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 dynamic resistance to the applied blast load. Thus, the dynamic ultimate
resistance of a member subjected to a blast load is greater than its static
ultimate resistance.
(2) Both the concrete and reinforcing steel exhibit greater strength
under rapid strain rates. The higher the strain rates, the higher the
compressive strength of concrete and the higher the yield and ultimate
strength of the reinforcement. This phenomenon is accounted for in the
design of a blast resistant structure by using the dynamic stresses to
calculate the ultimate resistance of the reinforced concrete members.
b.
Static Design Stresses.
(1) The materials of construction to be used for blast resistant
structures are given in NAVFAC P-397. The selection of the materials is
based primarily on the slab elements since, depending upon the anticipated
building usage, these elements may be permitted to attain large
deformations. Beams and columns are primary members and, as such, are not
permitted to attain large deformations.
(2) Reinforcing steel, designated by the American Society for
Testing and Materials (ASTM) as A615, grade 60, is recommended for use in
blast resistant structures. Since both beams and columns are not permitted
to attain large deflections, the reinforcement is stressed within its yield
range. The reinforcement is not stressed into its strain hardening region.
consequently, for the design of beams and columns, the static design stress
for the reinforcement is equal to its yield stress (fy = 60,000 psi).
(3) It is recommended that the minimum compressive strength of
concrete f'c be equal to at least 3,000 psi for members with small
rotations and 4,000 psi for members with large support rotations. This
provision applies primarily for slabs (exterior walls and roof), since beams
and columns are not permitted to attain large deformations. Consequently,
the concrete strength usually depends upon the design of the slab elements
which comprise the vast majority of the structure. The preferred concrete
strength for all blast resistant construction is equal to 4,000 psi.
c.
Dynamic Design Stresses.
(1) The increased strength of materials due to strain rate is
described by the dynamic increase factor, DIF. The DIF is equal to the
ratio of the dynamic to static stress, e.g., fdy/fy and f'dc/f'c.
The DIF depends on the material and the applied strain rate. For the design
of beams and columns subjected to low and intermediate blast pressures, a
DIF equal to 1.10 is used for reinforcement in bending and a DIF equal to
1.25 is used for concrete in compression. It is recommended that no DIF be
considered when determining shear or bond capacities nor when calculating
quantities of shear reinforcement.
(2) The dynamic design stress is obtained by multiplying the
appropriate static design stress by the appropriate DIF, where:
EQUATION:
fdy = (DIF) fy
(32)
2.08-70
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