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Old 06-12-2010, 01:27 PM   #1
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Strengthening Joists Before Basement Finish


So I'm going through my basement finishing project.

A little background information: the house is about 20 months old, I've lived in it for 10 months. The joists are 2 x 10s, 16 o.c. and span 15 feet. I believe the lumber is pine but can't say for sure.

I've done all the framing, plumbing and electrical rough in as well as insulation. Prior to my finishing the basement at least one small hole, like a half inch hole, was drilled in many of the joists for electrical wiring by an electrician I had hired to get me some light in the basement before I moved into the house. A few, like 3 or so, have 2 holes in them. The holes are generally about 1-2 inches above the bottom of the joists. I know the drilling should have been done at least 2 inches above the bottom of the joists but I can't change that now.

SO, I've been reading on joist strengthening. My floors are fine, no sagging, no bouncy floors, etc. I just don't want to cover that ceiling only to see some bouncy floors in 5,10 years or so. My main concerns are (1) the span of the joists is long, probably just within engineering specs for the minimum floor load required by code (2) the holes drilled in the joists have been somewhat concerned. I was looking at using joist bridging with wood or blocking to tie the joists together so they can share their loads and reduce deflection.

I just thought I'd get some opinions, am I worrying too much or not? Thanks.

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Old 06-12-2010, 01:35 PM   #2
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Strengthening Joists Before Basement Finish


You could measure the deflection [sag] in fractions of an inch at midspan before and after putting a concentrated known load [two or three people] at midspan.
From this the E value of your existing joists can be estimated and the whole thing reverse engineered, within some error.

Blocking prevents the joists from twisting and so you get the calc. value of stiffness. If a joist twists the formulas do not apply.

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Old 06-12-2010, 01:54 PM   #3
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Strengthening Joists Before Basement Finish


Millions and millions of homes have been built in N.America and have had holes drilled for electrical cables.
I doubt that there ever was one that had a floor collapse that was attributed to a hole being drilled in them.
If you wish to have a worry, think about the oil spill in the Caribbean.
Now, there's a problem!
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Old 06-12-2010, 02:23 PM   #4
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Strengthening Joists Before Basement Finish


There's really nothing you need to do.
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Old 06-12-2010, 03:30 PM   #5
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Strengthening Joists Before Basement Finish


If lower than 2", I would be a little concerned. Perhaps our resident engineer would comment on your using a truss mend plate or strapping on that joist as brought out for top plates here: http://www.ladbs.org/faq/info%20bull...20notching.pdf

Blocking or bridging would help distribute any load: http://books.google.com/books?id=DWs...olumns&f=false

Be safe, Gary








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Old 06-12-2010, 08:35 PM   #6
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Strengthening Joists Before Basement Finish


Quote:
Originally Posted by GBR in WA View Post
If lower than 2", I would be a little concerned. Perhaps our resident engineer would comment on your using a truss mend plate or strapping on that joist as brought out for top plates here: http://www.ladbs.org/faq/info%20bull...20notching.pdf

Blocking or bridging would help distribute any load: http://books.google.com/books?id=DWs...olumns&f=false

Be safe, Gary
The electrical code where I live requires that holes that are drilled for electrical cables, must be a minimum of 1 1/2 inches from edge of the wood.
If closer, protection against nail penetration, in the form of a metal plate must be used.
Its likely that a similar rule exists in most jurisdictions.
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Old 06-12-2010, 10:32 PM   #7
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Strengthening Joists Before Basement Finish


Yes, that rule is in effect here, also. I was referring to the strapping as a possible fix for the joist with a hole in the bottom 2" (tension force)- which could lose it's 2x10 rating and be way over-spanned for it's new 2x8 rating. Especially if in the middle 1/3 of span. The fix was shown on a plate, not a joist.

