# Screw Calculations/Math

At the risk of every woodworker in the world snickering at me, I'm curious how strong my wood joints are and want to know how much weight a table supports. I'm an aero engineer who ended up in EE, but I should remember how to do this, but some basic searches didn't show me how to answer:

• Given a thread depth, $$d$$ in wood with a some strength properties (I think modulus of elasticity (MOE) and modulus of rupture (MOR) would be relevant), what is the strength of the joint at varying screw lengths?
• How would I quantitively compare a butt-joined screw from a pocket screw for different loads?

I know one way to do this would be to use FEM. On the other hand there are probably some empirical tables with rough parameters, but I'm looking for a middle ground of some basic rules/math that help me get a feel for the strength of stuff I'm building. I am eager to get smart on how to model this with Fusion360, where I do have explicit modeling of the screws in my cad models.

Btw, this paper may have some insight and there are sites like this with calculators. It would be great to find some resources that break down the design trades.

• The problem with wood is the variability of the properties (mainly strength), as such you can always perform some calculations however the results can always be 50% or more away. At least in my experience. However someone here might be able to provide some useful resources/equations. Commented Jan 27, 2022 at 13:13
• Yep. Also wood species, preparation, grain direction, etc all come into play. There are still design rules you can follow (eg this size beam = may support this much load), but it's harder to avoid the complexity if you want to connect the design rules to the fundamentals. Esp. compared with metals, which are kindof the canonical situation for ME when talking about stuff like screws. Commented Jan 27, 2022 at 22:25