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Old 10-29-2010, 07:52 PM   #1
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knots in joists (engineers)


what is the differance between a knot in a joist a hole in a joist and a hole with a bolt through it
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Old 10-29-2010, 09:28 PM   #2
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Nothing, they all can create the same problems as the other.



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Old 10-29-2010, 10:34 PM   #3
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Actually they're entirely different.

The joist is graded according to the placement and size of the knots in that particular piece of wood, amongst other things. Knot placement and other defects are accounted for in the span tables that tell you how far the joist can span under a given load. A #2 will have larger (or looser) knots closer to the edges than a #1 for instance, so it is a bit weaker.

Drilling a hole in a joist is much like adding a knot, or worse. It isn't always a bad thing, and it doesn't always negatively affect the joist's performance. The code has specific requirements for where you can and can't drill holes. Stay within those requirements and you won't hurt the joist's performance, or at least an appreciable amount. Even though your #2 joist may have a knot near the edge, you can't drill a hole there.

It would be safe to say that a hole in a joist and a loose knot in a joist, in the same location and the same size, will affect the structural integrity of the joist in the same way.
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Old 10-29-2010, 11:13 PM   #4
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I am not an Engineer, but...

When placing Joists, try to have any edge knots placed UP on the joist.
That is the compression edge, so the knot will tighten up.
If it is on the bottom (tension) edge, it allows the joist to deflect more.

No holes within 2" of an edge. Ideally, place them in the middle third of a joist.
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Old 10-29-2010, 11:25 PM   #5
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If properly located and installed, bolts do not weaken the joist significantly. First of all, bolts tend to be small for wood framing, typically 1/2 inch diameter. A 3/4 inch bolt or larger is extremely rare in residential construction, might be seen in heavy timber industrial type buildings.

Second, the bolt is typically placed well away from the edge of the beam, relatively close to the neutral axis, so the loss of section is relatively unimportant.

Third, the bolt hole is filled with a steel or occasionally iron pin, which if tightly fitted actually increases the section modulus of the beam.
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Old 10-30-2010, 01:40 AM   #6
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Dan: How does a bolt increase the section mod, if that is easy to explain. thanks. john
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Old 10-30-2010, 07:54 AM   #7
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I have a joist 13 feet long 2x10 with a 3 foot cantiliver 4 knots and 2 holes (drlled) aprox 1.5" from the tension edge what I understand this should be sistered no way to get sister in without tacking down deck and removeng siding plywood and rim joist is there any thing else I can do?

I have another situation with joists bolted together in a staggered pattern with the bolts 5/8" aprox 2" to 2.5" from edge is this safe?
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Old 10-30-2010, 12:47 PM   #8
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Art: This came up once before, and a cat posted a link to some steel strapping that is easy to install. It runs from the top of the joist, along the side to the bottom, then back up along the joist to the other end. You can criss cross the straps if needed, if they will fit in your situation. If what you have is working, and you beef it up this way, I'd guess you'd be fine. Does your lumber company have any spec sheets on hole size and location? If you've ever looked at BCIs or TJIs, there are holes all over them, pre-punched. The web of the beam ("middle") primarily serves to separate the flanges (top and bottom) so they are far apart, one in tension, on in compression. If Dan gets back on here, he can clarify, elaborate, or refute that. That is what I remember my dad, an engineer, telling me. I've seen commercial buildings with webs made out of tubing, crossed in X patterns, so that ducting, wires, etc, can be run through them. Your 5/8" holes don't impress me for worrying about.
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Old 10-30-2010, 02:53 PM   #9
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http://www.finehomebuilding.com/PDF/Free/021184090.pdf

is this the link?
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Old 10-30-2010, 03:17 PM   #10
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Old 10-30-2010, 03:20 PM   #11
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Old 10-30-2010, 04:31 PM   #12
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How a bolt increases the section modulus of a wooden beam.
The modulus of elasticity E of wood varies with the species, but typically runs between 1 million psi and almost 2 million psi. The modulus relates stress and strain via the formula Stress = E x strain, where you have to watch the units. Typically in the U.S. we use stress in pounds per square inch (psi), and E is the modulus of elasticity in psi, and strain has no units.

The modulus of elasticity for steel is approximately 29 million psi, which is between 15 and 30 times that of wood. This means that for the same strain, steel develops 15 to 30 times the stress as wood. When computing the section modulus of a composite material such as a wood member with a steel plate bolted on, you need to convert the actual area of the steel to the equivalent area of wood. This is done by multiplying the area of the steel by the ratio of the modulus of elasticity of the steel to the modulus of the wood, which as I said is 15 to 30 times. This means that a half inch bolt is equivalent to anywhere from 7.5 to 30 inches of wood.

Things are not quite this simple, because the bolt is not going to develop tension even if it is located on the tension side of the beam, and the beam is loaded in bending. However, the bolt could very well develop compression loading force, which it would resist. Again, things are complicated because the bolt would only feel as much compression as could be delivered by the wood, so in practice it will not increase the section modulus significantly, but if it is tight in the bolt hole, it won't decrease the section modulus either.

It is a completely different story if you add a steel plate to the bottom of the beam or the sides, and connect it rigidly to the wood using bolts or lag screws. Then the steel acts compositely with the wood, and its equivalent wooden section is 7.5 to 15 times the area of steel. This is why a thin steel plate can add an enourmous amount of strength to a large wooden beam, if it is properly connected to the wood.
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Old 10-30-2010, 05:25 PM   #13
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jklingel


that was a web site I found with 6 ways to strenghten joists one on them uses steel straps thought that was what you were reffering to please this is a serious problem I need help
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Old 10-30-2010, 06:00 PM   #14
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The bolts that are 2-1/2 inches in from the edge are fine, for all the reasons I mentioned. As for the cantilevered joist, well that is far too complex for me or anyone else to analyze over the net, either you live with what you have or you hire an engineer for a personal investigation, no way to do a diagnosis via the web.
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Old 10-30-2010, 06:17 PM   #15
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it is the section between the bearing walls with the problems not the cantiliverd section is it possible to sister the bearing section and leave the cantiliver alone
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