External loads on a structure may be classiﬁed in several different ways. In one classiﬁcation in types of structural loads , they may be considered as static or dynamic.
- Static loads are forces that are applied slowly and then remain nearly constant. One example is the weight, or dead load, of a ﬂoor or roof system.
- Dynamic loads vary with time. They include repeated and impact loads.
- Repeated loads are forces tha tare applied an umber of times,causing avariation in the magnitude, and sometimes also in the sense, of the internal forces. A good example is an off-balance motor.
- Impact loads are forces that require a structure or its components to absorb energy in a short interval of time. An example is the dropping of a heavy weight on a ﬂoor slab, or the shock wave from an explosion striking the walls and roof of a building.
- External forces may also be classiﬁed as distributed and concentrated.
- Uniformly distributed loads are forces that are, or for practical purposes may be considered, constant over a surface area of the supporting member. Dead weight of a rolled-steel I beam is a good example.
- Concentrated loads are forces that have such a small contact area as to be negligible compared with the entire surface area of the supporting member. A beam supported on a girder, for example, may be considered, for all practical purposes, a concentrated load on the girder.
Another common classiﬁcation in types of structural loads for external forces labels them axial, eccentric, and torsional.
- An axial load is a force whose resultant passes through the centroid of a section under consideration and is perpendicular to the plane of the section.
- An eccentric load is a force perpendicular to the plane of the section under consideration but not passing through the centroid of the section, thus bending the supporting member
- Torsional loads are forces that are offset from the shear center of the section under consideration and are inclined to or in the plane of the section, thus twisting the supporting member
Also, building codes classify loads in accordance with the nature of the source. For example:
- Dead loads include materials, equipment, constructions, or other elements of weight supported in, on, or by a building, including its own weight, that are intended to remain permanently in place.
- Live loads include all occupants, materials, equipment, constructions, or other elements of weight supported in, on, or by a building and that will or are likely to be moved or relocated during the expected life of the building.
- Impact loads are a fraction of the live loads used to account for additional stresses and deﬂections resulting from movement of the live loads.
- Wind loads are maximum forces that may be applied to a building by wind in a mean recurrence interval, or a set of forces that will produce equivalent stresses.
- Snow loads are maximum forces that may be applied by snow accumulation in a mean recurrence interval.
- Seismic loads are forces that produce maximum stresses or deformations in a building during an earthquake.
- BUILDING DESIGN AND CONSTRUCTION HANDBOOK for Frederick S. Merritt & Jonathan T. Ricketts