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Old 02-27-2012, 08:08 PM   #1
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Removal of wall and sistering joists


Good Evening,

We're gutting and renovating a three-storey home. The existing joists are 2x8" (dimensional lumber) 16" OC and spanning 15'9". There is an existing wall on the ground floor running perpendicular to the joists, though it does not run the full length of the house. When we removed the plaster and lathe, it appears that the top of this stud wall is not even in contact with the joists above.

We're like to remove this wall and sister the existing 2x8" joists. Our building code indicates that for new construction, 2x8" joists with cross bracing and drywall ceiling can span 12'. The joists must support a 50 lb live load.
We're wondering if sistering with the additional 2x8" will be sufficient for a 15'9" span (essentially creating a 4 x 8") or if we'll need to do something different.

Thank you!

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Old 02-27-2012, 08:17 PM   #2
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Removal of wall and sistering joists


Going to have to get an on site engineer for this one. Sounds way to long a span to me.
http://www.awc.org/calculators/span/...rcalcstyle.asp
2 X 8's were barly stong enough for what to had, now you want to double it.

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Old 02-28-2012, 11:21 AM   #3
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Removal of wall and sistering joists


Welcome to the forums!

Doubled 2x8s rated at 1500fb will carry 1368# total load or 85# per sq.ft. at 16' span. Check with your local AHJ.

http://www.areforum.org/up/GeneralSt...nstruction.pdf

Gary
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Old 02-28-2012, 06:41 PM   #4
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Removal of wall and sistering joists


Beside bending stresses and shear stresses, the deflection needs to be checked.

Depending on the species of the wood, the deflection of the 15'-9" double 2 x 8 spaced at 16" O.C. supporting a 50 PSF live load, will deflect from 5/8" to 1", which exceeds the span/360 recommended maximum deflection for limiting cracking of the drywall.

With longer span joists, deflection criteria often is the controlling factor rather than the strength factor.
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Old 02-29-2012, 04:16 PM   #5
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Removal of wall and sistering joists


Thanks for the responses. Our thought was that sistering will help with both deflection and load, and we'll use solid blocking every 5-6ft to help with deflection as well.
Our other concern is whether we should try and get dimensional lumber or just use 2x10's and trim them so that the bottoms are flush. Would this be better than notching the 2x10 to fit onto the sill?

Thanks again.
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Old 02-29-2012, 04:26 PM   #6
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Removal of wall and sistering joists


Sistering certainly will help with both deflection and load. However, even after sistering, the L/D ratio is approximately 190, which is below normal code minimum for flooring. If your building inspector allows it, strength should be adequate, but you would need to resist the temptation to put stiffness sensitive flooring such as porcelain tile down. Sistering a 2x10 would give you a better L/D ratio, although of course it will drop the ceiling height by 1-1/2 inches.
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Old 02-29-2012, 04:35 PM   #7
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Removal of wall and sistering joists


We have enough ceiling height that 2x10's would not be a problem. Is it okay if we use a joist hanger or notch them at the sill plate?
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Old 02-29-2012, 05:09 PM   #8
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Removal of wall and sistering joists


Quote:
Originally Posted by mooret View Post
We have enough ceiling height that 2x10's would not be a problem. Is it okay if we use a joist hanger or notch them at the sill plate?
Notching the base of a joist member "essentially" negates that cut dimension...

I would highly suggest a beam at the midspan to accomodate the loading. At those deflection numbers simply walking around upstairs will make everything shake and rattle. It will get old quick and you'll be cursing as to why you didn't just put the beam in while you had the chance.
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Old 02-29-2012, 06:08 PM   #9
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Removal of wall and sistering joists


I disagree with AG about the notching part. If you only notch a few inches on the end to allow for installation on a wall, it has no effect on the moment of inertia of the joist at the center of the span, which is where you need the moment the most. So it is fine in my opinion. Or you could use a joist hanger, which would be my method of choice.
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Old 02-29-2012, 11:26 PM   #10
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Quote:
Originally Posted by House Engineer View Post
Beside bending stresses and shear stresses, the deflection needs to be checked.

Depending on the species of the wood, the deflection of the 15'-9" double 2 x 8 spaced at 16" O.C. supporting a 50 PSF live load, will deflect from 5/8" to 1", which exceeds the span/360 recommended maximum deflection for limiting cracking of the drywall.

With longer span joists, deflection criteria often is the controlling factor rather than the strength factor.
So the l/360 from this book for that fb (species that meet that rating) is not accurate? Page 107: http://www.awc.org/pdf/WSDD/C2C.pdf

How much deflection do you figure is there?

All the older houses Ive worked on had a 2x2 or 2x4 ledger cut-in to the joist, and it's still acceptable (d/4): http://arch.umd.edu/Tech/Structural_..._Guide_A11.pdf

Gary

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