No one was injured in a structure fire last Thursday morning in north Sioux Falls, according to a news release. Crews responded to a sprinkler system activation around 7:05 a.m. Thursday in the WestRock building at 100 E. Benson Road. A small fire was in the air handling equipment on the roof. Crews used an aerial ladder to access the roof and knock out the fire with a hose and control it within 10 minutes. The fire was started by an overheated electric motor on the air handling unit, according to the release. The fire damaged several hundred square feet of roofing material and caused light smoke inside the building. Activated sprinkler heads contained the fire until fire personnel arrived on scene. Workers were allowed back into the building within an hour of crews arriving on scene.