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#21845 - 02/11/03 11:55 PM More help for my renovation...  
Redsy  Offline
Joined: Mar 2001
Posts: 2,056
Bucks County PA
I truly appreciate all the help regarding my "drum room".
Now, if I may ask another:

My home currently has knee walls and the circuits are simply draped across the joists behind the walls.
The whole thing will be floored in 2 days. I have heard conflicting comments on drilling and/or notching floor joists,(standard joists, not "engineered wood products") but the prospect of pulling out and re routing existing wiring is much less appealing than simply cutting a small notch in the top of the joist and laying the existing cables in.

The book, "Electrical Wiring Residential" by Ray C. Mullin states that "notches on the top or bottom of joists shall not exceed one-sixth the depth of the joist, and shall not be located in the middle third of the span".
Does anyone have any comment, or can anyone cite a source for this criteria?

Thanks again. I owe you all a beer (or soda, or whatever the case may be). [Linked Image]

Work Gear for Electricians and the Trades

#21846 - 02/12/03 12:37 AM Re: More help for my renovation...  
rhagfo  Offline
Joined: Apr 2001
Posts: 43
Portland, OR. USA
Notching a floor joist will do more to weaken it than drilling a small (3/4" or less) in it's center. Engineered or standard lumber the worst thing you can do is cut the top or bottom edge. They both work the same way, only engineered products are more consistent. With a load on the floor, the top edge is in compression and the bottom edge is in tension, and the center is mostly free of stress. If you notch a 2x8 1" deep you just made a 2x7 out of it.
The size of hole you can drill without affecting the strength of the joist depends on it's size.

#21847 - 02/12/03 02:16 AM Re: More help for my renovation...  
Len_B  Offline
Joined: Jan 2003
Posts: 53
New Hampshire
The "criteria" is one of structural engineering.

The following is a bit of an oversimplification, but should serve the purpose.

A load placed on a wood joist deflects the joist into a slight curve. Viewed from the side: the top wood fibers compress; the bottom fibers stretch; the center fibers remain neutral(relaxed). This is why holes should be drilled in the middle of a joist and avoiding the middle third of the span. The allowed value of this bending is Fb(extreme fiber stress in bending). Horizontal shear failure occurs when the wood is bent to the point where the fibers slide over one another and become detached(split). The fibers are at their most stressed values in the very top and bottom of the joist at mid span. Note how I beams and truss joists exploit this concept. All the material is put to work at the top and bottom, where the stress is. Wood is stronger in tension(bottom of joist) than compression(top of joist). Notching the bottom, in the middle third of the span, is by far the worst thing to do. This can and does lead to catastrophic failure (breaking = modulus of rupture). The closer you get to either end of the joist the less the fibers are stressed lengthwise. Note how older joists and metal roof trusses are tapered where they rest on/in plates. The failure mode to worry about at the attachment end of the joist is vertical shear. This is a rare failure mode as wood joists that are sized properly for horizontal shear are usually, by default, overdesigned for vertical shear at the attachment ends. There are a number of other factors but these are the biggies.

If you are going to notch the top, try not to do it in the middle third if possible, and use metal plates over shallow notches. Several wide shallow notches for 2-3 single cables will weaken the joist much less than a single deep notch for a bundle of cables. The metal plate and subfloor, especially if glued, will help strenghten the joist where the material was removed. You could cut plywood reinforcements(glue and screw) for the joists where you notch them (24"-36" long running the depth of the joist) if the joists are marginally sized per span tables.
follow link to "tutorial for using the span table"

Hope this helps


#21848 - 02/12/03 08:09 AM Re: More help for my renovation...  
Redsy  Offline
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Thanks, guys.
I probably will have to notch at about the 6' area of a 25' joists. I am glad Len offered the comment that several shallow notches are better than one deeper one. I was curious about that.


The existing joists are 2 x 8s. (They will not need to be notched). They are going to be "sistered" with 2 x 10s per building specs. The 2 x 10s will need the notch. So I believe I will have a stronger joist, overall, than just the 2 x 10s. Is this correct?

