Civil Engineering Reference
In-Depth Information
Chapter 10
Rain-on-Snow
Surcharge Loads
It is not unusual for rain to fall while a building's roof is still covered with
snow. This can occur, for example, in spring before the winter snowpack has
melted completely. The rain-on-snow provisions in Section 7.10 of ASCE
7-10 are intended to cover such loading situations.
A rain-on-snow surcharge is occasionally one of the contributing factors
in a roof collapse, although the author is unaware of case histories wherein
rain-on-snow was the sole source of an overload that led to a collapse. For
example, a series of mixed-precipitation events over the 1996-1997 holiday
season led to a large number of structural collapses in the Pacifi c Northwest.
According to a report by SEAW (1998), a total of 1,663 buildings were dam-
aged by the holiday storms. Analysis of three of these 1996-1997 Pacifi c
Northwest collapses (one located in the mountains of central Washington and
two located in the greater Yakima area) showed that the structures were cold
rooms (intentionally kept at or below freezing) with measured roof snow loads
greater than the design roof snow load (57 lb/ft
2
versus 32 lb/ft
2
, 29 lb/ft
2
versus 22 lb/ft
2
, and 36 lb/ft
2
versus 30 lb/ft
2
). In all three cases, roof design
errors were discovered that were unrelated to the snow load. In these cases,
calculations suggest that the contribution of rain on snow to the total load was
modest. In two of the buildings, the surcharge at the eave for a rain-on-snow
scenario or for the rain simply refreezing in the roof snowpack was about 5%
of the total load. In the third building, the roof collapsed prior to the rain.
Four possible rain-on-snow scenarios are outlined in
Table G10-1
and
described below.
Scenario 1:
•
The rain refreezes in both the ground and roof snow-
packs. This is most likely to occur if both snowpacks are cold and
comparatively deep. In this case, there is a semipermanent increase
in both the ground and roof snow loads. Presumably this additional
load would be recorded as part of any ground snow load measure-
ment program.
Scenario 2:
•
The rain refreezes in the ground snowpack but perco-
lates through the roof snowpack. This is most likely to occur if the
roof snowpack is warmer or shallower than the ground snowpack,
