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Wrong Brace Makes Damage Worse

August 14, 1994

Regarding the article headlined "Securing Your Chimney in Case of Quake" (July 31), the Uniform Building Code, Section 3704(c) requires anchor straps to be connected to 2-by-4 ties crossing a minimum of four joists or rafters.

If done as shown (only one or two joists), this bracing could cause more damage to the house during an earthquake than before the remedial work suggested. The whole job is not for a novice; use only those experienced in wood-frame construction in the local area.

WILLIAM KRISEL, A.I.A

Los Angeles

The writer is an architect.

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As I looked at the diagram with the chimney-bracing article ("Securing Your Chimney in Case of Quake," July 31), certain obvious problems caught my eye.

Most likely, the rafters were 2-by-4s or 2-by-6s spaced 24 inches apart. Obviously, the two lag bolts shown could not be bolted to a typical rafter and a single bolt would be difficult to properly attach on a 1 1/2-inch rafter (the finished dimension of a 2-by-4), and even if connected, would be rendered ineffective in a significant earthquake by either splitting or breaking the rafter.

Further, most pitched roofs do not have solid sheathing nailed to the rafters. Over the years, spaced 1-by-6 boards have been the common construction practice in attic areas beneath roof shingles, so even if bolts were attached to the rafters, the roof system would be unable to offer much resistance to support the chimney.

A solution would be to place wood blocking beneath the bolts and between adjacent rafters and wood sheathing on the underside of the rafters.

FRANK L. BURKE

Los Angeles

The writer is a structural engineer.

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Abrams replies: It's my experience after the Northridge quake that a chimney brace bolted into even one rafter is better than nothing, but clearly it's preferable to bolt it to as many rafters as possible or to add blocking in the attic across several joists. However, in most modern Southern California homes, with vaulted ceilings and no attic, blocking or bracing is not possible.

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