Is stacking of F436 washers acceptable with an A325 bolted connection?
Yes, in some cases the stacking of F436 washers seems to be allowed, but not in all cases.
The Research Council on Structural Connections (RCSC) in Specification for Structural Joints Using High-Strength Bolts, December 31, 2009, page 16.2-12 says,
“If necessary, the next increment of bolt length can be specified with ASTM F436 washers in sufficient number to both exclude the threads from the shear plane and ensure that the assembly can be installed with adequate threads included in the grip for proper installation.”
They go on to say, later in the same section,
“To determine the required bolt length, the value shown in Table C-2.2 should be added to the grip (i.e., the total thickness of all connected material, exclusive of washers). For each ASTM F436 washer that is used, add 5/32 in.; for each beveled washer, add 5/16 in. The tabulated values provide appropriate allowances for manufacturing tolerances and also provide sufficient thread.”
One could reasonably assume based on the above passages that the use of multiple washers to achieve a purpose like keeping the threads out of the shear plane is acceptable.
I was able to find one instance where the stacking of washers is prohibited. In the same document, in Table 6.1 page 16.2-45, while discussing the requirements for 5/16” thick F436 washers to be used with A490s with oversized or slotted holes, they say,
“Multiple washers with a combined thickness of 5/16 in. or larger do not satisfy this requirement.”
So it appears, that when using F436 washers to compensate for bolt lengths, the stacking of washers is acceptable, however, if 5/16” thick F436 washers are specified with A490 bolts and oversized or slotted holes, simply stacking two regular F436 washers does not satisfy that requirement.