Shear Walls, Anchor Bolts & Hold Downs
Not a very exciting topic for residential architects to be talking about is probably what you’re thinking. We agree. Only, we want our clients to never be excited about these things, because they keep the homes we design safe and sound. The structural engineer, with whom we always coordinate, works together with us to determine the best locations for shear walls, the anchor bolt spacings and the shear wall hold downs.
What’s the difference between an anchor bolt and a hold down? An anchor bolt, as defined by the International Residential Code: anchor bolts resist lateral forces that could cause a building to lift or slide off the foundation. Anchor bolts must have sufficient embedment to resist pullout and must be spaced properly to secure the sill in place. Washers must be capable of distributing a load across the sill without it cracking or splitting.
We would also add that anchor bolts help resist vertical suction loads as well, such as during heavy winds. Many of our projects are in mountainous wind zones of between 110 mph to 130 mph. We would also modify the definition of the washer at the sill plate to primarily distribute a compression load across the wood plate.
Hold downs are essentially the anchor bolt’s big brother and are typically required to be installed adjacent to several studs, typically called a “studpack.” The hold down is a premanufactured item out of heavy gauge galvanized steel and its anchor bolt rod typically extends down all the way into the footing. The main purpose of hold downs are to secure the wood stud walls in the structure from dislocation during seismic shaking and wind.
We typically look at our floor plans and evaluate where the structural engineer would likely want to see shear walls. We normally accomplish this with manufactured wood sheathing panels securely attached to both sides of stud walls that run perpendicular to our “View Walls.” Because the view walls normally have large amounts of windows and glass, we look for every opportunity to brace these “normal” to their geometry, to strengthen them against horizontal wind and earthquake movement. According to what the contractors and building officials tell us, our designs normally have more bracing for wind and earthquakes than most home design plans coming through their offices. This is because we are architects and want it done correctly.
Within the shear walls, we locate the hold downs, near ends and at structural discontinuities like windows and doors, fireplaces and the like. Therefore, our hold downs brace the already panel-sheathing-braced shear walls, which results in a super-secure arrangement making the houses we design more solid than most in the United States and the world.
Once again, perhaps this does not sound very glamorous, but if you are sleeping and up gusts a high wind or a little tremor, you should be able to continue sleeping comfortably, knowing that your structural engineer and architect have resolved these issues.
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