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'[EE]: Best place to weaken a beam'
2007\10\10@231607 by Cedric Chang

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I am considering cutting some slots into some floor beams and  
reinforcing above the slots with OSB.  My question is ....... where  
is the safest place to cut a slot ????   near the supporting wall or  
out in the center.  I am thinking it would be better to cut the slot  
near the supporting wall ; I don't have my college texts on static  
systems anymore and I can't really support my thoughts.  Maybe it  
does not matter where the slot is cut  !

Best

Cedric!

2007\10\11@003212 by Dr Skip

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It's never a good idea to cut into floor joists - they are usually built
marginally to begin with. Builders tend to minimize cost where they can, and
overbuilding floor systems is not a usual occurrence.

That said, I can't quite picture your mod. I will assume you are cutting a slot
perpendicular to the joist, on the joist bottom, and above this slot, you will
nail/glue OSB on the sides of the joist. Am I close?

The nature of wood adds a few other concerns one doesn't usually encounter in
statics class. At the ends, the board is subject to shear. There are tables to
determine the capacity of a 2-by for shear. It is dependent on width of the
beam (bearing surface) and the height of the beam. At a minimum, if you cut at
the edge (and to some degree anywhere) you run the risk of going past the shear
capability of the beam.

The beam will also have a load supporting rating, based on length, height,
width, and species for some acceptable level of deflection. You are effectively
turning a 2x12 (for instance) into something less, like a 2x8 perhaps,
depending on your cut.

The joist at the center will have the greatest deflection. Depending on
loading, it may have little, or a lot (as when you move a refrigerator over
it). Wood being made of fibers, you've introduced a 'crack' in it and depending
on the grain and species, as well as all sorts of individual criteria, it could
 continue cracking under load. As it deflects, your cut will open more.
There's open grain or tight, knots to consider, and the way it was sawn. It
should not be a problem if the new, smaller beam size (after cut) fits within
the loading and shear tables. It won't though, because your builder would've
way overbuilt at excess cost in that case....

As for the specifics, the depth of cut is critical, as is the number of cuts
along the beam and through other beams. The load does distribute across other
joists, so the other nearby ones can take up some for the bad beam, but if many
are compromised, no good. OSB will get you little support, especially if only
nailed as a fish plate (on the sides).

It is customary, and best, to use additional 2-by lumber well past the cut.
Being _very_ familiar with wood structures, and if it were my house, I would
not use just nails if it were a cut of any real depth. I would use wood glue,
and to get the best bond, one always sands the wood to reveal clean, unoxidized
wood. It there is twist in the joists, more reason to avoid cutting them... I
would put the boards on both sides, 4+ feet past the cut, and glue, clamp, then
nail. Of course, even cutting at all would depend on depth, position, etc. first.

Steel is relatively cheap too. Fish plates of 1/4" steel bolted through the
joist would be very good... There are even more options if you go steel, but I
won't go there...

There are a few other options. One is to add supporting columns from below.
Another is to add perpendicular joists on either side of the cut(s) and use
steel brackets on the full sides of good joists and the full side of the new
cross joist(s). This would help distribute the weight to remaining joists in a
marginal situation. Only support from below, or maybe fish plates with long
2-by, will work if you go past the tables for whatever size joist you create
with your cut. Remember that beam height is more important than cross section
or width - ie, three 2x4s may be the same amount of wood as one 2x12, but is
much less strong and deflects more. One other alternative is to put
perpendicular joists under the current ones, but support _them_ at their ends
well beyond your cuts, assuming you need room.

I hope that helps. Lots of variables and one really has to see what's there. It
could be risky and isn't generally advisable. As such, do at your own risk,
this is not to be considered engineering advice, and I was never here... ;)




Cedric Chang wrote:
{Quote hidden}

2007\10\11@031252 by Forrest W Christian

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Cedric Chang wrote:
> I am considering cutting some slots into some floor beams and  
> reinforcing above the slots with OSB.  My question is ....... where  
> is the safest place to cut a slot ????  

Not at all....  :)

I am not a structural engineer, but I know that there are specific
places where it is somewhat ok to do.  BUT AGAIN I AM NOT A STRUCTURAL
ENGINEER - PLEASE CONSULT ONE OR AT LEAST A LOCAL INSPECTOR WHO HAS A CLUE.

About the only truly safe holes that I know of are holes in the middle
third (vertically) of a joist, and not within a couple of inches of any
other holes.  There's also a maximum size, but typically I'm only
running Romex or PEX so I'm not talking more than a 1" hole in a joist.

Slots scare me, simply because the load can't transfer "around" a
slot... When you drill a hole part of the load goes on either side of
the load and since it's supported on both sides (by the undrilled joist)
it has very little effect.

