


Thread Tools  Display Modes 
03172010, 04:30 PM  #1 
Newbie
Join Date: Mar 2010
Posts: 4
Rewards Points: 10

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? Advertisement 
03172010, 06:13 PM  #2 
Mod
Join Date: Mar 2010
Location: Kansas (NCK)
Posts: 7,988
Rewards Points: 2,232

How to calculate Joist span table for 3x8's
What are you going to be storing in that loft?
Advertisement 
03172010, 06:32 PM  #3 
Civil Engineer
Join Date: Mar 2009
Location: Boston
Posts: 4,348
Rewards Points: 2,274

How to calculate Joist span table for 3x8's
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. 
The Following User Says Thank You to Daniel Holzman For This Useful Post: 
drtbk4ever (03192010)

03172010, 07:14 PM  #4 
Member
Join Date: Nov 2009
Location: central virginia mountains
Posts: 1,857
Rewards Points: 1,000

How to calculate Joist span table for 3x8's
you the man daniel
__________________
The older I get the better I was 
03172010, 07:31 PM  #5 
Newbie
Join Date: Mar 2010
Posts: 4
Rewards Points: 10

How to calculate Joist span table for 3x8's 
03172010, 07:33 PM  #6  
Newbie
Join Date: Mar 2010
Posts: 4
Rewards Points: 10

How to calculate Joist span table for 3x8'sQuote:
You could have just said "I don't know". 

03172010, 07:38 PM  #7 
Member
Join Date: Nov 2009
Location: central virginia mountains
Posts: 1,857
Rewards Points: 1,000

How to calculate Joist span table for 3x8's
daniel gave alot of good info there sorry he did'nt have the reply you needed to do what your going to do anyways
__________________
The older I get the better I was 
03172010, 07:52 PM  #8 
Member
Join Date: May 2008
Location: Near Philly
Posts: 2,051
Rewards Points: 1,022

How to calculate Joist span table for 3x8's
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.

03172010, 08:38 PM  #9 
Member
Join Date: Dec 2009
Location: Canadian Rockies
Posts: 1,280
Rewards Points: 500

How to calculate Joist span table for 3x8's
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. 
03172010, 09:35 PM  #10 
Member
Join Date: Jan 2009
Location: South of Boston, MA
Posts: 17,248
Rewards Points: 2,000

How to calculate Joist span table for 3x8's
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 
03172010, 11:06 PM  #11 
I have gas!
Join Date: Mar 2007
Location: Massachusetts
Posts: 1,783
Rewards Points: 1,018

How to calculate Joist span table for 3x8's
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.
__________________
I tear things down and build them up. 
03182010, 03:23 AM  #12 
Member
Join Date: Feb 2010
Location: MI's Western UP
Posts: 599
Rewards Points: 500

How to calculate Joist span table for 3x8's
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?
Last edited by forresth; 03182010 at 03:25 AM. 
03182010, 07:59 AM  #13  
Member
Join Date: Jan 2009
Location: South of Boston, MA
Posts: 17,248
Rewards Points: 2,000

How to calculate Joist span table for 3x8'sQuote:
That's why floors sag 

03182010, 12:50 PM  #14 
Member
Join Date: Feb 2010
Location: MI's Western UP
Posts: 599
Rewards Points: 500

How to calculate Joist span table for 3x8's 
03182010, 01:22 PM  #15  
Member
Join Date: Jan 2009
Location: South of Boston, MA
Posts: 17,248
Rewards Points: 2,000

How to calculate Joist span table for 3x8'sQuote:
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 Without a proper span chart & calculation it is guessing Show me your math Advertisement 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Joist size and span under tile.  jackson_norman  Building & Construction  9  08122009 09:10 AM 
Max span length on 42x10 fir beam?  gadget463  Building & Construction  4  08032009 06:51 PM 
2x12 joist span  phillybr  Building & Construction  3  03312009 10:38 AM 
A 2x6 to span 21' as a ceiling joist???  Scuba_Dave  Building & Construction  2  03012009 02:58 PM 
joist span?  apowens  Carpentry  17  05042007 08:54 AM 