Continuous LVL

[Simlac450]

Hey guys,
The other I was designing a continuous beam LVL (3 support, 2 pins one roller) supporting floor joist in wood sizer, and I noticed that when the beam is flush (continuous support on top and bottom) the negative Mr was the same as positive Mr BUT if the beam was located under the joist (continuous support on top and bottom support only at support) then the negative Mr was largely reduced. I repeated the same operation for an built-up wooden beam in SPF an the Mr pos and neg was the same for both scenarios… so I was wondering what the hell lol

 

[XR250]

It's because when the beam is flush, it is continuously braced by the joists. The allowable moment is based on the unbraced length of the member.

 

[phamENG]

I agree with XR - but the thing about SPF not caring is strange. Check the other settings. Sizer has a lot of settings. Could be there's a default setting checked to allow the inflection point to be considered a brace for sawn lumber. This practice lingers in a few texts, but has largely been ruled incorrect and a poor design practice.

 

[XR250]

ah, did not notice it was different for the dimensional lumber beam. I suppose I should read more carefully Makes sense what Pham said.

 

[Celt83]

Depending on the built-up SPF member depth it's also possible for clause 3.3.3.1 to apply making C[sub]L[/sub] = 1.0 and no reduction for LTB.

3.3.3.1 When the depth of a bending member does
not exceed its breadth, d <= b, no lateral support is re-
quired and CL = 1.0.

 

[Simlac450]

That’s was my first thought but why a simply supported beam is not affected, only occur when it’s continuous..

 

[jayrod12]

Well there's no negative moment when simple span. So that would explain why the capacity reduction doesn't exist.

If you removed your top restraint for simple span, you'd likely end up with the same reduced Mr for the LVL lumber.

What size of SPF beam are you running versus size of LVL? As Celt indicated, if the beam is as wide as it is tall, then no reduction for lack of bracing is necessary. LVL maybe doesn't have that same allowance, and I've rarely seen beams out of LVL that are as wide as they are tall.

 

[DoubleStud]

Yeah, probably because when you use SPF, it is so wide that it does not need to be braced.

 

[kissymoose]

Better yet, what software are you using? Forte?

 

[Simlac450]

Yeah I know there is no negative moment on a simply supported beam, but I’ll try to remove the top restraints for a simply support beam and I let you know, for the size of the SPF I tried several dimensions and I always got the same results, I’m using wood sizer

 

[Simlac450]

Ok update, I tried several sections and it seems to be realated with the D/b ratio, but for the LVL its way more restrictive than for SPF. So problem solved tks everyone !