External loads on a structure may be
classified in several different ways. In one classification, 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 that are
applied a number of times, causing a variation 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 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.
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