Piling extensions over or under engineered

[Vincentc]

The existing pilings are not as tall as Lilly Marie's sheer of the boats at high tide. Wanting to add 3 feet to the existing piling height, we cut a 3 ft piece off of the butt end of a scrap piling then drilled 4 holes into the top of the existing piling and the 4 holes into the end of the scrap piece, then filled all 8 holes with thickened epoxy resin and inserted 5/8 x 16 inch rebar into the holes on one end, added more thickened epoxy to the top of the piling then put the 3 ft piece on top.

Problem is the holes and rebar are difficlut to line up even with the jig. We had difficulty with the first one and almost did not get it to come together. I would like to use one pair of holes and one piece of rebar per piling, located in the center of the piling. That would be much easier alignment, I wonder if one rebar provides enough strength?

An Internet site indicate that 5/8 rebar epoxied into concrete has a tensile strength of about 24,000 pounds. My shade tree engineering tells me that a 3 foot lever against an 8" piling base (4") radius against a length of rebar at center would be a 9 / 1 reduction ( 36" / 4" = 9) Thus the joint should stand a 2,666 lb pull to the side from the top of the piling.

Any thoughts on this?

 

[mike]

Have you thought about sleeving it with a steel pipe over the existing piling? You would only be increasing the OD of the piling by the wall thickness of the pipe.

 

[OBXTucker]

Sleeving the piling is a great idea!

 

[Vincentc]

Thanks,
I had not thought about sleeving it with steel pipe, sounds like an effective solution if I can put my hands on 8 to 11 diameter (the pilings' diameter varies) steel pipe that can be cut in 12-18 inch lengths. Any suggestions on where to find this without going overboard on cost?

I have been thinking about wrapping the joint with fibergalss cloth tape and epoxy, and could put some steel tabs underneath with screws through the tabs and into the pilings.

There are probably a lot of ways to increase the strength of the joint, but how much is enough. That's the engineering component of this post.

 

[Maynard Rupp]

You could put a steel pipe over those pilings long enough to sit on the sea bottom. Because you only need a 3' extension I would buy large PVC pipe, cut them to a length that would allow the PVC to sit on the bottom and extend 3' over the existing pilings. I would also glue a PVC cap over the top. Those tubes should be cheaper than steel, have adequate strength, and with the nice caps, should look a lot better than rusty steel. We do that trick in the Great Lakes over small diven steel pilings to avoid damage to a boat. Over steel pilings the PVC will actually roll as a boat slides its rubrail along the piling.

 

[SKYCHENEY]

Like Maynard suggested, you can sleeve them with PVC no matter what you use for the structural part of the extension. This way you could still do your fiberglass joint or some steel tabs or a steel sleeve or whatever. It won't matter how is looks because the PVC will finish it off nicely.

 

[Vincentc]

Thanks,
The idea of a PVC sleeve sounds like an excellent solution to put over the wood extension with one rebar dowel.

 

[George]

All I did was bolt a 3 X 12 to the inside of the piling. you can sink the bolts into the wood so they dont scratch your rub rail. so easy, even a caveman can do it

 

[REBrueckner]

I assume George means "outside" the piling....anyway, that's what I was thinking....
I'd place a pair of heavy such wooden pieces perpendicular to the hull, for added strength, and thru bolt the pieces...two bolts on top two on the bottom. If you have the beam room, you could also bolt a single thicker piece of womanized lumber directly to the existing pile to extend it.....that would also serve to keep the hull and fenders off the creosote soaked piling....

 

[SKYCHENEY]

you could also bolt a single thicker piece of womanized lumber directly to the existing pile .

So does womanized lumber come from this kind of a tree?

 

[Vincentc]

I ran into one issue on pvc pipe, the largest the plumbing supply had was 10" and several piling butts exceed that. In addition one length of 10" was >$250. Couplings were >$50 would guess end caps would be comparable. That comes to over $100 per 6' piling and I am doing 13.

Additional research indicates that fiberglass reinforced plastic has a tensile strength of 20,000 pounds per square inch. A few wraps of fiberglass cloth tape around the joint with epoxy should have the joint exceed the strength of the piling.

I think that will work best for me.

 

[Maynard Rupp]

Gee Vincent, I have seen PVC pipe being used for city water mains. This stuff was big enough to crawl through. That price is hard to handle for sure.

