Built up beam allowable stresses

[EnginerdNate]

Hi all,

Aero guy here doing some basic work on a beam for a personal project, wondering if anyone here can help me out. Pics attached show the beam dimensions and configuration, along with some napkin math. This was done to learn a bit more about wood construction as much as anything. I realize a TJI or gluelam would be easier to size for a real structure. That out of the way.

I built the box with SYP for caps and 22/32 OSB for the webs. Glued and screwed with PL Premium and Fastenmaster HeadLok fasteners on a 7" pattern, staggered on each side so the fasteners don't fight in the center of the cap.

I found strength values for SYP here:

https://www.southernpine.com/using-southern-pine/design-values/

My original napkin math assumed higher allowable stress than what's in that table (I was seeing strength values for SYP in the low 3000-4500psi range and MoR over 10k psi on less official sites before I found this resource) but I applied F.S.=5 to those initial calcs.

Using the values from that table with F.S.=5, I clearly need to limit load to much less than the 1000lb I've used to do the initial calculations. Are these intended to be "allowable" loads that you just have to stay below, or would a safety factor typically be applied on top of these values?

Any resources for fastener bearing strength in OSB and SYP, and panel shear strength of OSB would be helpful as well.

I'm already considering doubling the fastener count to 3.5" spacing just based on gut feel on fastener load in the OSB. The bond should be plenty to take the entire shear load but I don't trust it as much as a fastened joint so I'd planned on belt and suspenders.

Thanks,
Nathan

 

[EnginerdNate]

Looks like it only attached one of my sheets.

 

[ryaneng]

I'll bite, purely as a thought experiment! I would never spec something like this on a project and I would not recommend using this design. As an aside, the top and bottom chords would come very close to supporting this load alone if you just flipped them on their side and put them together.

I'm in Canada so the values I use have a reduction factor on the materials and a load factor on the loads, but there's not really a FoS.

SPF No1/No2 tension capacity: 800psi x 0.9 x 1.4 (Size Factor) = 1008 psi.

Load Factor: 1.5 --> Mf=1.5*1000*8'/4 = 3000lb-ft.

Tension in btm chord: (3000lb-ft)/(12"-1.5")=3428 lbs. Divide by the area of the btm chord gives you 508 psi.

Shear flow with 1.5 Factor = 71 lb/in.

#10 x 2" lg screws would have a capacity of about 180 lbs each, so screws at 5" o/c.

That gives no consideration to the overall stability or buckling considerations of this setup. It also doesn't consider how the load is locally applied. Essentially, this is just a homework problem so don't count on it to hold anything.

 

[EnginerdNate]

Good to see my numbers and yours line up when I apply your 1.5FS.

"I would never spec something like this on a project and I would not recommend using this design."

Any specifics there that make it substantially worse than a typical I beam?

As far as stability is concerned, my thought had been that the short span (8') and solid blocking mid-span would break the shear panels up into pretty small (1ftx4ft) areas that would be unlikely to experience any buckling and that as long as load is applied without significant torsional loading (Reasonably centered above/below beam) it wouldn't be driven into much of an issue torsionally either. I have not done any math to verify those assumptions yet though.

I purposefully chose a geometry that was homework level in it's complexity because I wanted easy references for unfamiliar connection types and materials it seems maybe that wasn't the best course of action.

Regarding the actual numbers, do the values in the link (I'm seeing 625psi for SYP 2x10s in compression for example) have the factors you've shown cooked in already?

Thanks,
Nathan

 

[ryaneng]

Main differences in my opinion are the lack of good construction quality. Are the screws being placed correctly to not split the top chord? Is the correct glue used and applied correctly? Were the surfaces dry and what was the temperature when the glue was applied? I think the I-joists with a groove are nice too in that they inherently bear on the web in addition to the glue.

 

[EnginerdNate]

That's a fair criticism. I guess other than "I built it myself" I don't have any guarantees. I did measure to make sure the screws were landing mid-thickness in the caps and spaced away from one another several D but once the second shear web goes on it is basically uninspectable.