Answers to Everyday Questions : How can I design a work table 12ft long 2ft wide and 2ft 6ins high to take 300lbs per sq ft UDL. Material wood

How can I design a work table 12ft long 2ft wide and 2ft 6ins high to take 300lbs per sq ft UDL. Material wood

4 Responses to “How can I design a work table 12ft long 2ft wide and 2ft 6ins high to take 300lbs per sq ft UDL. Material wood”

1. Jim N said :
   May 25th, 2010 at 10:27 am

   No engineering analysis here, but I would purchase two sheets of 3/4″ plywood NOT particle board and have the lumber yard rip them in half. This will give you 4 pieces 2′ wide by 8′ long. Glue and screw them together into a layer, overlapping the joints and cutting the excess. I’d made the legs out 2 x 4’s glued and screwed into rectangle for legs and put them on 4′ centers. This will give you legs at both ends and two in the middle and screw them into the bottom of the plywood. This should hold everything you can put on it. If you feel nervous, upgrade the 2 x4’s to 2 x 6’s.
2. andrew f said :
   May 25th, 2010 at 10:29 am

   That’s quite a load!

   The legs are probably the easiest bit, because timber is very strong when compressed along its grain. The more legs the better, I reckon, because you don’t want the top to sag. My copy of Machinery’s Handbook lists the strengths of various types of timber. The weakest pines and cedars (it’s an American book) will take 500 pounds per square inch in outside locations (a bit more more indoors, in the dry), so the minimum total cross-sectional area for 3 tonnes will be surprisingly little – 14 sq ins or so. Using legs 2 feet apart (7 each side), 2×2 or 3×2 should be very ample, but make sure it’s decent, straight-grained and knot-free wood.

   Then, you need a framework, (two sides, two ends and five intermediate cross-pieces) resting on the legs, to support the top. It is important that the legs should support both components in each joint in your framework from beneath. In the framework, there will be bending and shear stresses, and I don’t even pretend to understand the tables on these. All I can say is that doubling the width of the component will double its strength, but doubling its depth will quadruple the strength, so go for depth. Floor joists around 9 inches deep would do the trick, I expect, but a structural engineer’s advice would be handy.

   So, we now have 14 legs with a frame on top. Diagonal cross-braces will help prevent the legs from buckling under.

   As to the top surface, plywood would be best, but I’m not sure how thick – ask the structural engineer. The frame beneath will effectively divide the top into six sections, each two feet square, and each of those sections will have to support half a tonne. That equates to seven 12 stone men, if you could crowd them on to each two-foot square section. I guess – and it’s only a gueas – that you would need two layers of one inch plywood.,

   Build it in situ -I’m not going to come and help lift it into place.

   Finally, consider how strong the floor under this contraption will be – it isn’t going to rest on a suspended floor, is it?

   Good luck, and keep your toes out from under it.
3. ticketoride04 said :
   May 25th, 2010 at 10:49 am

   surprisingly, uprights can uphold more weight than you can imagine. place uprights w/4x4s equally at 4′ centers. bottom shelf is to be at 8″ off ground at btm and keyed into the uprights as NSEW bracing with a 3/4 7ply plywood notched in as shelf for stability.
   the top of the 4xs are to be keyed to 2x6s at the perimeter drilled, bolted/lagged and flush to the top to receive the top. Assure the inner uprights have keyed 4xs or dbled 2x6s, one on each side of the uprights.
   your top is to be two sheets of 7/8s ply, screwed & glued individually to prevent further movement. it nets less.
   place stud cross ties(metal) at the back,2, and on each side. leave the front open for storage if necessary.
   this’ll hold up your car.
   put push-down rollers on at all 4 perimeter legs for quick moving…then released for use.
4. robling_dwrdesign said :
   May 25th, 2010 at 11:00 am

   This a engineering question, wow. Know any engineers? Let’s see statics. Statics is easy, but the engineering of the materials is the hard part. Considering you increasing the load by 6 fold over typically residential design (50#-sq.ft.), I’d start by laminating 1″ plywood in two layers. The 300 #sqft is more critical than 3 tons, assuming tons is 2000 lb. The entire table must support 300 # sq.ft x 24 sq.ft = 7200 lbs. Then each long side of table must handle 3600 lbs, or 1800 lbs shear each end. And 10800 ft-lbs bending. That’s what the beams that support the long end need to support. Assuming 1′ spacing of the purlins or cross members, each member supports 300lbs shear, 300ft-lbs bending. SO if you build it out of wood, maybe LVL for the long ends, and 2×10 for the purlins. Maybe go to the local lumber yard, not Home Depot or Lowe’s, and ask where they get their engineered lumber, and take the numbers there. They can tell what size LVL to use. Considering a 2×10 can span 16′ depending on species it can handle 50lbs-sq.ft (10#sqft live, 40#sqft dead load) I’d think the 2×10 can safely handle the load you specify for that short span. The main problem is fasteners. Your point of failure is probably going to be joining the pieces. Use joist hangers and screw it together.

   Of course, I’m probably wrong. It’s been a long time since I took statics, so my numbers may be wrong.

   Edit: That’s just assuming support at the ends. If you have support in the middle, then the LVL’s can be smaller.