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Deck 05-12-2008 12:45 AM

Engineering help request: drilling holes in a deck joist, supported by a post
 
So I'm watching This Old House on PBS last night and Tom Silva is all like breaking down everything and altering my cozy world once again.

He describes how you can only notch or drill a hole in a joist according to certain conditions.

At one point he says that when you are drilling holes in a joist, you should follow the rule of "2 and 2": you shouldn't drill less than 2" from the bottom, or less than 2" from the top. Apparently doing this will affect the integrity of the wood.

Now I'm confused, because I was under the impression that if a post is supporting a joist, like for a deck, you could use two 1/2" carriage bolts to connect them together. Assuming the joist is a 2x6, if you follow that rule of 2 and 2 to drill your holes, your two holes will be...
5.5" - 4" (2&2) = 1.5" (working space in the middle) - 1" (two 1/2 bolts ) = .5" (leftover space)

.5" space between the bolts? That doesn't seem right to me, or safe.

I was planning on adding support for a joist by adding a notched post, so the joist could rest right on the post, and could also be secured to it. I was planning to secure using two 1/2" carriage bolts, staggered, and definitely spaced farther apart than half an inch.

I figured this would be the best and cheapest way since I already have the carriage bolts. Now I'm not so sure. I know I've seen pictures or I've seen it IRL the way I'm describing.

What do you guys think about spacing in a situation like this?

Can someone expand on why the integrity of the wood would be altered if I did it the way I intended?

Any general thoughts on this? Is it considered acceptable to secure using just one carriage bolt right in the middle?

Also, a carriage bolt question. Do you have to have a washer on both ends of the bolt? Right after the head of the carriage bolt there is a short square type thing before the shank starts. Should a 1/2" washer fit over that?

Thanks :)

MacRoadie 05-12-2008 01:21 AM

The general idea is that a joist, like any horizontal member is in compression along the top and tension along the bottom (in the long axis). These loads are most concentrated in the fibers along the top and bottom of the member. By Code, you are not allowed to notch a joist any deeper than 1/6 it's depth, and the notch must not be located with in the middle third of it's span. Any more than that and you have now reduced the effective dimension of the lumber to whatever is left (6" joist notched 2" leaves you with a virtual 4" member). You also can't notch a joist at it's point of bearing.

The Code allows you to drill up to a 2" hole in that same joist as long as you keep a minimum of 2" in from the edge of the joist with the logic being that you will preserve the critical wood fiber that would be removed by notching.

I think your confusion arises from the fact that you are applying a structural connection to a deck framing scenario. Through-bolting a joist to the side of a post with carriage bolts isn't a Code-approved connection for dimensional framing members, however common it may be in deck construction (and used to be in heavy timber construction). Tom is addressing a notch or hole drilled in a joist which is required by code to have "full bearing", a condition you can't achieve with through-bolting. In the bolting scenario, the connection is only as strong as the shear strength of the wood fibers and the fasteners, and those connections are typically calced out individually by the engineer (bolt size, hole size, hole spacing, bolt diameters to edges and end of member, etc)..

Termite 05-12-2008 07:55 AM

If you're adding a notched post for the joist in question to bear directly on, the fastening of the joist to the post is not such a major consideration. The holes you'd be drilling will be very small and will be located directly above the bearing point, so I'd have a hard time applying the 2" rule.

The 2" rule is in the code to prevent removal of the top or bottom tensile or compression chords of the joist. Inside that, you can drill a hole equivalent to 1/3 of the joist's depth.

If you want to use carriage bolts, I'd suggest washers at both the head and the nut. For what you're doing it will be 10x stronger than it needs to be, washers or not.

Deck 05-12-2008 11:36 PM

1 Attachment(s)
Thanks for the replies guys, some good info here.

Instead of notching, do you think it would be acceptable to do something like this:

http://img253.imageshack.us/img253/9058/djt14zyx8.jpg

I have several of this type of connector. Would this be substantially less secure? Simpson lists maximum acceptable load for this type of connection between 1000 and 1400 lbs.


What if I notched the post as originally planned and then drilled the joist for a single 5/8" or 3/4" or 1" bolt in the middle? More/less secure than, say, two 1/2" bolts 1 inch from the top and bottom of a 2x6 joist?


Also I just found out today that 1/2" carriage bolts require 5/8" washers at the head. The 1/2" washers don't fit over that square thing before the shank. Is it common to put 5/8" washers by the head?

Termite 05-13-2008 07:57 AM

I'll put it this way...

If your 2x6 joist has more than 1000 pounds of reaction at the bearing point, your deck is underdesigned. That's why you rarely see 2x6's used for joists on decks with spans in excess of 8-1/2 feet or so, if they're built by competent carpenters to code.

So yes, you're designing a very strong connection for a tiny joist that probably doesn't need a very strong connection. It will work great. You're overthinking this.

As for washers, yes, with carriage bolts it is just fine to use larger washers to clear the square portion under the head.

Deck 05-13-2008 09:48 PM

Thanks again. I'll try to relax =). I have time to continue working on this project tomorrow. I will use the DJT14Z connectors, and secure using nails and bolts.

