Live Chat
Lag Screw Shear Strength and Design
Answer: 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.
The American Wood Council has more information regarding wood construction and specific lag bolt recommendations. They also have an extensive wood connection calculator.
Posted by Greg Lindsay
Phone: (800) 599-0565 Email: greg@portlandbolt.com
View other posts by Greg Lindsay
Per IBC and other model codes, refer to American Forest & Paper Association (American Wood Council) National Design Standard (NDS) for wood structures, Chapter 11, for information on code-recognized design values for lag screws.
April 29, 2013 at 1:44 pm@Gene Eichel – Lag screws are typically made per SAE J429 grade 1, which is a low carbon steel with a minimum tensile of 60ksi and a minimum yield of 36ksi. Simpson SDWS screws, according to their website are made from heat treated carbon steel. It does not list any specifics in regards to their mechanical properties, but any heat treated carbon steel will be stronger than SAE J429 grade 1. So from a simple material standpoint, the Simpson screws will be stronger. However, the Simpson screws appear to max out at 0.220″ diameter, whereas the lag bolt you question is 3/8″. Without specifics about the mechanicals for the Simpson screws, I cannot give you a functional comparison. You may be able to contact an engineer with Simpson and they may be able to get you more specific information.
April 15, 2013 at 7:43 amWhich is stronger and a better fastner, 3/8×4 lag bolts or Simpson Strong Drive SDWS structural wood screw 4″ long.
April 13, 2013 at 3:31 amHi there Greg,
Great answer. Luckily I seen this post. And now I know what should I buy when buying a screw.
December 7, 2011 at 4:21 am