Structural Sustainability: Doing a Home Earthquake Retrofit

A house really isn’t very sustainable if it falls down and has to be rebuilt!  When upgrading an older house to 21st century sustainability standards, it makes sense to do an earthquake retrofit as part of the project.  While nothing can ever completely guarantee any building will survive an earthquake, doing a few things can make a big difference.

Hiring someone to do the work on an earthquake retrofit for you can cost many thousands of dollars, but could prevent significant damage to your home which may not be covered by your home owners insurance.  Or you can do the work yourself for a few hundred dollars in materials, plus some sweat and effort.  It can be dirty uncomfortable work, but it’s not difficult.

Back when craftsman bungalows were being built (and other classic houses), building designers made the false assumption that the downward forces of gravity alone are enough to keep a house on its foundation.  So there was nothing really connecting the wood frame of the house to the concrete foundation wall beneath it.  This is generally true for mid-century and earlier houses.  Unfortunately, during an earthquake other forces are at work — the shake, rattle, and roll of a seismic event — which could cause a house to fall off its  foundation.  When that happens, the structural damage throughout the house might be too much to try to repair it.  A total rebuild might be required.  NOT very green!

The first step in doing an earthquake retrofit is to connect the wood frame of the house to its foundation with anchor bolts.  Holes are drilled through the lower-most wood piece (the sill plate) and into the foundation wall with a rotohammer.  Then the anchor bolts are pounded down into the hole with a sledgehammer until they are snug against heavy-duty square washers.


earthquake retrofit anchor bolts
Anchor bolts and square washers installed in the sill plate of a basement pony wall. The bolts connect the house wood frame to the concrete foundation wall as a critical part of an earthquake retrofit.

The short “pony walls” of the basement need to be strengthened structurally, so the house won’t twist or sway on top of its foundation.  The most common and least expensive way to reinforce these walls is to cover them with plywood sheets.  Nailing every few inches on every part of the wood frame creates a stiff structural panel resistant to seismic motion.  Nails are used instead of screws because they flex and don’t break under stresses.

Structural reinforcement of basement walls

Plywood sheets nailed to wood framing creates a structural panel resistant to seismic stresses.

The first floor of the house needs to be connected more securely to the strengthened basement wall, so the first floor doesn’t fall off.  Metal framing clips are put on top of the wall, and nailed to both the top of the basement wall below, and the joist for the first floor.  A palm nailer is an extremely helpful tool in this process!

Reinforcement of structural beams and posts

Reinforcement of structural beams and posts

Other reinforcement include nailing metal framing clips to posts and beams to keep these important internal stuctural members connected during a seismic event.  Then connecting the water tank to something structural with metal straps to prevent it from falling over.  All the above steps I have been able to accomplish with some help from handy family members — with training and tools from the Phinney Neighborhood Association and with a couple of hours of consultation from a specialized contractor on design, plans, and permitting.  The project was done in two phases, with each phase taking two weekends of work.

The third and final phase will be to remove the furnace chimney from the center of the house.  Chimneys can topple during earthquakes, causing damage to the roof and anything and anyone below.  Plus after nearly 100 years, the brick mortar has deteriorated to the point of being problematic in other ways.  Originally built to exhaust an oil-burning furnace, it’s time for this relic of the oil-era to go!  But that will need to be done as part switching heating for the house and water to renewable energy