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3/4" holes in Floor Joists
2

3/4" holes in Floor Joists

3/4" holes in Floor Joists

(OP)
Hello, my floor joists have one 3/4" holes in 10 of the floor joists (in a row, with PEX tubing in them).  The problem is that they are located only 1" from the bottom of the joists! the Joists are 2x8 wood on 16" cetres.  They span 12 feet.

Do I have a structural issue? If so, how do I repair the problem?

thanks in advance for your advice/discussion!

RE: 3/4" holes in Floor Joists

Don't know the grade for your joists, but you are very near the maximum span for a HF#2 2X8 floor joist.  I would install metal straps, 3 foot long and 1.25" wide, across the underside of the joists just to be safe.

If these were 2X10's there would be no problem.  Normally, the holes should be in the center of the joist depth for no structural issues.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
 

RE: 3/4" holes in Floor Joists

You are close to the limit for a 40psf live/10psf dead loading. See the attached web site.

The 1" is closer than the 2" minimum edge distance recommended for all 2x joists by the WWPA (Western Wood Product Association) Notching and Boring Guide.

Now a metal strap will not transfer any load until the joist fails. Unless you can install the metal strap while it is under tension. But they may keep the floor from total collapse due to domino effect.

But whether your joists are structurally sound depends on too many factors to address here. (How far the hole is from the bearings. How close are knots and other holes to it. The grade and species of the joist. The stress in the joist at the hole. The location of the joists per the framing plan. Etc.)

Now there are many safety factors in wood design. So you may never have a problem show up because of the holes. But I cannot tell from the information given. I will say that the closer the holes are to the bearing the better.

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.

RE: 3/4" holes in Floor Joists

Your 2x8's are now 2x6's!!

Handle appropriately

RE: 3/4" holes in Floor Joists

(OP)
I appreciate all the chatter! here is an update.... the joists span only 11 feet (not 12).  My house has a 1.5 foot thick piled stone foundation.  There is a supporting beam in the middle (4 2x8's thick, spanning the length of the house- 30 feet long- douglas fir).  On each side of that are the joists (that span 11 feet with 16 inch centers). The joists are Douglas Fir (I believe, my house was built in 1930) above the joists is one inch thick tongue and groove (Douglas Fir I believe)in a diagonal pattern and above that is the original hard wood floor.  It is a 1.5 story brick home.

My friends think I'm crazy for worrying about the structural integrity of the house... but I am worried!  

thanks again.
   

RE: 3/4" holes in Floor Joists

Have these holes been there since 1930??  If so, and you not planning a massive orgy - find something else to worry about ... like climate warming or oil leaks or dolphins or something!!

RE: 3/4" holes in Floor Joists

(OP)
No, they are new holes... I can't believe you gueseed it, I was planning an orgy party and I was only going to invite people that are in the 200 pound + range, so I need to know if I have anyting to worry about.

From what I can tell, and correct me if I'm wrong.  The joists have now become 2x6's, which according to code, Douglas Fir 2x6 joists can span 11 feet? so everything is fine and I'm crazy and should redirect my energy?

RE: 3/4" holes in Floor Joists

The joists to be considered 2x6's (in just the area of the holes) would have to be regraded per the code. This is because the 2x8's are allowed bigger holes and other lesser grading requirements than a 2x6 does. So as a 2x6 the grade may drop.
Again you may or may not have a problem. Depending on many factors. Do you know the company that drilled the holes? If you do I have them fix it or provide an engineers calculation on it. If you do not know. Than I would recommend that you buy ten 2x6x10' Sel Struc or No.1 & Btr DF-L to nail to the joists centered as best you can over the holes. It should be cheaper than hiring your own engineer to look at it.

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.

RE: 3/4" holes in Floor Joists

According to my calcs DF #1 2x6 is about 35% over stressed and "over" deflected by about the same amount.

This assumes a 40 psf Live Load and 15 psf Dead.  Ratio it if you want.

So - the steel strap idea is not a bad one!!

RE: 3/4" holes in Floor Joists

(OP)
I agree Mike, the steal strap idea is a good plan 'just in case'. 1.25"x3'.... how thick should the straps be?

RE: 3/4" holes in Floor Joists

MiketheEngineer - In your calculation were you using 16" oc spacing or 8" oc accounting for the existing 2x8?

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.

RE: 3/4" holes in Floor Joists

In my opinion the strap is engaged after the joists have failed (unless you are going to jack up the joists prior to installation).  Therefore the strap really isn't going to do a whole lot for you, except in the event of a catastrophic failure.  If that's the purpose (and understand you'll likely have significant damage to the ceiling gyp and flooring at this point), I'd size the strap to take the full tension component of the moment at this point.  I'd calculate the maximum moment where the strap will be applied and divide by the joist depth (lazy way to avoid finding compression block).  I'd then size the strap for this force.

