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



## WillSTX (Mar 17, 2010)

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?


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## kwikfishron (Mar 11, 2010)

What are you going to be storing in that loft?


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## Daniel Holzman (Mar 10, 2009)

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.


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## tpolk (Nov 7, 2009)

you the man daniel


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## WillSTX (Mar 17, 2010)

kwikfishron said:


> 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.


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## WillSTX (Mar 17, 2010)

Daniel Holzman said:


> 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".


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## tpolk (Nov 7, 2009)

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


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## bob22 (May 28, 2008)

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.


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## jlhaslip (Dec 31, 2009)

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.


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## Scuba_Dave (Jan 16, 2009)

WillSTX said:


> 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


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## Clutchcargo (Mar 31, 2007)

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.


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## forresth (Feb 19, 2010)

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?


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## Scuba_Dave (Jan 16, 2009)

forresth said:


> 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


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## forresth (Feb 19, 2010)

Scuba_Dave said:


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


math and logic is not guessing


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## Scuba_Dave (Jan 16, 2009)

forresth said:


> 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




forresth said:


> math and logic is not guessing


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


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## WillSTX (Mar 17, 2010)

Found the answer, or rather the AWC sent it to me. Go to: http://www.awc.org/Standards/wsdd.html and download the Span Tables - Quick Finder

You will have to lookup the Fb for the type of wood you're using but then it's just following the table.


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## kwikfishron (Mar 11, 2010)

The guy is sitting on a pile of 3x8’s and wants a little storage space in the barn. What’s the big deal? It’s not like he’s building a apartment to store the in-laws up there or something. What ever happened to common sense? If the thing's sagging throw in another beam , add a post or something. It’s just storage in a barn.

I’m new here and see a lot of good information coming out of this site but also see a lot of very simple things get turned into complicated ones. 
Why couldn’t the “Engineer” ask another question or two and help the guy out with his “Storage Space”. 

I would think the purpose of a DIY site would be novices along with the pro’s helping each other getting through their projects and not “you must give your local Engineer a $100 per hr. before you can put your Xmas boxes up there”.


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## Daniel Holzman (Mar 10, 2009)

Since you ask, I will tell you why the Engineer does not design the job for someone on this site. I have a license as a professional engineer in multiple states. As a licensed professional, I have obligations and am subject to laws and regulations that do not apply to unlicensed individuals. Additionally, I am subject to penalties for errors and omissions that do not apply to unlicensed individuals offering their opinions on chat groups such as this.

So I avoid designing projects for individuals on this site, since I would be potentially liable for the consequences of my actions, even if it were as simple as the individual misunderstood the instructions and improperly installed the beam. I only design projects when I have a contract with the person for whom I am doing the design, and I only take on a contract when I can personally visit the site.

So occasionally I will offer what I believe to be factually accurate information that could be used by an individual to help with their design or installation. I don't design jobs over the internet. I leave that to the unlicensed folks who have no professional license at stake.


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## Scuba_Dave (Jan 16, 2009)

kwikfishron said:


> The guy is sitting on a pile of 3x8’s and wants a little storage space in the barn. What’s the big deal?


The big deal is when it becomes overloaded & collapses
40 live load is more then what a bedroom floor is usually designed to carry (30?)
My last house is over 100 years old
Many other people on here have houses that are just as old or older
Very few people live in their house for the lifespan of the house
Building to code means building it to last

Going to an engineer may not be needed
Having the beams sized for the span is

I have 6 sets of LVL beams in my house
All were sized by a lumber yard for free & my plans were sent out to be stamped - for FREE
I did buy all my LVL's from them & 2 major lumber orders
So a simple trip to a local lumber yard could answer the question & gice proper sizing & spacing

WillSTX did find a span table that lists a 3x8


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## kwikfishron (Mar 11, 2010)

Well since it’s clear that you guys will always give the best information you can on a subject to ensure that the DIY’R competes his or her’s project up to industry standards and code, that’s great, I completely agree. The last thing anyone wants is for a project to fail in any way that could result a failed inspection when trying to sell, creating more problems in the end than you started with or even worse, getting hurt. That’s why we have codes in the first place and that’s exactly why we have the “Permit Process”. You (or someone else) draws up a plan for a proposed project submits that plan to their local building department and they will either approve the plan, reject the plan or say the 3x8’s won’t do, revise to a 3x10 and we’ll approve.

I see time and time again a post with pictures of a project in process with a question like “Is this wall load bearing, can I take it out?” or any other structural or mechanical question for that matter that would not have had to been asked if they didn’t skip one little step, “The Permit Process”.

So my question is this. Is the purpose of this site to help the DIY’R to properly complete a project from idea to end or how to successfully outlaw in your little remodel without hurting someone or something?


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## Scuba_Dave (Jan 16, 2009)

kwikfishron said:


> I see time and time again a post with pictures of a project in process with a question like “Is this wall load bearing, can I take it out?” or any other structural or mechanical question for that matter that would not have had to been asked if they didn’t skip one little step, “The Permit Process”.
> 
> So my question is this. Is the purpose of this site to help the DIY’R to properly complete a project from idea to end or_* how to successfully outlaw*_ in your little remodel without hurting someone or something?


I'm not sure what the part I put in bold means ?
As much as possible I try to help people with everythig I can
But when they want to remove a wall & puit a beam in I will always say get the beam sized
I guess I'm lucky to have lumber yards that will do this without cost
Others have reported lumber yards in their area do not offer this

IMO things should be built to code or above
As many say...Code is the Min that you can build to & pass
I went oversize on my roof rafters & floor joists
Just for that one heavy snow/ice storm, that big party when there is a 100 people in the house, maybe that slate pool table, or the large saltwater aquarium I want to put in
I've put enough electric in that I could put a space heater in every room if the heat failed
...and as luck would have it my heat did go out & I heated with wood & 3 space heaters for 3 1/2 days
Or in case of a lot of people here each person could use a hair dryer in each bathroom

In some cases even with pictures its hard to reach a definite conclusion on if something will work or not
And in some cases people here have outlined the proper way to meet code & build/wire something
...only to have the person go & build/wire it with in such a way that it does not meet code

There'a lot of very talented people on here who offer their assistance
Usually if the answer is not provided there is a good reason


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## forresth (Feb 19, 2010)

Scuba_Dave said:


> 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
> ...


 what is the differnce between three 2x8s and two 3x8s? common sense says there is no difference apart from the load caried by the decking spanning the larger gaps when a load is aplied between the spans, but we are talking beams here, not decking. That is common sense not a wild guess. just because you cant find a table that has all the exact variables you are looking for doesn't make it unsafe. you are acting like a computer that recieved an invalid input and not a thinking human being.

as far as comparing 3x8s to 2x9s:
all other thing being identicale, all that is left is the formula based on cross sectional shape. I remember that formula to be 1/3 x base x hieght^3. all other factors will cancel out. If my memory serves on that formula (and I'm pretty sure it does), 3x8 is slightly stronger than 2x9 bending along the thick dimension. I don't know were my textbooks are stored, so I cant go and look up the complete formulas, but that should be easy enough for someone to verify if they are overly concerned. but it is irelevent because a 3x8 table is easier to find than a 2x9 table.


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## Scuba_Dave (Jan 16, 2009)

forresth said:


> what is the difference between three 2x8s and two 3x8s?


2x8 = 1.5" thick x 3 = 4.5"

3x8 = 2.5" thick (unless true 3x8) x 2 = 5"

You don't guess with beams, joists, rafters & spans


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