Page 225 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.9

determine the weight of snow is to extract a measured volume of a column of snow from
the structure and weigh it as snow, or later as water after it has melted. A 6- or 8-in-diameter
stovepipe driven into the snow effectively isolates a sample for weighing. Care should be
taken to obtain a sample of the full depth of the snow, since compaction, melting (some-
times from the bottom up on heated structures), or accumulation at different times can
cause the density of snow to vary with depth from the snow surface.
Often snow on a failed structure is too disrupted to be weighed reliably. In these cases,
it might be possible to measure snow on an adjacent surviving portion of the structure, or
on the ground near the structure. Frequently, a scarp remains on snow at the edge of a col-
lapse. Depth measurements and samples can be extracted at such locations. However,
before any measurements made away from the actual failure location can be used with con-
fidence, the investigator must assess whether differences in exposure, drifting characteris-
tics, heating and melting, or other factors affect the reliability of such measurements.
Ice weight is best evaluated by determining the surface area over which ice accumulated
and estimating the surface load by one of two means: (1) measuring the ice thickness at sev-
eral locations and (2) weighing an ice-covered object before and after the ice melts.
Usually, rain load is transient with duration similar to the length of a storm. As such, the
determination of the weight of rain frequently requires hydraulic flow rate calculations
based on the rate of precipitation and rate of draining from surfaces. Consideration should
be given to the possibility that drains were obstructed or, especially in the case of rain on
snow, that flow to drains was inhibited during a storm. Furthermore, on relatively flat roofs
with flexible structural systems, the potential for ponding should be evaluated (Fig. 7.5).

FIGURE 7.5 Partial roof collapse due to ponding. (Credit: D. Kelly, Simpson Gumpertz &
Heger Inc.)

Liquids in containers, granular solids, and other bulk materials that are handled or con-
veyed in structures require special analyses. Handling loads and membrane stresses in bins,
containers, and tanks are functions of the methods for handling, filling, and emptying, and
of the bulk properties of the material. Impact and vibratory loads commonly result from
handling. For fluids, it is possible that rapid drawdown of a tank can cause negative pres-
sure inside the tank. Volatile materials and materials subject to heating and cooling can
cause pressure changes in closed vessels.