3/8" bolt holding on the tounge. A 3/8-18 bolt has an effective cross sectional surface area of 0.0775 in^2
A grade 8 bolt of 3/8-18 is rated for 120,000 Psi. So 120,000psi* 0.0775= 9,300 Lbs
But keep in mind that there is really two surfaces that are providing this shear strength (the bolt touches the trailer on both sides of the tongue where it goes through, both left and right)
So the real shear strength of a 3/8-18 grade 8 bolt is 18,600 Lbs... Theoretically anyways
Below is taken from
http://www.derose.net/steve/resources/engtables/bolts.html
Size Nominal diameter, Stress Area, Grade, Proof load ksi, Clamp load lbs, Assembly torque( dry ft*lb), Assembly torque (lubricate ft*lb)
Size 5/16"
5/16-18 0.3125 0.0524 2 55 2162 11 8
5/16-18 0.3125 0.0524 5 85 3342 17 13
5/16-18 0.3125 0.0524 8 120 4718 24 18
5/16-18 0.3125 0.0524 9 145 5701 29 22
5/16-24 0.3125 0.0581 2 55 2395 11 8
5/16-24 0.3125 0.0581 5 85 3701 19 14
5/16-24 0.3125 0.0581 8 120 5225 27 20
5/16-24 0.3125 0.0581 9 145 6314 32 24
Size 3/8"
3/8-18 0.3750 0.0775 2 55 3196 19 14
3/8-18 0.3750 0.0775 5 85 4939 30 23
3/8-18 0.3750 0.0775 8 120 6974 43 32
3/8-18 0.3750 0.0775 9 145 8427 52 39
3/8-24 0.3750 0.0878 2 55 3622 22 16
3/8-24 0.3750 0.0878 5 85 5599 34 26
3/8-24 0.3750 0.0878 8 120 7904 49 37
3/8-24 0.3750 0.0878 9 145 9551 59 44