Lock washers on A325 bolts? 1

[radair]

I was recently told that lock washers (a.k.a. split washers) were not allowed on A325 structural bolts. Their reasoning was the following in AISC's 9th Ed. of ASD (page 5-266):

"(e) Washers. Flat circular washers and square or rectangular beveled washers shall conform to the current requirements of ASTM Standard Specification for Hardened Steel washers, ASTM F436"

This reads to me that FLAT & BEVELED washers must be hardened steel washers, but I don't read this as a prohibition on lock washers. I can't understand why a contractor should remove these washers, particularly after seeing a recent failure due to ineffective Anco (lock) nuts.

Any opinions, preferably backed by spec.?

 

[jen4950]

I don't have any specs here at home, but if I were going to guess I would say that you can't get a spring washer in the grade of steel required. Fundamentally this makes sense- if they made the spring washer out of high strength hardened (aka brittle) steel- it wouldn't behave as a spring, but more like a yielded steel member, or worse as a fractured brittle steel member. (just brainstorming here)

 

[jheidt2543]

radair:

"...seeing a recent failure due to ineffective Anco (lock) nuts."

Could you fill in the details or point us to the story on this failure?

 

[swall]

There was a pretty good discussion on split lockwashers on the Automotive Engineering Other Topics forum about six months ago .

 

[CoryPad]

thread108-114051
thread404-94197
thread725-85323
thread725-63343
thread725-57042

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[radair]

Thanks Cory. I did read a few of those threads prior to posting, but they do not address my question, which specifically is whether AISC allows lock washers on A325 bolts or not. I also searched AISC's site and Googled to no avail.

I have inspected close to 800 telecommunications towers and have seen lock washers in many, many instances. In this case, I'm not really interested in opinions on their effectiveness, but in the requirements of AISC.

jheidt2543 - sorry, I can't give you details at this time, other than the bolts were loaded in tension and appear to have developed fatigue cracks prior to failure.

 

[CoryPad]

Any opinions ...

In this case, I'm not really interested in opinions ...

Sorry that the previous threads weren't what you wanted. I don't think you will find what you want. The definition in your first post could be interpreted as excluding split washers.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[radair]

Sorry to not be more clear.

I am not interested in opinions on their effectiveness, but I welcome opinions/interpretation of AISC requirements.

Thanks.

 

[Screwman]

Radair,
My interpretation of AISC is that it allows only flat and angled washers to the exclusion of lock washers. One of the reasons for this is the high portential for SCC failures of the washers under environmental conditions in service. A broken lockwasher is a very serious condition.

The failure that you mention sounds like inadequate preload on installation. I have NEVER seen a properly tightened and designed A325 joint loosen in service. Lock washers will not prevent a failure due to improper installation.

Dick

 

[HgTX]

ASTM A 325-04b section 3.3 recommends F 436 or F 959 washers, but doesn't strictly require them.

Hg

Eng-Tips policies: faq731-376

 

[radair]

I found the requirements in EIA/TIA-222, Structural Standards for Steel Antenna Towers, to be interesting. Both revisions of this standard reference AISC Manual of Steel Construction.

Revision F of the standard has been in effect since 1996. It states: "Where high-strength bolts are used and tensioned in accordance with AISC "Specification for Structural Joints Using ASTM A325 or A490 Bolts", a nut-locking device is not required"

Revision G, put into effect on January 1, 2006 states: "4.9.2 Nut-Locking Devices - Bolts used to connect load-carrying members shall be provided with a nut locking device or mechanism such as, but not limited to, lock nuts, lock washers, or palnuts, to prevent loosening, except as provided in 4.9.3 (Pre-tensioned bolts)

This tells me that not only are lock washers allowed on this type of structure, they (or other locking device) are required, except for pre-tensioned connections.

Screwman - until two weeks ago, I would have completely agreed with you regarding inadequate preload. I too have never seen such loosening other than on this structure. I have since learned that these particular bolts were tightened 3 times in 5 years and repeatedly kept loosening. I talked to the mechanics who tightened them and they claim that they used 3/4" drive sockets and really "reefed on them". Thanks for your input.

