Page 245 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.29

public safety agencies. In addition, reliable information at specific sites often can be deter-
mined by establishing the high-water mark through examination of stains or debris left on
the structure or in its immediate vicinity. Once the water elevation is known, hydrostatic
pressures on surfaces can be calculated easily, using the density of water and the water
elevation.
Hydrodynamic loads due to water flow must be calculated based on estimates of the
velocity and direction of water flowing by the structure. This information is best obtained
from local, state, or federal government agencies that record such data, or through consul-
tation with engineers specializing in hydrology or hydraulics.
A simple way to account for water flow loads is to add a calculated equivalent head to
1
the hydrostatic head for the water elevation. The following formula can be used to calcu-
late the equivalent head to represent flowing water:
DV 2
h =
2 g
where h = equivalent head
V = velocity of the flowing water
g = acceleration due to gravity
D = drag coefficient for the shape of the object in the flow stream
For complicated wave action, more refined approaches are necessary.
Normally, it is sufficient to apply the equivalent head to the upstream face of the struc-
ture only, where the inertial effects of the flowing water on the building are most severe.
Sidewalls and the walls in the tailwater usually experience pressures similar to those asso-
ciated with the hydrostatic head.
Analysis for wave action must consider the effects of breaking waves, wave run-up, and
drag due to water flow. 24,25

BLAST LOADS

Nature and Consequences of Blast Loads

Blast loads arise from accidental and intentional detonations of explosives and other
volatile substances in or near structures. Such explosions generate hot gases, air pressur-
ization, and shock waves that radiate away from the detonation point. Shock waves that
strike rigid surfaces are reflected and proceed onward to the next surface. Shock waves
diffract around corners. Pressurized gases expand into confined spaces. Explosion-
generated “winds” pass by structures, causing negative pressures on sidewalls and roofs.
For severe explosions, a negative pressure phase often follows the initial positive pres-
sure phase, as overexpanded gases contract and air movement reverses back toward the
detonation point.
Structures near explosions receive a complex sequence of positive and negative pres-
surizations that typically occur in a fraction of a second. A roof first will be pushed down
by a shock wave that passes a building and then will be drawn upward by the rebound and
the negative pressure caused by moving air. The magnitude of the pressure can be orders
of magnitude larger than the pressures associated with environmental loads derived from
wind and gravity. The duration is very short: Often the entire loading sequence is complete
in a few tenths of a second. Often the loading sequence is so short that massive objects do
not receive enough momentum to respond significantly.