Geoscience Reference
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components (ropes and chains), and other structural elements. A common example of a static situ-
ation is shown in the bolt-and-plate assembly. The bolt is loaded in tension and holds two elements
together. One force acting on it is the load on the lower element (160-lb load plus 15 lb of suspending
elements). Another force is that caused by the tightened nut (25 lb). The total effective load on the bolt
is 200 lb (160 + 15 + 25). The plate will fail in shear if the head of the bolt pulls through the plate.
11.6.2 d ynamiCs
Dynamics (kinetics in mechanics) is the mathematical and physical study of the behavior of bodies
under the action of forces that produce changes of motion in them. In dynamics, certain properties
are important: displacement, velocity, acceleration, momentum, kinetic energy, potential energy,
work, and power. Environmental engineers work with these properties to determine, for example, if
rotating equipment will fly apart and cause injury to workers or to determine the distance required
to stop a vehicle in motion.
11.6.3 h ydrauliCs and p neumatiCs —F luid m eChaniCs
Hydraulics (liquids only) and pneumatics (gases only) make up the study of fluid mechanics, which
in turn is the study of forces acting on fluids (both liquids and gases). Environmental engineers
encounter many fluid mechanics problems and applications of fluid mechanics. In particular, envi-
ronmental engineers working in chemical industries, or in or around processes using or producing
chemicals, need to have an understanding of flowing liquids or gases to be able to predict and con-
trol their behavior.
11.6.4 W elds
Welding is a method of joining metals to achieve a more efficient use of the materials and faster
fabrication and erection. Welding also permits the designer to develop and use new and aesthetically
appealing designs, and it saves weight because connecting plates are not needed and allowances
need not be made for reduced load-carrying ability due to holes for rivets, bolts, and so on (Heisler,
1998). Simply put, the welding process joins two pieces of metal together by establishing a metal-
lurgical bond between them. Most processes use a fusion technique; the two most widely used are
arc welding and gas welding. In the welding process, where two pieces of metal are joined together,
the mechanical properties of metals are important, of course. The mechanical properties of metals
primarily determine how materials behave under applied loads—in other words, how strong a metal
is when it comes in contact with one or more forces. The important point is that if you apply knowl-
edge about the strength properties of a metal, you can build a structure that is both safe and sound.
The welder must know the strength of his weld as compared with the base metal to produce a weld-
ment that is strong enough to do the job. Thus, the welder is just as concerned with the mechanical
properties of metals as is the engineer.
11.6.5 m oment
Moment is a synonym for torque ; however, in many engineering applications, the two terms are not
interchangeable. Torque is usually used to describe a rotational force down a shaft (e.g., turning of a
pump shaft), whereas moment is more often used to describe a bending force on a beam. Moment is
the product of the force magnitude ( F ) and the distance from the point to its action line. The perpen-
dicular distance ( d ) is called the arm of the force, and the point is the origin or center of the moment.
The product is the measure of the tendency of the force to cause rotation (e.g., bending, twisting).
The unit of measurement for moment is a combination of the names of the force and distance units,
such as pound-foot (lb-ft), to distinguish it from the unit for work or energy, the foot-pound (ft-lb).
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