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Design question - long go-bar bench
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=47773		 Page 1 of 1
Author:  joshnothing [ Sat May 14, 2016 7:13 pm ]
Post subject:  Design question - long go-bar bench
Any engineers in the house? :P

I'm currently in the process of refitting my workshop a little with the goal of making things a bit more efficient and more suitable for my build process.

I use go-bars for bracing and for a few years I've been using a couple of the commonly-seen 24"x24" portable go-bar decks. These things take up a little too much space for my liking. They're either taking up space on the bench or taking up space sitting in the corner when the bench is used for other things. I'm after a more space-efficient solution.

The bench where the bracing work happens is 9' long and permanently attached to a wall of the shop. I want to be able to use the entire length of it for bracing tops. Just to give you a picture of things, this bench is built into a 'recess' where there is stud wall along its back and both sides.

That solution would seem to be to build a shelf above this bench and use its underside as the 'top' of the go-bar deck. That way the bench can be used as both go-bar deck as well as a general purpose bench space.

So far, so good.

But I've run up the limits of my engineering knowledge trying to design this shelf. It will need to be 9' long, 24" deep and able to deal with the load of potentially up to 80 go-bars at once without excessive deflection.

That seems like a pretty big and potentially heavy shelf.

So far I've considered:

1. Making the shelf integral to the bench eg. give the shelf "legs" along its front and attach them to the bench below. This seems simple and strong but the tall posts at the front of the bench would really get in the way, reduce elbow room and reduce the utility of the bench for non-gobar purposes. So this seems workable but non-optimal.

2. Torsion-box shelf attached only to the wall. This could be made light enough and stiff enough to minimise sagging under its own weight but I feel like the limiting factor will be its attachment to the wall. I can screw it to the studs along its sides and back edge but all that upwards load from the go-bars, particularly in the middle of the span, seems like it could cause problems. Valid concern? I suppose I could make angled support brackets to sit above the shelf and transfer this load back onto the wall.

Is there a better way? I'd really appreciate some design input.

PS. I'm almost convinced the real answer to this question is "switch to vacuum clamping/cam clamping your bracing and never worry about it again" :P But I like go-bars!
Author:  Jeff Highland [ Sat May 14, 2016 7:36 pm ]
Post subject:  Re: Design question - long go-bar bench
If you can support the front of the bench from a ceiling above, that would be ideal.
Author:  joshnothing [ Sat May 14, 2016 7:53 pm ]
Post subject:  Re: Design question - long go-bar bench
Jeff Highland wrote:
If you can support the front of the bench from a ceiling above, that would be ideal.


I hadn't actually considered that but it makes a lot of sense. Maybe I can run some posts from the shelf through the ceiling drywall and bolt to the rafters. I'll get up in the roof and see what I have to work with above this bench area. Thanks Jeff!
Author:  DennisK [ Sat May 14, 2016 7:53 pm ]
Post subject:  Re: Design question - long go-bar bench
I'd go with solution 2.

Sounds like the go-bar ceiling would make a nice shelf to set things on top of as well, so you might factor that into your design.

The weight of the shelf (and anything you put on top of it) will help to counteract the force of the go-bars, so that's convenient. I doubt you'll have trouble with it deflecting upward.

You could add some 2x2 posts or plywood dividers above the shelf that attach to the ceiling to help keep the long front edge of the shelf from deflecting either direction. Dividers would double as angle brackets, being attached to both ceiling and wall, but they'd add a lot of weight themselves so posts+brackets may be better. If you plan to put a lot of weight on top so upward deflection is a non-issue, then you could even use chains instead of posts.
Author:  meddlingfool [ Sat May 14, 2016 8:39 pm ]
Post subject:  Re: Design question - long go-bar bench
It's nice to have access to multiple sides of the go bar deck, which you would't have.

Perhaps consider designing a collapsible deck of a size more suited to your liking?
Author:  joshnothing [ Sun May 15, 2016 3:06 am ]
Post subject:  Re: Design question - long go-bar bench
Thanks for the advice guys. Meddling, I agree it is nice to have access to multiple sides of the deck but I've become accustomed to working this way so I don't feel like I'm restricting myself too much. I'll give it some further thought ...


