Lag bolt construction, is this a bad idea?

RichardBronosky

Here is the most basic outline of a lag bolt construction frame drafted and animated in Google Sketchup.

http://cl.ly/FNhw/Table with Lag Bolts.mov

As soon as I saw it, I thought the bolts would act as pivot points. This would require me to either add dowel pins, or put the lag bolts in from the (90°) other side on the bottom frame. (I'll sketch that out tomorrow.)

So, want do you think of this construction?

RichardBronosky

Okay, so I couldn't just go to sleep with this idea dancing in my head. I sketched it with opposing bolts as well.

http://cl.ly/FNim/Table with Lag Bolts 2.mov

I feel a lot better about that.

Please advise.

ttime4four

I like the simplicity, but I think they would move on the pivot.  Perhaps using two lags per attachment point or dowels, biscuits or dominos. 

Not the same as your design, but I followed the following plans ... http://sites.google.com/site/mybiggreenegg/home ... and used two carriage bolts per attachment point.

Here's a progress shot.

RichardBronosky

ttime4four said:
Here's a progress shot.

That wood is beautiful. Is it mahogany? I wish I had the tools and the skills to make a solid top like that.

I think I fixed the pivot problem in my second post/comment above. I'm going to finish up the plans tomorrow.

ttime4four

Richard - I think you have fixed your racking problem and I suspect it won't move.  I'd still put in dowels in the upper points though. 

Thanks for the comment - the wood is Philippine Mahogany and boy is it heavy.  I'm kinda wishing that I stuck with 1x4 for the majority of the framing.  The top and bottom are not solid, but individual planks clamped together.  I will be fastening them to the frame with Kreg screws from below.  I was debating leaving space between the boards, but I figured the table will be covered when not in use and I will be finishing all sides of each board before assembly.

The inset is black with gold speck granite (two pieces glued together with silicon adhesive) to make up the right height.  Home Depot - about $4.25 per 12" tile x 2.

I'll post a whole thread when I figure out how to start a new thread.

stike

richard, those opposing bolt locations won't fight the racking.  you have what are called 'moment' connections.  virtually impossible in wood.  as they are. you'd need to over-tighten them a tremendous amount to limit racking, and it would still eventually happen


two smaller screws would actually be better than a giant lag bolt.  even then, though, the plans are susceptible to the frame racking, going out of square.

easiest thing (but maybe not the best cosmetic solution) is a diagonal somewhere.  or a plywood back.  not the best look in either case.

the giant lags look strong, but the washer is what is doing the work in your design. the friction from the washer trying to fight any force from racking.

better to think small.  a pair of screws will work a bit better. be careful of three or more screws... the wood can split.  the frame isn't carrying much weight, including the egg.  coupla screws will take the deadweight easily, leave the end of the wood a little less likely to split, and their being separated creates just enough of a moment in the connection to fight anything but a deliberate attempt to rack the frame. 

[edited]  should be glued with waterproof glue, too

Hillbilly-Hightech

I agree w/ Stike. A "moment" is a fancy (Engineering) word for "torque" - it's basically the "pivoting" you speak of.

ANY connection w/ only 1 connection point can (will) produce a moment.

If I may be so presumptuous, I'd say you wanted to use the lag bolts because they were the only things long enough in order to be able to span the distance of, and connect, all the pieces of wood w/ 1 long bolt, right? And you thought the lag bolts would be "stronger" right?

Well, if that's the case, it looks like your "legs" are 4x4s? If so, you don't really need those, that's overkill. 2x4s will suffice. However, if you wish to use 4x4s, and you can't find normal screws that are long enough to span the 3 pieces of wood, then I'd suggest going w/ a smaller diameter lag bolt & doing 2 of them (2 connection points stops the "moment" force being generated).

And to piggyback what Stike said regarding the wood splitting - I am by NO means a carpenter, and my "finish" carpentry skills leave a LOT to be desired, but what I always do is drill pilot holes first, which helps decrease the chances of splitting the wood.

Anyway, HTH,
Rob

stike

HH... not to nitpick again hahaha

but you can have a pinned connection that has no moment.  just needs to be triangular (which is why a cross brace or wire would work. though it may not look ideal)

take three drinking straws forming a triangle.  stick a single pin through each connection.  the joints of the triangle won't be deformed (unless we get into bending, and he won't get any bending with the small loads we're dealing with). 

but now make a square of drinking straws, pin through each corner, and it will collapse easily.

the pilot holes will help the wood to keep from splitting simply from th screws going in.  i meant that too many screws (or nails) makes the connection weaker.  some folks put as many as four screws in those connections.  actually weaker than just two screws, under the right (or wrong, i guess) conditions.  because each pair is in-line.  can encourage the wood to split if it racks.  pilots keep it from splitting while he's making the connection and driving the screws.  but multiple screws there can weaken the wood itself

RichardBronosky

Bummer. I knew that the table in the first video would rack. I thought I solved it with the second video. I guess I just have no way to improve upon Naked Whiz's "Option B".

stike

if you built the thing from oak, and the wood wasn't compressible very much, you could probably pull that off.  but most other woods are much softer. think PT or cedar.  they'd shrink and/or crush as you tightened

honestly, better to build it just as you have it, but use wood dowels, in pairs, and glue them then sand it flush.

