galvanized A35 structural bolts 1

[superinspector]

I'm doing a job using A325 galvanized 3/4 inch diameter bolts for heavy equipment new construction. As required, I've tested them in the bolt tension calibrator and can get only about 14K lbs of tension maximum, before the galvanizing begins stripping off the thread. AISC states that this size bolt should require at least 28K min (plus 5%) for full tension. I don't know if I have defective bolts, or if bolts were supposed to be threaded oversized for the additional galvanizing thickness. Any thoughts?

 

[unclesyd]

It sounds like someone undercut the threads too much and you don't have enough meat left to hold the load.
If you can I would cut would section a bolt and nut longitudinally and check the amount of undercut.

Do you find a maker's mark on the fastener?

If not, do you know the country of origin?

 

[superinspector]

Thanks unclesyd. The bolt is marked with a capital "JH" on the bolt head, with "A325" and three equally spaced radial marks. Based on manufacturers I think it may be John Hassall Inc. out of New York.

 

[Screwman]

Were the nuts overtapped for use on galvanized bolts? Do the pitch diameters on the bolts measure within the allowable tolerance. Either the nuts need to be overtapped after coating or the bolts undersized prior to coating. Check out the A325 spec for all the details.

 

[SnTMan]

superinspector, what do you mean "the galvanizing begins stripping off the thread"?

Is the BOLT failing, or is the galv only failing?

Regards,

Mike

 

[Billybaru]

The bolt threads are made to Class 2A thread fit prior to galvanizing (required by A325). Per ASTM A325 the maximum allowable limit for the 3/4 bolt threads is 0.020 over the maximum Class 2A thread pitch diameter of 0.6832 (thus 0.7032 is max pitch diameter after HDG). The galvanized nuts used are supposed to be tapped oversize 0.020 after galvanize per ASTM A563 Table 5.

It could be a problem with either the nut or bolt. However, I have not heard of this type of problem being attributed to the bolt. If the nut used is not a hot-dip galvanized (HDG) nut, that is most likely the culprit. The HDG nut threads should be bare metal with the exception of possibly galvanize on the thread minor diameter. Both the nut and bolt should be checked for proper thread size.

Note the bolts were produced by a very reputable factory in Taiwan (Jinn Her).

 

[superinspector]

The reason I suspect the galvanizing portion is stripping is that the upper portion of the thread looks normal, whereas the lower portion of the thread looks shaved. I don't know if this makes it clearer or more confusing.

Also I found John Hassall Inc. doesn't make a 3/4 inch bolt, so this probably is NOT the manufacturer.

I don't know if the nuts were overtapped, but I suspect not.

 

[hogan666]

Do you have the ability to post a copy of the bolt MTR? I believe that would help us answer your questions.

 

[Billybaru]

I will assume the nut had been assembled on the bolt in question, but not fully up the bolt threads to the body. This would explain why the lower portion of the threads look shaved and the bolt threads above where the nut assembled look normal.

Since you suspect the nut is not tapped oversize, that is the first place to look. Maybe the manufacturer used a standard tap for the galvanized nuts by mistake. Note if the nut finish is plain or zinc electroplated, the threads will not be tapped oversize and this will be a problem. If the nut is galvanized, the threads should be tapped oversize.

 

[superinspector]

Bily: Thanks you are correct, the nut did not fully engage ALL the threads on the bolt prior to the stripping of lower threads. The nuts and bolts are galvanized, although now after being stripped, the galvanizing on both the interior of the nut and the bolt are gone.

 

[Billybaru]

The inside of the nut threads should not have any galvanize coating on them. Hot-dip galvanized (HDG) nuts are galvanized before tapping the threads. If there are unused nuts that have a coating on the threads, it may be the nuts were mechanically galvanized. If this is the case, those nuts should not be used except on mechanically galvanized bolts. Mechanical galvanize on structural bolts and nuts is not nearly as common as hot-dip galvanize, so I suspect the threads of the nuts may not be properly tapped oversize. You can check the test report on the nuts for the galvanize method.