Civil Engineering Reference
In-Depth Information
The resulting unbalanced load case for wind out of the west is sketched in
Figure G12-9
. For some roof components the unbalanced loading in
Figure
G12-8
or
Figure G12-9
would control. For other components the minimum
load of 11 lb/ft
2
would control.
Design Example 3
The structure in question is a ski chalet with an attached two-car garage as
shown in
Figure G12-10
. The chalet is heated and has a well-insulated, venti-
lated 4-on-12 roof, whereas the garage is unheated with a 4-on-12 roof. Both
have metal roof surfaces. The structural system for the chalet roof consists of
roof purlins spanning from ridge to eave, with a supporting ridge girder. The
structural system for the garage consists of roof purlins spanning from eave
to ridge with a ridge board and tension ties from eave to eave.
The site, near Freeland, Pennsylvania, is heavily forested, at an eleva-
tion of 1,750 ft. The structure is located in a small clearing, among a mix of
mature conifers and deciduous trees.
General Considerations
The site is in a Case Study (CS) region in eastern Pennsylvania. A request
to the Cold Regions Lab in Hanover, New Hampshire, produced the case
study shown in
Figure G2-2
. The 50 year ground snow load for the site at
an elevation of 1,750 ft is 46 lb/ft
2
. Since the site is wooded, it is in Terrain
Category B. Neither the distance from the structure to the tree line nor the
average height of the trees was given in the problem statement. However,
since the clearing is small and the trees mature, it is both reasonable and con-
servative to assume that the trees in fact serve as obstructions. Hence, both
the chalet and garage roofs are taken as sheltered. Finally, the chalet roof is
described as well insulated. It is both reasonable and conservative to assume
Figure G12-9
Unbalanced load case
for wind out of the
west for Design
Example 2.
1
.
9
LB
/
FT
2
27.3 LB/FT
2
6.2 LB/FT
2
