The washers prevent galling. Without the washers you may not achieve the required installation tension. If you really needed a slip-critical installation, you probably needed the washers. However, read all the fine print. High strength bolts may be used in "bearing" connections with snug-tight installation only. The washers really shouldn't matter much. The character of the load is important. t's got to be essentially static in a "bearing" connection. The direction of the force is also important. It's got to be normal to the slot within plus or minus x degrees. Direct tension is not allowed. See the specification for high-strength bolts in the back of your steel manual for details. Use a recent edition. They are always fiddling with this document.
For after the fact justification, you could try a Skidmore-Wilhelm test to show that the tension is achievable without washers. Not likely with short 490s, though.
>>> Charlie Carter <carter(--nospam--at)aiscmail.com> 04/16 5:37 AM >>>
>On one of our projects, 1-1/2" dia. A490N bolts were specified for shear
>tab beam connection. Because of fit-up, the contractor made the connection
>using short slotted holes in the beam web. However, the requirements for
>minimum 5/16" thick washers were overlooked. The beam is in place now and
>because of the existing conditions, the connection can not be accessed to
>replace the bolts and the washers. What is the reason for requiring 5/16
Thicker washers are required for pretensioned applications with A490s of
diameter greater than 1 in.because of the tendency of standard F436 washers
to dish into the hole, thereby reducing the clamping force induced using an
otherwise acceptable installation procedure.
>Can one use some reduced capacity when standard thickness washers are used
>instead of 5/16 inch washers?
One possible solution is to consider whether pretension is actually required
in the first place. If the bolts are in shear, snug-tightened installation
of A490s is often acceptable (pretension is often not required). And per
RCSC Specification Section 6.1.2, for snug-tightened bolts in slotted holes
in exterior plies, a standard ASTM F436 washer is suitable.
Hope this helps.
P.S. If you don;t already have it, you can get the 2000 RCSC Specification
as a free download from http://www.boltcouncil.org.
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