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Old 07-03-2010, 06:44 PM   #1
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Offset bearing wall


How do I calculate the permissible offset for a bearing wall? The wall I'm moving is currently offset about 18" from the main girder in the basement.


Last edited by benjamincall; 07-03-2010 at 06:49 PM.
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Old 07-03-2010, 07:18 PM   #2
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Offset bearing wall


Within the depth of the joist: http://books.google.com/books?id=iwS...joists&f=false

Page #32, Fig. 5

Be safe, Gary

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Old 07-03-2010, 09:43 PM   #3
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Offset bearing wall


Figuring out the general case for
a distributed load plus a concentrated load [your wall] on a joist
shouldn't be too difficult, but you should find out your joist material and span plus the vertical force that the wall has to withstand.
The midspan joist deflection will be more due to this extra load but the floor stiffness feeling may increase and the max PSF will decrease.
If your design stiffness for the floor joists is L/360 at a certain PSF, then the joists would have to be reinforced somewhat to maintain the same stiffness at the same PSF.

Last edited by Yoyizit; 07-03-2010 at 09:46 PM.
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Old 07-03-2010, 11:44 PM   #4
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Offset bearing wall


Quote:
Originally Posted by GBR in WA View Post
Within the depth of the joist: http://books.google.com/books?id=iwS...joists&f=false

Page #32, Fig. 5

Be safe, Gary
Wow, only eight inches based on that guidline. If I went to say 24" and transferred some of the load to a wall in the basement, would I have to consider cutting the basement floor for a new footing, or would that load be shared sufficiently between the new wall and the main girder?

Quote:
Originally Posted by Yoyizit View Post
Figuring out the general case for
a distributed load plus a concentrated load [your wall] on a joist
shouldn't be too difficult, but you should find out your joist material and span plus the vertical force that the wall has to withstand.
The midspan joist deflection will be more due to this extra load but the floor stiffness feeling may increase and the max PSF will decrease.
If your design stiffness for the floor joists is L/360 at a certain PSF, then the joists would have to be reinforced somewhat to maintain the same stiffness at the same PSF.
I can't identify the lumber, but I have 2x8s 16" on center with an unsupported span of 10'6". 30lb snow load, 5/12 roof pitch, sheathing over the ceiling joists for attic storage, and an exterior house width of 22'. Now, I'm not really sure where to start with those figures.
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Old 07-04-2010, 11:30 AM   #5
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Offset bearing wall


Quote:
Originally Posted by benjamincall View Post
Wow, only eight inches based on that guidline. If I went to say 24" and transferred some of the load to a wall in the basement, would I have to consider cutting the basement floor for a new footing, or would that load be shared sufficiently between the new wall and the main girder?



I can't identify the lumber, but I have 2x8s 16" on center with an unsupported span of 10'6". 30lb snow load, 5/12 roof pitch, sheathing over the ceiling joists for attic storage, and an exterior house width of 22'. Now, I'm not really sure where to start with those figures.
http://www.engineersedge.com/beam-deflection-menu.htm
You need to reduce the geometry and forces on your joists to something that can be handled by existing formulas. For the dead load figure 35#/cu. ft for the wood density.
The biggest problem I can see is figuring out or measuring the vertical downward force that that wall needs to resist. If you post a sideview diagram I guess we can step through the repetitive calculations for calculating this force.
Or, you could measure current joist midspan deflections, put your wall in place, and then reinforce the joists until you get the same deflection.
Without knowing the wood type we can assume an E, the modulus of elasticity, of between 1 and 2 million PSI so we'll go with the low value for safety.
I guess I should pull my spreadsheets for this calc. out of the recycle bin.

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Old 07-04-2010, 06:20 PM   #6
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Offset bearing wall


Quote:
Originally Posted by benjamincall View Post
Wow, only eight inches based on that guidline. If I went to say 24" and transferred some of the load to a wall in the basement, would I have to consider cutting the basement floor for a new footing, or would that load be shared sufficiently between the new wall and the main girder?



I can't identify the lumber, but I have 2x8s 16" on center with an unsupported span of 10'6". 30lb snow load, 5/12 roof pitch, sheathing over the ceiling joists for attic storage, and an exterior house width of 22'. Now, I'm not really sure where to start with those figures.
That link Gary posted is not true for NJ. You can offset a load bearing wall more than the depth of the joist. Ive been framing in NJ for 27 years and have offset walls more than the depth of the joist many times. Every job I do is drawn by an Architect and approved by the town to frame this way.

