Screws in steel as per Canadian Wood Design Manual

[NorthCivil]

Page 252 of the 2010 wood design manual provides tables for selection of screws for shear.

The tables have sections on screw shear in light gauge steel (20 and 14 ga) as well as 4.76mm (3/16") A36 steel.

A handy addition - though I don't understand how there is a variation for values of N'rNs depending on species (Dfir, Hemfir etc)

Should these not all be the same? I do not understand how wood grade comes into effect designing for screw shear in a steel side plate.

Forgive me if I'm missing something, my experience in wood is limited to what I've learned in school.

 

[dhengr]

NorthCivil:
With a reasonably thick stl. side pl., the shear strength of a screw or nail depends almost exclusively on the compression and bearing strengths of the wood adjacent to the side pl., both parallel and perpendicular to the grain. The fastener cantilevers out of the wood and is loaded by the stl. pl., and the fastener will bend and crush the wood, like a point loaded cantilever beam on an elastic foundation. This assumes sufficient penetration into the wood to hold to fastener from pulling out. Look at a good Wood Design textbook (or the NDS) for modes of failure of bolted (or dowel pin type) connections. Thus, the mechanical properties and various allowable stresses for the different species of woods come into play.

 

[EdStainless]

And the more screws or nails that you put through the plate the stronger the joint is.
In desperate cases I have seen the fastener heads touching.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[dhengr]

EdStainless:
Well, you’re right to a point, but I think fastener heads touching is stretching it a bit. Before you get to that point, you will face edge distance, row spacing, and spacing btwn. fasteners in a row limitations, and wood splitting, etc. You also have to pay attention to the wood grain orientation w.r.t. the direction of loading, and you might finally end up with some net area problems with stl. side pls. which are too thin.

 

[CELinOttawa]

Well I'm with Ed... If he's including the fender washer. I have seen details with less than a few mms between fender washers in splice connection.

Not personally a fan of fender washers, but they have the advantage of making is nearly impossible for the contractor to put the fasteners too close together.