Civil Engineering Reference
In-Depth Information
Figure G6-3
Space available for
gable roof drift
accumulation with
the wind blowing
from left to right.
In the ASCE 7-10 provisions, unbalanced or across-the-ridge drift loads
must be considered for roofs with slopes of ½ on 12 through 7 on 12. The
lower-bound slope of ½ on 12 recognizes the fact that low-sloped roofs (spe-
cifi cally dead fl at through
3
⁄
8
on 12) are fl at enough that signifi cant drifts
do not form. In other words, the change in angle from the windward
3
⁄
8
on
12 roof (1.79º) to the
3
⁄
8
on 12 leeward roof is small enough (a total of 3.58º)
that there is no region of aerodynamic shade beyond the ridgeline. This, in
turn, is consistent with the mechanical design of Venturi tubes.
The upper bound roof slope of 7 on 12 is based upon two types of obser-
vations. The fi rst set is full-scale observations of unbalanced loads on gable
roof structures in the Lake Tahoe region (personal communication, Tahoe
Truckee Engineers Association, 2007). The second set are observations of the
Figure G6-4
Full drift on small arch,
with wind blowing
from right to left.