 

[unkamiko]

The ASD manual pg. 5-272 details the requirements for washer application. Typically an A325 -A490 bolt when fully tensioned will destroy a lock washer. The washers are not needed in a fully tensioned connection. When bolts are place in a connection that is required to slip the threads are distressed or the nut is tacked to prevent loosening.

If the bolts do not require full tension there should be no problem with a lock washer, but why use one?

 

[stumetals]

AISC build their specifications from experts in the field. The experts on structural bolting are RCSC Research Council on Structural Connections. The basis for excluding lock washers comes from the fact that A325 Bolts are intended to be tensioned for pretensioned or slip critical joints in steel structures. Two facts that stand out on the use of lock washers are: When a high strengh bolt is pretensioned it will not loosen due to its plasticity factures I'll not go into here, of which the need for a lock washer doen't exist. Secondly a lock washer would no bear the tension stress applied to it, it would simply spread out. Lock washers are not allowed on structures such buildings and bridges but are not addressed for other structures. I do know that AASHTO allows them on Hwy sign structures etc.
Hope this helps.

 

[Tunalover]

I have been unable to find any reliable source (backed by test data) that says that the helical split washer helps at all. In fact, the prevailing opinion I have received from this forum (different threads) is that they are actually detrimental to the bolted joint. Did you look at other threads?



Tunalover

 

[Torquester]

The straight-forward answer relative to the RCSC is that we don't recognize helical spring lock washers in any way. That simply means we don't contemplate their use, and I've never heard a fellow committee member question this lack of mentioning them.

Although the section wasn't contemplated as such, paragraph 2.6.2 pertaining to 'alternative washer type indicating devices' might be read to recognize lock washers IF they are approved by the EOR. (It would be a stretch to suggest anyone had that in mind when that language was incorporated.)

Perhaps the SAE Fasteners Committee said it best when the following language was written in SAEJ489: "It should be noted that the word 'lock' appearing in the names of the products in this standard is a generic term historicially associated with their identification and is not intended to imply an indefinate permenancy of fixity in attachments where they are used."

Or, as Hollywood would say it, "This story was inspired by a true locking fastener."

Ausgang
SAE/RCSC/ASTM Member

 

[radair]

Thank you all for your input. I have read all of the other threads and there seems to be a consensus that split washers are not effective. I'm not sure I can agree, however.

I routinely inspect telecommunication towers, which obviously see cyclical wind loads to an extreme degree. Hundreds or thousands of load cycles per day would not be unusual in many cases. Lately I have observed a number of loose bolts which used Anco nuts (these are the nuts with a lock wire that runs through the nut and "catches" on the threads of the bolt).

Although split washers are somewhat less common, I have observed no loose bolts with this type of locking device (nor have I observed any broken washers). I'm wondering what thermal effects may have to do with this phenomenon, as here in New England we see temperature variations greater than 100 degrees F over the course of a year. I've not been able to find anything definitive as yet on that subject.

My question was really on the requirements of AISC, not to reopen the debate on the effectiveness of split washers. Nonetheless, my observations seem counter to the prevailing consensus.

Thanks again.

 

[diamondjim]

My guess is that you are torquing the bolts
and that the lock washers having less surface area
is allowing you to have greater tension in
the bolts and that is what is helping your
situation. If you are using a tension devise,
then forget this comment.

 

[evelrod]

I talked to the mechanics who tightened them and they claim that they used 3/4" drive sockets and really "reefed on them".

Having built a few towers in my youth and since most of the fasteners were 7/8" or larger in diameter A325 or A490 galvanized exclusively with hardened washers and Anco nuts...the above quote tells me a lot. I recall we used a hydraulic multiplier on the larger bolts and a pleumatic impact wrench that was dutifully calibrated on a Skidmore each morning before we mounted the tower!!! I know it was a long time ago, but...the last tower I worked on was the KNX radio tower in Hawthorn, CA. in 1966...it's still there!

Rod