Building Structures

STRUCTURAL FUNCTIONS 17
Figure 1.24 Development of bending.
resistance to dimensional deformation. It must also be
inherently stable, both internally and externally. These three
characteristics—strength, stiffness, and stability—are the
principal functional requirements of structures.
Stresses and Strains
There are three basic types of stress: tension, compression,
and shear. Tension and compression are similar in nature
although opposite in sign or direction. Both tension and
compression produce a linear type of strain and can be
visualized as pressure effects perpendicular to the surface of
a stressed cross section. Because of these similarities, tension
and compression are referred to as direct stresses; one is
considered positive and the other negative.
Shear stress occurring in the plane of a cross section is
similar to the frictional sliding effect. Strain due to shear takes
the form of angular distortion, rather than the lengthening
or shortening due to direct stress.
Dynamic Effects
Vibrations, moving loads, and sudden changes in the state
of motion, such as the jolt of braking or rapid acceleration
of vehicles, cause force effects that result in stresses and
strains in structures. The study of dynamic forces and their
effects is complex, although some of the basic concepts can
be illustrated simply.
For structural investigation and design the significant
distinction between static and dynamic effects has to do with
response of the structure to the loading. If the principal
response of the structure can be effectively evaluated in static
terms (force, stress, linear deformation, etc.), the effect on
the structure is essentially static. If, however, the structure’s
response can be evaluated effectively only in terms such as
energy capacity, work accomplished, cyclic movement, and
so on, the effect of the loading is of a true dynamic character.
Judgments made in this regard must be made in consideration
of the adequate performance of the structure in its role in
the building system. Performance relates to both structural
responses and effects on the building and its occupants.
A critical factor in the evaluation of a structure’s response
to dynamic loads is the fundamental period of the structure’s
cyclic motion or vibration. This is the time required for
one full cycle of motion, in the form of a bounce or a
continuing vibration. The relation of this time to the time
of buildup of the load is a major factor in the determination
of the relative degree of the dynamic effect on the structure.
A structure’s fundamental period may vary from a fraction
of a second to several seconds, depending on the size, shape,
mass, materials, stiffness, support conditions, and possible
presence of damping effects.
Design for dynamic effects begins with an evaluation of
possible dynamic load sources and their potential actions.
The response of the structure is then considered using the
variables of its dynamic character. Once the dynamic behavior
is understood, the designer can consider how to manipulate
the variables to improve the structure’s behavior or to reduce
the load effects.
Design for Structural Behavior
In professional design practice the investigation of structural
behavior is an important part of the design process. To