Foundation Bolts and Bearing Plate Washers

 Foundation Bolts and Bearing Plate Washers

Proper Bolt Installation in Residential Seismic Strengthening

Research on foundation bolts and seismic upgrades as well as their function in the bracing of cripple walls has been done by the American Wood Council, the American Plywood Association, CUREE, and the Structural Engineer’s Association of Southern California.  Research done by all of these agencies demonstrated that bolts are more effective when bearing plate washers are placed under the nut. The main benefit stems from their ability prevent or greatly reduce 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 below greatly reduce this tendency, especially if the bolts are closely spaced.

Earthquake induced damaged caused when plywood pulls up on the foundation bolts,putting the shear wall into Cross Grain Bending. Bearing Plate Washers Prevent This

Plywood Tears up on the where the foundation bolts are.

Example of a shear wall overturning because of an earthquake and the shear wall did not have Bearing Plate Washers

 

 

The effects of plywood pulling up on the mudsill and cross grain bending damage.

 

In this photo from a test done by the American Plywood Association the shear wall was “loaded” until cross grain bending occurred.  If the shear wall bolts have small washers and /or insufficient bolts at the shear wall locations (they should be a minimum of 2 feet apart) the base of the shear wall will cross grain bend as shown here.

Another way Plate Washers Prevent Splitting of the Bolted Wood

The first experiments with plate washers and foundation bolts were done by the the Structural Engineer’s Association of Southern California.  They discovered bolts bend slightly when an earthquake pushes on them and would slice into the bolt like a dull bolt.  They discovered that  plate washers prevented the bolt from digging into and splitting the sill.  Below is a drawing of a bolt and plate washer made by one of the structural engineers who actually did the tests.

Drawing Plate Washer on Foundation Bolt

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.”

Plaste washer plus chart

 

 

 

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