V4 vs. V8 GLB question 1

[beam4u]

I recently evaluated an existing GLB that is undersized. There are some limitations as far as reinforcement of the situation, but the easiest fix might be to add a post (it is a floor over a garage). In all likelihood since the original beam was clear span it is a V4 beam. As a two-span I would need to evaluate it as a V8. Am I correct here??

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[msquared48]

Yes, but a 24f V4 is still OK if the new negative moment is less than 1200 psi.

Also, you could drill a series of horizontal holes or cut the beam at the new post location to maintain the V4 status...

Mike McCann, PE, SE (WA)

 

[cal91]

or cut the beam at the new post location to maintain the V4 status...

This made me think of a question. Why cut it at all? The cut is to relieve the negative moment at the post. Well, if we just leave it and the moment is too great, then tension fibers yield and the compressive fibers crush, decreasing the moment until the moment can be resisted.

I'm curious what people think about this. Any thoughts?

 

[msquared48]

Yes, but by cutting, you control where the "yielding" occurs. That is good!

Mike McCann, PE, SE (WA)

 

[dhengr]

Cal91:
That’s not bad reasoning, except... In steel, for example, which is a pretty homogeneous material, we have a pretty good control and understanding of the mechanics of this plastic hinge formation, and the moment redistribution as long as that section of the beam is prevented from bucking/rollover during the yielding process. Wood, even though in GlueLam form, is less consistent and homogeneous, so it will be (might be) a less uniform and controlled moment redistribution and yielding, ripping, whatever. I’m more concerned with the uncontrolled tensile ripping/fracture than I am with the relatively controlled crushing in compression. So, I like Mike’s idea of doing something to force/control the location of the tensile failure.

 

[cal91]

That makes sense. Now that I think about it I can see the tension ripping 6" to 12" away from the support and unzipping until the section fails catastrophically in shear.

 

[dhengr]

exactly!

 

[msquared48]

Yuppers!

Mike McCann, PE, SE (WA)

 

[Ron]

....maybe you got lucky and the V4 was installed upside down

 

[msquared48]

Not that that has ever happened before!

Mike McCann, PE, SE (WA)

 

[structure567]

Hi Msquared48,
What do you mean by 1200 psi moment? Did you mean 1200 lb-in?
I have also come across a v4 beam that is cantilevered and trying to see if the size is ok without replacing it.

 

[msquared48]

1200 psi would be the maximum BENDING STRESS seen in the negative moment region.

Mike McCann, PE, SE (WA)