On Sat, 19 Jan 2008 02:56:38 -0500, "Ed Huntress"
wrote:

Either the bars are weaker (the bolt will tear through one of the bars
when the joint fails), or the bolt is weaker (the bolt will fail in
shear).
=============
And herein lies the rub.

Proper lap joints *NEVER* rely of fastener shear strength. The
proper function of the fastener is to clamp the surfaces together
such that friction between the two is what resists the load. For
location in such a situation you would use dowel pins, but still
rely *ONLY* on the friction the fasteners generate by clamping
the surfaces together for strength.

In correctly designed and assembled lap joints, any axial failure
should be in the base/joint material. If this is not the case
(i.e. fastener shear failure), using more but smaller fasteners
to more evenly distribute the clamping (and friction) across the
joint, or "washers" to distribute the clamping force is
indicated, not harder bolts.

While stronger bolts may allow increased assembly torque and
therefore higher clamping force/pressure on the joint members at
assembly and thus "solve" the problem, it is due to the higher
clamping force and not the "harder" bolt.

Fine v course threads, assembly techniques [thread
lube/antiseize] and applied torque are also factors. Harder
bolts are also less prone to cold flow/creep under stress, and
thus may maintain joint clamping pressure better than lower grade
fasteners. Joint member material can also be a problem in this
regard if enough cold flow occurs to reduce the clamping pressure
and thus the friction, and periodic re-torquing may be required
to maintain joint strength.