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 SECTION 3.
BEAMS AND COLUMNS IN REINFORCED CONCRETE STRUCTURES
1.
INTRODUCTION.
a. Blast resistant concrete buildings subjected to external blast
pressures are generally shear wall structures rather than rigid frame
structures. Shear wall structures respond to lateral loads in a somewhat
different manner than rigid frame structures; the basic difference being the
manner in which the lateral loads are transferred to the foundation. In
rigid frame structures the lateral loads are transmitted to the foundation
through bending of the columns. In shear wall structures, the lateral
forces are transmitted to the foundation through both bending and shearing
action of the shear walls. Shear walls are inherently strong and will
resist large lateral forces. Consequently, shear wall structures are
inherently capable of resisting blast loads and can be designed to resist
substantially large blast loads whereas rigid frame structures cannot be
economically designed to resist significant blast loads.
b. In shear wall structures, beams and columns are usually provided
between shear walls to carry the vertical loads including blast loads on the
roof and not to transmit lateral loads to the foundation. For example,
blast loads applied to the front wall of a two-story shear wall structure
are transmitted through the roof and intermediate floor slabs to the shear
walls (perpendicular walls) and thus to the foundation. The front wall
spans vertically between the foundation, the floor, and the roof slab. The
upper floor and roof slabs act as deep beams, and, in turn, transmit the
front wall reactions to the shear walls. The roof and floor beams are not
subjected to significant axial loads due to the diaphragm action of the
slabs. The interior columns are usually not subjected to significant
bending moments since there is no sidesway due to the extreme stiffness of
the shear walls. However, significant moments can result from unsymmetrical
loading conditions. Columns which are monolithic with the exterior walls
may be required for severe load conditions. These exterior columns are
subjected to both significant axial load and moment. The axial load results
from the direct transfer of the roof and floor beam reactions while the
moments are caused by the lateral blast load acting on the exterior wall.
c. The design of slab elements has been extensively discussed in NAVFAC
P-397. This chapter is concerned solely with the design of beams and
columns in a receiver structure subjected to low and intermediate blast
pressures resulting from an explosion in a donor structure.
2.
DYNAMIC STRENGTH OF MATERIALS.
a.
Introduction.
(1) A structural element subjected to a blast loading exhibits a
higher strength than a similar element subjected to a static loading. This
increase in strength for both concrete and reinforcement is attributed to
the rapid rates of strain that occur in dynamically loaded members. These
increased stresses or dynamic strengths are used to calculate the element's
2.08-69
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