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Old 06-04-2015, 10:40   #1
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Dinette Table Hung off of Compression Post

Hi Folks

I'm rethinking the table in the center of my (horseshoe-shaped) dinette.

interior-layout

The table currently has it's own independent set of legs and getting into the seats is pretty uncomfortable because the table legs get in your way.

This boat has compression posts to bear the load of the (deck-stepped) mast. One post runs right alongside the outside edge of my dinette table and I'm thinking I could just hang the table off of that compression post and eliminate the table's legs. See attached drawing.

The post is 1.5", sched 80, seamless steel pipe. When replacing these compression posts, I reviewed Phil Rhodes' calculations for sizing them - buckling failure was the weakest link and sizing the pipes appropriately involved euler's bucking equation to calculate the "critical load" where various pipe configurations (diameters/thicknesses/materials) would fail due to buckling from force being applied in the direction of the column. That critical load for this pipe is something like 15,000 pounds.

For the table as I've described it here, it seems some significant sideways forces will be applied to the post if say someone were to sit on the table. That feels like it could induce buckling failure far easier than force applied in the direction of the pipe. Anyone have any idea how to quantify that?

I'm no mechanical engineer... and I'm a chicken... so I'm not going to do this project unless I can convince myself its a safe plan... But I don't know how to do that.
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Old 06-04-2015, 10:56   #2
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Re: Dinette Table Hung off of Compression Post

1.5" seems awful small, although in direct compression I suppose it's fine, but any side load could cause problems with assuming the calculations are good. I mounted the table on the 31 footer I built on the support. But that support was 3.5" polished brass pipe.
At any rate I seriously doubt the pipe will buckle, but seems marginal to me.
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Old 06-04-2015, 18:05   #3
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Re: Dinette Table Hung off of Compression Post

What would the length and weight of the table be. It looks like a long lever arm to be working on that little compression post. Perhaps, you could go to a central pedestal (which will also be in feet's ways, but it's a boat, you know). That might make entrance and egress easier.

Ann
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Old 06-04-2015, 23:41   #4
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Re: Dinette Table Hung off of Compression Post

Great boat, interesting design. Looks like the compression post per say is not directly supporting the mast load but sharing it with the bulkhead between main and forward cabin. I'm sure reason post is smaller than expected. Any way to beef up post maybe by installing either larger pipe sleeve over post or stitch welding flat bar edgewise on sides of compression post to stiffen side load?

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Old 06-04-2015, 23:52   #5
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Re: Dinette Table Hung off of Compression Post

although 1.5" ss tube is a lot stronger than it looks, the leverage put on it by a cantilevered table could be too much. I'd think about using the compression post at one side and another post/leg towards the inner part of the table - no leverage, just a small amount of extra compression.
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Old 07-04-2015, 01:46   #6
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Re: Dinette Table Hung off of Compression Post

IMO, unless you have really heavy dishes (joke), the geometry that you show wouldn't seem to put so much bending load on the tube. But if it worries you, simply taking the lower brace that you show down to near the base of the tube would reduce the bending load markedly, and would not interfere with feet very much.

Also, seems likely that you would never be loading up the table during sailing under high mast load conditions, 'cause stuff would be sliding off the table faster than you could pile it on! Really, I think it a non-problem...

Jim, using Ann's computer
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Old 07-04-2015, 08:34   #7
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Re: Dinette Table Hung off of Compression Post

Size it for your heaviest friend slipping and landing full weight on it's extremity. It's STRUCTURAL, don't mess around with it.

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Old 07-04-2015, 08:35   #8
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Re: Dinette Table Hung off of Compression Post

If anything, hang it from the roof.
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Old 07-04-2015, 09:22   #9
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Re: Dinette Table Hung off of Compression Post

The compression load from the mast would happen when you are in very high winds with lots of sail drawing. You most likely would not be sitting down to dinner at such a time, but there is a chance somebody could get thrown against the table at a time like that. You might consider something that folds up out of the way at times like that or when you want to have more room to move around. I would say when you build your attachments for the table, build them in such a way that it strengthens the center section of the pole, that is where the failure would happen in a buckling situation. Just make a two piece tube that completely encases the original pole for the center 1/3 of the poles length. I am a mechanical engineer, taught that along with physics and electrical engineering for 30 years.
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Old 07-04-2015, 09:32   #10
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Re: Dinette Table Hung off of Compression Post

The mechanical engineer has the right idea. You don't ever want that compression post to be flexed very far out of column, or it could be disastrous.
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Old 07-04-2015, 12:07   #11
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Re: Dinette Table Hung off of Compression Post

Thanks everyone - really good points.