Sent from my iPhone using Tapatalk
Author:  printer2 [ Sun May 15, 2016 7:08 am ]
Post subject:  Re: Design question - long go-bar bench
How much lb's force do you think is being put on the top during gluing using a gobar deck? I measured 2 lbs for one of the fiberglass road marker rods.
Author:  Terence Kennedy [ Sun May 15, 2016 7:20 am ]
Post subject:  Re: Design question - long go-bar bench
This has worked well for 14years. I used four posts screwed into the joists of the ceiling with hanger screws. You can use it to store your go bars. I used one piece of Melamine. Two would be better.

Image
Author:  Imbler [ Sun May 15, 2016 10:57 am ]
Post subject:  Re: Design question - long go-bar bench
printer2 wrote:
How much lb's force do you think is being put on the top during gluing using a gobar deck? I measured 2 lbs for one of the fiberglass road marker rods.


I've read it is 7 or 8 lb each for the size LMI sells and that sounds about what I would estimate from using them. So if the original poster really used 80, that would be around 600 lb. A small block chevy v8 is 535 pounds so that would give you a feel for how strong you need to make it,
Mike
Author:  doncaparker [ Sun May 15, 2016 1:14 pm ]
Post subject:  Re: Design question - long go-bar bench
I think having all the vertical supports for the shelves coming down from the ceiling is the best choice. I would use 3/4" plywood for the shelves themselves, and maybe wall studs cut in half (making 2X2's) for the vertical supports. You can use something smaller for the vertical supports and they will probably work fine, but you want something that will handle both tension (weight from what you store on the shelves) and compression (go bar rod pressure) without going all wobbly.

You are working with a finished ceiling, so you might have to get a little creative about attaching up there. That all starts with using a stud finder to locate and mark the ceiling joists. If I wanted to keep the ceiling drywall mostly intact, I would probably build a freestanding go bar deck of the desired height, with both a top and bottom of 3/4" plywood, and just screw it to the ceiling joists after assembly. The only lasting damage would be screw holes in the ceiling. On the other hand, if you are OK removing the ceiling drywall, attaching directly to the ceiling joists is easier, obviously.
Author:  printer2 [ Sun May 15, 2016 2:25 pm ]
Post subject:  Re: Design question - long go-bar bench
Imbler wrote:
printer2 wrote:
How much lb's force do you think is being put on the top during gluing using a gobar deck? I measured 2 lbs for one of the fiberglass road marker rods.


I've read it is 7 or 8 lb each for the size LMI sells and that sounds about what I would estimate from using them. So if the original poster really used 80, that would be around 600 lb. A small block chevy v8 is 535 pounds so that would give you a feel for how strong you need to make it,
Mike


600/18 = 33 lbs/sq ft. Half the load on the wall side and half the load on the outside (side?) 2'x9'. Not all that bad, store stuff on the shelf and be more concerned on the downward load.
Author:  Barry Daniels [ Sun May 15, 2016 2:47 pm ]
Post subject:  Re: Design question - long go-bar bench
You do not need to remove the sheetrock to attach the frame to studs or joists. Just run some braces up to a 1x4 screwed to the ceiling, or run them at an angle to the wall. Don't overthink this and keep the engineers away or they will start using their status and dynamics formulas to figure out the stresses involved. I know this because I am an engineer.
Author:  bcombs510 [ Sun May 15, 2016 3:00 pm ]
Post subject:  Re: Design question - long go-bar bench
Terence Kennedy wrote:
This has worked well for 14years. I used four posts screwed into the joists of the ceiling with hanger screws. You can use it to store your go bars. I used one piece of Melamine. Two would be better.

Image


Terence,

With a setup like this, how do you deal with height changes that would be needed to switch between a body and a radius dish, for example. Is the base sitting on the bench height adjustable?

