Page 244 - Forensic Structural Engineering Handbook

7.28 CAUSES OF FAILURES

Modal Analyses. Also available in NEHRP and other reference standards are modal-
analysis approaches. The basic NEHRP modal-analysis approach uses elastic properties
(calculated for cracked concrete, and with panel zone deformations accounted for in steel
moment frames) and assumes that the building base is fixed. By using appropriate spectral
response curves, the modal shears, moments, and drifts are calculated for a sufficient num-
ber of modes to accurately represent the participation of at least 90 percent of the actual
mass in each orthogonal direction. Overall responses are calculated by combining the
modal participations by the square root of the sum of the squares method.
In separate procedures, the effects of soil-structure interaction and P-delta amplifica-
tions can be estimated. However, empirical approaches are used in both cases, with the cau-
tion that inclusion of soil-structure interaction effects will decrease the calculated shears
and moments, but increase the calculated drifts.

FLOOD LOADS

Nature and Consequences of Flood Loads
Flood loads can be classified as hydrostatic loads and hydrodynamic loads. Hydrostatic
loads result from the static pressure of a head of water acting against a surface. For bodies
that are completely surrounded by water at a constant depth, the net hydrostatic load on the
body as a whole equals zero. Hydrostatic forces are important if the water depth is not the
same all around a body. This occurs in floods when building components, such as walls,
retain floodwaters, or if water depths on opposite sides of a building are not the same.
Hydrodynamic loads have two primary forms: (1) loads that originate from flowing
water and (2) loads due to waves. In both cases, pressures on buildings result from the iner-
tial effects, or drag, caused when the flow of the water is diverted by the object in its path.
Flowing water most often influences loads on buildings in floodplains near river channels.
Wave action and flowing water both are important at coasts.
Flood loads can cause collapse of walls that tend to restrain water and, in very severe
conditions, displacement of entire buildings that are in strong storm surge and wave action
during a coastal storm, or in the strong current of a flooding river. Collapse can occur when
structural elements are overloaded by water pressures (flood loads easily can exceed pres-
sures due to wind by severalfold, even when water depths are only a few feet) or by top-
pling structural elements that have insufficient foundations to support overturning forces.
Floods also can induce impact loads on structures if heavy debris is being carried by river
flow. Strong currents can induce collateral damage, by undermining foundations by scour
or by inducing movements of earth slopes and scarps. To the extent that these soil move-
ments affect foundations for buildings and bridges, the effects of floods can be severe even
when water pressures are small. Floods also can cause uplift on structures if the structures
prevent water from finding a level inside that matches the level outside. Uplift also is pos-
sible if flowing water is able to pass under a structure, in contact with the underside of the
lowest floor. This is particularly true if the water flow becomes constrained as it passes
under the structure. In this case, inertial forces can pressurize the water slightly, and induce
upward forces.

Determination of Flood Loads
Hydrostatic loads can be determined relatively easily once the flood level is known.
These important data often can be determined from records on flood levels kept by