How do I determine the shear capacity and strength of lag screws?
Unfortunately, we have no specific data to answer this question. First, “standard” lag bolts that are readily available in the marketplace are ungraded, meaning they are not manufactured to any ASTM specification and have no verifiable mechanical requirements. Therefore, it is impossible to determine the strength characteristics of a lag screw unless they are custom manufactured or the specific lot of bolts is tested. Second, the forces that are acting on these lag screws will affect their ability to hold the structure in place. The shear strength of a fastener is approximately 60% of its tensile strength, but this is only an approximation and unless you have graded lag screws, there is no way to determine shear or tensile strengths. Additionally, the type of wood, grain structure, etc. will determine the ability for a lag screw to withstand the forces that are acting against it. Finally, we have no engineers on staff and due to liability issues; we are not in a position to answer these types of questions. We would recommend contacting a structural engineer.