Civil Engineering Reference
In-Depth Information
The pattern in Case 2 could be analogous to Case 1 except the wind
event is shorter in duration and is traveling from left to right. In Case 2, the
wind event is only long enough to clean the driftable snow (half the balanced
load in the scenario under discussion) from the fi rst upwind span.
The pattern in Case 3 could occur on the structure shown in
Figure G5-1.
The structure has a monoslope roof (sloping toward the east at ½ on 12)
without a parapet and with a pediment located near the center of the west
wall. Since the pediment would be an obstruction, one would classify the
whole roof as partially exposed and use the appropriate
C
e
factor from Table
7-2 to establish the balanced load for the entire roof. For a strong wind from
the west, snow would be removed from the roof via wind scour for the north-
ern and southern portions of the roof; however, little to no wind scour would
occur directly behind the pediment, leaving a pattern similar to Case 3.
Note that the prescribed cases for the investigation of continuous-beam
systems do not cover all permutations or patterns of partial roof loading (for
example, the Breffny's Soups sign in
Figure G5-1
could correspond to three
spans as opposed to the two spans presented in Case 3). The representative
cases were chosen to cover situations that could reasonably be expected to
occur and to avoid burdening the designer with numerous partial load cases
that, although conceivable, are unlikely to govern the design. For example,
Cases 1 and 2 specifi cally target the end span that would have the largest
midspan bending moment given equal span lengths and uniform load from
span to span. Finally, partial load provisions are
not
required for structural
members (e.g., a frame girder in metal building systems) that span perpen-
dicular to the ridgeline of gable roofs with slopes of ½ on 12 and greater.
As we will see later, such gable roofs need to be designed for unbalanced
loads due to across-the-ridge drifting. Although it is conceivable that there
could be true partial loading on structural components spanning from eave
to eave (as shown in
Figure G5-2A
), the more common distribution is an
unbalanced or gable roof drift load (as shown in
Figure G5-2B
). Since a true
partial load as shown in
Figure G5-2A
is uncommon and an unbalanced
load likely governs, the designer is not burdened with a partial load check
for those members.
Figure G5-1
West elevation of a
building with a pedi-
ment. Wind out of
the west results in
nonuniform loading.
