Diy large shelf span workbench - Page 1
Forum section: electronics / projects

author: Wartex
author_rank: Frequent Contributor
timestamp: January 07, 2013, 05:23:35 am
content: Wanted to build an easy-to-disassemble, long-spanning shelf workbench. Here's a work in progress:

author: SeanB
author_rank: Super Contributor
timestamp: January 07, 2013, 06:17:45 am
content: Very nice shelves. Only criticism is the pipes will cut through the shelves with heavy load. Take the plug and glue it into the pipe with epoxy and then use a fender washer ( large surface washer) underneath to act as a force spreader to stop this. Chopode one that fits the rod and is larger than the tube by a few mm, either bought or home made with a hole saw and some 2mm steel or stainless steel plate. Use a lot of cutting paste or other lubricant on the stainless cutting and a low speed.

author: Wartex
author_rank: Frequent Contributor
timestamp: January 07, 2013, 10:44:21 pm
content: Quote from: SeanB on January 07, 2013, 06:17:45 amThe shelves are already under 100-150 lbs of stress with the shelf being empty, they are under stress by design. Construction scaffolding uses same type of plywood and can withstand much higher pressures with smaller contact areas. You are underestimating the plywood strength.

author: Kibi
author_rank: Unknown
timestamp: January 07, 2013, 10:48:34 pm
content: Very nice. That will be an awesome bench.Tell me, what depth is your work surface - back to front?

author: robrenz
author_rank: Super Contributor
timestamp: January 08, 2013, 12:26:58 am
content: Very nice job!I especially like the Ghetto lathe.

author: G7PSK
author_rank: Super Contributor
timestamp: January 08, 2013, 05:58:49 pm
content: It's going to be a nice bench, if you cut a grove front and back on the underside of the shelves and put a steel wire into the grove stapled down the shelves wont bend at all, works even better to drill the slots at either end of the board and bring the wires through before fixing while applying tension to the wire.

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2013, 06:22:19 pm
content: I have built a lot of shelving from shutterply, using an aluminium u channel as a back brace to hold it to the wall or a backing board. Handles me walking on it with no problem.If you want the front to look nice when ripping the boards look for the best edges ( no inclusions) and use them for the fronts. The bracing is good, though it is stiffer if you cut a strip around 4 times the thickness and attach to the back as either a top stop or a under brace, either way it makes it a lot stiffer. Attach with good wood glue and a lot of long 42mm drywall screws into predrilled and countersunk 1.5mm holes. This makes it incredibly stiff and is very quick to make.I generally sand with a 120 grit paper and then dust them off, then wipe with thinners before a coat or two of sanding sealer or clear polyurethane varnish. Matt is my preference, does not show imperfections and is scuff resistant.I generally try to use the sheet dimensions as well, as I prefer to have the board cut according to a cutting plan I make, and which i give when buying the board. If there are a lot of offcuts keep them and make a set of small pidgeon hole shelves, very useful. You just need 2 sides, top and bottom with middle shelves and if there is not enough for a full back then a back stop for each shelf will work.

author: robrenz
author_rank: Super Contributor
timestamp: January 08, 2013, 06:42:02 pm
content: Quote from: G7PSK on January 08, 2013, 05:58:49 pmNo disrespect meant with the following.I think you are suggesting the idea that is used in pretensioned concrete beams. That only works if the ends of the tension member/s is completely immobilized at the ends of the beam or has very good full length bond to the concrete with a bond shear strength that is greater than what is generated by full load. The tension member/s also has to be big enough that its yield strength is not exceeded under full load. And most importantly, the rest of the beam material has to have no significant long term creep under compressive loading or it will still sag. Wood shelves have long term creep as we are all aware of. Straight grain solid wood shelf would have the least, plywood would be next and MDF is horrible.A layer of unidirectional fiberglass laminated to the top and bottom of the shelves would make them very stiff and would eliminate any long term change in sag. Then the shelf core could be foam because all of the tension and compression forces are in the glass fibers and the foam just maintains the separation of the skins. This is the principle of the I beam, the top part gets loaded in compression the bottom part gets loaded in tension and the vertical web keeps them separated and sees a gradient from tension at the top to compression at the bottom. In the same way the more distance between the top and bottom parts (thicker shelf material)the stiffer the beam.Sorry, I got carried away

