Joist calculators WTF?

[BK777]

Has anyone come across this before? I'm trying to work out a design and am getting very different answers.

Given the same parameters, AWG.org says 10' 4" and WCLIB.org says 14' 6". That's a HUGE difference. One is thumbs up for me and the other is thumbs down.

Screenshots below. What gives?

Here are the links:
https://www.awc.org/codes-standards/calculators-software/spancalc
https://www.wclib.org/resources/calculators/maximum-span/

 

[TurboMustang370]

It seems to me that your need some more professional guidance.
There is information missing.
What are you trying to accomplish? Is this for a roof rafter, floor joist or partition?
First thing I notice is you have two different wood species in the drop down box, minor, yes, but does make a lot of difference.
If you plan on having any load, I would recommend using Number 2 or better lumber, not stud grade.

 

[spudley]

Enter your numbers again. I just came up with 17'4" for Doug fir.

 

[BK777]

Maybe I'm missing something. The two screenshots show identical choices. The only difference being that one was a little more specific on the species. The same exact choice wasn't available on the other. And that should really only make a minor difference.

Also I agree Stud may not be the best choice but again, it shouldn't make a difference because the same choice was made in both calculators. They should show results within (maybe) a few inches.

For the record I'm trying to size a ceiling joist to span 14'.

I guess my main question was have others seen a similar disparity between calculators? And the other being, which one would you recommend as the most accurate?

 

[firebirdparts]

That's a lot of live load. If it's truly a ceiling, then the 2 x 10's are bigger than you really need. If the loads are correct, then the 2 x 10 is just right.

 

[BK777]

Again, the point was... why are two different calculators giving very different answers with the same exact (almost) input???

Here is another pair of screenshots with the same exact values.

 

[Skiff Builder]

Again, the point was... why are two different calculators giving very different answers with the same exact (almost) input???

Here is another pair of screenshots with the same exact values.

Interesting.
I played with the two calcs. Got them to match at 13'10" if I called it a floor joist on WCL....

Looks like you'll have to contact the two orgs. You'll get a better answer than us guessing here.

Let us know what you find.

 

[NeubCont]

Interesting thread
Was looking for a similar calculator for LVL beams but couldn't really find one
Any recommendations??
Thanx

 

[TurboMustang370]

I understand your point and your concern but the only true way is to contact your source company and ask them what they rate/certify their lumber for.
I purchased the lumber for my "clean room" from the orange big box store. I contacted the lumber supplier they use right from the pro desk with the pro desk sales man for them to size the lumber for the project.
My clean room build is 20'x20' with no center supports. They recommended No. 2 2"x12"x20', 16" on center with a 2"x4" stud wall, 16" on center.
I went with their recommendation in the hopes that if it fails, the insurance company will cover the loss.

 

[SGKent]

DIY calc and you are complaining that one rounds down and one rounds up? I'll bet an engineer would give you a different number.

 

[strutaeng]

Again, the point was... why are two different calculators giving very different answers with the same exact (almost) input???

Here is another pair of screenshots with the same exact values.

Read the AWC "Limits of use." The equations in the code have additional "adjustment factors." I'm thinking one of the 2 is not taking into account some of these.

If you meet the criteria on the AWC, I would use that, or, just use the tables.

How did you determine 30 psf live load is required?

 

[BK777]

DIY calc and you are complaining that one rounds down and one rounds up? I'll bet an engineer would give you a different number.

Come on, 13' 10" vs 15' 11" is not "rounding".

 

[BK777]

Interesting.
I played with the two calcs. Got them to match at 13'10" if I called it a floor joist on WCL....

Looks like you'll have to contact the two orgs. You'll get a better answer than us guessing here.

Let us know what you find.

Ah, that sounds like a bug.

I'm not going to bother contacting the orgs so they can just point fingers at each other or challenge me to prove them wrong. What's scary is that one of these is wrong, and people are designing things based on wrong data.

I'd just like to find a reliable calculator. When I want the square root of 28.5 I don't second guess my math calendar -- maybe I should...

If anyone has a suggestion, that's essentially why I started this thread. Sure I can figure out how to use the tables but I seriously doubt that many DIYers do that. Hell, most of them just go with what sounds good & I don't want to do that either.

The specifics of my project don't really matter for this thread. I'm just playing with numbers right now. Of course I may do other projects in my lifetime. Thus the need for a reliable calculator...

 

[JoeMcGov]

I'd just like to find a reliable calculator. When I want the square root of 28.5 I don't second guess my math calendar -- maybe I should...

Yes you should.

 

[firebirdparts]

I guess you figured this out already, but the "west coast lumber" gives different answers for a ceiling joist than it does for a floor joist with the same loads. The "american wood council" does not.

If they have the same wood parameters then they match if you set them to floor joist.

 

[mcbane]

A couple of considerations:
1) "Douglas fir" and "DFL-North" are different wood and have different design values
2) Software, especially free software, tends to be tested more thoroughly for values that are realistic. You will not see 2x10 stud grade in official lumber design value tables. So the software should have given you an error message or asked what design parameters you wanted to use for Fb, elasticity and compression perpendicular to grain once you specified wood that wasnt in reference tables. Instead it looks like each app picked a different random value.

 

[SGKent]

I worked in the IT industry for years - software is always OOPs. You are the beta tester. Look at what Boeing is going thru with the 737 Max and their software. Why anyone would think they can build a house by relying on software is somewhat beyond me. Does the software company have an IT help number you can call to ask about it? Is this freeware or a paid for product that has support.