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Definitions of Loads
A load in a building works to destroy the gravity resistance system of the structure.
According to NFPA 5000: Building Construction and Safety Code, loads are “forces or other actions that result from the weight of all building materials, occupants and their possessions, environmental effects, differential movement, and restrained dimensional changes.”
Specific terms are used to describe the various types of loads and the way they are placed on a building.
It is important to understand and use these terms correctly.
Dictionary definitions are not always identical to the definitions used in the construction field.
In addition, the various terms are not always mutually exclusive. For example, a load may be a live load and an impact load at the same time.
Stress and Strain
Before we proceed any further, we must first understand the concepts of stress and strain, the result of forces applied to a structural member.
In the common vernacular, stress and strain are considered synonyms; however, a clear understanding of the technical definitions of these terms is critical for fire fighters.
As we have discussed, an external force that acts on a structure is called a load.
The internal forces that resist the load are called stress and strain.
Stress is the force per unit area that produces a deformation.
It is usually measured in pounds per square inch (psi), although occasional references to pounds per square foot (psf) are also found.
The unit area measurement is at the discretion of the person making the calculations.
Always be careful to note the unit area when examining any calculations.
KIP, a term meaning 1,000 pounds, is used in engineering calculations where the number would be so large as to be unwieldy.
Strain, in contrast, is the actual percentage of elongation (deformation) that occurs when a material is stressed. It is measured in fractions of an inch of deformation per inch of original length of the material.
Compression, Tension, Torsion, and Shear Forces
Generally, four types of forces can be applied to a structural member: compression, tension, torsion, and shear Figure 2-2.
In essence, a compressive force is one in which the force squeezes a structural member, such as a concrete column supporting a floor.
Tensile forces stretch a member such as a steel cable that is supporting a suspended walkway.
A torsional force is a twisting force such as that created by turning a screwdriver on a screw; it is the result of torque (the measurable turning force applied to a structural member).
Shear forces occur within a building member when opposing forces pull the member in opposite directions and are often the result of wind loads.
Figure 2-2 The design of a suspension bridge anticipates three separate forces: compression, torsion, and tension.