I don't think the deflection being over the allowable is a concern.  You have a "2x6" only locally at the hole, so for deflection purposes the beam is really a 2x8.

Personally, if you are concerned, I'd stitch nail another joist as recommended, the strap in my opinion is just a bandaid...

RE: 3/4" holes in Floor Joists

(OP)
Thanks jdg.... so in your opinion, my structure is fine apart from a few localized areas (where the holes exists). But the localized areas are supported by 2x8's so the odds of having a problem is close to nothing (because deflection is not the problem).

I realize this is your opinion based only on the information given.... I appriciate the input.  

RE: 3/4" holes in Floor Joists

16'' oc and Fbrep.  If installing any extra support - like straps or sisters - a little upward shoring would be most helpful - as noted.  Just don't push hard enough to break anything above!!

RE: 3/4" holes in Floor Joists

I haven't run any calculations nor have I looked at your specific spans/loading. I was merely suggesting that if you are concerned I don't really like the strap unless you shore the beam. In my experience it usually takes a fair amount of movement before the strap engages.

If Mikes calculations are correct and your beam is 30% overstressed I say this puts you in a grey area. As an engineer, yes I would recommend you repair it and if it was your plumber who made the mistake it should be on his dime. Now with that said, we've worked on remodels of a lot of houses where the existing floor joists or rafters should be significantly overstressed but don't fail. Now if this is because they never reach their design loading or if there is significant safety factors implied in our design I'm not sure. From what I remember from school I believe the wood allowable stresses are based on the average minus a standard deviation or two so there is a good chance your wood is significantly stronger than assumed. Just my two cents.

In the end if it's uncovered. Seems like it woul be fairly easy to stitch nail another joist on...

RE: 3/4" holes in Floor Joists

Typically, wood joists have a safety factor of around two - but it is not called that.  I think it is based on a standard deviation algorithm.  Just done't remember exactly.

Regardless - at a 40 psf live loading - a 20'x 20' room could support 16,000 lbs of load.  Assume the average person weighs about 175 lbs (including women and children - not just us fat boys), you are looking at 90 people.  Try putting 90 people in this space and you would have a riot on your hands!!

So, part of the answer is it will probably NEVER reach design load!!

RE: 3/4" holes in Floor Joists

MiketheEngineer - NO NO NO!!!! It is not the room but each joist you have to look at. The possibility of one or more joists receiving the loading is the concern. Such as from a water-bed or exercise equipment or entertain center or sofa with six people on it, etc. Then you have someone moving the entertain center into the bedroom with the water-bed and across the same joists. Or the six people on the sofa and having a large keg or two being moved to the deck across the same joists. Now at 16" oc the 40psf live load is 53 plf times the span of 11' you get a total load of 583# live load on each joist. I can see many situations where this might happen to one or more of the joists, depending on their locations.

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.

RE: 3/4" holes in Floor Joists

For the purposes of discussion (not that I am recommending using higher allowable stresses by any means), it appears that the NDS allowable values are based on the 5% Exclusion Values.  Therefore, when they give an allowable stress (with exception of F'c perp) the value they quote should be statistically calibrated so that 95% of members in that species grade should fail at or above this value.

So for the purposes of determining the allowable stresses in the NDS they take the average of a group of species and then subtract out 2 standard deviations.  According to Breyer's Book the purpose of this is to provide an AVERAGE safety factor of about 2.5.

How's this relevant to the discussion? Probably not at all, just perked my interest so I decided to look into it, and this was the thread that started it.  For the OP's question -- I wouldn't consider any of this if it was my engineering license on the line.  If it was my house... well my answer may/may not be different.

RE: 3/4" holes in Floor Joists

Woodman -

I do tend to agree but with 3/4'' plywood and 16'' oc you are going to get a lot of load sharing unless you are putting all your load through the tip of a crow bar.

Everything ALWAYS needs to be checked.

RE: 3/4" holes in Floor Joists

Woodman, I'm with MiketheEng on this.  I've seen way too many situations where someone condemns an existing structure that can't meet a new use under a current code.  Be realistic about loading.  I have a favourite little scenario to demonstrate the ludicrous requirement here in Ontario that an existing building being changed to assembly occupancy must carry 100psf.  The number of historic houses converted to restaurants is legion, yet they pretty well always fail the test.  100 psf is 1600# on a 4'x4' square.  Assume your table is 2x2, & weighs 200#.  You have 4 chairs at 50# each & 4 guys sitting in them at 200# each. The waiter standing there is another 200#, which only leaves you 200# for the beer pitcher & 4 glasses.  Just remember, those 4 guys & the waiter are in a 12" wide rectangle around the table.  Now you might think this is still possible, but do it in every 4'X4' square around the first one.  Physically impossible.  Period.