 

[CapetaniosG]

Vincent: In several of your posts you're using the term "tensil strenght" of rebar or epoxy as an indication of the strength of the proposed joint. The problem is your piling joint will be subjected to shear (pounds/sq.in.) and bending (ft-lbs) forces and not tensil forces, generated by the boat impacting the piling. Impact forces are quite different than static forces. Impact forces generate much higher stresses than the equivelant static forces. Driviving a nail with a hummer into a wood block is much easier than driving the same nail by just pushing it down even with a static force 2 or 3 times greater than that generated by the hummer impacting the nail. So, in order to understand how a structural joint will behave, under load, you first have to have a good undestanding of the type of forces acting on that joint. The tensil strength of the joint is normally associated with a force acting along the long axis of the pile or rebar putting either one in tension until it fails (breaks).

Now I'm wondering why the hell I wrote what I wrote above. Does it serve any purpose? Well take it for what is worth, which maybe not much.

CapetaniosG

 

[krush]

The problem I see with the original design is when your boat rub rail pushes on the pile far away from the joint. This will result in a very strong moment at the joint which will cause it to "pivot" on one side and pull the rebar out of the join OR will just bend the rebar.

Rebar is designed to be strong in tension only. If you can somehow be sure the rebar is bonded to the wood and won't release, then when a moment (push on the top of pile) is applied, the rebar will only be loaded in tension.

Shear loading will occur if your boat contacts the piling very near the joint....but I wouldn't worry about this scenario as I do not think it will cause failure.

One option I can think of that would address the above loading problems would be to bore out the center of the pile on the top and the bottom for a 4x4 or something similar. Then you can through bolt through the piling and the 4x4 on both the top and the bottom (and countersink). A large dowel would work too.

Another option is to just use doweling and then use strapping on the outside of the pile running from the top to the bottom. The strapping will take the tension and the dowel will take the shear---and the combination of the two will solve the moment problem.

And even another option is to drill continuous holes vertically in both the top and the bottom and use all thread. Drill near the circumference to allow for cutting a notch out to on put in a plate/washer and a nut. This will satisfy the loading the same way as the dowel and straps.

But really, I think ya'll should just debate shadow ghost forces and loadings on the internet LOL. Whatever you do, good luck.

Disclaimer: no expertize was used in making the above post. Everything I know I learned from Wikipedia.org. My comments are worth exactly what you paid for them.

 

[sofarelli]

Vincent,
You have created a " hinge " connection. Setting a sister pile
along the existing pile is the best solution. An alternate would
be the splint idea with 2"x material attached to the side as mentioned
earlier. A 3' extension will require a 6' overlap to offset the cantalever.
That's your 2 cent consultation from a humble Architect.
Mike

 

[fissioneng]

All I did was bolt a 3 X 12 to the inside of the piling. you can sink the bolts into the wood so they dont scratch your rub rail. so easy, even a caveman can do it

I had the same problem in my slip and solved it this way using 2x6s. Very easy and quick, just countersink the bolt heads. Then I bought a fancy Taylor foam bumper and screwed it to the inside which covered the bolts. Works great and is cheap.

 

[fissioneng]

So does womanized lumber come from this kind of a tree?

Sky, that is just tooo funny.

 

[REBrueckner]

That SURE is a great tree....reminds me..............................................................damn, can't remember now....Oh well, never mind...

"Additional research indicates that fiberglass reinforced plastic has a tensile strength of 20,000 pounds per square inch. A few wraps of fiberglass cloth tape around the joint with epoxy should have the joint exceed the strength of the piling."

I think post #13 covers it....you CANNOT count on a few fiberglass wraps to hold your boat in high winds....just not gonna work....use bolts and lumber as described above....maybe copy proven pile extensions in use in your area if you can find any examples....

 

[Maynard Rupp]

WOW..You guys are over-engineering the daylights out of this simple and very crude job.

 

[Vincentc]

Crude,
Maynard, now I feel insulted, but you did answer the question. It is over engineered. But considering Krush's disclaimers. . .

This probably is Chevy rather than Jaguar engineering ( See Richard Prior's answer to Gene Wilder in SILVER STREAK ) I appreciate all the analysis and it has led me to further thought and a final solution.

A 10" x 3/4" hole was drilled in the center at one end of each piling. 10" of a 20" #5 rebar was embedded in the top piece and glued in place with filled epoxy resin. After set up, the extension piling was placed on top of the base piling with the other 10" of the rebar embedded and glued into the base piling hole. The top and hole in the base piling was coated/filled with more filled epoxy. The joint is then wrapped with 3 layers of fiberglass tape and epoxy.

As shown in the diagram below, the joint is a composite structure. When force is applied to one side of the extension, the rebar (Green) is placed in tension and one side of the piling joint fiberglass cloth (Purple) is in tension and the other compression. Everything is attached to the piling with epoxy (Red) including the rebar. As I understand it, builders secure rebar and bolts into existing concrete slabs with epoxy and the joint is stronger than the rebar. Wouldn't wood be the same, yielding a joint as strong as the weaker of either the rebar or the piling?