The simplest major building systems consists of only two components. One component is a ﬂoor, a ﬂat, horizontal surface on which human activities can take place. The other component is an enclosure that extends over the ﬂoor and generally also around it to provide shelter from the weather for human activities.
The ground may serve as the ﬂoor in primitive buildings. In better buildings, however, the ﬂoor may be a structural deck laid on the ground or supported above ground on structural members, such as the joist and walls in above Figure. Use of a deck and structural members adds at least two different types of components, or two subsystems, to the simplest building system. Also, often, the enclosure over the ﬂoor requires supports, and the walls, in turn, are seated on foundations in the ground. Additionally, footings are required at the base of the foundations to spread the load over a large area of the ground, to prevent the building from sinking . Consequently, even slight improvements in a primitive building introduce numerous additional components, or subsystems, into a building.
More advanced major building systems consist of numerous subsystems, which are referred to as systems in this book when they are major components. Major subsystems generally include structural framing and foundations, enclosure systems, plumbing, lighting, acoustics, safety systems, vertical-circulation elements, electric power and signal systems, and heating, ventilation, and air conditioning (HVAC).
Structural System. The portion of a building that extends above the ground level outside it is called the superstructure. The portion below the outside ground level is called the substructure. The parts of the substructure that distribute building loads to the ground are known as foundations.
Foundations may take the form of walls. When the ground under the building is excavated for a cellar, or basement, the foundation walls have the additional task of retaining the earth along the outside of the building . The superstructure in such cases is erected atop the foundation walls.
- BUILDING DESIGN AND CONSTRUCTION HANDBOOK for Frederick S. Merritt & Jonathan T. Ricketts