Stacking Washers for Fasteners

Question

Stacking Washers for Fasteners

gmoney731 (Structural)

(OP)

9 Feb 21 15:20

Hello,

We specified 0.25" thick washers to be used for a fastening application. We are fastening the flange of a HDPE pipe to concrete.

However, the contractor stated those washers won't be available to be bought until April.

The question from the contractor is whether he can stack (4) 1/16" thick washers together to create a combined thickness of 0.25".

My answer here would be yes, they can stack (4) washers together to create the 0.25” thickness. I don’t think the stack is too large (I found online that if you’re going to stack, the “thickness” of the washer stack – 0.25” - needs to be less than the diameter of the washer used – 2”). Since we meet this guidance, we are good.

I know AISC is only for steel connections, but there is some guidance in there that mentions that you cannot stack washers together for a combined thickness of 5/16” or larger for slotted or oversized holes only.

I don’t see any guidance aside from that.

Any advice would be appreciated.

dauwerda (Structural) · Answer 1 · 2021-02-09 17:31:23

Why do you need them to be 1/4" thick? Are the holes oversized?
If the holes are oversized and you need the anchor/nut/washer to resist uplift you need the full thickness washer. (4) 1/16" washers stacked will not provide the same bending capacity that (1) 1/4" washer will. I have a very hard time believing it would take that long to get washers. If nothing else, go to a fab shop and have them cut them out of plate for you.

dvd (Mechanical) · Answer 2 · 2021-02-10 00:24:46

From Specification for Structural Joints Using High-Strength Bolts, June 11, 2020

Tmoose (Mechanical) · Answer 3 · 2021-02-10 13:11:07

regrettably the OP did not identify what fastener size and washer OD is needed.
Nor what kind of loading the plastic pipe flange can be asked to endure.
I'm guessing possibly stainless washers against plastic pipe.

The point about equivalent bending/cupping stiffness is mighty important on steel/iron flanges and motor feet.
I'm thinking load distribution into a plastic flange might be pretty important too.

I'd discreetly take a look at McMaster Carr and Fastenal before accepting the contractor's announcement the washers are "not available."

IFRs (Petroleum) · Answer 4 · 2021-02-13 16:52:47

Any issues with corrosion between the washers? Any shop with a waterjet can make you washers of any thickness in any material. The ones I use could typically turn them around quickly.

sdz (Structural) · Answer 5 · 2021-03-08 00:36:28

Reading that it is a connection for a HDPE pipe flange I would expect the requirement is for stiffness to spread the bolt load. In this case to get the same stiffness you use the sum of stiffnesses of the individual washers and since that is proportional to t^3 you're going to need a very thick stack of thin washers to get the same stiffness.

Review the reasons for specifying the 1/4" washers in the first place before making a decision.

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Stacking Washers for Fasteners