Most
of the material in the preceding chapters has pertained to the design of
reinforced concrete structural elements, eg; slabs, columns, beams, and
footings. These elements are combined in various ways to create structural
system for buildings and other construction. An important part of the total
responsibility of the structural engineer is to select, from many alternatives,
the best structural system for the given condition. The wise choice of
structural system is fare more important, in its effect on overall economy and
serviceability, then refinements is proportioning the individual members. Close
cooperation with the architect in the early stage of project is essential in
developing a structure that not only meets functional and esthetic requirements
but exploits to the fullest the special advantages of reinforced concrete,
which include the following.
Versatility
of from. Usually placed in the structure in the fluid state, the material is
readily adaptable to a wide variety of architectural and functional
requirements.
Durability.
With proper concrete protection for the steel reinforcement, the structure will
have long life, even under highly adverse climatic or environmental conditions.
Fire
resistance. With proper protection for the reinforcement, a reinforced concrete
structure provides the maximum in fire protection.
Speed
of construction. In terms of the entire period, from the date of approval of
the concrete drawings to the date of completion, a concrete building can often
be completed in less time than a steel structure. Although the field erection
of a steel building is more rapid, this phase must necessarily be preceded by
prefabrication of all parts in the shop.
Cost.
In many cases the first cost of a concrete structure is less then that of a
comparable steel structure. In almost every case, maintenance costs are less.
Availability
of labor and material. It is always possible to make use of local souses of
labor, and in many inaccessible areas, a nearby source of good aggregate can be
found, so that only the cement and reinforcement need to be brought in from a
remote source.
Floor
and Roof System:
The
types of concrete floor and roof systems are so numerous as to defy concise
classification. In steel construction, the designer usually is limited to using
structural shapes that have been standardized in form and size by the relatively
few prodcers in the field. In reinforced concrete, on the other hand, the
engineer has almost complete control over the form of the structurl parts of
building. In addition, many small producers of reinforced concrete structural
elements and accessories can compete profitably in this field, since plant and
equipment requirement are not excessive. This has resulted in the development
of a wide variety of concrete system. Only the more common types can be
mentioned in this text.
In
general, the commonly used reinforced concrete floor and roof system can be
classified as one-way system, in which the main reinforcement in each
structural element runs in one direction only, and two-way systems, in which
the main reinforcement in at least one of the structurl elements runs in
perpendicular directions. Systems of each type can be identified in the
following list:
(a) One-way slab supported by monolithic concrete beams
(b) One-way slab supported by steel beams (shear concrete
are used for composite action in the direction of the beam span)
(c) One-way slab with cold-formed steel decking as form
and reinforcement
(d) One-way joist floor (also know as ribbed slab)
(e) Two-way slab supported by edge beams for each panel
(f) Flat slabs, with column capitals for drop panels or
both, but without beams
(g) Flat plates, without beams and with no drop panels or
column capitals
(h) Two-way joist floors, with or without beams on the
column lines
Each
of these types will be described briefly in the following sections. In addition
to the cast-in-place floor and roof system described in this section, a great
variety of precast concrete systems has been devised.
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