Thanks!
Brad
Author:  Mark Mc [ Sun May 15, 2016 5:48 pm ]
Post subject:  Re: Design question - long go-bar bench
You don't need to worry about height adjustment, within certain limits. A go bar rod delivers the same downward force whether it is flexed slightly. or quite a lot. So, as long as your rod is long enough to do the "lowest" job (e.g. gluing braces on a soundboard in a radius dish) it will be able to just flex a bit more to accommodate a "taller" job (like gluing the back onto the sides).

Since my bench is up against a wall I just placed some overhead cupboards (standard kitchen type) above the bench. The cupboards are very useful for storage, but also really sturdy so that the underside of the cupboard can support the top surface for the go bars. I screwed a double thickness of plywood sheeting to the underside of the overhead cupboard. It extends out from the wall to the full width of the bench (so pokes out from below the overhead cupboard, but the doors still swing open over it). When you are not using the go-bars you just have an unencumbered bench, and a useful cupboard. Open shelving would work just as well.
Author:  joshnothing [ Sun May 15, 2016 6:10 pm ]
Post subject:  Re: Design question - long go-bar bench
Thanks for all the input guys! Supporting from the ceiling seems to be the way to go.

Quote:

I've read it is 7 or 8 lb each for the size LMI sells and that sounds about what I would estimate from using them. So if the original poster really used 80, that would be around 600 lb. A small block chevy v8 is 535 pounds so that would give you a feel for how strong you need to make it,
Mike


Yup, those are the numbers that had me concerned. The 80 go-bars would be the absolute peak load, so it wouldn't happen all the time, but it would need to be able to handle that from time to time. However...

Quote:
Don't overthink this and keep the engineers away or they will start using their status and dynamics formulas to figure out the stresses involved. I know this because I am an engineer.


Not overthinking is some very useful advice. I'm prone to overthinking these kinds of questions instead of just getting the job done. You could charitably call this 'attention to detail' but at times its closer to 'paralysing obsession'. And while this approach is helpful with the minutiae of guitarmaking, it makes me the world's slowest carpenter. :P :P
Author:  Terence Kennedy [ Sun May 15, 2016 6:26 pm ]
Post subject:  Re: Design question - long go-bar bench
bcombs510 wrote:
Terence Kennedy wrote:
This has worked well for 14years. I used four posts screwed into the joists of the ceiling with hanger screws. You can use it to store your go bars. I used one piece of Melamine. Two would be better.

Image


Terence,

With a setup like this, how do you deal with height changes that would be needed to switch between a body and a radius dish, for example. Is the base sitting on the bench height adjustable?

Thanks!
Brad


As mentioned, you have some flexibility with the go bar. I have different length bars too. Shorter 1/4" for closing the box and longer 5/16" for bracing on a radius dish. You can also put a piece of plywood or particle board under the radius dish or the mold for height adjustment.
Author:  douglas ingram [ Sun May 15, 2016 7:52 pm ]
Post subject:  Re: Design question - long go-bar bench
I run my bars right off the ceiling. I can't imagine doing it any other way now. Mind you, my ceiling is only 8', so the bars only need to go from the ceiling to the bench top.
Author:  Woodie G [ Sun May 15, 2016 9:57 pm ]
Post subject:  Re: Design question - long go-bar bench
The reason to have either height adjustment or other way to accommodate changes in the thickness of the work is because a bar stores spring energy when deflected, which is released when allowed to straighten out. More bend = more energy stored, and it does not take much to punch a hole through a top or side, or travel the length of the shop to injure a coworker.

We minimize deflection by using a 28" distance between top and bottom surfaces of the deck and a 4" platform plus a few 1/2" and 3/4" 2' x 2' (with clipped corners) MDF height adjustors...4" platform plus radius dish gives us an inch of deflection in our 24" x 3/16" bars for bracing work, while just the radius dish and maybe a height shim lets us close bodies with about the same deflection.

