Page 101 - Forensic Structural Engineering Handbook

DESIGN CODES AND STANDARDS 2.7

Subsequent to the 1982 edition of ANSI A58.1, the American National Standards Institute
(ANSI) and the ASCE Board of Direction approved ASCE rules for the standards commit-
tee to govern the writing and maintenance of the ANSI A58.1 standard. The current docu-
ment prescribes load combinations, dead loads, live loads, soil and hydrostatic pressures,
wind loads, snow loads, rain loads, and earthquake loads. Like earlier editions of the ANSI
standard, ASCE 7 has significantly influenced the development and revision of other build-
ing codes.

Wind Loads. Because of the complexity involved in defining both the dynamic wind load
and the behavior of an indeterminate structure when subjected to wind loads, the design cri-
teria adopted by ASCE 7 are based on the application of an equivalent static wind pressure.
This equivalent static design wind pressure p is defined in a general sense by
p = qGC
p
2
where q = velocity pressure, lb/ft
G = gust response factor to account for fluctuations in wind speed
C = pressure coefficient or shape factor that reflects influence of windon various
p
parts of a structure
Velocity pressure is computed from
2
q = K (IV)
z
where K = velocity exposure coefficient that accounts for variation of velocity with height
z
and ground roughness
I = importance factor associated with type of occupancy
V = basic wind speed, mi/h
The 1982 ANSI standard was a major revision of the 1972 version of that standard. A
new wind speed map for annual, extreme, fastest-mile wind, based on an annual probabil-
ity of exceedance of 0.02 (fifty-year mean recurrence interval), was introduced with 70 mph
as the minimum design basic wind speed. This one map replaced maps for twenty-five-,
fifty-, or one-hundred-year mean recurrence intervals that were used as a measure of the
importance of the facility (anticipated use, life, hazard to personnel, acceptable risk, and
other judgment factors). The importance coefficient I was introduced in 1982 to account,
in a more consistent manner than by selecting from three different maps, for the twenty-
five-year and one-hundred-year winds by using a multiplier for the wind speed provided on
the one map.
The ASCE 7-98 edition and the more recent editions of the wind load standard provide
three methods that can be used to calculate wind loads: a “simplified method” where wind
pressures can be selected directly without any calculation when the building meets all the
requirements for the application of this procedure; and two other methods (analytical
method and wind tunnel procedure), which are essentially the same methods that appeared
previously in the standard with some changes.

Snow Loads. In ASCE 7, the design roof snow load is determined from
P = 0.7C C Ip g
t
e
f
where C = wind exposure factor
e
C = thermal effects factor
t
I = importance factor for end use
p = maximum ground snow load
g