Previous earthquakes had shown that round washers, which can be found on all bolted houses in California built before July of 1999, did little to keep the mudsills from splitting. The first experiments with plate washers were done by the the Structural Engineer’s Association of Southern California. The author personally knew a member of the research team, and the drawing below was sketched by him. As shown in the drawing, the team discovered bolts bend slightly when an earthquake pushes on them, and would slice into the bolt like a “dull knife,” as this engineer described it. They discovered that plate washers prevented the bolt from digging into and splitting the sill.
This is what this engineer had to say about their experiments:
“The code requirement for bearing plates came out of the observation by the Structural Engineers Existing Building Committee. The bolting research was done after after the City of Los Angeles demolished 30 or so houses. The tests were done after the houses had already been removed. We isolated bolted pieces of wood so that we could use a jack to apply a horizontal force on the end of a wood piece that would result in a shear force to various anchor devices: existing anchor bolts, retrofit anchor bolts of various types, steel side plates, etc. For anchor bolts, we observed the failure happened by bending of the anchor bolt as it cut into the wood like a dull knife, splitting the wood. When we installed a plate washer on the bolt, the plate would restrain against excessive bending by holding the nut, and therefore the bolt, upright.”
Cross Grain Bending
Further research done by the American Plywood Association and the American Would Counsel demonstrated that plate washers helped tremendously in reducing cross grain bending. All shear walls, no matter how tall, try to overturn when subjected to earthquake forces. The taller the wall, the greater the overturning, and the greater tendency to cross grain bend. Cross grain bending will ruin a shear wall. Large plate washers as shown above greatly reduce this tendency, especially if the bolts are closely spaced.
In the photo below from a test done by the American Plywood Association, the shear wall was “loaded” until cross grain bending occurred. A research scientist with the American Plywood Association told the author of this article that as long as there were enough bolts (approximately 2 feet apart) with plate washers cross grain bending did not occur.