Page 223 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.7

Sometimes the most reliable information about live loads is established by interviewing
persons familiar with the use of the building. The recollections of witnesses, while often
imperfect, should be considered a useful resource.
Snow, Rain, and Ice Loads. The weights of snow, rain, and ice (and other fluids)
often are difficult to assess. Melting, dissipation, evaporation, and additional precipita-
tion after a failure can obscure the amount of fluids supported by a structure at the time
of failure.
It is important always to search for records of precipitation in the vicinity of the fail-
ure site. Such data often can be obtained in the United States from the National Climatic
4
Data Center. In addition, local airports, television and radio stations, colleges and uni-
versities, municipal public safety agencies, departments of public works, and hobbyists
sometimes accumulate data on precipitation. The National Climatic Data Center and
some other data-recording entities sometimes maintain continuous records of ground
snow accumulation, which often is more important than the amount of precipitation that
fell during a single storm immediately before a failure. In all cases, the accuracy and
applicability of data accumulated off-site and by entities that are not universally
acknowledged as reliable should be accepted only with consideration and evaluation of
the potential of error.
Data that are collected off-site often must be adjusted to account for differences in con-
ditions at the location of data acquisition compared to those at the site of the failure.
Storms often deposit precipitation that varies significantly, even within small local
regions. In addition, a building’s proximity to hills, trees, wooded areas, and other build-
ings can affect snow and ice accumulation. If wind is channeled by terrain or nearby con-
struction, the resulting locally high wind speed might sweep snow from its path. Likewise,
a sudden decrease in wind speed and associated turbulence, such as might occur at the
edge of a roof, can cause drifting on lower surfaces (Figs. 7.2 and 7.3). Snow accumula-
tion on a roof often is less than that on the adjacent ground because heat from within a

FIGURE 7.2 Roof failure due to drifting snow, Seaboard Building, Danvers, Massachusetts.
(Credit: C. Russo, Simpson Gumpertz & Heger Inc.)