Buildings can be classified based on occupancy and structure. Residential, educational, institutional, assembly, business, mercantile, industrial, storage, and hazardous are common occupancy classifications. Structures can be load bearing, framed, or composite. Load bearing structures transmit loads through walls to foundations, while framed structures use columns and beams to separate loads from walls. Composite structures combine load bearing walls with framed supports. Buildings must be designed to support dead, live, snow, rain, wind, and earthquake loads. Foundations transmit loads and come in shallow, deep, pile, pier, well, caisson, spread, strap, combined, and mat types. Doors, windows, and stairs are essential building components.