Hillbilly-Hightech

HH... not to nitpick again hahaha
but you can have a pinned connection that has no moment.  just needs to be triangular (which is why a cross brace or wire would work. though it may not look ideal)

Stike - well, technically, true. Sorry, should have been more precise. Was talking about 2 members pinned by 1 connection, wasn't talking about 3 members w/ 3 pinned joints which form a rigid constraint because of the geometry of them all being aligned in that manner. The leg & the horizontal board in the table would be 2 members, and would have 1 pinned connection (in the OP's example).

Adding in a 3rd member & aligning them in that geometry, and you're basically talking about a strut, which, because of the geometry, restricts the degrees of freedom. Each pinned joint, acting by itself, will allow the moment - however, when placed in the configuration you're describing, the moment is restricted because it's being pinned by the other one and it's rotation is being restricted - it's not the "pins" of any one joint restricting the movement, it's the geometry of all the joints acting together.

Take your 3 straws example, and, instead of putting it in the shape of triangle, put it in the shape of a "Z." In that case, there will still be a torsional degree of freedom allowed on the upper & lower "legs" of the "Z."

You can also take those 3 straws, and stack them on top of 1 another such that 1 pin can go through all 3. That pin would allow rotation of all 3 around the single pinned joint.

So, agreed, I should have been more precise w/ my statement that "any" pinned joint will allow rotation, but agreed that you're also being nitpicky ;-)

^5 - good catch!!

stike

m'kay. thanks

Hillbilly-Hightech

Richard - Just to make sure it's clear, you have at least 2 choices:

1.) Use your original design

2.) Redesign the look of your table

If you wish to use the design you originally submitted, a way to do that is by using 2 screws (or, as Stike suggested, dowels) instead of 1.

I "assumed" you wanted to use that design (because that's what you submitted), and thus the advice was given based off that assumption.

If you wish to change your design, then you can utilize the diagonal cross-bracing suggestion (or even guy wires, cementing the legs into the ground, adding gussets, making it out of welded steel, etc). Heck, if you're really worried, you can spend more on your table than you did your Egg to ensure it's stability.

Finally, while it's still a possibility for the "racking" using 2 screws, my personal opinion is that it's a SLIGHT possibility based off of the function of the table.

After all, it's going to be holding a grill & possibly being moved infrequently - it's NOT going to be holding nuclear bombs and being pushed & pulled on w/ any great force or regularity. More than likely, if/when the "racking" occurs, it's going to be several years down the road, at which time the wood will probably be faded & in need of repair/replacement anyway.

Sometimes the best becomes the enemy of good enough.

HTH,
Rob

Bjorg

Richard - I think you have fixed your racking problem and I suspect it won't move.  I'd still put in dowels in the upper points though. 

Thanks for the comment - the wood is Philippine Mahogany and boy is it heavy.  I'm kinda wishing that I stuck with 1x4 for the majority of the framing.  The top and bottom are not solid, but individual planks clamped together.  I will be fastening them to the frame with Kreg screws from below.  I was debating leaving space between the boards, but I figured the table will be covered when not in use and I will be finishing all sides of each board before assembly.

The inset is black with gold speck granite (two pieces glued together with silicon adhesive) to make up the right height.  Home Depot - about $4.25 per 12" tile x 2.

I'll post a whole thread when I figure out how to start a new thread.

Great tip for the 1x4 for the framing, you made me change the design of the IPE table I am working on to lighten it up a little. 

I will share another good tip: BGE released a new product called the table nest available for 25$. This elevates the egg 2 inches. It is similar to a nest (black metal crossbar with legs) but for a table. Therefore, there is no more need for ugly slab or firebrick under the egg base. 

RichardBronosky

I found a great resource for calculating capacities for this application.

http://www.pdhcenter.com/courses/s167/s167content.pdf

It's looking more and more like I will be going to pocket screws. I'll post a new design soon. Thanks all!

stike

no need for that much work.  the stresses here (loads) are nothing close to what would strain the wood, even if you dropped down to 3x3 posts and a 1x frame.

the question of the lag bolt isn't whether it is strong enough. it's more than strong enough.

but the joint itself isn't a strong joint, and tightening the lag more and more isn't a solution.

take an axe by the very end of the handle and hold it straight out to your side with the axe head as far from you as possible. your hand in front of you below the waist. although your grip is strong, i'm betting you can't hold it with one hand.

now put two hands side by side, again right at the very end, and it's no problem.  you just overcame 'moment' .  it's not that the other hand doubled the strength of the joint, it's that two connections (hands, screws, bolts) can resist that overturning better than one.

you'll be fine.  you're overthinking.  and probably because we are too

RichardBronosky

no need for that much work.

I didn't actually do the work (math), but just looking at it was enlightening.

Anyway, I redesigned it with pocket screws as I said I would. I figured it was worth a new thread.

http://eggheadforum.com/discussion/1138287/pocket-screw-contruction