Abstract: An improved building structure and method of use comprising separately fabricated floor, wall and ceiling structures capable of withstanding shear and seismic forces. The wall structure comprises an elongate track and a plurality of posts rigidly attached to a foundation structure of the building. Disposed between the posts are a plurality of interlocking foam wall sections which encapsulate the posts of the wall structure. Attached to the top ends of the posts are upper and lower header beams which serve as a support structure for the roof and ceiling structures. Brace members are attached to the posts and header beam to provide the wall structure with increased seismic stability.