Well Defined Bolt Grip Lengths

[jasoncwells]

Where do I look to find structural bolts with a well defined grip lengths? I have good access to industry specs so a referal to the right docs would serve.

I am familiar with aircraft bolts. Aircraft bolts have grip lengths in 1/16 inch increments (e.g. NAS6604-12). As I looked through ASME B18 series, I don't really see a similar control of grip lengths. What I've found is that bolt length is specified, thread length is plus/minus a football field, and grip length is completely random. It seems to me that in the structural steel world, there must also be a need for bolts with well defined grip lengths so that the engineer can avoid placing threads in bearing.

I am working on some structural steel stuff used to test airframes and the engineers (from a sub-tier supplier) have provided me with designs with large washer stack-ups to avoid threads in bearing. It's sooooo ugly! I'd like to be able to respond with something more productive than, "I really don't like this design."

Perhaps the answer is, "Use aircraft bolts." That's OK. I just want to be sure I am not overlooking something from the steel construction discipline.

Regards,
Jason C. Wells

 

[dvd]

See Section 2.3 Heavy-Hex Structural Bolts Specification for Structural Joints Using High-Strength Bolts free publication by RESEARCH COUNCIL ON STRUCTURAL CONNECTIONS

Adjust your connection thickness if aesthetic appearance is important, or maybe metric structural fasteners could get you where you need to be.

 

[azcats]

To my knowledge, structural bolts designed per AISC and the RCSC document above don't differentiate between threads in bearing and not. They do differentiate between threads in the shear plane or not. If you do specify a bolted connection with the threads excluded from the shear plane, the threads will generally terminate somewhere in the ply against the nut.

That RCSC document does specify the typical thread length, head size, etc. and I believe 1/4" length increments are readily available.

 

[jasoncwells]

Thanks. That got me down the right path. For other people's future reference:

RCSC got me to the ASTM specs. ASTM A325 and A490 bolts were rolled into ASTM F3125. ASTM F3125 table 2 provides dimensions in the form of a reference to ASME B18.2.6. ASME B18.2.6 table 2.1.9.2-1 specifies grip lengths. For others coming from the aircraft world, grip length in the steel construction world has a different meaning. Grip length of a bolt includes imperfect threads per ASME B18.2.6. Body length of a bolt excludes imperfect threads. A person familiar with aero terminology should take care to understand what grip length means in the steel construction world. Also understand the difference between threads in bearing in the aero world and threads in the shear plane in the steel construction world. Steel practice allows some threads "in the hole". Aero practice does not.

Regards,
Jason

 

[MintJulep]

That path will tell you what the standards allow (require).

You may find that not all of the standard permutations are readily available.

I suggest you check fastenal.com or supplier of your favor for what can be purchased off the shelf.

 

[CWB1]

Steel practice allows some threads "in the hole". Aero practice does not.

Am I misunderstanding or are you saying that aircraft do not use fully threaded fasteners?

I concur with above in that you should review available COTS products rather than waste time searching standards few outside of the federal govt use.

 

[RolMec]

Large washer stacks are imo a no-go, that renders the bolted connection de facto incalculable.


Roland Heilmann

 

[jasoncwells]

CWB1: Aircraft do not use fully threaded fasteners in most (all?) structural applications like skins, stringers, frames, and single pin connections.

NAS screws are used in many places and are fully threaded, but these application tend to be attaching a piece of equipment to the aircraft and carry only body acceleration type loads. E.G. a radio screwed into the instrument panel.

Also, I am not an aircraft structures engineer. The structures guys make 'em. I break 'em.