author: robrenz
author_rank: Super Contributor
timestamp: January 08, 2013, 07:01:32 pm
content: I use moveable shelf supports made from pieces of 3/4" dia. tube or dowel that is the same as the inside height of the shelves. They are not attached and can be freely moved between instruments. I use this with MDF shelves and have no sag problems because the max span between supports is around 13" max. for what is on my shelves. Obviously the lowest shelf or table top has to be rigid enough to handle the transferred forces without sagging itself.

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2013, 07:15:58 pm
content: I see you also have a back bracing angle that provides stiffening. Nice with the adjustable dowel brace.

author: robrenz
author_rank: Super Contributor
timestamp: January 08, 2013, 07:50:43 pm
content: Quote from: SeanB on January 08, 2013, 07:15:58 pmIt does look like a bracing angle but it is just a lighting artifact from the flash, the shelf has no edge bracing. I initially did not have the dowel brace in there and the shelf sagged about 1/4" over a 36" span in a months time. I then made the dowel braces (actually black phenolic tubing) and put them in place with the bowed shelf. A month later the shelf flattened out so the end supports touched again

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2013, 08:05:04 pm
content: MDF board shelves? Chipboard shelves ( I call them shyteboard) do bend but do not ever come back. Worst are metal Dexion and clone shelves, they are good for light loads, but anything heavier than a few mostly empty hard files and they bend. I have a lot, and double them up to prevent them bowing.

author: Wartex
author_rank: Frequent Contributor
timestamp: January 10, 2013, 12:01:55 am
content: To answer some questions and comments: The table depth is 3.5 feet. The 0.5 feet was used to double the thickness of the front/sides to do the roundover.I am aware that cutting a slot in the shelf and inserting a metal bar or attaching hardwood to the edge makes the shelf much stiffer, but my budget is already busted as is since I spent nearly $1000 for my Fluke PM6304 LCR meter and any further electronics expenses might result in my wife castrating me in my sleep.The tension from upper shelves also keeps the lower ones from sagging.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 11, 2013, 09:16:44 am
content: Quote from: Wartex on January 10, 2013, 12:01:55 amwhat tension? I don't think the upper shelves are going to hold up the lower shelves in any way.Also, check out the sagulator for calculating shelving sag, given the thickness, span, material types, load, etc.http://www.woodbin.com/calcs/sagulator.htmApparently, our eye can detect 1/32" of sag per running foot.

author: lapm
author_rank: Frequent Contributor
timestamp: January 11, 2013, 04:06:56 pm
content: Quote from: Wartex on January 10, 2013, 12:01:55 amBut honey, i need to build this shelf system so my expensive things dont fall into flooor and brake. It actually saves moneu in long run, since i dont need to buy them again....

author: Wartex
author_rank: Frequent Contributor
timestamp: January 12, 2013, 02:16:03 am
content: Quote from: codeboy2k on January 11, 2013, 09:16:44 amYes they will - in order for the lower level shelf to sag, it has to pull the pipes closer to the center of the shelf. The upper shelves will prevent that. I just went and sat on it (I'm 200 lbs) dead center and it sagged about 1/4".