Gulfstar - You are right, there are two separate compression posts, one at the table, one directly in front of the bulkhead you mentioned. The mast sits on a sort of "truss" assembly on deck whose ends are bolted to each of those compression posts.

Ann + Jim - Table probably spans 3 feet from the post, so a decent lever arm. I like the idea of maximizing the span (height) of the brace - I can't quite get my head around it but it seems the lower I move the bottom of the brace, the more downward the resultant force applied to the column (with respect to weight on the table) . Not sure if that affects whats happening with the top brace.

JHeld- Your wording got me on a different line of reasoning - Maybe make the bracing wimpy enough to make sure it would break long before imposing any significant force on the compression post. Although that brings its own dangers.

Anyways, thanks again folks - I think I'll just come up with an independent support like Ann suggests. Or just keep putting this one on the bottom of the todo list.
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Old 07-04-2015, 12:24   #12
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Re: Dinette Table Hung off of Compression Post

I'm not sure this is a big deal, especially if the table is not heavy...but the small compression post makes me wonder a bit. The force is eased a bit by having the two posts...
The triangular support... the longer the hypoteneuse of that triangle.. take it to near the floor?... the less force on the post...
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Old 07-04-2015, 14:36   #13
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Re: Dinette Table Hung off of Compression Post

Quote:
Originally Posted by chris95040 View Post
Table probably spans 3 feet from the post, so a decent lever arm. I like the idea of maximizing the span (height) of the brace - I can't quite get my head around it but it seems the lower I move the bottom of the brace, the more downward the resultant force applied to the column (with respect to weight on the table) . Not sure if that affects whats happening with the top brace.
see if this makes sense, place a weight on the far edge of the table, it creates a moment (force times distance from the pole) if the weight is in the middle of the table the moment is half as much. That moment tends to bend the table down, it is resisted by the structure holding the table upright by 2 forces one at the bottom of the structure pushing toward the table and one at the top of the structure pulling away. The location of the top and bottom of the structure, and those forces, makes no difference on the magnitude of the forces, only the distance between the top and bottom is important. These forces are equal and opposite, they are the moment divided by the distance between them. Those forces tend to bend the compression post, the closer they are to the center the worse. Also the larger the worse. I would make the structure centered on the post up and down, and make it out of a pipe that reinforces the post in the center 1/3. Nothing magic about 1/3 just a good compromise, personally I would suspect you could attach the table like you want without reinforcing the pole and you would never have a problem, never be in a storm with sails up and have somebody thrown against the table. You don't have a diagonal brace on top of the table running to the top of end the pipe, it should be stiff enough to carry the moment and force to the top end of the pipe, just a diagonal brace underneath would be OK. The person flying into the table does more than just pushes down on it, they also push sideways, that also tends to bend the pole, but that happens if the table if there or not and they fly into the pole, that should be in the initial design, this is why I suspect you would not have a problem with anything you do, but better safe then sorry. Everything is a compromise.
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Old 07-04-2015, 15:01   #14
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Re: Dinette Table Hung off of Compression Post

Take the horizontal support out to 3/4 of the total length of the table, make the bottom of the support, as deep as you feel comfortable and place the collars at the top and bottom to spread the load over the pipe, then make the web (the line between the end of the horizontal support and the lower collar, a fair curve (steeper at the base, narrower at the end of the table). Drill great big holes, or fabricate the web like a gossamer wing to keep it light and strong. I do a lot of this when making cantilever beams. Sadly, I have no pics to show for it. It's also how I will be doing my sterncastle table. It can be an extraordinarily stiff and strong structure, that looks way cool and weighs very little.
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Old 07-04-2015, 15:47   #15
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Re: Dinette Table Hung off of Compression Post

When a relatively minor force gets the post out of line ... the compression will do the rest.

Can you perhaps deploy another post in alternative location and hang glide the table from there?

We have such a table, it is a great solution.

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