Ledger attachment to joists thru wood siding

Question

Ledger attachment to joists thru wood siding

EricMasters (Civil/Environmental)

(OP)

25 Jun 06 23:05

I am attaching a ledger for a 2nd floor framing system to a ballon framed farm house cira 1840, with rough cut 2x4 studs with german lap siding on the exterior. The design was to attach the 2x12 ledger thru the siding with 3/8" lag screws 2 per stud. the studs are 16" o.c. 20' engineered wood joists frame into the ledger with simpson or simillar hangers, Using 10 lbs dead and 40 lbs live The load on the ledger is 500 lbs/lf. During permit review the county has specified that the ledger be attached to "blocking" and fastened 6" o.c. with 1/2" lag bolts. I don't see the need for the blocking nor do I see the need for the 1/2" lags 6" o.c.

Are the 3/8" lags sufficient? With 4" embedment? the diameter was choosen to minimize the size of the fasteners and to allow for a little error in placement.

Does anybody know of alternative attachemnt methods? Screws? I don't think nails would be appropriate given the siding. One of the major architectural reasons for the design is to keep the exterior siding exposed. It will become the interior finish on one wall of the room. Other wise we would just frame a wall under the joists.
Thanks in advance for any responses

Ron (Structural) · Answer 1 · 2006-06-26 22:26:32

I would not attach the ledger through the siding. Since the siding is not a structural component and its fastening is not required to handle significant shear loading, you are taking a chance by running the lag bolts through the siding. The lag bolts will now be subjected to a bending stress in addition to the shear they should be carrying.

Cut the siding at the attachment points and fasten the ledger with appropriate fasteners. If your plan review group wants 1/2" lag bolts and you have no engineering to prove otherwise, then do it. If you want to prove that 3/8" lag bolts are sufficient, then do so but don't guess.

EricMasters (Civil/Environmental) · Answer 2 · 2006-06-26 23:08:49

thanks for the input. I agree that guessing is not good engineering. The plan review board requires that the 1/2" lags be 6" oc. into blocking? How does the load transfer from the blocking into the studs into the foundation? They did not give any specification for the blocking attachement into the studs nor do they require any jack studs under the blocking. I am not trying to ignore the advice of the plan review just trying to understand it. The wood siding will take a compressive load so if the bolt is fastened properly why will the bolt be subject to bending? Wouldn't the siding and the ledger be a composite member?

Ron (Structural) · Answer 3 · 2006-06-27 10:43:02

For balloon framing, the ledger need only be attached at the studs, assuming the studs are laterally braced against buckling in the weak direction.

Check the capacity of the lag bolts into the wood you'll be embedding into, including appropriate safety factors. With your loading (500x1.33=665 lb/stud), you might be OK with two, 3/8" lag bolts, but you do need to specifically check it.

No, the siding and the ledger would not act as a composite section unless you nailed or glued them together to assure full shear transfer. In any case your siding is not likely a structural grade of lumber, thus would likely have low compression resistance right at the fastener (as the load bears onto the fastener, the "softer" lumber will compress, thus offering little resistance to the load, while the load it being applied about 3/4 of an inch out from the embedment point on the stud, thus causing a rotation on the lag bolt). Since you will have two lag bolts, one on top of the other, you will then have pullout to contend with on the top lag bolt. So your top lag bolt will be in shear and in tension, so interaction will apply.

EricMasters (Civil/Environmental) · Answer 4 · 2006-06-27 16:20:09

Thanks for the reply,
You have defined the issues clearly for me. The siding is actually very dense lumber, given that it is 140 yrs. old and well seasoned. If you hit with your hammer you don't dent it at all! I will however consider the issue of bending on the lags and probably cut the wood siding out. I think it is probably hem fur. The studs are either fur or oak. Do you know where to get some empirical data on fasteners into old wood studs? NDS has some info but it doesn't address the age of the lumber. It can actually be difficult to drive a 16 penny nail into them but I am concerned that the dryer the lumber it has a signficant probablity of splintering under load. I have considered using Simpson structural screws but do not have any experience with them either from an engineering perspective or from a field perspective. Any idea what is the best type of fastener for old lumber given the approximate loading involved? thanks

Ron (Structural) · Answer 5 · 2006-06-28 08:18:58

Eric...I would try to take a sample of the wood from an inconspicuous location and have it tested for structural properties.  Even if you remove one entire stud and replace it with a similar piece, you won't compromise the structure nor its authenticity, but the info you'll gain will help you tremendously.

One other approach to consider is to "sandwich" the existing studs between two pieces of 2x6 that have been ripped to the same depth as the studs.  The 2x6's would then be "thru-bolted" through the studs to provide you with a "receiver" for your ledgers that would allow you to use more bolts to reduce the contact stresses and reduce creep.

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A poor schematic, but hopefully you get the idea.

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