Dynamic/earthquake analysis - Computational Analysis and Design of Bridge Structures - page 524

Bridge deck displacements

2

Linear THA
Nonlinear THA

1.5

1

0.5

0

0 1 2 3 4 5 6 7 8 9 10

−0.5

−1

−1.5

−2

Time

Figure 17.9 Linear versus nonlinear time-history analysis for a nine-span bridge model.

many times greater than the normal design loads, and blast winds can be
much more severe than hurricanes.
Blast waves are produced whenever an explosion takes place. These
waves propagate in the form of spherical waves, resulting in discontinuities
in the structures. Some of these waves transfer across the structures while
remaining are reflected back. During this wave propagation, high pressure
and high temperature are generated, which travel across the least resistance
path of the structure. This entire process of the wave generation and propa-
gation last for a few milliseconds.
The initial step in blast design or analysis is the determination of the
blast loads. The factors that consider attention are energy absorption, load
combinations, critical elements, and structural redundancy to prevent pro-
gressive collapse of the structure.
If an explosion occurs on the top of the bridge, bridge deck will experi-
ence the downward thrust of the overpressure, which will be transmitted to
other bridge components such as hangers, cables, and towers. Foundation
will experience blast-induced vertical and overturning forces. If the blast
load is applied at the bottom of the bridge, deck slab and the supporting
girders will experience an upward pressure for which they are generally not
designed. When they are subjected to vertical upward forces, the bottom
of the deck member is subjected to compression and the top is subjected to
tension, for which they are not normally designed for. Towers and founda-
tions are also subjected to vertically upward lateral forces and overturn-
ing moments. Failure of the system is obvious unless otherwise they are
designed for the vertical upward forces.
Several structural analysis options are available for blast-resistant design:

1. Equivalent static analysis (ESA). This method is generally for simple
system to determine the equivalent static design load conservatively
and neglects the inertial effects of members in motion.