Boston firefighters who responded to a false alarm at a 5-story brick apartment building in Chinatown today evacuated at least 20 residents after finding issues with the structure’s integrity. Fire crews arrived at 25 Harrison Ave. around 3:25 p.m. but found no fire in the building, fire department spokesman Steve MacDonald said. While investigating the call, some firefighters raised concerns about “structural issues” with beams and framing, he said. Floors one and two were vacant, but fire personnel evacuated between 20 and 30 residents from the third, fourth and fifth floors, MacDonald said.