On a slot, you are effectively reducing the width of the joist for a
much wider area than the slot itself.  I realize you're talking about
"reinforcing" it, but that's a lot of load to transfer and I'd be
skeptical about OSB being able to handle the additional load.   Really,
talk to a structural engineer.

-forrest

2007\10\11@090247 by Carl Denk

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Others have said they are not a structural engineer, well I am, but this
advice is without liability on my part! Without knowing more of the
loading, member (beam or joist, wood, steel, or other material)
material, one cannot advise at a distance what would be safe! There
could be concentrated loads above like a wall or bathtub, etc. But the
more likely safe location would be around 20% of the span length from an
end, and the hole at mid height of the member. The should not be a slot,
nor should it be near any other holes or defects like knots in wood. For
wood, the most common modes of failure are:
1: Horizontal shear: Near a support (end) the wood can shear (slide)
parallel to the grain usually near mid height. This is why your hole
doesn't want to be near the supports.
2: Bending at mid span: This is what most people think of beam failure,
with for a beam supported on the ends and loaded on the  top. Tension on
the bottom fibers pulling fibers apart, and compression on the top
fibers crushing the wood. The strength is related to the 3rd power of
the depth so one does not want to cut into the top or bottom fibers.

I'm not familiar with the engineering properties of OSB, but think the
likelihood of application here is poor. Some of the other ways of
reinforcing, might be a steel or wood(including plywood) plate, adding a
same sized piece of wood joist next, or adding bridging (solid blocking
perpendicular to the span) to move some of the load to adjacent joists.
Also the reinforcing might not be required at all, or just a minor piece
of wood with a few screws or nails.

The bottom line is you need local expertise in the form of a structural
engineer to look at the structure, do some simple calculations probably
and advise you. I would hesitate at contacting the local government
inspectors, though once you have determined properly what to do, you may
need a building permit, without could effect your house insurance.

Forrest W Christian wrote:
{Quote hidden}

2007\10\11@095349 by 556RECON

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Cedric Chang wrote:

>I am considering cutting some slots into some floor beams and  
>reinforcing above the slots with OSB.  My question is ....... where  
>is the safest place to cut a slot ????   near the supporting wall or  
>out in the center.  I am thinking it would be better to cut the slot  
>near the supporting wall ; I don't have my college texts on static  
>systems anymore and I can't really support my thoughts.  Maybe it  
>does not matter where the slot is cut  !
>
>Best
>
>Cedric!
>  
>
What are the details of this? Beam size?   Beam length?  slot width and
depth?  Slot distance from supported end?

Recon

2007\10\11@120034 by Cedric Chang

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I really appreciate the replies and will let you all know what I do  
and what happens.
One further comment:  I have a lot of faith in OSB since I have seen  
many I-beams
built out of them.
Learning about the 3rd power of the cut was most instructive.

Best

Cedric

Cedric Chang wrote:
{Quote hidden}

2007\10\11@122806 by Peter Todd

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On Thu, Oct 11, 2007 at 10:00:22AM -0600, Cedric Chang wrote:
> I really appreciate the replies and will let you all know what I do  
> and what happens.
> One further comment:  I have a lot of faith in OSB since I have seen  
> many I-beams
> built out of them.

I have a lot of respect for engineers because they manage to build I-beams
out of OSB. :)

- --
http://petertodd.org
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2007\10\11@141059 by Dr Skip

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In an engineered beam like that, the load is distributed along the ends of the
OSB top/bottom through the solid parts. It is evenly distributed. Nailing puts
the load at points, with some residual friction between surfaces. Bear in mind,
OSB is not as strong as real wood, but it's cheaper and of uniform properties
(though weaker) so it's cheaper to make the engineered beam and manufacturing
doesn't depend on working around any natural imperfections. They can also be
made longer than would be easily obtainable in real wood. It is _not_ good for
your application.


Cedric Chang wrote:
> I really appreciate the replies and will let you all know what I do  
> and what happens.
> One further comment:  I have a lot of faith in OSB since I have seen  
> many I-beams
> built out of them.

2007\10\11@195700 by Jim Korman

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Cedric Chang wrote:
> I am considering cutting some slots into some floor beams and  
> reinforcing above the slots with OSB.  My question is ....... where  
> is the safest place to cut a slot ????   near the supporting wall or  
> out in the center.  I am thinking it would be better to cut the slot  
> near the supporting wall ; I don't have my college texts on static  
> systems anymore and I can't really support my thoughts.  Maybe it  
> does not matter where the slot is cut  !
>
> Best
>
> Cedric!
>  
Give this pdf a look.....

http://www.haringey.gov.uk/note07_-_domestic_timber_floors.pdf

Note that they show NO cuts of any kind on the bottom edge of
the beam. Several others I looked at were similar.

Jim

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