#21849 - 02/12/03 08:25 AM Re: More help for my renovation...  
walrus  Offline
Joined: Jul 2002
Posts: 680
Bangor Me. USA
By sistering the 2 by 10s to the 2 by 8s, you mean they are going to be place side by side? and nailed together.
The strength of a joist is in its depth and not in its width. The extra width will add some stability but little to the overall bending stress. I'd have to find an old statics book to show you mathematically why this is so.

In your case I wouldn't worry much about it but I would limit any notches that you can.

[This message has been edited by walrus (edited 02-12-2003).]

#21850 - 02/12/03 12:51 PM Re: More help for my renovation...  
Len_B  Offline
Joined: Jan 2003
Posts: 53
New Hampshire
I probably will have to notch at about the 6' area of a 25' joists.

I assume there must be a load bearing wall or beam somewhere around midspan underneath this floor, as dimensional lumber this size will not carry 25' clear span loads at allowable code deflections(there would be, at minimum, serious deflection problems). Based on this assumption your 6' measurement would place your notches in the middle of a 12' supported span. This isn't going to be a problem, especially with the sistered joists---I just brought it up for clarification.

Joist width does affect strength. The depth of a joist affects its stiffness(deflection). Allowed loads, for a constant joist depth and deflection at a certain span, are based upon formula that yields a maximum load per inch of joist width. This must then be factored in with joist spacing, etc, to achieve the desired performance values for the floor. From a lumber usage standpoint deeper and narrower is more efficient - but there is a point at which lateral bowing, lack of deflection, nailing, and other factors make it a poor design choice. The equation, of course, can be configured to give allowable loads per inch of joist depth based on a constant width.
The best examples of this compromise between depth and width are found in timber framed structures(homes, bridges, ships, etc.)
Standard dimensional lumber dictates the use of 2x material, and most of us have simply become accustomed to going no further than the span table.


[This message has been edited by Len_B (edited 02-12-2003).]

#21851 - 02/12/03 02:31 PM Re: More help for my renovation...  
HotLine1  Offline

Joined: Apr 2002
Posts: 6,853
Brick, NJ USA
For what it's worth, can you install running boards at right angle to the joists??

If you're in the "knee wall" area, and are going to use it for storage, the additional 3/4" floor elevation may not bother you.

The "strips" will protect the NMC, and will comply with "protection from physical damage"

Using this method, you will not have to notch or drill.



#21852 - 02/12/03 09:20 PM Re: More help for my renovation...  
ThinkGood  Offline
Joined: Aug 2002
Posts: 1,081
Milwaukee, WI
I live in a house built somewhere between 1915-1920. There are some notches in the bottom of a few joists. (They were used for BX, which has since been removed.)

Len_B, is this what you are saying to do--use plywood rather than 2x lumber:
[Linked Image]

...Sometimes it's easier to paint a picture [Linked Image]

In this case, I mean for when there is no cable going through the notch...

[This message has been edited by ThinkGood (edited 02-12-2003).]

#21853 - 02/12/03 09:42 PM Re: More help for my renovation...  
Len_B  Offline
Joined: Jan 2003
Posts: 53
New Hampshire
Even with cable going through plywood with a notch, the plywood adds lots of strength due to the cross grain and increased joist width. It strengthens what may be a possible stress riser(weak point at which forces become concentrated.) Try to deflect plywood on edge - it ain't easy. Thats why it makes great box beams. I had to do this under a kitchen remodel(my old house). I just cut 1/2" plywood the depth of the joists, three feet long and glued and screwed it to one side. Two would probably have been better, 4 feet long would have been nicer, but I am "frugal"(the plywood was in the scrap pile) and lazy...


[This message has been edited by Len_B (edited 02-12-2003).]

#21854 - 02/14/03 11:17 AM Re: More help for my renovation...  
Redsy  Offline
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Sorry, the joists are 14', with a support at 12' or so.
All notches will be at the first 3-4'.

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