A plate washer look like a large metal disc. It solves a problem regular washers can’t address. If you’re tightening a bolt down on concrete or wood, the force is concentrated into a small circle of force. That material will then compress under that force and because there’s not enough surface area the joint will loosen with time. The way a plate washer work is that it spreads same force out over wide area so it doesn’t fail due to compression.
Building codes now require them for all anchor bolts securing a sill plate to the foundation. Different plates fits different needs, as shown in the chart above. Round heavy washers is better for round columns when the load comes from all directions. The flat sides of square washers make them a good choice for a timber frame which uses post and beam construction with the square washer sitting flush on both sides of the post and beam. Bridge washers covers multiple bolts in long pieces of steel or heavy equipment bases while slotted ones let you move them into place first before locking down the nut. Different shapes are used because inspectors quickly find weak spots in the wrong ones.
How to Choose the Right Washer
Beyond the shape of the washer itself, it’s also important to pay attention to the grade. Everyday wood framing can be sufficiently supported by standard F844 washers; however, when you have structural steel bolts exerting extreme force against them, those washers deform. Hardened F436 washers don’t deform as easy, which is why they’re used in all slip-critical steel connections. For the fabricator who cuts their own custom sizes in the field, there’s A36 plate stock, a compromise between the two.
Whatever you do, use the right washer for your bolt specification and not whatever happens to be on the shelf. The same goes for finishes: Use plain steel for indoor use and drywall; if it’s exposed to weather, pressure-treated lumber, or anything else, use a hot-dip galvanized finish. This have a heavier coating of zinc which resists the corrosive chemicals used today in wood preservatives that eat through thin electroplated finishes in just a couple seasons. Even so, stainless steel are the better long-term bet in salty coastal locations or where de-icing salts are used (though pricier up front).
Check if plate is the right size. Does it cover at least the minimum amount of material? And is it coated appropriately? Getting that wrong can ruin the entire load path all the way from your foundation up to the roof. Good framers carry different sizes in their truck since no single size fit every bolt.
You should of checked if they is correct. It would of been a big mistake to use thin washer for heavy loads. Actualy, one moddern project had issues because of wrong sized plate based off bad info. I think it was more better to use hardened ones. They dissapears into the background when installed properley.
