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-   -   How to calculate Joist span table for 3x8's (http://www.diychatroom.com/f19/how-calculate-joist-span-table-3x8s-67036/)

WillSTX 03-17-2010 03:30 PM

How to calculate Joist span table for 3x8's
 
I want to put a loft in a barn. The span is 16 foot. I have a bunch of 16 foot 3x8's I also have a bunch of Simpson HU38's to hang them.

The problem: All span tables I've seen are for 2 bys.

Since a 3x8 is 50% bigger than a 2x8 can I multiply the maximum length a 2x8 can span by 1.5 to get the manimum length a 3x8 can span?

If so; I can span the 16 feet using the 3x8's on a 12 inch center.

This sound like a good idea?

kwikfishron 03-17-2010 05:13 PM

What are you going to be storing in that loft?

Daniel Holzman 03-17-2010 05:32 PM

No, you cannot multiply the length by 1.5. The allowable span for a beam is governed by the moment of inertia of the beam (depends on the size of the beam), the maximum allowable fiber bending stress (depends on the type of wood and the grade), the modulus of elasticity of the material (depends on the species of wood), and the loading (dead plus live load, often governed by code).

Maximum span is governed by the more restrictive of the deflection of the beam at the center, or the bending strength of the beam. Deflection is related to the fourth power of the span length, while maximum fiber bending stress is related to the square of the length, so neither factor scales directly with the beam moment of inertia.

If none of this makes any sense, let me make it a little simpler. Any structural engineer worth a nickel can tell you based upon an inspection of your site and a review of the loading what size beam you need, or in your case how much you can span with a specific size beam. There is more to the design than simply sizing the beam, there are connection details and an analysis of the flex of the beam that should also be done at the same time the beam is evaluated for strength.

tpolk 03-17-2010 06:14 PM

you the man daniel

WillSTX 03-17-2010 06:31 PM

Quote:

Originally Posted by kwikfishron (Post 416129)
What are you going to be storing in that loft?

Just general storage stuff. Nothing too heavy to carry up a ladder. I figured if I could get to a "40 live load" it would do the trick.

WillSTX 03-17-2010 06:33 PM

Quote:

Originally Posted by Daniel Holzman (Post 416141)
No, you cannot multiply the length by 1.5. The allowable span for a beam is governed by the moment of inertia of the beam (depends on the size of the beam), the maximum allowable fiber bending stress (depends on the type of wood and the grade), the modulus of elasticity of the material (depends on the species of wood), and the loading (dead plus live load, often governed by code).

Maximum span is governed by the more restrictive of the deflection of the beam at the center, or the bending strength of the beam. Deflection is related to the fourth power of the span length, while maximum fiber bending stress is related to the square of the length, so neither factor scales directly with the beam moment of inertia.

If none of this makes any sense, let me make it a little simpler. Any structural engineer worth a nickel can tell you based upon an inspection of your site and a review of the loading what size beam you need, or in your case how much you can span with a specific size beam. There is more to the design than simply sizing the beam, there are connection details and an analysis of the flex of the beam that should also be done at the same time the beam is evaluated for strength.


You could have just said "I don't know".

tpolk 03-17-2010 06:38 PM

daniel gave alot of good info there sorry he did'nt have the reply you needed to do what your going to do anyways

bob22 03-17-2010 06:52 PM

I know Daniel knows how to do it but he isn't where you are and therefore can't know the issues he outlined, he can't say. There is a difference.

jlhaslip 03-17-2010 07:38 PM

What he is trying to say, in layman terms, is NO, you can not simply multiple by 1.5.

It should be inspected and decided by a professional.

Scuba_Dave 03-17-2010 08:35 PM

Quote:

Originally Posted by WillSTX (Post 416167)
You could have just said "I don't know".

If he was on site & could see what you are doing, existing support, existing structure, WHERE you are located, local codes, etc etc etc then he would be able to run the numbers

What you want to do is beyond the normal span tables
As such you need someone to evaluate YOUR specific installation & base the decision on that

Clutchcargo 03-17-2010 10:06 PM

According to this website calculator, if you double the 3x8s (4.5 x 7.25 actual) 16" OC, you can span 16' and get an L / 417.
But I agree with Daniel Holzman, there's more to it than just sizing the beam.

forresth 03-18-2010 02:23 AM

if you can find a table for a 2x9, use that. 3x8 is a little bit stronger than 2x9 (maybe 3/4s of a 2x10, so dont use that chart), but will have to carry more of it own weight too. another option would be to take a 2x8 chart, and space them out further. 6 3x8s should be as strong as 9 2x8s covering the same area. anyone disagree?

Scuba_Dave 03-18-2010 06:59 AM

Quote:

Originally Posted by forresth (Post 416357)
if you can find a table for a 2x9, use that. 3x8 is a little bit stronger than 2x9 (maybe 3/4s of a 2x10, so dont use that chart), but will have to carry more of it own weight too. another option would be to take a 2x8 chart, and space them out further. 6 3x8s should be as strong as 9 2x8s covering the same area. anyone disagree?

Yes, I disagree with guessing
That's why floors sag

forresth 03-18-2010 11:50 AM

Quote:

Originally Posted by Scuba_Dave (Post 416397)
Yes, I disagree with guessing
That's why floors sag

math and logic is not guessing

Scuba_Dave 03-18-2010 12:22 PM

Quote:

Originally Posted by forresth (Post 416357)
if you can find a table for a 2x9, use that. 3x8 is a little bit stronger than 2x9 (maybe 3/4s of a 2x10, so dont use that chart), but will have to carry more of it own weight too. another option would be to take a 2x8 chart, and space them out further. 6 3x8s should be as strong as 9 2x8s covering the same area. anyone disagree?

You are taking one size & trying to guess at what a 3x8 will carry
A 2x9 is not the same as a 3x8
Its not 3/4 of a 2x10
a 2x8 is not the same as a 3x8
"should be" That is guessing


Quote:

Originally Posted by forresth (Post 416487)
math and logic is not guessing

Without a proper span chart & calculation it is guessing
Show me your math


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