Page 221 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.5

investigator should refer to the references listed at the end of this chapter and to other
authoritative references on loads and load effects.

GRAVITY LOADS

Nature and Consequences of Gravity Loads
Gravity loads result from the weights of structures and their contents. Normally such loads
are classified as dead (fixed weight of the structure), live (variable loads resulting from the
use of the structure), snow, ice, or rain. In addition, live loads, sliding snow or ice, bulk
materials handling, and any other moving loads have the potential to impose impact loads
that can magnify the load effect on the structure.
While gravity loads normally create no net horizontal loads on a structure, they can gen-
erate horizontal forces in individual members and at the foundation level. An example is a
rigid-frame structure that experiences balanced horizontal thrusts at opposite footings as it
resists vertical loads on its horizontal or inclined members. Gravity loads also can generate
horizontal loads when sliding snow strikes a surface with significant horizontal velocity. In
this case, the horizontal load is a transient that imparts momentum that must be arrested by
the structure for it to survive.
Gravity loads can cause overstresses and instabilities in structural elements and con-
nections (Fig. 7.1), overall structural instability, and foundation settlement.

FIGURE 7.1 Collapse of L’Ambiance Plaza, Bridgeport, Connecticut, 1987. (Credit: D. Dusenberry,
Simpson Gumpertz & Heger Inc.)

Determination of Gravity Loads

The best way to obtain reliable information about gravity loads on a structure is through
direct measurement. When possible and practical for an investigation, objects on a struc-
ture at the time of a failure should be extracted and weighed. Then, to the extent that their
locations on the structure can be determined after the failure, the influence of measured
weights can be calculated with high reliability.