Cantilevers also have been 2' on every single house and addition I've framed with dimensional lumber. In your case you should have it looked at since it's not new framing and a cantilever.
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Old 07-04-2010, 08:40 PM   #7
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Quote:
Originally Posted by Joe Carola View Post
is drawn by an Architect
And he or his software may have used this method to calc. stresses.
http://www.codecogs.com/reference/en...lay_method.php
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Old 07-04-2010, 08:49 PM   #8
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Offset bearing wall


"Wow, only eight inches based on that guidline. If I went to say 24" and transferred some of the load to a wall in the basement, would I have to consider cutting the basement floor for a new footing, or would that load be shared sufficiently between the new wall and the main girder?"--------------------- You should really consult a Structural Engineer to offset more than the UBC or IRC standard minimum Code that I referenced before. Ask your local Building Inspector first-- before spending money possibly unnecessarily. A new footing below a new wall would be needed for the loads. OR, possibly new floor joists (or other strengthening) after figuring the loads from a professional with a degree in that field.

Be safe, Gary
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Old 07-05-2010, 05:10 PM   #9
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Old 07-05-2010, 05:24 PM   #10
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Offset bearing wall


Now that I think about it, the portion of the bearing wall I'm removing supports only two ceiling joists.

A section of bearing wall that runs most of the length of the house is offset almost 28 inches from the girder, and another portion (approximately 9 feet) of bearing wall is offset about 6 inches from the girder.

I want to remove a 3-foot section of the 9-foot wall that currently supports two ceiling joists. The plan is to extend the portion of the bearing wall that is offset 28 inches to support these two joists.

Last edited by benjamincall; 07-05-2010 at 06:59 PM.
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Old 07-05-2010, 08:29 PM   #11
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Offset bearing wall


Quote:
Originally Posted by benjamincall View Post
Now that I think about it, the portion of the bearing wall I'm removing supports only two ceiling joists.
Which should limit how much max weight is on this new wall.

This could possibly be calculated by knowing the design PSF for the attic floor,
or
you could measure the weight using some spare lumber, a vertical 4x6, a known weight [yourself] and a lever. An assistant would look for the point where the ceiling below just these two joists is lifted ever so slightly.

Then the reinforcement necessary for the two floor joists could be calculated and tested, all this before a single nail is pulled.

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Old 07-05-2010, 10:29 PM   #12
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Offset bearing wall


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Originally Posted by Yoyizit View Post
. . . you could measure the weight using some spare lumber, a vertical 4x6, a known weight [yourself] and a lever. An assistant would look for the point where the ceiling below just these two joists is lifted ever so slightly.

Then the reinforcement necessary for the two floor joists could be calculated and tested, all this before a single nail is pulled.
Sounds like a job for McGyver. I'm having a hard time visualizing the mechanics of this method. How do I apply the known weight?
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Old 07-06-2010, 07:40 AM   #13
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Offset bearing wall


I'm pretty sure if I tripled each joist I wouldn't experience significant deflection.

In the past, I've taken a diagonal notch out of the end of a sister joist, reducing the overall height, then I pound the wedge back in. I seldom seem to get the new joists jacked up the level of the existing joists, though. Problem?
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Old 07-06-2010, 02:14 PM   #14
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Quote:
Originally Posted by benjamincall View Post
Sounds like a job for McGyver. I'm having a hard time visualizing the mechanics of this method. How do I apply the known weight?
"The story arc of MacGyver follows the intelligent, optimistic, laid-back, resourceful secret agent Angus MacGyver, played by Richard Dean Anderson. "

He's not returning my calls. I think he is still mad because I stole one of his women*.

You step on one end of the lever, with a scale underfoot or not, then use the formulas.
http://www.engineeringtoolbox.com/levers-d_1304.html
The other end of the lever applies upward force to the attic joists.
For more precision with more calculating effort we can take into account the weight of the scale and lumber used in the lever system.


But,
let's say you want the affected floor joists to be deflected no more than 10% of what they already are.

You make a false wall out of a lally column and 2 ea. 36" wide 4x6s.
Put the false wall in place and adjust its height so that it is barely carrying weight. Measure the existing floor joist deflection.

Crank up the lally column height by, dunno', 1/8" or 1/4". At this point, depending on the stiffness of your house structural lumber the false wall is now carrying virtually all of the weight that the old wall carried. Do not extend the false wall height beyond this point. Measure the new floor joist deflection.

Reinforce the floor joists if necessary and repeat the wall experiment.

Doubling or tripling the thickness of a floor joist doubles or triples its stiffness. Doubling the height of a floor joist multiplies its stiffness by 8x.



*


Last edited by Yoyizit; 07-06-2010 at 02:35 PM.
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