From my notes, changes in length and diameter of the rod affect the applied force by the 4th power of the diameter and 2d power (the square) of the length, so a 24" x 3/16" rod, a 43" x 1/4" rod. and a 66" x 5/16" rod of the same material will all apply the same force once deflected to buckling load. Cutting any of those bars in half will quadruple the force applied, and doubling the length will reduce force by a factor of 4 (2^2), so there are lots of ways to adjust deck height and rod diameter to keep things under control.

The line of action for a go-bar (which passes through the point of application and is aligned with the applied clamping force) is always between the tips, no matter how bent the bar, so keeping the tips in line vertically requires that we either place the bar tips precisely each and every time, or allow easy adjustment once they are loaded in the deck. For my boss, using the ceiling as the upper surface of the deck would not be an issue - at 6'2", he can easily touch an 8' ceiling. For me, that would be a real issue, so the lower deck height we use here works well in a shop where we don't want to have a Disneyesque minimum height sign at the shop door...the work is low enough to access and clean up efficiently, and the upper surface is high enough and small enough (2' x 2') so that taller builders don't have to stoop too much.
Author:  David Collins [ Sun May 15, 2016 10:58 pm ]
Post subject:  Re: Design question - long go-bar bench
Terence Kennedy wrote:
This has worked well for 14years. I used four posts screwed into the joists of the ceiling with hanger screws. You can use it to store your go bars. I used one piece of Melamine. Two would be better.

Image


I worked with a harpsichord / fortepiano maker for several years, and this is very similar to how his go-bar setup was laid out. Of course it was a bit larger (4'x8'), but similar concept. With the increased size, he did have a hefty butcher block bench, and the upper plate mounted to the ceiling was double layer 3/4 MDF with 12" on center supports above it, but it worked wonderfully. The advantage of having access from all sides is really a nice touch.

For go-bars, he simply used poplar strips, probably 1/4" x 3/4". With bars so cheap and easy to make, it was quite easy to keep a large assortment of different length increments to choose from.

And the spaces between supports of the upper plate made for convenient storage shelves for trim and purfling.
Author:  Pat Foster [ Mon May 16, 2016 11:59 am ]
Post subject:  Re: Design question - long go-bar bench
I first had a deck with 3/16 x 24" FG rods that gave about 7 lbs of force each. I've switched to 1/4 x 36" FG, which give about 12 lbs each, which I'm happy with. Another idea I saw here perhaps, was carpeting on the upper surface to reduce the chance of slippage. It's not at the top of my to-do list at the moment. Eat Drink

Pat

P.S. Woodie, that's an excellent book in your sig!
Author:  Woodie G [ Mon May 16, 2016 6:57 pm ]
Post subject:  Re: Design question - long go-bar bench
Thanks, Mr. Foster - it was a good read! Part of the reading list here...thought I was done with book reports...no such luck.
Author:  Bryan Bear [ Tue May 17, 2016 11:05 am ]
Post subject:  Re: Design question - long go-bar bench
I haven't read all the posts here (yet) so forgive me if I am repeating anything.

My go bar deck is really just my bench top and ceiling. I screwed a piece of plywood into the joists to protect the drywall on the ceiling. The ceilings are low in my basement workshop and my bench is high so the bars aren't too long but they are much longer than 24 inches. This extra headroom is nice to be sure but it does mean you lose clamping force from having the bars longer. This is not a problem if you make your own wooden bars as you can just make them thicker. I use fibereglass driveway markers available form the box stores. They are thicker therefore stiffer and it works out well with my length.

A couple other thoughts:

- I don't think I would like my deck with walls on three sides. mine is against the wall and I sometimes wish I could access the other side. This is where having the added headroom is nice, it is easier to get in to the far side to clean up if you don't have to squeezes into a 24 inch box.

- Using the ceiling is not adjustable for height. This is not that huge of a deal especially if you have a high deck. Once a bar is flexed enough, the clamping force doesn't really increase so it is not that big of a deal to have the bars bend more than normal. The few inches in height that you would adjust a short deck is a much smaller percentage of the total on a long deck. . . I do confess that I have a few shorter bars for certain things though.
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