Q: We searched for ways to brace our chimney and were referred to a chimney builder by an earthquake retrofitting company.
I don’t recall the specifics, but the job involved steel rods inside all four corners of the chimney extending the length of the chimney top to bottom, with a lightweight substance (cement-like polymer, if I remember right) poured into the space between the flue and the bricks on the outside, to bond with all and hold all together.
How does this sound to you? Your recent article sounded as if you thought chimneys were hopeless to retrofit.
A: We’ve seen one masonry chimney built from the ground up by a master mason. Four long pieces of rebar were bent at 90-degree angles and embedded in a cross pattern into a concrete footing. The remaining 14 feet or so of the rebar formed reinforcement between the flue tiles and the chimney bricks.
This chimney was a freestanding structure, independent of the house. Your retrofit is not anchored to a footing, so our gut reaction is the retrofit is not going to perform in a quake. But rather than go with our gut, we consulted with an expert.
A while back we were taken to task by Richmond, Calif., structural engineer Ralph Hueston Kratz for an answer we gave a reader about diagonal bracing in wood-framed walls. The email string morphed into a discussion about earthquake retrofitting, even though our initial column was about mitigating cracked wall finishes.
After a number of email exchanges, Kratz shared his chagrin at seeing all of the residential brick chimneys during his travels around communities on the eastern side of San Francisco Bay. Rather than masonry chimneys, his trained eye saw a catastrophe in the making when the next big quake hits the Hayward Fault.
We asked Kratz’s opinion on our reader’s chimney retrofit. Here’s his answer:
"First, I really don’t believe it’s feasible to adequately brace an old, unreinforced masonry chimney to resist the strong lateral force of an earthquake that can easily exceed the full force of gravity.
"Second, even if the chimney is strengthened from top to bottom, as proposed by your reader’s advisers, it simply creates a monolith that will fall as a unit. This is more dangerous than if the chimney simply crumbles brick by brick.
"A typical old wood-framed house is barely strong enough to keep itself intact, much less have enough strength to keep a very heavy brick chimney upright. Bracing it to an old roof consisting of 2-by-4 rafters is practically impossible. This type of roof structure is barely strong enough to hold itself up without badly sagging and certainly not strong enough to resist a huge earthquake while bracing a chimney.
"I have to laugh — sadly — when I see a tiny little horizontal steel rod, maybe 3/4 inches in diameter, extending from an old roof to a big brick chimney. And I practically cry when I see four big steel angles, one at each corner of a brick chimney, but extending only above the roof.
"This completely ignores the most dangerous point — where the chimney meets the roof. These steel angles only make the chimney more dangerous.
"Finally, if you do strengthen a brick chimney and fasten it properly to a house, you’ve made the house even more susceptible to significant damage in an earthquake because you’ve added the chimney’s earthquake forces to the house’s earthquake forces. You’ve made a bad situation worse."
Kratz concludes that, "There really isn’t a practical way to strengthen a heavy brick chimney and make one’s home safer. Tear it down at least to the top of the fireplace, and preferably to the ground, and replace it with a lightweight fireplace and chimney."