(1) Precast concrete construction can consist of either prestressed
or conventionally reinforced members. Prestressing is advantageous in
conventional construction, for members subjected to high flexural stresses
such as long span or heavily loaded slabs and beams. Other advantages of
precast concrete construction include: a) completion time for precast
construction will be significantly less than the required for cast-in-place
concrete, b) precast construction will provide protection against primary
and secondary fragments not usually afforded by steel construction, and c)
precast work is generally more economical than cast-in-place concrete
construction especially when standard precast shapes can be used. The
overriding disadvantage of precast construction is that the use of precast
members is limited to buildings located at relatively low pressure levels of
1 to 2 psi. For slightly higher pressure levels, cast-in-place concrete or
structural steel construction becomes the more economical means of
construction. However, for even higher pressures, cast-in-place concrete is
the only means available to economically withstand the applied load.
(2) Precast structures are of the shear wall type, rigid frame
structures being economically impractical (see the discussion of
connections, paragraph 2.g. below). Conventionally designed precast
structures may be multi-story, but for blast design it is recommended that
they be limited to single story buildings.
(3) Some of the most common precast sections are shown in Figure 60.
The single tee and double tee sections are used for wall panels and roof
panels. All the other sections are beam and girder elements. In addition,
a modified flat slab section will be used as a wall panel around door
openings. All of the sections shown can be prestressed or conventionally
reinforced. In general though, for blast design, beams and roof panels are
prestressed, while columns and wall panels are not. For conventional
design, prestressing wall panels and columns is advantageous in tall
multi-story buildings, and thus of no benefit for blast resistant design
which uses only single story buildings. In fact, in the design of a wall
panel the blast load is from the opposite direction of conventional loads
and hence prestressing a wall panel decreases rather than increases the
capacity of section.
Static Strength of Materials.
(1) Concrete. Generally, the minimum compressive strength of the
concrete, f'c, used in precast elements is 4000 to 5000 psi. High
early-strength cement is usually used in prestressed element to ensure
adequate concrete strength is developed before the prestress is introduced.
(2) Reinforcing Bars. Steel reinforcing bars are used for rebound
and shear reinforcement in prestressed members as well as for flexural
reinforcement in non-prestressed members. For use in blast design, bars
designated by the American Society for Testing and Materials (ASTM) as A615,
grade 60, are recommended. As only small deflections are permitted in
precast members, the reinforcement is not stressed into its strain hardening
region and thus the static design strength of the reinforcement is equal to
its yield stress, fy = 60,000 psi.