The shear strength of grade 5 and grade 8 hex head bolts is 60% of their minimum ultimate tensile strength. Designers prefer to use a partially threaded hex head bolts in applications that encounter shear loading. The best designs make sure that any shear forces are exerted across the body and not across the threads. The body has a larger cross-sectional area than the threads and therefore the body provides a higher shear strength than the threads. On average, the load required to shear a hex head bolt is about 1.2 to 1.5 times higher across the body than it is across the threads, for coarse threaded hex head bolt in diameters from 1/4” to 1 1/4”.

Note that in applications where shear forces are encountered and the shear loads are exerted across the threads, then the use of a partially threaded hex head bolt provides the same shear strength that a fully threaded hex set screw provides.

In answer to the original question, a partially threaded hex head bolt should be used in applications where shear forces are exerted across the shoulder, and either a hex head bolt or a hex set screw can be used in applications that encounter tensile loads.