Be safe, Gary
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Old 06-12-2010, 11:12 PM   #8
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Strengthening Joists Before Basement Finish


Quote:
Originally Posted by GBR in WA View Post
If lower than 2", I would be a little concerned. Perhaps our resident engineer would comment on your using a truss mend plate or strapping on that joist as brought out for top plates here: http://www.ladbs.org/faq/info%20bull...20notching.pdf

Blocking or bridging would help distribute any load: http://books.google.com/books?id=DWs...olumns&f=false


Yes I am going off the exact same boring and notching standards and that was what raised my concern that some of my holes are within the no-touch 2" area of the joist. That's what had me wondering about using blocking to help distribute any load. I would appreciate more input on how to remedy this situation, if it is necessary.

Quote:
Originally Posted by Yoyizit View Post
You could measure the deflection [sag] in fractions of an inch at midspan before and after putting a concentrated known load [two or three people] at midspan.
From this the E value of your existing joists can be estimated and the whole thing reverse engineered, within some error.

Blocking prevents the joists from twisting and so you get the calc. value of stiffness. If a joist twists the formulas do not apply.
I like this idea. I can even compare the deflection on one joist that was drilled with another that wasn't. Should be interesting.
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Old 06-13-2010, 10:29 AM   #9
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Strengthening Joists Before Basement Finish


In theory, they drilled the holes in the wrong place, and in fact they violated code. That said, I would leave it be if it only affects a few joists. Blocking will not stiffen individuals joists, as noted it simply reduces twisting and effectively eliminates the potential for lateral buckling, which never happens in residential applications anyway.

If the floors feel OK, have another martini and don't get too concerned. There should be blocking or X bracing between the joists, if you are saying there is none now, well then I would add it, but if you are saying you want to add more, I would not bother. I can't tell you how many houses I have looked at with electrical and plumbing holes in the wrong place, as previously noted, none of them have fallen down.
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Old 06-13-2010, 10:48 AM   #10
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Strengthening Joists Before Basement Finish


I know nothing will fall down, I'm just concerned with bouncy spots on the floor in the future as it has been used and abused. There's no bracing or blocking there right now at all. As I recall it is only required when the depth of the joists exceed 12". None of the houses built here by this builder has bracing or blocking in the engineering plans (I have the full blueprint for my house).
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Old 06-13-2010, 11:17 AM   #11
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Strengthening Joists Before Basement Finish


Quote:
Originally Posted by braverichard View Post
I know nothing will fall down, I'm just concerned with bouncy spots on the floor in the future as it has been used and abused. There's no bracing or blocking there right now at all. As I recall it is only required when the depth of the joists exceed 12". None of the houses built here by this builder has bracing or blocking in the engineering plans (I have the full blueprint for my house).
If you have no bracing, I would heartily recommend it! Sounds to me like the builder was building to minimum code.
Even if the electrician drilled in the wrong place, bracing would help to negate this damage by transferring the load over the weakened joist to the adjacent ones.
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Old 06-13-2010, 11:20 AM   #12
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Strengthening Joists Before Basement Finish


Yeah nothing structurally is above code. In fact, that 15 foot span seems to be just right for 40psf live load and 10psf dead load. Not even 1% above code.
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Old 06-13-2010, 12:02 PM   #13
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Strengthening Joists Before Basement Finish


Pretty sure that blocking/bracing would be required to meet code here on a 15 foot span.
At least 1 set, possibly 2 lines of blocking.
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Old 06-13-2010, 12:28 PM   #14
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Originally Posted by jlhaslip View Post
Pretty sure that blocking/bracing would be required to meet code here on a 15 foot span.
At least 1 set, possibly 2 lines of blocking.
And it does the most good at midspan.

The next time you hold a party see if any joists twist at midspan, using a level vertically or a plumb line.

Last edited by Yoyizit; 06-13-2010 at 12:36 PM.
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Old 06-13-2010, 02:08 PM   #15
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Strengthening Joists Before Basement Finish


The bottom of page #718 and top of 719: http://books.google.com/books?id=DWs...olumns&f=false

Be safe, Gary

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