Timothy A. Philpot - Mechanics of materials _ an integrated learning system-John Wiley (2017)

the distributed uniform area loading. In the analysis, the effects of such concentrated loads
at various potentially critical locations must be investigated.
A load suddenly applied to a structure is termed impact. A crate dropped on the floor
of a warehouse or a truck bouncing on uneven pavement creates a greater force in a structure
than would normally occur if the load were applied slowly and gradually. Specified
live loads generally include an appropriate allowance for impact effects of normal use and
traffic. Special impact consideration may be necessary for structures supporting elevator
machinery, large reciprocating or rotating machinery, and cranes.
By their nature, live loads are known with much less certainty than dead loads. Live
loads vary in intensity and location throughout the lifetime of the structure. In a building,
for example, unanticipated crowding of people in a space may occur on occasion or perhaps
a space may be subjected to unusually large loads during renovation as furnishings or
other materials are temporarily relocated.
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SAFETy
Snow Load
In colder climates, snow load may be a significant design consideration for roof elements.
The magnitude and duration of snow loads cannot be known with great certainty. Further,
the distribution of snow generally will not be uniform on a roof structure because wind will
cause snow to drift. Large accumulations of snow often will occur near locations where a
roof changes height, creating additional loading effects.
Wind Loads
Wind exerts pressure on a building in proportion to the square of its velocity. At any given
moment, wind velocities consist of an average velocity plus a superimposed turbulence
known as a wind gust. Wind pressures are distributed over a building’s exterior surfaces,
both as positive pressures that push on walls or roof surfaces and as negative pressures (or
suction) that uplift roofs and pull walls outward. Wind load magnitudes acting on structures
vary with geographic location, heights above ground, surrounding terrain characteristics,
building shape and features, and other factors. Wind is capable of striking a structure from
any direction. Altogether, these characteristics make it very difficult to predict the magnitude
and distribution of wind loading accurately.
4.3 Safety
Engineers seek to produce objects that are sufficiently strong to perform their intended
function safely. To achieve safety in design with respect to strength, structures and machines
are always designed to withstand loads above what would be expected under ordinary
conditions. (Such loads are termed overload). While this reserve capacity is needed to
ensure safety in response to an extreme load event, it also allows the structure or machine
to be used in ways not originally anticipated during design.
The crucial question, however, is “How safe is safe enough?” On the one hand, if a
structure or machine does not have enough extra capacity, there is a significant probability
that an overload could cause failure, where failure is defined as breakage, rupture, or collapse.
On the other hand, if too much reserve capacity is incorporated into the design of a
component, the potential for failure may be slight but the object may be unnecessarily
bulky, heavy, or expensive to build. The best designs strike a balance between economy and
a conservative, but reasonable, margin of safety against failure.
Two philosophies for addressing safety are commonly used in current engineering
design practice for structures and machines. These two approaches are called allowable
stress design and load and resistance factor design.