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

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dESIgN CONCEPTS
• Although their chemical composition may be the same, the materials used in prototypes
or test components may differ from those used in production components because of
such factors as microstructure, size, rolling or forming effects, and surface finish.
• Stresses may be created in a component during the fabrication process, and poor
workmanship could diminish the strength of a design.
• Models and methods used in analysis may oversimplify or incorrectly idealize a
structure and thereby inadequately represent its true behavior.
Textbook problems may convey the impression that analysis and design are processes of
applying rigorous calculation procedures to perfectly defined structures and machines in
order to obtain definitive results. In practice, however, design procedures must make allowances
for many factors that cannot be quantified with great certainty.
4.2 Types of Loads
The forces that act on a structure or machine are called loads. The specific types of load
that act on a structure or machine depend on the particular application. Several types of
load that act on building structures are discussed next.
Dead Loads
Dead loads consist of the weight of various structural members and the weights of objects
that are permanently attached to a structure. For a building, the self-weight of the structure
includes items such as beams, columns, floor slabs, walls, plumbing, electrical fixtures,
permanent mechanical equipment, and the roof. The magnitudes and locations of these
loads are unchanging throughout the lifetime of the structure.
In designing a structure, the size of each individual beam, floor, column, and other
component is unknown at the outset. An analysis of the structure must be performed before
final member sizes can be determined; however, the analysis must include the weight of the
members. Consequently, it is often necessary to perform design calculations iteratively:
estimating the weight of various components; performing an analysis; selecting appropriate
member sizes; and, if significant differences are present, repeating the analysis with
improved estimates for the member weights.
Although the self-weight of a structure is generally well defined, the dead load may be
underestimated because of the uncertainty of other dead-load components, such as the
weight of permanent equipment, room partitions, roofing materials, floor coverings, fixed
service equipment, and other immovable fixtures. Future modifications to the structure may
also need to be considered. For instance, additional highway paving materials may be
added to the deck of a bridge structure at a future time.
Live Loads
Live loads are loads in which the magnitude, duration, and location of the loading vary
throughout the lifetime of the structure. They may be caused by the weight of objects temporarily
placed on the structure, moving vehicles or people, or natural forces. The live load
on floors and decks is typically modeled as a uniformly distributed area loading that accounts
for items normally associated with the intended use of the space. For typical office
and residential structures, these items include occupants, furnishings, and storage.
For structures such as bridges and parking garages, a concentrated live load (or loads)
representing the weight of vehicles or other heavy items must be considered in addition to