In
civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewage ...
, specified loads are the best estimate of the actual loads a structure is expected to carry. These loads come in many different forms, such as people, equipment, vehicles, wind, rain, snow, earthquakes, the building materials themselves, etc. Specified Loads also known as Characteristic loads in many cases.
Buildings will be subject to loads from various sources. The principal ones can be classified as
live load
A structural load or structural action is a force, deformation, or acceleration applied to structural elements. A load causes stress, deformation, and displacement in a structure. Structural analysis, a discipline in engineering, analyzes the effe ...
s (loads which are not always present in the structure),
dead load
A structural load or structural action is a force, deformation, or acceleration applied to structural elements. A load causes stress, deformation, and displacement in a structure. Structural analysis, a discipline in engineering, analyzes the e ...
s (loads which are permanent and immovable excepting redesign or renovation) and wind load, as described below. In some cases structures may be subject to other loads, such as those due to earthquakes or pressures from retained material. The expected maximum magnitude of each is referred to as the characteristic load.
Dead loads are those representing the self weight of the building; their magnitude can be estimated on the basis of material densities and component sizes.
Dead loads are those due to the self weight of the structure and any permanent fittings and finishes. It is generally possible to quantify the magnitude of dead loads with a reasonable degree of confidence.
Imposed loads are those associated with occupation and use of the building; their magnitude is less clearly defined and is generally related to the use of the building.
A good example of specified loads would be the following simplified
floor to ceiling sandwich load table (based on the National Building Code of Canada standards):
Floor Finish (
Terrazzo
Terrazzo is a composite material, poured in place or precast, which is used for floor and wall treatments. It consists of chips of marble, quartz, granite, glass, or other suitable material, poured with a cementitious binder (for chemical bindi ...
) per 10 mm thickness = 0.24 kN/m^2
Reinforced Concrete per 10 mm thickness = 0.24 kN/m^2
Mechanical Services = 0.14 kN/m^2
Electrical Services = 0.10 kN/m^2
Floor Area (110 mm thickness) = 8 m^2
Total Dead Load = (0.24 + 11*0.24 + 0.14 + 0.10)*8 = 24.96 kN
In order to design to these loads, one would need to convert them to
design load
In a general sense, the design load is the maximum amount of something a system is designed to handle or the maximum amount of something that the system can produce, which are very different meanings. For example, a crane with a design load of 20 ...
s by applying
Load factors or, more generally, a form of
safety factor
In engineering, a factor of safety (FoS), also known as (and used interchangeably with) safety factor (SF), expresses how much stronger a system is than it needs to be for an intended load. Safety factors are often calculated using detailed analy ...
s to them. In the case of limit states design, the resulting factored load is then called a
Design load
In a general sense, the design load is the maximum amount of something a system is designed to handle or the maximum amount of something that the system can produce, which are very different meanings. For example, a crane with a design load of 20 ...
. Note that in this case of Limit states design, we would refer to the factor as a load factor rather than a safety factor to try to eliminate possible confusion between
Limit states design
Limit State Design (LSD), also known as Load And Resistance Factor Design (LRFD), refers to a design method used in structural engineering. A limit state is a condition of a structure beyond which it no longer fulfills the relevant design criteria ...
and the older
Allowable stress design
Permissible stress design is a design philosophy used by mechanical engineers and civil engineers.
The civil designer ensures that the stresses developed in a structure due to service loads do not exceed the elastic limit. This limit is usually ...
.
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Structural engineering