Civil Engineering Reference
In-Depth Information
Chapter 3
Flat Roof
Snow Loads
Section 7.3 in ASCE 7-10 provides for calculation of the fl at roof snow load,
p
f
, and prescribes a minimum roof snow load. The fl at roof snow load for-
mula incorporates effects of wind exposure, thermal conditions of the roof
system, and the importance of the facility. The fl at roof snow load is com-
bined with the slope factor,
C
s
, from Section 7.4 to form a design snow
load for which all roofs must be checked. The minimum roof snow load is a
separate load case intended to cover situations where the “typical” wind and
thermal effects are not applicable.
3.1
Measured Conversion Factors
Case histories have shown that the snow load on a roof is typically less
than that on the ground provided drifting is absent. To establish a rational
basis for roof snow loads, CRREL sponsored a program in the late 1970s in
which university researchers made simultaneous measurements of the ground
and roof snow loads. The researchers measured a group of structures with
varied wind and thermal environments over the course of a few winters, and
O'Rourke, Koch, and Redfi eld (1983) subsequently analyzed the data.
Figure G3-1
shows the measured loads for one structure over the course
of the 1976-1977 winter. For this structure, the ground and roof loads were
nominally the same on the three sampling dates in December; however, the
ground load was four or fi ve times greater than the roof load for both the
mid-February and early March samples. Although there are a number of
values for the ratio of simultaneous roof to ground loads (ranging from
about 1.0 prior to January to about 0.23 in February and March), the ratio
of the maximum annual roof load (26 lb/ft
2
on or about New Year's Day) to
the maximum annual ground load (50 lb/ft
2
in mid-February),
C
m
, is of most
interest to structural engineers. In this example,
C
m
is 0.52 (i.e., 26/50), and
the maximum annual loads do not occur on the same day. The ratio
C
m
is
referred to as the ground-to-roof conversion factor or simply the conversion
