Construction Lessons from the Napa Earthquake

Early in the morning on August 24, Napa Valley residents were rudely awakened by a 6.0-magnitude earthquake that shook hard enough to rattle residents’ teeth—and their homes.

Thankfully, no one was injured… but many people either lost their homes or are facing major reconstruction. Engineers red-tagged more than 150 houses that were unsafe to enter because of chimney collapses or damaged foundations. Another 1,000 homes got yellow tags, allowing only limited access due to structural damage.

California banned unreinforced masonry buildings way back in the 1930s, and many communities have started requiring retrofits in an attempt to reduce danger and damage in an earthquake. But not all of those retrofits held up under the quake’s punishment. According to ENR California, several recently retrofitted and newer buildings were damaged, too.

So how can California’s homeowners and construction professionals protect both lives and property from the next quake?

Performance of Current Retrofits

The August 24 quake taught many residents and building owners one painful lesson: Current retrofit regulations don’t provide as much protection as they were intended to.

Home to many historic buildings built with beautiful (but less than earthquake-proof) unreinforced brick walls, Napa requires older brick structures to have seismic retrofitting. In most cases, this means the walls are bolted to ceilings and floors, which provides some extra stability.

However, this retrofit doesn’t address the major problem brick walls face during a quake — namely, the weak mortar joints between bricks. When the mortar gives way, entire sections of wall can crumble loose and send tons of bricks crashing to the ground.

While the current retrofit regulations do save lives by keeping buildings from pancaking down when the ground shakes, these buildings can still sustain major damage.

715-6-damaged_buliding
Photo of Napa earthquake damage by Erol Kalkan, via USGS

Stronger…but More Expensive

Stronger retrofitting options include applying reinforced concrete or polymers to the walls, or installing diagonal braces. Officials also say that part of brick building retrofitting should include a mortar test to make sure joints are strong enough.

But these fixes come with a much steeper price tag than current retrofits. Since most communities only require the less expensive option, many building owners find it hard to justify the cost.

Still, there are various step-ups from the basic retrofit that can further strengthen a building and protect those inside it.

For example, many brick buildings were built with wood floors and ceilings that flex during a quake, pulling and pushing on walls to create even more stress. So the next level of protection, after bolting walls to ceilings and floors, is to increase the horizontal stiffness of the floors and roof. This can be as simple as adding a layer of plywood or oriented-strand board sheathing.

Next, owners can add columns that will hold up floors and the roof, even if parts of the bearing wall crumble. This won’t prevent all property damage, but adds an extra layer of protection for people inside the building.

Anchored supports attached to the walls can act as braces that keep the walls from bowing in and out during the quake. Another method of dampening these vibrations is to build interior partitions that stiffen the overall structure of the building.

One more method of stabilizing a building is to drill a hole down through the wall from top to bottom, install a steel bar, then fill the hole with grout.

Seismic retrofitting can be an expensive undertaking, especially when a building’s construction requires one of these more advanced methods. But as Napa taught us, not all retrofits are created equal. Taking on more difficult and pricey options may be critical to preventing injuries — and worse, loss of lives — in the next big quake.

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