Will this work?

I recently picked up two solid core wooden doors 36" x80". They are very he avy and I am planning on laying them end to end for the top of a heavy duty
work bench I want to construct in my garage. I also have several six inch in diameter PVC tubes that I am planning on using as stands for the workben ch. I will have six of these with two on each end corner and the two doors setting on two pillars in the middle. I will be filling them with premixed concrete from LOWES or one of the big box stores. I work from a wheel chair so I am not going to build as high as most people would need. Have any of you ever used a concrete leg for a bench? If so did you like it or would yo u go the traditional wood frame route if you had to do it again? I may even mount a 9" x 20" metal lathe on the top somewhere so I need to make sure v ibration is to a minimum. All advice appreciated. Gun Nut
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On 03/11/2015 9:33 AM, CertifiedGunNut wrote:

No, can't say as have but don't see any reason it wouldn't work ok. The only difficulty I see is that the mounting for rigidity may require some more creative thinking than construction w/ lumber would...I don't have a great idea for that otomh. Metal pipe threaded flanges are used quite a lot; I guess a glued PVC flange may be available; not sure how rigid it would be. Shorter is easier/better from that standpoint, of course, so that's an advantage over taller.
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On Wednesday, March 11, 2015 at 10:34:02 AM UTC-4, CertifiedGunNut wrote:

I would put the legs about a foot in from the corner. I would also put in some rebar inside the PVC legs. Concrete is great in compression. Not so good in tension. I am assuming you can get the rebar at the local scrap ya rd. Might skip the rebar if you have to buy it retail at Home Depot.
A long time ago the company I was working for built some concrete tables. We put poly on the concrete floor and framed up some tops just a bit bigger than 4 feet by 8 feet. Put in rebar and made some nice flat table tops. But we had a forklift to lift them.
Dan
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On 03/11/2015 3:03 PM, snipped-for-privacy@krl.org wrote:

I don't see any tension to speak of here; a _tiny_ bit of bending moment but not even much of that. Think the rebar is wasted...
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On Wed, 11 Mar 2015 13:03:13 -0700 (PDT), " snipped-for-privacy@krl.org"

To add some to that, the engineering assumption for the tensile strength of concrete, in most applications, is zero. Metal or other reinforcement only adds to the tensile strength if it is near the tension side. In a table leg, that would mean all around the outside of the concrete -- a difficult proposition. It would be stronger if you made tubes of welded-wire mesh -- even fine mesh -- that you could roll into a tube and stuff down into the PVC tubes before pouring in the concrete. Then you'd have ferrocement, with the strength dependent on how much steel you could get into the tubes and still be able to pour concrete down the middle.
In other words, it's not a good deal unless you want to go to that trouble. Concrete-filled *steel* tubes would be a good deal -- but then, you probably wouldn't need the concrete <g>.. With PVC, the modulus (stiffness) is so low that it will add almost nothing to the tensile strength of the concrete. It will crack inside with any substantial bending load on the legs. The stiffness of the whole affair will be mostly based on the stiffness of the PVC, which is very low, once you get a crack in the concrete.
Now, that's all looking at it from the standpoint of strength to resist bending of the tubes. If you're going to be gentle with it, or only pound down on it, it could work out great. Just don't bump into it with anything heavy.
I used pairs of 2 x 4s nailed together for legs. That was 59 years ago, and I've pounded and pushed the heck out of my workbench. I've dropped car engines onto it. It's still standing.
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On Wed, 11 Mar 2015 16:25:04 -0400, Ed Huntress

Correction -- that was 50 years ago. It just *feels* like 59 years ago. <g>
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On Wednesday, March 11, 2015 at 4:25:06 PM UTC-4, Ed Huntress wrote:

I agree it is best if the rebar is placed close to the outside of the leg. But even if the rebar is in the center , it will help., It adds to the tensile strength when it is in the center.
Now if you want to go to the effort , you could add a threaded portion on the end of the rebar and tighten it after the concrete has set. Giving you post stressed concrete. If you are thinking of doing this do some research on the internet.
Dan
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On Wed, 11 Mar 2015 14:10:42 -0700 (PDT), " snipped-for-privacy@krl.org"

Dan, that's an addition to tensile strength only if you're pulling on the ends. The normal load on legs like this, aside from the straight-down compression load, would be bending.
In that case, rebar in the center is right on the neutral axis -- it is neither in compression nor in tension when the leg is being bent or pushed from the side. It adds nothing to strength or stiffness.
This is like designing concrete beams. The entire tensile load is on the extreme tension side. In this case, that would be all around the tube.

That would work. And you don't have to do a lot of engineering. A piece of threaded rod, a couple of really large washers, and a couple of nuts would do it. Guess at, say, 500 pounds of tension. That would be plenty, and it's not enough to compromise the compression strength of the concrete.
Practical post-tensioned beams have been made with a piece of PVC or other plastic tube just large enough to enclose the threaded rod and a simple calculation to measure tension from the extension of the rod as you turn the nut(s).
When these jobs are engineered, the rod or bar thickness is calculated to produce the proper post-tensioning at the point at which the rod just starts to stretch -- its yield point. But you don't have to get that fancy.
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On Wednesday, March 11, 2015 at 5:28:10 PM UTC-4, Ed Huntress wrote:

g.

tensile strength when it is in the center.

