, East Greenwich, RI
Building codes refer to the National Design Specification
as the technical resource for capacities of various parts and pieces. Machine bolts are rated in the NDS
, but carriage bolts are not. It seems that technically, carriage bolts might not be permitted by code. Is there a difference in the strength of like-size carriage bolts compared to machine bolts with washers?
Rob Munach, a structural engineer in Carrboro, N.C., replies: Yes, there’s a difference in their relative strengths, mostly because the bearing surface of a carriage-bolt head is smaller than that of a machine bolt with a washer. This means that the machine bolt and washer can resist a higher tension load before overstressing the wood. Avoid using carriage bolts where they could be subjected to significant tension loads, such as guardrail-post attachments.
For guardrails, use machine bolts with washers. The greater surface area of the washer used with a machine bolt provides more holding power than you get with a carriage bolt.
Photo by: Krysta S. DoerflerShear strength is a different story. The NDS bases the shear capacity of bolted connections on the diameter and yield strength of the fastener and the density of the wood.
Appendix I of the NDS addresses the diameter of the fastener and the percentage of the bolt that is threaded. The bottom line is that while a partially threaded bolt (less than 25% of its length) is stronger than a fully threaded rod or carriage bolt, the difference isn’t significant. A representative of the American Wood Council confirmed that the reduction in shear capacity is 10% to 15%. As such, it appears that fully threaded fasteners, such as carriage bolts and threaded rod, could likely be substituted for machine bolts in typical residential shear applications.