The fire broke out shortly before 2 p.m. at the BSI Inc. building next to the interchange of the 118 Freeway and Interstate 5. It took 50 firefighters about 30 minutes to extinguish it, said Fire Department spokesman Brian Humphrey.
``The fire was sparked by a welder who inadvertently ignited flammable liquids and refuse, which had collected in a permanent trench in the floor,'' Humphrey said.
It burned so intensely that it melted large steel girders in the building's roof.
``They were twisted like toys,'' Humphrey said.
The injured person was a 61-year-old man who had been working at the building and whose name was not immediately available. He was hospitalized in fair condition.
The blaze sent a huge plume of smoke spiraling into the sky above Interstate 5, a major route connecting Los Angeles with San Francisco, but didn't appear to disrupt traffic.
A damage amount was not immediately available, but Humphrey said the sprawling single-story building in the northeast San Fernando Valley was heavily damaged.
