Civil Engineering Reference
In-Depth Information
6
Serviceability
6.1
INTRODUCTION
Today the structural design profession is concerned with a
limit states
philosophy. The
term
limit state
is used to describe a condition at which a structure or some part of a struc-
ture ceases to perform its intended function. There are two categories of limit states:
strength and serviceability.
Strength limit states
are based on the safety or load-carrying capacity of structures
and include buckling, fracture, fatigue, overturning, and so on. Chapters 3 to 5 have been
concerned with the bending limit state of various members.
Serviceability limit states
refer to the performance of structures under normal service
loads and are concerned with the uses and/or occupancy of structures. Serviceability is
measured by considering the magnitudes of deflections, cracks, and vibrations of struc-
tures as well as by considering the amounts of surface deterioration of the concrete and
corrosion of the reinforcing. You will note that these items may disrupt the use of struc-
tures but do not usually involve collapse.
This chapter is concerned with serviceability limits for deflections and crack widths.
The ACI Code contains very specific requirements relating to the strength limit states of
reinforced concrete members but allows the designer some freedom of judgment in the
serviceability areas. This doesn't mean that the serviceability limit states are not signifi-
cant, but by far the most important consideration (as in all structural specifications) is the
life and property of the public. As a result, public safety is not left up to the judgment of
the individual designer.
Vertical vibration for bridge and building floors, as well as lateral and torsional vibra-
tion in tall buildings, can be quite annoying to users of these structures. Vibrations, how-
ever, are not usually a problem in the average size reinforced concrete building, but we
should be on the lookout for the situations where they can be objectionable.
The deterioration of concrete surfaces can be greatly minimized by exercising good
control of the mixing, placing, and curing of the concrete. When those surfaces are sub-
jected to harsh chemicals, special cements with special additives can be used to protect
the surfaces. The corrosion of reinforcing can be greatly minimized by giving careful at-
tention to concrete quality, using good vibration of the concrete, using adequate cover
thickness for the bars, and limiting crack sizes.
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