Civil Engineering Reference
In-Depth Information
Chapter 5
Partial Loads
The partial loading provisions of Section 7.5 of ASCE 7-10 take into account
patterned or checkerboard loading. The Commentary (reference Section C7.5)
explains that partial loading is required for continuous structural members,
such as roof purlins in metal building systems, where a
reduction
in snow
loading on one span results in an
increase
in stress or defl ection in an adjacent
span. The Standard requires the designer to apply half of the full-balanced
load to the partially loaded spans and the entire full-balanced load to the
remainder of the spans. As noted in Chapter 4, the balanced snow load is
derived from Equation 7-2 (
p
s
=
0.7
C
e
C
t
C
s
I
s
p
g
) and not from the mini-
mum snow load for low sloped roofs,
p
m
, described in Section 7.3.4.
Note that partial loads are different from unbalanced or drift loads,
which are discussed in Chapter 6. Unbalanced and drift loading arises when
snow is removed from one portion of the roof and accumulates on another
portion. With partial loading, snow is removed from one portion of the
roof (for example, via wind scour or thermal effects), but it is not added to
another portion.
C
s
p
f
=
5.1
Continuous-Beam Systems
Although partial loading for a roof can be compared to the checkerboard
loading pattern used for fl oor design, an actual checkerboard pattern is
not
prescribed for roof snow. Instead, continuous-beam systems are investigated
for the effects of three loading cases as described in Section 7.5.1 and as
illustrated in Figure 7-4 of ASCE 7-10. Case 1 may occur when two sepa-
rate snow events, both corresponding to half the balanced design load, are
separated by an intervening event (such as sleet, freezing rain, or crust for-
mation) which prevents the lower half of the roof snowpack from drifting.
The second snowfall is then followed by a strong wind blowing from right to
left. As it blows across the roof, it removes snow from all of the spans while
simultaneously depositing snow on all of the downwind spans. Over time,
all the driftable snow is blown off some upwind spans. Before the driftable
snow is blown off the last downwind span, the wind stops, leaving a pattern
corresponding to Case 1.
