The blaze blocked Interstate 15 in both directions in San Bernardino County, the California Highway Patrol said. The highway is the main route between Los Angeles and Las Vegas.
The fire in Hesperia near Victorville was reported at about 1:30 p.m. local time. Investigators say blowtorches ignited wooden supports and, as the blaze spread, first wood and then metal began fallling to the roadway below.
Construction workers were trying to dismantle the bridge to allow firefighters access to the interior, while workers struggled to clear debris from the highway.
“We’re doing everything we can to get this roadway open as quickly as possible,” Matthew Hunt, a highway patrol spokesman, said. “We understand how important it is to get this roadway cleaned up.”
Transportation officials said they hoped to reopen the northbound side of the highway by 4 p.m. to ease the evening commute out of Los Angeles. The southbound lanes were scheduled to reopen by rush hour Wednesday morning.