I have checked many hardware stores within 10 miles of me (including Home Depots) and not one of them carries 5/8" HDG machine bolts (hex bolts). Home Depot's website says that they carry 5/8" HDG stuff but they do not. Even when I chose to "shop locally" it says they have 5/8" dia HDG stuff in store stock, but they don't. I have physically checked two stores and I have also asked.

Also, the DJT14Z connector won't take 5/8" carriage bolts because of the square thing before the shank. I can't find 5/8" HDG carriage anyway. As of right now I plan to use 1/2" HDG carriage bolts, because that's what I have. The square thing before the shank on 1/2" carriage bolts just makes it through the connector's hole.

Termite 05-14-2008 07:43 AM

You could use carriage bolts...Just stack and extra washer or two under the head to compensate for the square thing.

Or, see if you can find 5/8" galvanized threaded rod. Cut it with a hacksaw or sawzall and just use a nut and a washer on each end.

ncgrogan 05-15-2008 02:04 PM

Bolts are supposed to be 4d (4*diameter) from each edge and each other

Termite 05-15-2008 02:51 PM

Quote:

Originally Posted by ncgrogan (Post 123391)
Bolts are supposed to be 4d (4*diameter) from each edge and each other

That might be good practice, but it is not in the code. I'd also be careful about the 4x bolt diameter from the edges...If that were true you wouldn't be able to use less than a 1/2" bolt and still meet the code's hole boring requirement.

Deck 05-15-2008 07:52 PM

I haven't worked on this part of the project yet. Just to confirm do you guys think that using the DJT14Z connector is ok? The joist hole in the connector is centered at about 1 3/4" I think. So let's say I drill for a half-inch carriage bolt using the connector's hole as a guide. The actual hole in the joist would be between 1 1/2" and 2" from the bottom. These are rough numbers, I don't have the connector in front of me.

Quote:

Originally Posted by thekctermite (Post 123397)
I'd also be careful about the 4x bolt diameter from the edges...If that were true you wouldn't be able to use less than a 1/2" bolt and still meet the code's hole boring requirement.

Can you explain this please.. you mean I should be careful or he should be careful about saying that (because it's not always true?)?

Thanks

Termite 05-15-2008 09:00 PM

You aren't supposed to drill within 2" of the bottom or the top of the joist, since these are the tension and compression chords. This is very important over the span, but not such a big deal right over the bearing point. The code has the 2" rule, but doesn't really give any leeway on drilling the bearing point.

Once again, don't worry about it!

ncgrogan 05-16-2008 05:54 AM

Quote:

Originally Posted by thekctermite (Post 123397)
That might be good practice, but it is not in the code. I'd also be careful about the 4x bolt diameter from the edges...If that were true you wouldn't be able to use less than a 1/2" bolt and still meet the code's hole boring requirement.

I wouldn't use less than a 1/2 bolt on anything. If you are why not just use nails. 4d comes from the NDS

Termite 05-16-2008 07:59 AM

I agree that the national design spec has that info, however the code requirements will always override, so I felt it was worth mentioning to add the code requirement to the discussion. 3/8" dipped galvanized bolts are commonly used in deck construction for ledger attachment, etc., although I'm with you in that I wouldn't use anything less than 1/2" for most connections.

Deck 05-21-2008 09:08 PM

1 Attachment(s)
Hi guys, I have an update. I wrote Simpson and they said this:
Quote:

You are correct about the 4D edge distance requirement for the DJT14Z, but that requirement only pertains to the loaded edge. In chapter 11 of the NDS, table 11.5.1A, it states that the “loaded edge” edge distance requirement is 4D whereas the “unloaded edge” edge distance is 1.5D. The DJT14Z will be installed on the unloaded edge so the 5/8” bolt will be well outside the critical edge distance.

So the loaded edge is always on top?
Disregarding the connector for a sec, does that NDS info mean if you have a 2x6 joist you can drill two 9/16" carriage bolt holes by doing this.....

Code:

1.5D = 1.5 * 9/16 = 27/32 from bottom critical

27/32 from bottom edge + 9/16 hole dia = 1 13/32, so...

first hole centered between 27/32" - 1 13/32" up from bottom

Code:

4D = 4 * 9/16 = 2 1/4 from top critical
2 1/4 from top edge + 9/16 hole dia = 2 13/16

second hole centered between 2 1/4" - 2 13/16" down from top

Now... 1 13/32 + 2 13/16 = 4 7/32
So that's the sum of the outer edges of the hole to the edges of the wood...

And... 5 1/2 (width of 2x6) - 4 7/32 (amount of width used) = 1 9/32" = distance between the two holes.

If you've kept up so far thank you!

http://img528.imageshack.us/img528/6...ipboardis0.jpg

My Q:
Is all of this correct, and is 1 9/32" enough distance between two 9/16" holes according to the NDS?

Thanks guys :)

Deck 05-31-2008 10:16 PM

Quote:

Originally Posted by Deck (Post 124923)
My Q:
Is all of this correct, and is 1 9/32" enough distance between two 9/16" holes according to the NDS?

:bump:

Hi, I'm writing to follow up on this post. Any information on this topic would be helpful. I'd also like to know what the minimum NDS edge requirement is, like from the left or right edge of a joist. I have two 4x4s I'm using for railings that I plan to place at the edge of the joist by the joist hanger. I wonder if it's OK to drill through these joists, similar to what is described in the pic I made.

Thanks


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