author: codeboy2k
author_rank: Super Contributor
timestamp: January 12, 2013, 11:27:41 am
content: Quote from: Wartex on January 12, 2013, 02:16:03 amI'm not saying it won't support your weight, it will probably support 50lbs/sq.ft. equally distributed, which you might have when you load it up with equipment.But.... I don't think you understand elasticity of materials....Consider a steel bridge anchored to the earth on both sides of a river. The bridge sags, flexes and bends under its own static load (the weight of the bridge itself) and further sags, flexes and bends under the dynamic loads of the vehicles crossing it. The bridge is elastic. It deforms, but the deformation is not permanent, and it will return to it's original shape. However, the earth did not have to flex inward to provide this sagging. The bridge material itself stretches and returns, and this is called elasticity. All materials exhibit some elasticity.If a material stretches too much, it leaves the elastic region and crosses over into the plastic region (changes from elastic deformation to plastic deformation). This is called the yield point, and it is here when it stretches and does not return to it's original shape. You've seen metals do that before. Eventually, if you keep stretching it, it will break. At this point it has reached it's plastic limit and will fracture. Disaster occurs. Some materials are brittle and don't have a plastic region, or have a very small plastic region, and these materials usually go almost immediately from their elastic limit to a fracture. Dry natural wood is like this (wet wood behaves differently, as does plywoods and LSL/OSB)In the case of a spanning beam, like your equipment shelf, the bottom side of the beam is under tension and is stretching elastically, the top side of the beam is under compression. The part at the bottom under tension will usually fracture first.However, the end supports are often the most rigid, and do not have to bend inward to permit the spanning beam to stretch elastically, and it is this elastic stretching that causes any sag you see.Note, however, that some specific civil engineering projects do in fact allow a certain amount of the support posts to bend inward or outward, adding to the engineered flexibility of the system as a whole. Flexibility is usually good, as it makes the system less brittle, and this prevents fracturing. And just like in EE, flexibility, resonance and feedback all have to be managed or disaster will occur... ref. Tacoma Narrows.

author: GeoffS
author_rank: Unknown
timestamp: January 12, 2013, 12:31:26 pm
content: A good tool for calculating sag in timber shelves is theSagulator

author: G7PSK
author_rank: Super Contributor
timestamp: January 12, 2013, 04:54:57 pm
content: Quote from: robrenz on January 08, 2013, 06:42:02 pmI have used wire to reinforce timber many times all that is required is the wire is pulled tight a good pair of pliers will do or a claw hammer and the wire stapled down while still pulling on it, not my own idea I first saw it on a wooden ladder where a wire was run down the back of the stiles (side rails) and under each rung in order to give greater strength.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 12, 2013, 09:03:35 pm
content: Quote from: G7PSK on January 12, 2013, 04:54:57 pmAs in my explanation above, with a normal loading, the bottom of the timber beam is in tension before it fractures, and the top is in compression. The goal of the wire you put at the bottom of the lumber is to pull the bottom of the beam into compression, thereby forcing the top of the beam into tension. Then when the design load is placed on the beam, it will balance out and normalize the beam into having neither of the top nor bottom surfaces in tension or compression. The beam can then takeadditionalload , and thus is stronger.The same thing is done with pre-stressed / pre-tensioned concrete, almost every horizontal span in concrete is done this way.You have to use strong steel wire (copper won't work like we EE's have lying around )You have to put enough tension on the wire to actually make a difference in the beam's properties....and the anchors have to be good. That's a weak point, anchoring a tensioned cable to wood.

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2013, 09:11:41 pm
content: Ladders use a 6mm steel wire threaded at the ends, into a large formed steel washer to spread the load. They serve a more important function in that they keep the rungs in the side grooves, as wear is going to cause movement there and failure. The wire does not do much other than ensure the wood is always in compression, it does nothing for the load carry capacity but is there to provide stability against sway.Wire on the stiles and under is more for preventing splitting of the wood at the bottom. All wooden ladders fail from splitting wood and wearing out of hinges.

author: G7PSK
author_rank: Super Contributor
timestamp: January 12, 2013, 10:52:38 pm
content: I use fencing wire, the wire not only puts some compression on the bottom of the plank it also increases the tensile strength of the bottom edge of the wood, probably the main way that such reinforcing works when used in this fashion as the requirements of pre-tensioning is that the total internal forces shall be equal or greater then the external forces.