Limber Hole Advice

[hokiefan]

I started out talking about notching framing members and somehow we got to holes but I am correct that notching anything in the center weakens it more than any other location and the building code is very clear about that also.

Sorry, but this is just not correct. If you look at the stress profile of a beam, it is at its max on one side, goes to zero in the middle, and back to the opposite max on the other side. One side will be in tension, the other in compression. The center will have zero stress.

The building code may say that, but that doesn't make it correct.

You are all forgetting that the skin, when properly fastened, forms the flange of the T bar, moving the neutral axis of the frame closer to the faying surface with the limber holes.

You are correct on this. The center of the T-beam formed is not the center of the stringer. You would have the calculate the new moment of inertia to find the resulting center, but it would be closer to the skin.

Bobby

[Jamesh]

IMHO
I have never heard of any one putting lumber holes in stringers only in frames, pitch is often used to fill the wedge uphill of stringers, epoxy+filler might well replace it
As to frames I would be tempted to put a 2" radius semi circle down the middle, and then staple 6" wide glass tape to the frames so that when you come to turn the hull over the cloth is ready in place to epoxy even through the lumber holes.

James

[htom]

I am not letting this one go yet. I still believe I am correct in my statement that a hole in the center of a beam or joist weakens it more than if put in other areas (outer thirds). I have read the code again and although it refers more to notching I think it is common sense. Bobby, I understand what function the web in a beam serves but it again seems like holes in the center third would cause failure before holes in the outer third. If you are an engineer, I am listening but still not convinced. If I could draw on the computer I could make a better argument, hell I can't even do that quotation thing you all do!

I wish I had a photo of the joists I had to replace because someone believed like you do. Drilled a bunch of 1" holes centered maybe an inch from the lower edge, years ago. Eventually, BANG!

As a very rough guide, you can drill a hole the diameter of 1/4 of the height of the beam, if you put the center of the hole on the centerline of the beam. (For example, a 2" hole centered in an 8" joist.) You can drill smaller holes -- but they must remain totally outside of those two strips that are the outer 3/8 of the beam, top and bottom.

The Code -- if it actually says what you say it says -- was probably written by politicians, not engineers.

[Ron Williamson]

Is it possible that waltwood's code is actually referring to the center of the span as opposed to the center of the beam?
Here's a pic
http://www.google.ca/imgres?imgurl=h...ed=0CFkQ9QEwAQ
The centers of these joists are almost nonexistent.
R

[seo]

I hope that the picture linked in #55 resolves this question. I went back and read my post at #22 which I think started this colloquium on the subject of mechanical engineering of wooden structures, and I continue to think that I'm right that a stringer attached to a plank is a laminated beam, and a hole bored through the lower side of the stringer is in fact near the center of the laminated beam, and won't have any structural effect. It may very well be better to simply fill the low spots with goo, so that water won't pool.

[Breakaway]

and I continue to think that I'm right that a stringer attached to a plank is a laminated beam,

Your making me think, which is always scary

If that's so, then would we call a wood framed house wall a laminated beam, since the sheathing and sheetrock sandwich sticks on edge? A floor a joists a laminated structure?

Just thinking

Kevin

[boattruck]

Gents, Lets be clear, the Universal Building Code is for the construction of buildings and houses, it is a fantastically useful and important collection of documents to guide and control the safe, orderly, construction standards for these structures, it says nothing about the boats and their unique engineering situations. Some if the information contained within it may, with careful interpretation, be useful to a boat-builder, but only after careful soul searching as to the applicability to your current challenges. Ours is a trade and craft that is largely unregulated and uncharted, ABYC and Lloyds, and Skene's being a more likely source of guidance, as well as the thoughts of the oldest and most experienced boat-right you can find. And then you will still have a cluster of opinions to try and assimilate into some sort of plan, good luck! Cheers, Steve

[Peerie Maa]

I wish I had a photo of the joists I had to replace because someone believed like you do. Drilled a bunch of 1" holes centered maybe an inch from the lower edge, years ago. Eventually, BANG!