No you are not thinking this through. If the rebar is exactly in the cente r, and the concrete has no strength in tension, then the neutral axis is be tween the rebar and the outside. If you try to bend the tube, the concrete resists compressing and the rebar is put into tension. The neutral axis mo ves depending on which way you try to bend the tube.
Dan
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On Wed, 11 Mar 2015 15:46:53 -0700 (PDT), " snipped-for-privacy@krl.org"

Ah, right you are, Dan. I was thinking only about the concrete breaking in tension. As long as it's intact, aside from the crack, it will still have compression strength, and the steel in the center will provide tensile strengh.
The bending couple is now shorter -- half of the diameter -- so the strength is much less than if the steel was around the periphery of the tube, but it won't fail just because the concrete is cracked.
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On 03/11/2015 6:36 PM, Ed Huntress wrote: ...

W/o actually working it out I'd venture even w/o any steel the stiffness of a 6"D solid block inside the PVC at only 26-28" L is going to be more than adequate... :)
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The stiffness is good. The tensile strength -- and therefore the bending strength -- is low.
If you don't hit it hard from the side, it should be Ok.
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On 03/11/2015 7:09 PM, Ed Huntress wrote:

It's the leg for a bloody garage shop workbench for a guy in a wheelchair for heaven's sakes...and I submit it'll take a fair amount more than that to make even a dent...now if ram it w/ a forklift headon, yeah, you can do some damage but I'd submit the leg will detach from the top before it actually breaks in two since the bottom isn't attached to anything.
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On Wednesday, March 11, 2015 at 7:36:11 PM UTC-4, Ed Huntress wrote:

leg.

he tensile strength when it is in the center.

nter, and the concrete has no strength in tension, then the neutral axis is between the rebar and the outside. If you try to bend the tube, the concr ete resists compressing and the rebar is put into tension. The neutral axis moves depending on which way you try to bend the tube.

Give the man a cigar.
I tried to send you some questions about health insurance. Did you get it? I included my email address. The one google knows has not been good for decades.
Dan
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On Wed, 11 Mar 2015 19:08:27 -0700 (PDT), " snipped-for-privacy@krl.org"

Sorry, I didn't get it. My real email address is the one above ( snipped-for-privacy@optonline.net), with the 3 removed. Maybe that was it.
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Ed Huntress

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On Wed, 11 Mar 2015 14:10:42 -0700, snipped-for-privacy@krl.org wrote:

Post-stressed post?
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Tim Wescott
Wescott Design Services
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On 3/11/2015 9:33 AM, CertifiedGunNut wrote:

Everyone missed the obvious. When you pour the concrete set it up to install 1, 2 or 4 studs. Then you can mount a 2" x 6" countersunk for a washer and nut(s). Lots of directions to go with the 2x6. You could even drill the table top then counter sink for the washers and and nuts, then fill the hole. It is obvious the large the diameter of the leg, the farther apart you can put the studs, and the more lateral support you get. Can you adapt a 4" pipe to a 6" just to get the extra width, and use much less concrete? These ideas could be worth more than you paid for them, Good Luck, Mikek
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CertifiedGunNut wrote:

Use a short piece of 2" pipe screwed into a flange that's either bolted or screwed to the underside of the top . Also , you can drill a hole in the side of the PVC and insert a piece of pipe from leg to leg - an elbow or tee screwed onto the end of that crossbar will lock it into place . The PVC is basically just a form , and will add no strength to the unit . The suggestion above to add some rebar is also a very good idea , as is using some stiffeners to the underside to prevent sag .
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On Wed, 11 Mar 2015 07:33:59 -0700 (PDT), CertifiedGunNut

Solid core doors are not always extremely stiff, so you may need some stiffeners underneath. Maybe lag a few steel channels, like Superstrut, under them for that purpose. http://www.homedepot.com/s/unistrut?NCNI-5
I like both wooden and steel legs, so I'd be more apt to go that route. Also, when you move, taking those heavyarse cement legs with you will be a total bear. Drill and screw the legs to the cement if you need more stability. LeeValley has some really nice cast iron leg systems for sale, if 23" will work for you. http://www.leevalley.com/US/hardware/page.aspx?p@575&cat=3,40993,46344,40575 27" wall-mount legs http://www.leevalley.com/US/wood/page.aspx?p@578&cat=1,41637 The only problem with them is the CHA_CHING!
Crossbraced 4x4s with Simpson angles lagged to the ground via these is probably the route I'd go, cuz I'm so che^H^H^Hfrugal: http://tinyurl.com/n87rxg5
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Thanks for all of the ideas guys. Some good information from all. I measured my wheelchair and it is 28 inches high at the arm rests. So I fi gure 29 Inches to the bottom side of the bench should allow wiggle room. My chair is probably one of the highest on the market and even most motorized chairs aren't any higher. I do more wood working and I use almost all han d tools with the exception of my drill press and it isn't going to be mount ed on the bench. Vibration shouldn't be an issue. I don't use the BFH much anymore so it won't get much abuse. My choice of metal lathe will be the determining factor in extra reinforcem ent. Right now I am leaning towards a Grizzly with 10" x 22". I think it w eighs in at 600 pounds or less so I will probably put extra reinforcement directly under it. After check prices on small batches of concrete I have d ecided to go with some of the old wood a friend of mine got from a 100 year old barn he tore down. That stuff is about as hard as a cowboy at the Long Branch in Dodge City on a Saturday night in 1876. The wood was a freebie i n payment for a favor I did for him many years ago. I have also decided to do an eight foot bench instead of sixteen.So I will glue up the doors and r eserve one end of the bench for my Metal Lathe with my drill press next to it on its stand.by going with wood I will also have more options with vises and hold downs. I have a feeling this bench will outlast me and maybe even my only Nephew who will probably inherit some days. Thanks again all.
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