Page 311 - Civil Engineering Formulas

BUILDING AND STRUCTURES FORMULAS 245

Snow Loads
Determination of designing snow loads for roofs is often based on the maxi-
mum ground snow load in 50-year mean recurrence period (2% probability of
being exceeded in any year). This load or data for computing it from an
extreme-value statistical analysis of weather records of snow on the ground
may be obtained from the local building code or the National Weather Service.
Some building codes and ASCE 7-95 specify an equation that takes into
account the consequences of a structural failure in view of the end use of the
building to be constructed and the wind exposure of the roof:

p f 0.7C e C t Ip g (9.134)
where C wind exposure factor (range 0.8 to 1.3)
e
C thermal effects factor (range 1.0 to 1.2)
t
I importance factor for end use (range 0.8 to 1.2)
p roof snow load, lb per ft 2
f
p ground snow load for 50-year recurrence period, lb per ft 2
g
The “Low-Rise Building systems Manual,” Metal Building Manufacturers
Association, Cleveland, Ohio, based on a modified form of ASCE 7, recom-
mends that the design of roof snow load be determined from
(9.135)
p f I s Cp g
where I is an importance factor and C reflects the roof type.
s

Wind Loads
Wind loads are randomly applied dynamic loads. The intensity of the wind
pressure on the surface of a structure depends on wind velocity, air density, ori-
entation of the structure, area of contact surface, and shape of the structure.
Because of the complexity involved in defining both the dynamic wind load
and the behavior of an indeterminate steel structure when subjected to wind
loads, the design criteria adopted by building codes and standards have been
based on the application of an equivalent static wind pressure. This equivalent
static design wind pressure p (psf) is defined in a general sense by

(9.136)
p qGC p
where q velocity pressure, psf
G gust response factor to account for fluctuations in wind speed
C pressure coefficient or shape factor that reflects the influence of the
p
wind on the various parts of a structure
Velocity pressure is computed from

q z 0.00256 K z K zt K d V I (9.137)
2