previous
  • Hot Water Now
    Hot Water Now
  • Remodeling Articles
    Remodeling Articles
  • Master Carpenter Videos
    Master Carpenter Videos
  • Tips & Techniques for Painting
    Tips & Techniques for Painting
  • Basement Remodeling Tips
    Basement Remodeling Tips
  • 7 Small Bathroom Layouts
    7 Small Bathroom Layouts
  • 7 Smart Kitchen Solutions
    7 Smart Kitchen Solutions
  • 7 Trim Carpentry Secrets
    7 Trim Carpentry Secrets
  • Pro Tool Rental. Learn More.
    Pro Tool Rental. Learn More.
  • Video: Install a Fence
    Video: Install a Fence
  • Clever daily tip in your inbox
    Clever daily tip in your inbox
  • 9 Concrete Countertops Ideas
    9 Concrete Countertops Ideas
  • Custom Flooring Inspiration
    Custom Flooring Inspiration
  • Magazine Departments
    Magazine Departments
  • All about Roofing
    All about Roofing
  • Read FHB on Your iPad
    Read FHB on Your iPad
  • Video Series: Tile a Bathroom
    Video Series: Tile a Bathroom
  • Energy-Smart Details
    Energy-Smart Details
  • Classic Cabinets
    Classic Cabinets
  • Design Inspiration
    Design Inspiration
next
Pin It

Is a carriage bolt as strong as a machine bolt?

Q: 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?





A: 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.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. Doerfler
Shear 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.


From Fine Homebuilding 193, pp. 98 January 17, 2008