Story of the Lag Bolt

[Marcel R. Cyr]

Story of A Lag Bolt

How many have used one of these and never thought twice about it.
Me for one. Until I came across this article.
Kind of interesting.

Contributors;


Portland Bolt and Manufacturing Company



First, “standard” lag screws that are readily available in the marketplace are ungraded, meaning they are not manufactured to any ASTM specification and have no 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.
We would recommend contacting a structural engineer.


Recommended Lead Hole for Lag Screws


There are a number of variables that may change the required lead hole dimensions including: bolt length, diameter, and wood species. Consult an engineer for questions regarding specific applications.

Nominal Diameter
of Lag Bolt, inches
Shank (Unthreaded)
Portion, inches
Diameter of Lead Hole,
inches (Thread Portion)
3/8
3/8
15/64
7/16
7/16
9/32
1/2
1/2
5/16
9/16
9/16
23/64
5/8
5/8
13/32
3/4
3/4
1/2
7/8
7/8
39/64
1
1
23/32
1-1/8
1-1/8
53/64
1-1/4
1-1/4
15/16

Table provided by OBEC Consulting Engineers

• Design provisions and design values in the National Design Specification® for Wood Construction (NDS®)are applicable to lag screws conforming to ANSI/ASME Standard B18.2.1-1981. Tabulated design values are based on lag screws conforming to ANSI/ASME Standard B18.2.1-1981 and having assumed bending yield strengths provided in the table footnotes. Note that self-tapping lag screws are not addressed in ANSI/ASME B18.2.1 and are not specifically covered by provisions of the NDS. Specifically, the NDS does not address fabrication and assembly requirements, withdrawal design values, or lateral design values for self-tapping lag screws.

For self-tapping lags screws with dimensions similar to those provided in ANSI/ASME B18.2.1, the general form of the yield equations should apply for determining lateral design values. Accordingly, tabulated design values would also apply provided that the self-tapping lag screw dimensions meet or exceed the dimensions ASME B18.2.1 and the bending yield strength equals or exceeds the assumed bending yield strength in the table footnotes. In order to use lateral design provisions of the NDS, it must be assumed that fabrication and assembly of connections using self-tapping screws permits the development of the full bearing strength of the wood beneath the lag screw or permits yielding of the lag screw (i.e. installation does not damage the wood member or connection).

Finally, it should be noted that NDS Section 7.1.1.4 indicates that connections, other than those covered in the provisions, are not precluded from use where it is demonstrated by analysis, tests, or extensive experience that the connections will perform satisfactorily in their intended end use.

[William Warner]

Thanks Marcel.
I don't know how many "standard" lag bolts I've snapped with very little effort and the sheared area looks nothing more than pot metal.

[Gary Farnsworth]

Standards.

Just like home inspection laws, testing, SOP's, codes, all basic, all BS.

[Wayne B. Wilson]

could we not look for the grade marks to check?

[Marcel R. Cyr]

That is right Wayne, no BS there, that is fact.
No markings is = to SAE Grade 0 Strength and hardness is not specified

Medium carbon steel grade 3.
SAE Grade 0[22][25]Strength and hardness is not specified