As a very rough guide, you can drill a hole the diameter of 1/4 of the height of the beam, if you put the center of the hole on the centerline of the beam. (For example, a 2" hole centered in an 8" joist.) You can drill smaller holes -- but they must remain totally outside of those two strips that are the outer 3/8 of the beam, top and bottom.

The Code -- if it actually says what you say it says -- was probably written by politicians, not engineers.

Think of it this way. If you have an 8" floor joist, and cut a 2" hole in the top or bottom edge, you are left with a 6" joist, with a big stress raiser.

If you were then to glue a plywood flange on the edge with the hole you would put back a lot of the lost strength.

Here is a good illustration of making holes without losing strength.

[Soundman67]

Your making me think, which is always scary

If that's so, then would we call a wood framed house wall a laminated beam, since the sheathing and sheetrock sandwich sticks on edge? A floor a joists a laminated structure?

Just thinking

Kevin

A house wall could be considered a torsion box. so can a joisted floor if it is skinned on both sides. A torsion box is very much like the beam with the holes in it except it is more 3D now. If you think of a single sheet of plywood it will bend quite easily. now separate 2 sheets of plywood with 2x4s and glue and fasten them securely. Very Very much more difficult to bend. just as in a beam the ply on one side is in tension and the ply on the other side is in compression. The 2x4s are there to make the lever bigger. or keep the 2 sheets apart.

[waltwood]

I have been talking about the center of the span not the center of the joist the whole time. It had to do with this boat I am building, I was going to put the limbers closer to the floors rather than the center of the span between the floors. Many Building Codes are written by politicians but this one is practical.

[seo]

I pretty much agree with "Boatruck" in #58, I'd point out that Herreshof's Rules of Construction is a pretty comprehensive "building code" for a certain type of wooden hull. It was written by a very well-trained engineer with a lot of experience in wooden construction (sort of an understatement). Other "Scantling Rules" of this type include Nevins' rules, which I think were pretty much a rewrite of Herreshof's rules. I have been told that Nevins rules winds you up with a heavier hull, but it better recognizes the realities of the materials and workmanship available to everybody who isn't named Herreshof. Lloyd's also has rules for wooden construction. Historically, American Bureau of Ships (ABS) published rules for wooden construction, but I think they're mainly applicable to much larger vessels. ABS has published rules for FRP construction, which I think are the first new comprehensive rules for that material since the Dawn of Time, otherwise known as the publication of the Gibbs and Cox manual of FRP construction.
All that said, the fact remains that what's been debated up above is only peripherally about building codes. It's more about the basic characteristics of materials and structures.

[waltwood]

We have been debating over a point that has been misinterpreted by most everyone and I don't know how. I am still correct in my original statement which was a notch in the stringers closer to the floors weakens it less than a notch in the center of the span between the floors. It was never indicated that I was going to place a hole in the center of the stringer which does not seem useful or smart. I was using the Common Sense rule here but with no technical information to refer to other than the Building Code, which I am very well read and it was pertinent to the situation, that is what I used. If the debate continues I have devised a test that will prove my point but hopefully this clarification of what I am talking about will suffice.

[wollybugger]

I am not letting this one go yet. I still believe I am correct in my statement that a hole in the center of a beam or joist weakens it more than if put in other areas (outer thirds). I have read the code again and although it refers more to notching I think it is common sense. Bobby, I understand what function the web in a beam serves but it again seems like holes in the center third would cause failure before holes in the outer third. If you are an engineer, I am listening but still not convinced. If I could draw on the computer I could make a better argument, hell I can't even do that quotation thing you all do!

I have to concur with the Hokie engineer. Look at the link focusing on the "green" stress distribution http://www.ejsong.com/MDME/MEMmods/M...ng_Stress.html. Note how it goes to zero in the center. This is where you want to remove material, where it sees no stress.

