Civil Engineering Reference
In-Depth Information
Figure G8-3
Parapet wall drift at
southwest corner for
Example 8-1.
If the roof is a continuous-beam system (for example, a metal building
system roof with lapped purlins), then the roof also needs to be checked for
the partial load provisions in Section 7.5. The resultant partial load would
be considered a separate load case from the balanced-plus-drift load case
determined above.
Example 8-2
Rooftop Unit (RTU) Drift
Problem
Solve the same problem as Example 8-1, except a 4 ft
×
12 ft
×
20 ft RTU is
located as shown in
Figure G8-4
and the roof has no parapets.
Solution
Balanced Load
Although the parapets have been removed, the stand of conifers is still in close
proximity, so
C
e
,
C
t
,
I
s
, and
p
f
are the same as in Example 8-1 (
p
s
=
21 lb/ft
2
and
h
b
=
1.17 ft).
Drift
The clear height to the top of the RTU is
h
c
=
4.0 ft − 1.17 ft
=
2.8 ft and
h
c
/
h
b
0.2. Therefore, a roof projection drift needs to be considered. Since
the cross-wind dimension of the RTU is only 12 ft, which is less than the
15 ft minimum for an east-west wind, drifting along the east and west sides
of the RTU need not be considered. For a north-south wind, the larger of the
upwind fetch distances is 160 ft. Hence,
>
(
)
(
)
3
h
d
=
0.75 (0.43)
160
30
+
10
1.5
=
3.3 ft
4
