Civil Engineering Reference
In-Depth Information
heights as opposed to the
sum
of the two drift heights) is specifi cally men-
tioned in Section 7.7.1. Based on this design approach, one might assume that
wind only blows from one direction throughout the winter season; however,
that is not the case. In fact, it is possible to have a 180° shift in wind direction
during a single storm event. For example, consider a storm that passes from
west to east over a site. Due to the counterclockwise rotation of the wind
around the low pressure point, the site initially experiences the wind coming
from the south (when the low is located to the west of the site); then as the
low pressure point moves over the site, the site experiences the wind coming
from the north (when the low is located to the east).
So it is possible to have both windward and leeward contributions to
the same drift formation. The approach of choosing the larger independent
value for the design drift loading illustrates the empirical nature of the roof
step drift provisions. That is, the leeward case-history drifts, upon which the
provisions are based, are due to either all leeward drifting or some combina-
tion of leeward and windward drifting. Hence, the extent to which leeward
and windward drifting are both present is already refl ected in the
observed
drift height. Adding the
design
leeward values to the
design
windward values
would result in unrealistic drifts that are much larger than those observed.
Finally, to evaluate windward drifts, the full upwind fetch is used as
opposed to some effective fetch that accounts for the space occupied by the drift
itself. This is also a result of the empirical nature of the drift relations. That is,
the observed drift heights were regressed against the full upwind fetch.
7.3
Adjacent Roofs
The leeward roof step drift discussed above envisions lower and upper level
roofs that are adjoining. If there is a single column line at the roof step, those
columns carry the balanced plus drift surcharge from the lower level roof,
plus the balanced load from the upper level roof. Note that the upper level
roof need not be blown clear of snow for a substantial drift to form on the
lower level roof.
Although the adjoining roof step case is more common, occasionally the
lower level roof is simply adjacent to the upper level. If the two roofs are close
enough (separation distance less than 20 feet), Section 7.7.2 requires that the
lower roof be designed for a drift if it is in the wind shadow (aerodynamic
shade region) of the upper roof. Based upon Tabler (1994), the wind shadow
is assumed to trail from the upper roof (top of the parapet if one is present)
at a 1
6 slope as sketched in Figure C7-2 of the ASCE 7-10 Commentary. For
simplicity, the rise-to-run of the adjacent roof drift is also taken to be 1
6.
The height of the adjacent drift surcharge is the smaller of
h
d
and (6
h
−
s
)/6. The fi rst of these limits (
h
d
) recognizes the fact that the height of the
adjacent roof drift would not be larger than the height of the adjoining roof
drift. The second limit [(6
h
s
)/6] is related to the horizontal extent of the
wind shadow region. For an elevation difference of
h
(upper roof including
any parapet to lower roof excluding any parapet), the horizontal extent of
the wind shadow at the lower roof elevation is 6
h
. Hence the maximum
−
