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Re: stacked floor joists

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Another way to look at it is by comparing the section modulus and moment of inertia for 2 identical beams side by side or stacked vertically with no connection between the two with a beam of the same width but with doubled depth.

For 2 beams side by side:

S = 2 beams * 1/6 * b *d^2  = 1/3 bd^2
I = 2 beams * 1/12 * b *d^3 = 1/6 bd^3

For a beam with doubled depth:

S = 1/6 * b * (2d)^2 = 2/3 bd^2 or twice of the parallel beams
I= 1/12 *b * (2d)^3 = 2/3 bd^3 or 4 times of the parallel beams


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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