Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL THEORY 5.15
loaded with p s, and also with the windward wide unloaded and the leeward side
carrying 1.5p s.
For curved roofs, the snow load on the portion that is steeper than 70p� may
be taken as zero. For the less-steep portion, the load p s may be computed as for a
sloped roof, with � taken as the angle with the horizontal of a line from the crown
to points on the roof where the slope starts to exceed 70�. Curved roofs should be
designed with the whole area fully loaded with p s. They also should be designed
for the case of snow only on the leeward side, with the load varying uniformly
from 0.5p s at the crown to 2p s at points where the roof slope starts to exceed 30�
and then decreasing to zero at points where the slope starts to exceed 70�.
Multiple folded-plate, sawtooth, and barrel-vault roofs similarly should be
designed for unbalanced loads increasing from 0.5p s at ridges to 3p s in valleys.
Snow drifts may form on a roof near a higher roof that is less than 20 ft
horizontally away. The reason for this is that wind may blow snow from the higher
roof onto the lower roof. Drifts also may accumulate at projections above roofs,
such as at parapets, solar collectors, and penthouse walls. Drift loads accordingly
should be taken into account when:
1. The ground snow load p g exceeds 10 lb/ft 2 .
2. A higher roof exists (or may be built in the future) within 20 ft of the building,
if the height differential, ft, exceeds 1.2p ƒ/�, where p ƒ is computed from Eq.
(5.16) and � is the snow density, lb/ft 3 .
3. A projection extends a distance, ft, exceeding 1.2p ƒ/� above the roof and is
more than 15 ft long.
In computation of drift loads, the snow density �, lb/ft 3 , may be taken as follows:
p � 11–30 31–60 60 or more
g
� � 15 20 25
The drift may be assumed to be a triangular prism with maximum height, located
adjacent to a higher roof or along a projection, taken as h d � 2p g/�, modified by
factors for risk and exposure, described for flat roofs. Width of the prism should
be at least 10 ft and may be taken as 3h d for projections up to 50 ft long and as
4h d for projections more than 50 ft long. Accordingly, the load varies uniformly
with distance from a projection, from h d� at the projection to zero. For drifts due
to snow load from a higher roof at a horizontal distance S, fit, away horizontally
(S � 20 ft), the maximum drift intensity may be taken as h d�(20 � S)/20.
Rain-Snow Load Combination. In roof design, account should be taken of the
combination of the design snow load with a temporary water load from an intense
rainstorm, including the effects of roof deflection on ponding. The added water load
depends on the drainage characteristics of the roof, which, in turn, depend on the
roof slope. For a flat roof, the rain surcharge may be taken as 8 lb/ft 2 for slopes
less 1 ⁄4 in/ft and as 5 lb/ft 2 for steeper slopes, except where the minimum allowable
design snow load p exceeds p computed from Eq. (5.16). In such cases, these
min ƒ
water surcharges may be reduced by p � p .
min ƒ
(W. Tobiasson and R. Redfield, ‘‘Snow Loads for the United States,’’ Part II,
and S. C. Colbeck, ‘‘Snow Loads Resulting from Rain on Snow,’’ U.S. Army Cold
Regions Research and Engineering Laboratory, Hanover, N.H.)