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Lag Bolt Specification

Lag Bolt Specification

Lag Bolt Specification

I am working on a project that involves the design of a connection subject to shear and withdrawal loading. I want to check the capacity of the lag bolt in tension. I am using the AISC 13th edition and am unsure which section to use. I have found threads on this forum that reference using section 5 for tension members but is seems that section J3 in the specification may be more applicable to the check of a fastener.

Additionally I have found that A307 grade A lag bolts and SAE J429 grade 2 lag bolts seem to be readily available. Which of theses is commonly specified, or another grade?

If specifying A307 how do you determine the bending yield stress for use in the NDS yield limit equations when checking the connection for shear? It appears that the A307 specification does not required the yield stress to be published as part of the material specification. Section J3 in the steel manual has the published nominal stress to check the fastener for A307 using the method in J3 but how does that relate the bending stress required in the NDS yield limit equations for a shear connection?

Any guidance or reference material suggestions would be greatly appreciated.

RE: Lag Bolt Specification

AISC is not for timber design.

RE: Lag Bolt Specification


As far as my understanding goes, it is not typical to specify wood lag screw mechanical properties in building construction, unless you really need a specific Fyb. I have never had to worry about that, as wood limit states usually dominate. My specs call for ASTM A307 or SAE J429 lags (with no specific grade), and I use Fyb = 45,000 psi in my designs. Maybe Bridgebuster can confirm this, but I believe that AASHTO has a lag screw specification which is equivalent to other standards.

Take a look at NDS 2012 Appendix I - page 169
Also, the Portland Bolt company has a lot of information.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Lag Bolt Specification

Here's what AASHTO LRFD 7th Ed. says about lag screws:

Bolts and lag screws shall comply with the
dimensional and material quality requirements of
ANSI/ASME B18.2.1, Square and Hex Bolts and
Screws—Inch Series. Strengths for low-carbon steel bolts,
Grade 1 through Grade 8, shall be as specified in Society
of Automotive Engineers Specification SAE-429,
Mechanical and Material Requirements for Externally
Threaded Fasteners. Bolt and lag screw grades not given
in SAE-429 shall have a minimum tensile yield strength
of 33.0 ksi

The design of timber connections using mechanical
fasteners including, wood screws, nails, bolts, lag screws,
drift bolts, drift pins, shear plates, split rings, and timber
rivets shall be in accordance with the 2005 NDS®

RE: Lag Bolt Specification

I understand that the connection between wood and fastener is governed by NDS for withdrawal and that for shear connections NDS checks lateral loads for the connection between wood and fastener and bending in the fastener.

My specific situation has an 1400lb ASD Live load applied in withdrawal to a 1/4" lag bolt. There is also a 100 lb shear load. In checking the withdrawal I was unable to get a standard 1/4" lag bolt with enough threaded length to resist pull out in the wood. My client has decided to have the fasteners made, 8" long with a 6 1/2" threaded length.

I am concerned with the metal fastener failing not the connection between the wood and the fastener. I apologize for not being more clear in my original post. Any further thoughts would be appreciated.

RE: Lag Bolt Specification


"It is imperative Cunth doesn't get his hands on those codes."

RE: Lag Bolt Specification

Quote (ryholde)

I am concerned with the metal fastener failing not the connection between the wood and the fastener. I apologize for not being more clear in my original post. Any further thoughts would be appreciated.

So are you concerned with the fastener yielding with a 1400lb tensile load applied at the head, or with the side plate yielding due to shear?

First case, wood is going to govern. If you still want to check it run 1400lbs axial on the cross sectional area of the steel. Typically critical section is at the chamfer where the shank meets the head.

Second case, check your side plate for yielding with that 100lbs shear. Don't really think this is a concern though.

If you are looking for a good design spec to go off of, Canadian Safety Agency's OS86 has a great section on designing lags. And most other wood connectors for that matter.

RE: Lag Bolt Specification

From my experience, the long small diameter lags seem to break very easily. Once we spec'd 3/8" diameter lags with similar lengths and about half broke during installation. Snapped right off.

RE: Lag Bolt Specification

There's good manufacturers and there are lousy ones. Been there too.

Mike McCann, PE, SE (WA)

RE: Lag Bolt Specification


That is weird, haven't experienced that personally. Were they screwing the lags or hammering them? I guess weak steel & not hitting them true could lead to bent lags. Ive always been told lags aren't meant to be screwed in at all, not designed to withstand installation torque.

RE: Lag Bolt Specification

Screwed in small battery powered Impact Wrench. I have sheared off a few 1/4" lags by hand with a socket wrench. I even pre-drilled the holes, but I guess I am just that powerful. Either that or Home Depot sells crap, which is more likely.

How would you install them if not screwed?

RE: Lag Bolt Specification

Every heavy duty lag bolt I have used requires pre drill to ~5/8 shaft dia and hammer in. Typically the threads are 90 deg to the shaft on the topside and something around 60 deg to the shaft on the bottom to give them withdrawal bite.

I did a quick google search and lots of the shiny fancy shmancy 1/4" screws they are marketing as lags now seem more like heavy duty hex drive screws to me.

When I think lag, I think course threads (maybe 3 thread rotations per inch), cast steel screws you hammer home. Hex head is just to withdraw if you need to take it out down the road.

This could just be the industry I was working in at the time (utility lineman) but it did the trick and the screws worked like a dream. Anything around the 1/4" mark and you would just as well specify Simpson SDWs or Trussloks

RE: Lag Bolt Specification

Quote (Siginous)

Anything around the 1/4" mark and you would just as well specify Simpson SDWs or Trussloks

Those are a standard at our firm. Much more reliable installation.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Lag Bolt Specification

Refer to NDS. Both the angle of load (withdrawal + shear) is addressed....I believe it is Harkinsons formula, and the lag design parameters are specified. Look in the footnotes of the connection tables for the bending yield stress and in the code itself for the bearing stresses. The AISC does not apply in this situation if you are only connection wood towood. I don't have my NDS in front of me, and sadly don't have it memorized, but I know it is in there.

I think the only two books you need for 95% of wood design are Breyer and the NDS, with a check on the awc.org and apawood.org websites.

RE: Lag Bolt Specification

Google hits for "lags" that are driven in are pretty rare.
The ones that come up are either this post, or another reference to mounting stuff on utility poles.

posts 5 and 7 here -

there are "drive type" lags shown here (along with conventional screw in types) - page 5-43 in a pole line hardware catalog

they seem to start at 3 inches long

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