Capacity of a 2x12 joist with a large hole 1

[keyPitsimplE]

Can anyone provide a good design example to calculate the shear and moment capacity of a 2x12 joist with a 4" hole drilled 2.25" up from the bottom? I have figured the max shear and moment at that location, but I am not sure how to get a correct capacity. I did calculate an Sx = 24.8 in3, based on conservatively allowing a 2.25" tall "flange" top and bottom like an I-beam (so they would be equal and easier to calculate), but I'm not sure that is correct, nor am I seeing how to properly figure a shear capapcity. It is a continuous joist 8' + 12.5' span, with the hole occuring 4' from the center support into the 12.5' span. Thanks.

 

[msquared48]

Moment and shear might be minimal in this case.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[keyPitsimplE]

OK, minimal, but how much? I cannot seem to find a way to calculate it. Calculating Sx with the "I-beam section assumption" must be incorrect because it is 78% of what a solid 2x12 is.

 

[woodman88]

The problem with wood is not only the size of the known hole. The problem is what other deflects are near this hole. A 2x12 (per the WWPA Western Lumber Grading Rules 2011) is allowed the following

Grade / Knots at edge / Knots at centerline / Holes any clause / Per lin ft.
#3 / 5.5” / 6.5” / 3.5” / 1'
#2 / 3.75” / 4.75” / 3” / 2'
#1 / 3” / 3.75” / 1.5” / 3'
SS / 2.25” / 3” / 1.25” / 4'

With a 4” dia. hole you have a piece of lumber that can not be regraded as a structural grade.

Per the attached NOTCHING & BORING GUIDE FOR FLOOR JOISTS & STUD WALLS IN CONVENTIONAL LIGHT-FRAME CONSTRUCTION (older guide that was not up dated to the IBC/IRC) from the WWPA at

http://www2.wwpa.org/.

My attached copy was a free download, now they want $2 to download a NOTCHING AND BORING GUIDE (A-11). So maybe it has been updated to the IBC/IRC.
A 2x12 is allowed a maximum 3.75” hole (minimum of 2” from edge of member) if the joist meet the conventional light-frame construction requirements. Personally I have never agreed with this and have always added a note that no other holes or knots shall be within the above “Per lin. ft.” of the hole.

As your hole is greater than the 3.75” for the conventional light-frame construction requirements or the grading rules sizes. I would replace the 2x12 or add another to it and have them drill an allowable size hole in it if needed. If you decide to “own” the hole. You will need to decide how to calculate it, what CYA notes to use and what personal safety factor to apply.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[nac521]

The reduced Sx is 24.8 in3, using a 2.25" rectangular section top and bottom.
For the shear, I would use the net section.

 

[keyPitsimplE]

nac521, you got the answer I got. I was going to go a step further and take a 50% capacity reduction (or so) to consider stress concentrations. Then i was going to use only the portion above the hole for the shear area. Seems a reasonable way to approach it, especially with my knowledge and experience with older homes. I do agree with Woodman88 that i should also consider any knots/holes in the vicinity of the 4" hole. And THANK YOU Woodman88 for the lengthy and informative consideration. Very much appreciated.

On a final note, I found these,

http://www.metwood.com/products/reinforcer

, which the plumber is choosing to use as a repair. They are $30 each (plus S & H) and come with engineering and an ICC report. They are impressively small with few screws and allow a 6" hole in the center of a 2x10!

 

[msquared48]

Well a couple of things here for the Joist Reinforcer:

1. The ES Report is under the old BOCA Code, not the IBC or IRC. Might be a problem in certain jurisdictions although it has been updated to 2005. Why the IBC was not mentioned at that time is a concedrn.

2. Make sure the reinforcer is installed before the piping is installed.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

Funny, I logged onto the ICC website (

http://www.icc-es.org/reports/index.cfm)

and I could not find the report. Hopefully someone else can find the most current report for this there.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[keyPitsimplE]

Indeed it is not up to date or per IBC/IRC. I spoke with the Vice President, Al Smith 540-537-2348, who said he is waiting for current approval and has been waiting for a long time after spending a lot of money. Seems to be the same story I have heard from multiple people trying to get a current ICC report.

Many of us, especially in the residential world, use older books to create simpler designs from simpler times, that work. It seems to me that it is not unreasonable to assume that the testing, which is described in the ES report, would still apply to real world applications. I guess it is up to the EOR to decide his level of risk tolerance.

 

[woodman88]

Up to date or not may or may not be a concern. But not being able to download the most current report from the ICC website does concern me. I would send a copy of the report from the manufacturer to ICC and ask them if it is the most current one or if they could send you it or tell you where to find it on their website.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.