Civil Engineering Reference
In-Depth Information
tion from wooded terrain to more open terrain, the following approximate
reductions in wind velocity were noted: 60% reduction in velocity at fi ve
tree heights downwind, 45% reduction at 10 tree heights, and ranging from
10% to 30% at 20 tree heights. A rule of thumb, often used by sailors and
windsurfers, is that you need to be six tree heights downwind in order to get
“good wind.” Based on the above, the 10
h
o
requirement in ASCE 7-10 is
reasonable, if a bit conservative.
Although Table 7-2 provides an obstruction distance of 10
h
o
, it does
not establish where the measurement is taken. For example, if a potential
obstruction is located north of a roof, it is unclear if 10
h
o
is measured from
the building's north wall, south wall, or other reference point. When faced
with such a situation in practice, the author suggests measuring from the
roof's geometric center.
3.4
Thermal Factor (
C
t
)
The ASCE 7-10 thermal factors are shown in Table 7-3. They range from 0.85
for specifi ed greenhouses to 1.3 for structures intentionally kept below freez-
ing. A
C
t
factor of 1.1 is assigned to structures kept just above freezing and to
certain well-insulated “cold roofs.” A cold roof is one in which air passage-
ways allow suffi cient infl ow at the eaves and outfl ow through ridge vents.
The intent of using thermal factors is to quantify the differences in
ground-to-roof conversion factors due to heat loss through the roof layer.
Driving through a suburban area four or fi ve days after a signifi cant snowfall,
one can see this effect. The roof snow on the heated homes will be notice-
ably less than the roof snow present on the unheated garages. The
C
t
values
in Table 7-3 suggest that the roof snow load atop an unheated structure
would typically be about 20% greater than for a heated structure, whereas
the observed conversion factor data in
Figure G3-6
suggest a slightly larger
value of 27% (i.e., unheated mean/heated mean 0.70/0.55
1.27).
It should be noted that for a freezer building (structure intentionally
kept below freezing) of ordinary importance (
C
t
is 1.3 and
I
s
=
=
1.0), the fl at
roof snow load, as given in Equation 7-1, could be larger than the ground
snow load. That is, for a wind exposure factor,
C
e
, of 1.2, the fl at roof
design load equals 1.09
p
g
. At fi rst glance, it seems odd that the roof snow
load would be larger than the ground snow load absent drifting or slid-
ing. Nevertheless, a number of case-history observations demonstrated this
exact phenomenon.
One such set of case histories came from the 1996-1997 holiday storms
in the Pacifi c Northwest. A report by SEAW (1998) presented measured ground
and roof snow load data for the Yakima area. The annual maximum ground
snow load was reported to be 32 lb/ft
2
on December 29, 1996, whereas the
annual maximum fl at roof snow load atop a number of freezers and cold
rooms was reportedly 36 lb/ft
2
. Thus, the ground-to-roof conversion factor
for the 1996-1997 winter was 1.13 for these facilities. In general, all the
air below the insulated roof layer at these facilities was kept at or below
freezing. These structures did not simply house a freezer unit; they were,
