Wood Beam / Pipe Penetration / Net Section Modulus

[CrabbyT]

I'm feeling super dumb right now, I had a real heavy loss over the weekend and I just feel like I'm not thinking clearly. Can someone please help me check my work?

I've got a 5.25x9.25 wood beam (three ply LVL). Contractor wants to drill a 2.125" hole through the middle of it for a pipe.

Am I doing this right?? Is this the right way to calculate the net section modulus for a beam with a hole in it? The hole is dead center, right through the neutral axis.

Inet = Igross - Ihole
Inet = bd[sup]3[/sup]/12 - bD[sub]hole[/sub][sup]3[/sup]/12
Inet = (5.25)(9.25)[sup]3[/sup]/12 - (5.25)(2.125)[sup]3[/sup]/12
Inet = 346.26 in[sup]4[/sup] - 4.20 in[sup]4[/sup]
Inet = 342.06 in[sup]4[/sup]

OK - I checked it in AutoCAD using the getmassproperties command, and this result agrees (Inet = 342.06). Now for the section modulus.

Snet = Inet / c
Snet = Inet / (d/2)
Snet = 342.06 / (9.25/2)
Snet = 73.96 in[sup]3[/sup]

For context, I'm evaluating the moment capacity in accordance with APA Technical Note V900.

https://www.apawood.org/publication-search?q=v900

 

[Luceid]

Haven't done a penetration like this in wood - but I checked your section property numbers and they all look good - no calc mistakes.

 

[kjm93]

Yes this is what I've typically done. As you note the Inet equation is only valid for a hole through the neutral axis, otherwise use parallel axis theorem (or autocad) to get Inet and c.

The APA tech note you cited is what I use.

 

[driftLimiter]

Does this penetration fall outside of MFG specs? Weyerhauser TJ-9000 lists allowable penetration zones I normally follow that.