Next big quake could be around the corner
Next big quake could be around the corner
Bill and Kevin Burnett
A while back, we responded to a real estate agent who wanted to know how to fix an old two-story home with severely sloping floors. The home had just been newly drywalled and painted to get it ready for sale, but all the sprucing up in the world could not mask its major structural deficiencies.
We stopped short of suggesting the owner tear it down and start anew. But our list of proposed fixes was truly a parade of horribles, from replacing the foundation to possibly fixing leaking water and gas lines to re-rocking and painting. Our advice for an agent representing a buyer was to "walk away," and if representing the seller, to "disclose, disclose, disclose."
Our answer triggered an email from a structural engineer who has taken issue with some of our past suggestions, but not this time. He wrote, "I've been pretty critical of some of your evaluations/recommendations in the past, so I just wanted to let you know that I think today's column is spot on! Every word is right on the mark!"
We thanked him and agreed to address a serious concern he has about a widespread condition he sees in his travels around the San Francisco Bay Area. We paraphrased his comments:
Masonry chimneys are perhaps the most urgent earthquake hazard in older homes. The problem is that they're likely to fall in even a modest shake. A rule of thumb is that brick chimneys extending more than 1 1/2 times their least width above the roof pose a hazard of collapsing above the roof, not to mention any possible hazard they may pose below the roof level. That's less than 2 feet for a typical 14-inch-wide chimney.
If 3 feet of bricks fall toward the house, it's very likely to go through not only the roof, but also the ceiling below, taking down everything, and everybody, near it.
"I can't drive around my neighborhood without seeing many examples of deathly tall brick chimneys. They remind me of the people-squashing falling safes in old "Road Runner" cartoons. The attempts at bracing them are a joke!"
It's impossible to adequately brace an old brick chimney. First problem, what do you brace it to? The hand-stacked old roof framed with two-by-fours from eave to ridge can barely support itself, let alone brace thousands of pounds of bricks.
Problem two: How do you fasten braces to a chimney that may extend 10 feet above the roof (at an eave) and is as skinny as Twiggy? Bracing such a chimney might consist of a quartet of structural steel angles, one at each corner, extending continuously the full height of the chimney above the roof, well fastened together, and then braced to strengthened roof framing. This is not only ugly but is a leak problem in the making.
Chimneys inside tall attics are especially problematic. These chimneys can easily be 12 feet tall at the ridge. Buckling inside the attic is a real possibility, which would rain down a "ton of bricks" onto the unsuspecting occupants below.
Our structural engineer has a solution: Take the chimney down to the roof line or the attic floor and build it up with a new, lightweight, self-insulating metal chimney. If the metal look doesn't work with your sense of design, wrap the new metal in a plywood box faced with thin brick veneer.
According to the Association of Bay Area Governments (ABAG), approximately 60,000 masonry chimneys were destroyed or damaged beyond repair in the 1994 Northridge earthquake.
ABAG -- the regional planning agency for the nine-county San Francisco Bay Area -- advises that homeowners make chimney safety "a high priority."
"Most chimneys tend to break at the roof line and fall away from the home," ABAG notes. "However, some chimneys can fall into the home, causing serious injury and death."
ABAG has concluded that "retrofitting masonry chimneys with bracing or strapping is not an effective safety measure," and advises homeowners to consider replacing masonry chimneys with modern stud-framed ones around a metal flue.
Another option is a partial chimney replacement in which all of the bricks above the firebox are removed. This partial replacement technique is used by the city of Los Angeles.
We agree with these comments. Brick chimneys are easy to ignore. As the horrors of the 1989 Loma Prieta earthquake fade from memory, we go on with our lives. But, sure as the sun rises, another big quake will come.
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