3/4" holes in Floor Joists 2

[Jim5000]

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!

 

[msquared48]

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

 

[woodman88]

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.

 

[MiketheEngineer]

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

Handle appropriately

 

[Jim5000]

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.

 

[MiketheEngineer]

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!!

 

[Jim5000]

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?

 

[woodman88]

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.

 

[MiketheEngineer]

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!!

 

[Jim5000]

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

 

[woodman88]

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.

 

[jdgengineer]

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...

 

[Jim5000]

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.

 

[MiketheEngineer]

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!!

 

[jdgengineer]

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...

 

[MiketheEngineer]

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!!

 

[woodman88]

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.

 

[jdgengineer]

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.

 

[MiketheEngineer]

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.

 

[shobroco]

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.