RE: 3/4" holes in Floor Joists

shobroco - In the US and Canada the building codes are the people, by way of their elected officials, way of setting the standard of safety that they are willing to accept for structures to be built and maintained by. As engineers we are obligated to follow these standards, whether we agree with them or not. If you feel your argument to be valid then you should present it to the code officials to change the code.

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.

RE: 3/4" holes in Floor Joists

I have presented it as a proposed change, more than once.  The last time, I was told that the introduction of performace standards in addition to prescriptive standards would deal with it, however it depends how you read them.  The real problem is that codes are always written broad brush, to cover the worst case, and usually do not permit qualified individuals to use professional judgement.  An exemption in our code actually demonstrates the validity of my argument: conversion of an existing building into a restaurant is permitted to use a live load of 2.4 kPa (50psf) if the building is under X sq.m. (I forget the exact number)  Obviously if you have such an exemption, you have to have a cut-off point, but what possible scenario would double the floor loading  in a building 1 sq.m. larger than another of exactly the same use?   

RE: 3/4" holes in Floor Joists

Of course the straps will work and they will work long before failure occurs.  The design accommodates 40psf....which is not there yet.  So unless you load those joists to 40 psf +DL and then put the straps in place, you won't have to wait for failure for them to work!

Put the straps in place like Mike McCann says...then have your orgy.

RE: 3/4" holes in Floor Joists

Ron - For the straps to work. The nails have to achieve the required critical slip on each side of the hole. Are you saying that the bottom edge of the joist will elongate enough to meet the critical slip of the nails before failure?

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.

RE: 3/4" holes in Floor Joists

Look at it this way. Given the number of times that plumbers and electricians have drilled holes in the bottom of joists. If the metal strap worked then there should be tons of information about it on the Internet and structural magazines. If you can find any information please share it with us.

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.

RE: 3/4" holes in Floor Joists

Actually, I would think that there is no literature on straps being used on the bottom of joists because no one cares about joists.  It's pretty common to have holes drilled through them without any consideration from plumbers, etc. and without the engineers ever seeing the modifications. And I haven't read anything on joist failures due to holes because of plumbers.

I think Ron might have a point about the extra 40psf live actually getting the strap to engage though.  I think a quick check would be able to see the validity of that assumption.

See what the 40psf live load will cause in deflection, if the joists are long enough the elongation along the bottom of the joist might be enough to engage the nails and strap.

Although, off the to top of my head I am not sure what a 10d nail will slip in a condition like this... So maybe this check would prove the strap useless.

 

RE: 3/4" holes in Floor Joists

I do CARE about joists - have had too many massacred too many times by electricians and more massacred by plumbers with their 2'' pipes!!

Had a million dollar plus home with rich Italian marble on the floor that was highly mis-treated by an electrician.  I read him the the riot act and made them fix the joists.  NOBODY is going to crack my marble!!  And it hasn't in 15 years!!

RE: 3/4" holes in Floor Joists

woodman88...while I agree with your excellent advice most of the time, in this case I'll have to disagree.  Repair and strengthening straps can be used in tension applications such as this or on the sides of top or bottom plates.

Most nailings of straps that I've seen don't hit the center of the hole.  Further, the holes are sized to use either 10d or 16d nails.  If 16d nails are used, there's little slip required to mobilize the shear in the nails and then the tension in the strap.

Strains between steel and wood are so incompatible that I feel mobilization would occur long before it could be predicted by analysis.

RE: 3/4" holes in Floor Joists

Ron - If you have $15 you can get a pdf of the NTIS report ADA013987 "Evaluation of Expedient Techniques for Strengthening Floor Joist Systems in Residential Dwellings." In the attached link it gives some information about the results. It is from 1975 but I do not think that the lumber, straps and nails have changed enough to invalidate the results.

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.

RE: 3/4" holes in Floor Joists

Garth...I tried.  Not available.  Synopsis I found seems to support the use of straps, although they were the full length of the span.

RE: 3/4" holes in Floor Joists

I'll try another route to get that paper.  I agree it might be useful, even if a bit older.

RE: 3/4" holes in Floor Joists

Ron - Have you tried this link?
http://www.ntis.gov/
If you type in "ADA013987" it comes up.

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.

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