Yield, shear and tensile strengths are all different things. Basically (meaning there is much more to it than this), the yield strength is the stress level at which a permanent deformation occurs in tension. The tensile strength is the highest strength a bolt achieves before failure in tension. The shear strength is the strength of the fastener under shear loading, which is normal to the axis of the bolt. There are different types of shear stress, depending on the installed application.
In general, the tensile strength of a grade 5 fastener is 120,000 psi, and the strength of a grade 8 fastener is 150,000 psi. While usually referred to as tensile strength, these numbers are actually tensile stress levels. The shear strength is usually about 50-60% of the tensile strength. So to get the tensile strength of a 1/2" bolt, you would multiply the tensile strength by the cross sectional area of the bolt, which would be :
Tensile Stress x bolt radius squared x Pi
-or-
Grade 5 - 120000 x .0625 x 3.14 = 23550 lbs.
Grade 8 - 150000 x .0625 x 3.14 = 29452 lbs.
The shear strength of this same 1/2" bolt would then be:
Grade 5 - 23550 lbs x .6 = 14130 lbs
Grade 8 - 29452 lbs x .6 = 17671 lbs.
However, the tensile stress area of a fastener is somewhat less than it's nominal diameter would suggest. For a 1/2" bolt, the tensile stress area would actually be .142 for a coarse thread bolt and .160 for a fine thread bolt. So a more accurate tensile strength would be 17040 lbs for a coarse grade 5 bolt, 19200 for a fine thread grade 5 bolt, 21300 for a coarse grade 8 bolt and 24000 for a fine thread.
I'll let you calculate the shear strength of these bolts. Sounds like your bolt will be in double shear vs single shear, which changes the overall strength also. The real problem I see here is accurately determining the real load that these bolts will see.
