I suggest you go to Grade 8 Bolts. Please go to this web site:
http://www.boltdepot.com/fastener-in...ade-Chart.aspx The Grade of a fastener is marked on the head. If you have no marks it is a grade #2, If 3 marks you have a grade 5, if you have 5 marks it is a grade 8. Grade 8 is a higher strength fastener. Here is an on line calulator :
http://www.engineersedge.com/beam_be...m_bending2.htm
This is a simply supported beam with the load at the center. In your case the beam size is 3/8" dia. with the properties of the bolt head markings in the above web site from Bolt Depot.
My team also bent a grade #2 Bolt crossing the 4x6 bump. We replaced it with Grade #8. No problems since. It is not the Andy Mark Bolt it is the application and using the correct strength fastener for the job,
Norman Collier