WoodWorks Sizer - Built Up Column Design

[phamENG]

Bought Wood Works sizer and decided to test it out by spot checking some stuff I've done for built up columns. I was shocked to find that Sizer thinks all of my columns are undersized. I dug into a bit and discovered that the d/6 eccentricity (when the resulting moment is about the axis parallel to the plies) gets split up to each ply proportional to area, not applied to the section as a whole. In other words, it seems to ignore shear flow for the purposes of bending.

In one particular case, I have fc=256psi. For e/6, fb should also equal 256psi. But the program tells me I have 763psi fb. Well for (3)2x4, d=4.5" and A=15.75" so P=4032lbs, M=3024lb-in. For Sx=11.8125in3 for the full section, that works out to 256psi. If I divide that moment by three, I get 1008lb-in. For an Sx of 1.3125 (a single ply in flatwise bending), fb=768psi (0.65% error - close enough). So I'm confident I know what it's doing.

The question: why? Is there a rule that I've been ignorant of that says the connecting fasteners for built up columns can be used to augment Fc' but not contribute to shear flow for Fb'? Is the program just being hyper conservative? If so, is there a way to change it? (I have a tech support ticket in, but it's late in the afternoon on a Friday and I don't expect to hear back until next week.)

 

[Mike Mike]

skeleton thanks for the references, Koot K. and Celt get deep. I agree this discussion doesn't have many practical applications in my day job but it is interesting none the less. I agree applying some minimum eccentricity is good practice, but if I'm going to start throwing arbitrary additional safety factors at my posts like d/6, I would also want to take account of all the factors that help a typical light frame wood structure that are usually ignored like wall sheathing.

 

[phamENG]

Got the response. It's just a program limitation. The program doesn't look specifically at shear flow between plies, so they assume there is none. Simple as that.