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Re: Cut Plate Washers

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In the early 90's, the SEAOSC Existing Building Committee, Cripple Wall
Retrofit Subcommittee tested alternative ways to retrofit unbolted cripple
wall mudsills for seismic hazard reduction.  We tested 1/2" diameter
retrofit bolts in some old residential concrete foundations that were
scheduled to be demolished.

We added plates between the nut on the bolt and the mudsill after shear
to failure of mudsills with retrofit shear anchor bolts resulted in damage
to the mudsills.  The damage was caused by the anchor bolts being pushed
over and bending away from the
load, cutting into the mudsills, causing them to split.  The ordinary
round machine washers didn't help avoid that kind of damage.  We thought
that larger steel plates might tend to hold the head up so that the bolt
could not bend far enough to cut into the mudsill.  At the time, a
seat-of-the-pants call was made as to size of plate to try -- or maybe it
was something one of the guys had in the back of his pickup.  Anyway, the
plates we tried worked pretty well, so we didn't try larger plates; it
seemed like a reasonable size, so we didn't try smaller plates; square
plates were what we had, so we didn't try round plates -- or rectangular or
octagonal plates.  We wrote the square plate
size that we had tested into our recommendations.  We then decided we should
recommend plate sizes for different sizes of bolts, so we selected plate
sizes in reasonable proportion to the bolt sizes.  Before long, Simpson
Strong-Tie was manufacturing the plates that we had reported in our
recommendations, and listing them in their catalog.  Now our recommendations
are carved into the stone wall of the building code.

It makes me tremble to speculate where that experience has put me (and the
rest of the guys and a gal on the cripple-wall subcommittee of the Existing
Building Committee) in the
code writing process.  We certainly had no hope that other engineers would
(apologies to Keith Fix)
"abdicate their authority as specialists to" us
because our test had some amount of success in solving a problem.

Go ahead and use round plate washers if that's what you've got in the back
of your pickup.

Nels Roselund
Structural Engineer