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Deck 05-12-2008 12:19 AM

Engineering help request: a pre-drilling story, and some questions
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I have 3" hot-dipped galvanized (HDG) lag bolts. They are Simpson SDS type screws. They have 1" of shank, 2" of thread, and are 1/4" in diameter.

I have 2x10x10s. I want to attach them together and make 20' long pieces for a ramp. I have 2x10x5s as well. What I want to do is have half of a 2x10x5 attached to one 2x10x10 and the other half attached to another 2x10x10. I plan to predrill for the lag bolts/screws. I currently estimate about 50 for each assembly; this according to Simpson's most conservative spacing and stagger pattern.

The ramp will be for my dog. The ramp will be at 20 degrees, or a slope of about 1:3. Again, this is a dog ramp. It's possible I might need to walk him up the ramp during the transition. He cannot make it up and down the stairs anymore, which is why I am building the ramp. I plan on only 18.5" width, since it's a dog ramp.

NDS says pre-drill 40% to 70% of size (or so I'm told -- I don't have a copy of NDS). I had posted this question on another board, and I was told that when you are connecting two pieces of wood you want a clamping effect: The screw should move freely in the first piece and not in the second piece.

On a related subject, I pre-drilled when I made cross-bracing for two posts. I predrilled the 2x4 bracing pieces only, so that the screw would move freely. I used a 9/32 bit and drilled all the way through the bracing (1.5"). Below is close-up of the cross-bracing attached to a post. I later trimmed the excess. The actual structure is best explained in another post.

So do you guys think I weakened the structure by predrilling at 9/32 past the shank length? Or is it correct that the screw should move freely in the first piece (the 2x4)? Or is it one of those situations with no clear answer?

More importantly, how do you think I should predrill for the ramp pieces? Should I predrill holes in the 2x10x5' pieces at 9/32? Do I want that clamping effect?

Thanks :)

Termite 05-12-2008 08:00 AM

I think you're overthinking, given the fact that this is a dog ramp and not a deck you'll be throwing parties on. You're ok to pre-drill a small hole to allow the two pieces to draw together.

Deck 05-12-2008 01:42 PM

OK thanks. The slope is rather steep (for me) and I figured that shear and tension would change depending on the actual angle of the ramp. I have a small test ramp at 20 degrees that I've been using so he can get on the couch, and it works well.

If I have to walk him up the ramp it's me at 160 lbs and him at 100 lbs (he's in his weight class) so let's say 300 lbs. Obviously according to the engineering specifications for their screws, I really don't need so many to secure the pieces. But 45 mph wind like there is in NY today, and my general need to make sure everything is totally rock solid and won't collapse, leads me to really overbuild everything.

I will predrill the 2x10x5's using a 9/32 bit so that the pieces will draw together. For anyone else reading this post, when you predrill this way it is imperative that you don't overtighten. I usually finish them off manually -- with a small wrench -- to just tight, where I can't turn anymore without a greater force.

On an somewhat related subject, does anyone here use a torque wrench? I asked about them at Home Depot and this guy was like nobody does that, they just turn until tight. When I bought a plumbing rubber sleeve for a plumbing repair, it said something like tighten to 60 lbs? of torque. How important do you think it is that you're actually at the suggested number?

Termite 05-12-2008 02:17 PM

Just use a ratchet and don't crank too hard. Just go by feel. No need to get too scientific and use a torque wrench. Torque wrenches are great for setting valves on an engine, but aren't needed for deck construction or most plumbing connections. They probably give torque settings so people using impact guns to drive the SDS screws don't overtighten.

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