Drilling holes for wiring in laminate joist 1

[AAlpha]

Hi,
I need to drill holes in floor joists to rout wiring. I noticed that the builder (or electrician) of this house did not rout any existing wiring through a laminate joist which goes across halve of the basement. It almost looks like that he avoided going through the laminate joist. The beam in question consists of 2 "sistered" laminate 2" x 10" in a residential house.

Is there any rule that there should not be any holes drilled into laminated joists for electrical wiring?

 

[Ron]

No. As with any flexural structural member, keep the hole small and as near to the center as possible.

 

[JAE]

Go to this site:

http://www.ilevel.com/literature/TJ-9000.pdf

for a catalog on a particular brand of joist - look at page 36 for allowable holes.

Also see attached image for a screen shot of that page.

 

[msquared48]

As Ron stated "... keep the hole small and as near to the center as possible."

This applies to the overall depth of the joist as well as the joist span. Too close to the end of the joist could affect the joist in shear, and too or high in the depth of joist will affect the moment. Moreover, there is a size limitation on the diameter of the hole that can be drilled, particularly with manufactured joists. Some joists have knockouts that can be used as opposed to drilling in the field.

If these are manufactured joists, check the catalog for more specific guidance, that if not followed to the letter, will invalicate any warranty on the joists.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

What size hole does the electrician require? I'm guessing that they can get away with a 1" hole so I would position it in the center of the 2x10" and locate the hole in a region of low bending and shear stresses.

 

[a2mfk]

Do you guys have lots of situations where shear stress is a problem in beams in standard wood framed structures (especially residential)? I never have, which is why I would want it as close to the support as possible where the flexural stresses are lowest... Of course it depends on the size of the notch or hole.

 

[MiketheEngineer]

Shear stress is normally highest at or near the supports.

Depending on the loading conditions, load types, bearing locations, etc. all this can change. So follow the mfg or seek professional help.

 

[hawkaz]

General rule of thumb: Keep holes in the middle third of length and middle third of depth for most situations.

 

[woodman88]

A2mfk, The flexural stresses are mainly resisted by the top 1/3 and bottom 1/3 of the solid wood member. Usually, for a simple span plf loaded, this location for holes will have less than a 1% effect on the load capacity of the member. This is the reason for placing holes in the middle third of the span and in the middle third of the depth rule for simple span joists/beams. Notches, on the other hand, should be placed within 1/4 of the span ends but will have a much greater effect on the member.

Garth Dreger PE
AZ Phoenix area

 

[rscassar]

I always look at shear design with wood. Generally with wood I design in the order:

1. Deflections
2. Connections
3. Bending
4. Shear

Despite shear being at the bottom of the list it does become critical in the design from time to time. Particularly when using a material such as hardwood which has a low characteristic shear strength when compared to it's bending strength.

 

[a2mfk]

All interesting and good points.

I don't do tons of wood design, but I have never had a beam even close to not working in shear. I would think this is more of a problem with heavily loaded glulam girders, point loaded beams, etc..

Deflection usually controls in my experience, and with residential, the spacing of the beams is critical to get a good, serviceable floor. I have also never given drilling a 1" hole a second thought since I am not usually "cutting" it that close on my designs...