Using that, imagine you a 4x4" with 16' span resting on cinder blocks at the ends. Now drll a 1" hole located mid span and so close to the bottom that only thin strip (say 1/16") of material remains between the hole and the bottom edge. Get your biggest buddy and have him stand over the hole and start handing him barbells. At some weight, that thin strip will tear (which will then allow the whole board to fail). It will tear because that thin strip has to take all the stresses from your buddy/barbells

Repeat the experiment but move the hole up so the thin strip is 1/8" from the bottom. Get your buddy back up there and hand him same amount of barbells. You'll see the board doesn't break because the thicker strip can take more stress.

Now move the hole until it's in the center (equal strips top/bottom of 1-1/2"). The strips are the largest they can be, therefoer can take the most stress.

Now move the hole closer to the top (say 1/16" from top). When your buddy gets up there, the strip will fail at the same weight as when the hole was on the bottom. But in this instance, the strip will crush instead of tear.

Hope this helps ... but fun to think about this stuff huh?


Robb
Pitt Mechanical Engineering circa long time ago

[Peerie Maa]

Will yous guys please stop obsessing about limber holes in the stringers. Those stringers are so skinny that the only way you could make useful limbers is to cut out a short section and bridge the gap with a doubler.

[wollybugger]

Will yous guys please stop obsessing about limber holes in the stringers. Those stringers are so skinny that the only way you could make useful limbers is to cut out a short section and bridge the gap with a doubler.

Yeah

1 - do you need limbers? Yeah
2 - Do you have enough cross-section for large limbers? No

So the options are:
1 - live with small limbers
2 - beef up the cross-section (doublers)

[Peerie Maa]

Yeah

1 - do you need limbers? Yeah
2 - Do you have enough cross-section for large limbers? No

So the options are:
1 - live with small limbers
2 - beef up the cross-section (doublers)

Or save the owner a lot of poking with a stick through skinny limbers and pour pitch where the water will pool.

[waltwood]

Robb, I can't understand the link but I do get the explanation about the 4 x 4. I agree with that and always have but my original point was different. Using the 4 x 4 x 16' again spanning 2 concrete blocks with a 1" x 1" notch in the bottom of it at 8'. Load the 4 x 4 evenly and at some weight it will fail. Now put the notch anywhere from 5' 4"(one third of 16') from the end to the where it bears on the concrete block and load it and it will be able to carry more weight before failing than if the notch were in the center. That is all I have been trying to say. I am also somewhat enjoying this process.

Walt

[wollybugger]

Hey Walt, your absolutely right!

And I think we should better specify what "center" is. You case refers to center "span" and your are spot on in your thinking. I think some of us have been talking about center of the stringer's "cross section" or "web" (like Peerie shows, the holes are in the web).

If you have a small hole in the center of the web, it really has no affect on the max stresses the beam sees. So you can put a small hole anywhere along the "span" as long it's in the center of the "web".

For your specific case, your web is the combination of your stringer plus the plank it is attached to. I know it's confusing, but if you put a small notch on the bottom of your stringer, and if you attach (say glue for the sake of argument) a logitudnal plank to your stringer, you have made a laminate (with a small notched hole in the center of your web).

However, if that small hole grows (because you would like leaves and fishing sinkers, etc to not clog it up) your hole gets too high on your laminate which will lead to an area of high stress. If the hole gets too big, the stress too high, your stringer will break.

Your stress gets too high, because you don't have enough meat on top of the hole. So if you make a big hole, you need to add more meat in the form of a doubler (a 3rd layer of laminate ... plank + stringer + doubler)

I really hope I didn't confuse things for you,
Robb

[waltwood]

Yes Robb, I think we understand what each other was trying to say now. The planking is acting like the outer part of the beam.

I want to notch these stringers but the forward 2 sections are curved and under some tension, enough that they could break if notched much. I do not want to add a doubler on top for a few reasons so I guess that leads me to putting goop in those areas. I am still going to notch the floors.