Civil Engineering Reference
In-Depth Information
in essence, large freezers. It should be noted that snow drifting was not a
signifi cant factor at the facilities. Therefore, although the same total amount
of precipitation fell on the cold-room roofs as on the ground, the roofs accu-
mulated more snow because the snow on the roof melted at a slower rate
between storms than the snow on the ground. For heated structures, absent
drifting, sliding, and eave ice dams, the roof load never exceeds the ground
snow load.
Sack et al. (1984) presents another case history with a conversion factor
greater than 1.0. Periodic ground and roof snow measurements were made
over a two-year period at a U.S. Forest Service facility in McCall, Idaho.
The structure was located at a relatively sheltered site, and a roof covered a
loading dock area on one side of the building. Since the loading dock was
not enclosed, the bottom of the “overhang” roof was open to ambient air
temperatures, which on average were below freezing. The observed conver-
sion factors for the overhang roof were 1.27 and 1.28 for the 1982-1983
and 1983-1984 winters, respectively. As explained by Sack et al., the com-
paratively large conversion factors for the overhang are due, at least in part,
to effects of the ground heat fl ux. In early winter, particularly before frost
has set in, the earth's surface is comparatively warm, and heat fl ux from the
earth melts some of the ground snow. The roof snow located on the over-
hang, however, is not subject to the ground heat fl ux since it has ambient air
directly below it.
3.5
Importance Factor (
I
s
)
An importance factor is used to determine all the environmental live loads
(i.e., fl ood, wind, snow, and earthquake). Its purpose is to increase environ-
mental live loads for structures that are particularly important and to allow
a reduction for structures that are not. The various categories of structures
are identifi ed in Table 1-1 and are based on the nature of the building's occu-
pancy or intended use. Structures are deemed important when the potential
for loss of human life is particularly high (e.g., elementary schools with capac-
ity greater than 150 and explosives-manufacturing facilities are both in Cat-
egory III) or if they are essential in time of disaster (e.g., emergency shelters,
fi re stations, and hospitals are in Category IV). Unimportant structures are
those where the potential for loss of human life is particularly low (e.g., minor
storage facilities and agricultural facilities are in Category I). Ordinary struc-
tures (Category II) are by defi nition those not in Categories I, III, or IV. Note
that a structure's importance, as defi ned in ASCE 7-10, is unrelated to its
initial cost, replacement cost, economic value, or the value of its contents. For
example, although the contents value would be high for an unmanned stor-
age warehouse for Ming vases, the importance in terms of life safety would
be low. That is, a structure's importance relates, directly or indirectly, to the
hazard potential to human life in the event of failure.
The mapped values in Figure 7-1 (the map of 50-year ground snow
loads) are intended for ordinary structures (Category II;
I
s
=
1.0). For struc-
tures in Category I, the
I
s
factor is 0.8, which corresponds to a 25-year ground
