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Old 05-02-2012, 10:34 AM   #1
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using 2x4's for joists


Can I use 2x4,s for joists on 16" centers if the span is 4'1"? The joists will sit on top of the cross beams.

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Old 05-02-2012, 10:54 AM   #2
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Can I use 2x4,s for joists on 16" centers if the span is 4'1"? The joists will sit on top of the cross beams.
The problem you'll have with trying to use 2x4's as floor joists is that most span tables don't include 2x4's. This means that it will be hard for you to convince your planning department to approve your plans without some sort of proof that the 2x4's are adequate.

I've used 2x4's for floor joists before, but I had to prove that they were adequate for the design loads.

I suggest that you try to use a standard size floor joist and then refer to the span tables in your local building code. If using a standard size floor joist creates a problem with your finished floor elevation, look into notching the joist at the supports (read your local building code for notching requirements).

Finally, if you can't get a standard size joist (2x6 or larger) to work, you'll have to find a way to prove to your building department will accept 2x4's. You might try this, but I doubt they'll accept an online calculator as proof.

http://www.awc.org/calculators/span/...rcalcstyle.asp

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Old 05-02-2012, 11:23 AM   #3
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using 2x4's for joists


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Can I use 2x4,s for joists on 16" centers if the span is 4'1"? The joists will sit on top of the cross beams.
I don't know what cross beams are, do you want to use the 2X4s as ceiling joists or floor joists? 2X4s for either in a 4' span will work and carry the load but if in a house they may not pass inspection.
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Old 05-02-2012, 12:27 PM   #4
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using 2x4's for joists


What is your Code's stipulation for domestic floor loading per sq.ft.?
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Old 05-02-2012, 03:57 PM   #5
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using 2x4's for joists


It should be OK, but I'd go closer than 16" OC. 2x4s are cheap.

another option might be to put plywood below the joist as well as above. This would make the plywood the stressed member and the 2x4s would be acting more like the webbing of an I beam.
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Old 05-02-2012, 04:34 PM   #6
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using 2x4's for joists


you don't use 2x4's for floor joists.
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Old 05-02-2012, 04:39 PM   #7
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When you add floor joists you are increasing the deflection and "bounce" that is not reflected in the typical tables that only address strength and not comfort or livability. It will deflect more than you expect since you are adding the deflections of each layer.

Not specifically covered by the minimal codes, but sounds like a mistake waiting to be discovered.

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Old 05-02-2012, 04:44 PM   #8
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Quote:
Originally Posted by tyrral
Can I use 2x4,s for joists on 16" centers if the span is 4'1"? The joists will sit on top of the cross beams.
The answer.is no. Anyone here who tells you that.you can has no idea what they are talking about and giving you useless and dangerous information.

All you have to do is call your building department and ask them.
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Old 05-02-2012, 05:24 PM   #9
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On a 50" span, assuming live load 40 and dead load 10 psf, 2x4s would be ok @ 16.25"c/s.
The bending stress would be below 350psi, which is very low. Deflection, shear and bearing would all be well within normally accepted limits - see attached figs. for stress.
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Old 05-02-2012, 05:34 PM   #10
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Not specifically covered by the minimal codes, but sounds like a mistake waiting to be discovered.

Dick

Jeez, it was just a question, you don't have to call him a dick about it.
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Old 05-02-2012, 05:43 PM   #11
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The responder (Dick) was a registered engineer with over 30 years experience and is aware of the practicality of designing a suitable structure and just not being a nit-picker and being precise with calculations based on assumptions and minimums.

Dick
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Old 05-02-2012, 06:03 PM   #12
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To Tony.g, your computations are incorrect. Specifically, in the last line where you compute the bending stress, you use the formula Stress = M/I, which is incorrect. The correct formula is Stress = My/I, where M is the moment, y is the distance from the centroid of the beam to the edge, and I is the moment of inertia. If you want to get rid of the y, you can use Stress = M/S, where S is the section modulus, however that is not what you did.

The correct answer for maximum stress should be approximately 570 psi. Still allowable for most any species of lumber, but substantially higher than what you presented.
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Old 05-02-2012, 06:09 PM   #13
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Check that, I see you used the correct formula, however a 2x4 is not 2 inches wide and 4 inches deep, it is typically 1.5 inches wide and 3.5 inches deep, so the correct maximum stress is about 570 psi as I noted, however it appears the error was due to incorrect dimensions of the joist. Unless you think the OPS has access to full size lumber, pretty unlikely.
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Old 05-02-2012, 06:28 PM   #14
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using 2x4's for joists


for ceiling joists using 2x4 http://awc.org/pdf/STJR_2012.pdf

see page 11 and 12
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Old 05-03-2012, 05:01 AM   #15
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To Tony.g, your computations are incorrect. Specifically, in the last line where you compute the bending stress, you use the formula Stress = M/I, which is incorrect.
Hi Daniel,
I think my writing may have been at fault as what you referred to as 'I' was in fact 'Z', though I think you spotted that later.
As a matter of interest, something I came across quite recently in steel construction is that in the US, you use 'S' for elastic mod. and 'Z' for plastic mod. Confusingly, here and in Aus, its the other way round!
On your second post, you noted the timber section-size as being 1/2" less in width and depth. Again as a matter of interest, is it really that much? Timber sizes here are typically between 3/16" and 1/4" less than stated size, but they let us do calcs based on the stock size(!)
If your finished size is 1/2" less, I agree it would have an effect on the stress, and probably deflection as well.

cheers.

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