Primary Loads are divided into two broad categories according to the way in which they act upon the structure or structural element. These are DEAD LOADS and LIVE LOADS. When considering the possible combinations of these two categories of loading, the odds of certain loads occuring simultaneously are assumed to be null. One such combination would be heavy snow, a typhoon, a raging fire and an earthquake. It is possible that two of the first three could occur as the earth quakes, but not that all four would be present at the same time. Thus, one need only consider reasonable loading combinations.
DEAD LOADS
Dead Loads are those loads which are considered to act permanently; they are "dead," stationary, and unable to be removed. The self-weight of the structural members normally provides the largest portion of the dead load of a building. This will clearly vary with the actual materials chosen. Permanent non-structural elements such as roofing, concrete, flooring, pipes, ducts, interior partition walls, Environmental Control Systems machinery, elevator machinery and all other construction systems within a building must also be included in the calculation of the total dead load. These loads are represented by the red arrow in the illustration.
The magnitude of the dead loads of a building can usually be determined with only a 5% margin of error. Properties of building materials are often tabulated and published by authors of textbooks and by the manufacturers. It is always very important to be sure to keep up-to-date on the changes in building materials. Properties of construction materials often vary due to the rapidly changing marketplace. The load due to these materials is often expressed as kN/m^3 or lbs/ft^3. These are normally converted to load/area or load/length for further consideration.
LIVE LOADS
Live Loads are not permanent and can change in magnitude. They include items found within a building such as furniture, pianos, safes, people, books, cars, computers, machinery, or stored materials, as well as environmental effects such as loads due to the sun, earth or weather. Wind and earthquakes loads are put into the special category of lateral live loads due to the severity of their action upon a building and their potential to cause failure.
Most buildings have a working life that extends beyond the initially perceived use of the building. Thus, it is almost impossible to predict all of the potential uses that any given structure will experience before it is demolished. If, and when, a building is to be used for a purpose other than its original design, the capacity of the building for its new use must be determined. Since the body of knowledge about the behaviour of buildings is always increasing, a building which might have been designed according to the latest loading information in one year, might not satisfy the requirements a few years later. This has especially been the case with the effects of earthquake loading.
The magnitudes of live loads are difficult to determine with the same degree of accuracy that is possible with dead loads. The probable maximum value of live loads has been determined by research and is included in national building codes. These are usually a minimum design load per area. Building codes also provide for load reductions under certain conditions. As an example, full live loads will not occur on every floor of a multi-story building at the same time. Therefore, the design live load for some of the columns and the foundation can be reduced. Building codes around the world do not concur on the magnitude of the appropriate design live load values. It is critical that the designer take the time to determine the values set down in the local building codes. These are legal documents and MUST be followed.