applying a factor to the results that are shown in the IRC.

It appears that the NDS approach is more conservative than the IRC values.

In addition, as someone pointed out in a previous post, you have twice as many lag bolts as the IRC directs. If you meet the spacing & edge distance requirements you are probably OK.

How high is the deck off of the ground below?

 

[CEmonkee]

Hi OldPaperMaker,

Thanks for your reply - I found the article and I can see your point. I guess it's just not possible to replicate the IRC table with calculations using the NDS equations.

The deck is a little over 30" off the ground, so I don't think the inspector will get too picky about the ledger connection, but you never know.

Anyway, thanks to everyone for their input!

 

[RHTPE]

CEmonkee: Don't like decks with joists framing into a ledger attached to the wall (be it rim joist or studs) ever since I had to repair a deck constructed that way. I thought it would be a simple deck rebuild - until I took off the ledger board and found a colony of carpenter ants who were loving the moist environment. Seems the ledger was attached against the sheathing, the ledger was flashed when the siding was installed, but then the first deck board (running parallel with the wall) was nailed through the flashing and held tight to the wall. The flashing was poorly done and between that and snow accumulation through the winter, water managed to continually infiltrate between the ledger and the sheathing. Soaked the sheathing quite thoroughly and ultimately the ants found it quite hospitable. We were lucky to catch it before the ants had done some major structural damage.

I'm a fan of an independently supported deck, tied to the house structure only for lateral support (if necessary). I would run the decking boards perpendicular to the wall and hold them back from the finished wall enough to insure that leaves and debris fall through and do not accumulate (thus trapping moisture or snow).


Ralph
Structures Consulting
Northeast USA