holes in wood joists

[rittz]

The electrician drilled 3 3/4" holes an inch or so apart through a series of 2x10 joists. There is about 1-1/2" from the bottom of the holes to the bottom of the joist. There is more than one set of three holes and the distance from the end of the joists varies but they all appear to be in the middle third of the length of the joists. How about a 10 gage strap 1.5" x 18" or so screwed to each side of the joist under the holes. This is a house. 2x10 are at 16" oc and span 15 ft

 

[msquared48]

Why does an electrician need to drill 3.75" holes in a residential floor joist. I can see 1.5" or 2", but 3.75"?

I usually try to restrict the hole size to no more than 1/3 the depth of the joist (3" here), centered on the centerline of the joist, and in the middle third of the span for shear. You need to check the S value at each hole and the associated moment for that location. Shear is mopre than likely not an issue, unless it is a transfer beam.

I don't like what this guy did - he should know better. If I had my druthers, the joists would be sistered with new ones, and properly re-drilled under your direct supervision. But, I'm not the engineer.

Bottom line is don't try to patch the situation with a gerry-rigged fix. Do it right if possible.

Mike McCann
MMC Engineering

 

[rday]

I read it as (3) 3/4" holes.

I agree though, patch it right.

 

[msquared48]

I guess the OP needs to clarify this.

Mike McCann
MMC Engineering

 

[msquared48]

If it is (3) 3/4" holes, I would not worry about it.

Mike McCann
MMC Engineering

 

[rittz]

There are 3 holes 1inch apart each being 3/4 “ diameter. Looks like
the consensus is “forget it”.
Thanks guys
J

 

[FSS]

Lucky for you the plumbers did not get involved. Those guys can put in some big holes!

 

[jhoulette]

I don't agree to forget about this.

First, 2x10 joists at 16"o.c. for a span of 15ft is pushing the limit for 40psf LL/ 10psf DL.

Second, the diameter of the holes may be acceptable, but the location is in the bottom 1/3 of the depth and the holes are spaced too close. This is against code. Are you comfortable accepting a code violation?

I would reinforce the joists with OSB, LVL lumber or metal strapping. I have liked the performance using OSB glued and nailed to one or maybe each side.

Lastly, this electrician should not get away with this. He's just going to keep doing it until he has to start paying for his mistakes.



Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info

 

[scottiesei]

It would be much easier to add a strap now, rather then pull down the drywall ceiling and repair a damaged joist later. I know with my luck, someone would drop the leg of a baby grand on one of those joists and I would be buying a new ceiling!
Scott

http://www.dbssinc.com

 

[csd72]

rittz,

personally I think your strap idea should be adequate.

But what I would do is to calculate the stress on the section allowing for a stress concentration of 2 at the holes and then if that is less than allowable then I would accept it. 3/4" holes get put in joists all the time without the engineer knowing about it.

 

[RVSWA]

The strap can be sized to provide the strength necessary, but if you consider the nail slip I think you will find the deflection would be beyond a tolerable limit. The glue and wood option is better, though again the glue one chooses is important. As an example, floor adhesive, while strong is not designed to "dry" or really harden and could exhibit intolerable deflection. Perhaps performing a quick M=fb*sx calc is in order...

 

[bylar]

I'm surprised that nobody sited the code requirements.
The IBC specifies where and how large holes and notches can be in a joist.

 

[rittz]

W did check the code ok But the question is: what is the best way to fix the joists after the holes are already drilled and the wires pulled

 

[jhoulette]

This is far beyond what the code allows. The holes are not in the middle depth, in the middle length and adequately spaced.
I agree with RVSWA. Straps with nails will slip and when I previously mentioned the joists are already poorly designed for the span.
If it was one joist and the span was shorter, I would consider a strap. But this is a series of joists. You will have one angry homeowner and I predict if straps are used, you will be dealing with this later, and paying for the repair.

Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info

 

[pepe1234]

For these types of repairs - What type of glue do you recommend for your OSB/LVL repair, Jim? Are we talking wood glue, construction adhesive, something else?

Thanks

 

[jhoulette]

The repair is a nail and glue joint. Nails providing most of the strength for an easy calculation and the glue acting more as a stiffener. However, the glue being the stiffer element will take the load so you have to be smart about the design.

I typically recommend a structural glue that is epoxy based and has a wood lap shear strength of at least 1,500psi. But I have also allowed typical 'liquid nail' glue that may not meet the needed ASTM requirements for true structural glue. Just depends on the repair needed.

Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info

 

[bdruehl]

what about the effect of the plywood on the top of the joists? similar to composite analysis stud + plywood to withstand the effects of wiind on "high walls"... certainly this might add enough strength so as not to really worry about it? Especially if you can add screws on the plywood to the holey joists? was it a glued and screwed floor?

 

[jhoulette]

Sure you should always take into account composite action of systems when possible. However, for this situation the bottom of the joists was cut/damaged. The bottom of the joist providing tension if assuming a simple single span. The playwood/OSB on top of the joists provides very little to no additional strength when considering tension forces in the bottom of the joists.
Adding plywood or OSB to the bottom of the joists would give you the strength needed if designed correctly (composite action with the needed connection between materials to act together). I've had a builder do this and it really made for a strong floor, you can then think of it as a SIP (structural insulated panel) floor system.

Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info