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Using double washers in structural applications?

Using double washers in structural applications?

(OP)
Hi there, I am working on a bridge repair project and the bolts used are being pretensioned with the turn of the nut method (1/3 turn). The contractor was meant to use fully threaded M20s but they ran out so they decided to use M22s with a shank. The shank was too long so in order to space the bolt correctly they used two square spacers under the bolt head and two washers on the other side. My boss has told me not to allow double washers. When I was asked on site by the workers why not, I wasn't able to give them a proper answer. Can you please provide an explanation as to why double washers should never be used?

RE: Using double washers in structural applications?

If it is a pure bearing connection, I cant think of a reason not to allow it. In fact, I believe this is a fix recommended in an old issue of Modern Steel Construction. If there is shear in the anchor rod, you would need to weld the washers to the baseplate. In this case, I would suggest that the upper washer be smaller than the lower washer so that you would have enough space for a weld bead between the washers.

RE: Using double washers in structural applications?

(OP)
The connection is slip critical, would that change your opinion on using double washers?

RE: Using double washers in structural applications?

RStars,

What assumptions were made when you worked out the pretensioning 1/3 turn of the nut?

--
JHG

RE: Using double washers in structural applications?

A longer bolt will stretch more than a shorter bolt under the same load, so the amount the nut must be turned will increase to get the same load.

RE: Using double washers in structural applications?

I could sure use a picture or ten..

I'm wondering about things like -
- how an M22 with a "shank" can replace an M20 , and especially still be threaded M22 on both ends
- the size of the fastener passing through the holes in the steel, and what size the holes are.
- how thick the washers are

Has your boss offered his explanation for not wanting doubled washers ?

How thick are the washers being used now?

These are ~ 3/8 ~ 10 mm thick
https://www.mcmaster.com/#91117a249/=16kjo7q

Extra thick specials could made from bar stock pretty economically.

RE: Using double washers in structural applications?

(OP)
Hi @drawoh I used the standard turn of the nut equation for A325 bolts shown in the code. If the length of the bolt is up to or less than 4 bolt diameters then a 1/3 turn is used. The bolt lengths were 80mm, with a diameter of 20mm.

RE: Using double washers in structural applications?

Since these are slip critical and for bridge construction (fatigue sensitive), I would probably not allow as many washers as you described. The Research Council on Structural Connections allows stacking washers to keep the threads out of the shear plane; however, their logic escapes me!

I would be concerned as well about what CompositePro noted....longer bolts will likely need more than the 1/3 turn you described.

RCSC also gives criteria for plate washers (5/16" minimum thickness). Hardened washers must be used under the turned part.

RE: Using double washers in structural applications?

I have to worry about the hole size. I'm a buildings guy, not a bridge guy, but CSAS16 for buildings allows max 2mm hole size over bolt size or you have to use hardened washers. If an M22 replaces an M20, the holes for the M20s must be more than 2mm oversize, so what's up with the original washers?

RE: Using double washers in structural applications?

You are on metric terrain, so there's surely a standard to the execution of the bolted connection. If it is EN 14399 then you can derive permitted clamping length' and regulations concerning the application of washers.
In my opinion double washers would work, however you would imo be outside of the application rules as per your applicable standard and this might not be permitted as per the AHJ.
But:
There's a company distributing locking washers (Nord - Lock, no rel.) also for structural purposes, perhaps you contact them at
http://www.nord-lock.com/nord-lock/wedge-locking/w...
Or:
Why not (worst case) have manufactured something like a spacer sleeve, making up for the surplus length so that you can apply the bolts / nuts as per textbook?!
Regards

Roland Heilmann
Lpz FRG

RE: Using double washers in structural applications?

The count is up to almost 10 questions, with most unanswered by OP.

RE: Using double washers in structural applications?

My question is what did the bridge inspector have to say about the substituted bolts?

RE: Using double washers in structural applications?

Keep in mind that RCSC requires a pre-installation test to confirm the turn-of-nut rules. Also, the turn-of-nut requirement may not be the same with a metric bolt as with an imperial measurement bolt, (different thread pitches, etc.). Again, a test per RCSC Section 7 would establish what is the appropriate amount of turn. The test should be conducted with the actual grip range including the washers to get the proper value. It seems a little strange just to arbitrarily upsize the bolt rather than going out and buying the correct bolts (or checking with the original supplier to get more of the same).

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