QUESTION: After reading the Daily Union Democrat articles about the collapse of a stacked rock wall of a building in downtown Sonora, I'm wondering how many buildings in Sonora are constructed in this same way? Has anything been done to shore these buildings up? It seems like they might be particularly deadly places during an earthquake.
ANSWER: City Administrator Tim Miller said the city doesn’t have an inventory of buildings with stacked rock walls, also known as unreinforced masonry buildings or URMs.
He said that usually refers to brick buildings, but it also includes those with rock walls built before regulations were enacted for construction to resist earthquakes.
Miller said the state adopted a law in 1986 that required URM buildings be inventoried and retrofitted in earthquake-prone areas.The law also required this information to be reported to the state.
“The city was not located in a seismic zone in which this was required,” he said. So no inventory was done.
Last April, an outside wall at 89 N. Washington St. collapsed after a heavy rain and the back side of the building was demolished soon after. The wall had been bulging for several months, so there was some notice that the wall could fail.
The back part, which fronts on North Stewart Street, included two apartments and a studio.
Daren Wardell, the city’s building inspector, told The Union Democrat last year that building owners who are concerned about rock walls should consult a structural engineer.
“When you get into this unreinforced masonry, whether it’s stacked rock or brick, you have a risk of it coming apart,” Wardell said.
Joseph Gee, who owns the building, said he did not know how old the building was but likely dated to the Gold Rush era — 1850 or so.
“That kind of construction is pretty common in Sonora and I didn’t expect something like this would happen,” Gee told The Union Democrat last year. “I thought it was solidly built with granite rocks as a foundation.”