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Re: stacked floor joists

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They should "share" the load, but they should not work compositely
together (i.e. act as nominally the equivalent of a 2x8) unless there is
some ability to transfer the shear.  If there is no shear transfer (at
enough to transfer enough shear for composite action), then they should
share the load, similar to if they were placed side-by-side.  This is
independent of the type of load.  The only way the "type" (or amount) of
load would really change things is if one "type" (or amount) of load is
small enough that just friction shear (or maybe some minimal nailing of
some type) was enough to generate enough friction to allow composite
action.

If you want to check how much shear would needed, then get out the good
old calculation (or spreadsheet) and use shear stress = V*Q/(I*b) and see
if your shear stress exceeds what you believe that you can develop at the
interface between the two 2xs.

Regards,

Scott
Adrian, MI

On Fri, 23 Mar 2007, IRV FRUCHTMAN wrote:

> Some house details: the house is about 100 years old,
> the floor does not feel bouncy when I jump on it, I
> measure a deflection of about .8" @ span center and
> the flooring looks like 1x3 pine.
>
> Thanks to those who responded. Apparently all agree
> that stacked members should not be considered as
> combined. While this is clear in my head for lateral &
> axial loads, but why so for gravity loads? If the
> joists are in contact along their length their
> deflections are equal, by definition. Assuming equal
> size & material properties, shouldn't they experience
> identical moment and shear?
>
> In any event I'll make sure shear is transferred.
>
> Irv
>
>
>
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