Page 102 - Forensic Structural Engineering Handbook

2.8 DESIGN AND CONSTRUCTION PRACTICES

The ASCE 7 snow load provisions were developed for an extreme-value statistical
analysis of weather records of snow on the ground. The log normal distribution was
selected to estimate maximum ground snow loads, which have a 2 percent annual proba-
bility of being exceeded (fifty-year mean recurrence interval).
In 1965, the National Building Code of Canada first published comprehensive provi-
sions for loads due to snow drifting, together with a commentary, based on prior research;
these provisions were reaffirmed in the 1970 edition of the National Building Code of
Canada and were adopted by the American National Standards Institute in 1972, in ANSI
A58.1-1972, and by the BOCA Code in 1975. 11–13 The drift provisions were again revised
in ANSI A58.1-82 and the 1987 BOCA code.
The 1982 ANSI criteria are based on a statistical analysis of snowdrifts with height of
drift less than the height of the upper roof. 14 The criteria used an empirical equation to
determine the height of snowdrift and provide an empirical relationship between ground
snow load and the density of snow in the drift. When the elevation of the top of the drift, as
calculated by the empirical equation, is higher than the elevation of the upper roof, the drift
height is adjusted so that the top of the drift is at the same elevation as the upper roof. The
formula recognizes that the greater the length of the upwind higher roof, the greater is the
available snow that can be picked up by the wind and then deposited downstream in the drift
at the lower adjoining roof. 15,16
Earthquake Loads. The history of seismic design codes in the United States is fairly
brief. During the 1960s, research was conducted to demonstrate that ductile moment-resisting
concrete frames could be designed and constructed in seismic zones. This effort was in
response to the Structural Engineers Association of California (SEAOC) requirement that
buildings greater than 170 feet in height have a complete moment-resisting space frame.
Design of Multistory Reinforced Concrete Buildings for Earthquake Motions was pub-
lished in 1961 and provided initial material for the SEAOC Seismology Committee to
develop ductile reinforced-concrete provisions. 17 The SEAOC committee subsequently
published requirements for reinforced-concrete ductile moment-resisting frames and rein-
forced-concrete shear walls in the 1966 revision to the SEAOC Bluebook. 18
In 1974, a cooperative effort was undertaken by the Applied Technology Council
(ATC) to develop nationally applicable seismic design provisions for buildings. The pro-
ject, known as ATC-3, was part of the Cooperative Federal Program in Building Practices
for Disaster Mitigation initiated in 1972. The primary basis of the provisions was to pro-
vide life safety and to ensure continued functioning of essential facilities needed during and
after a catastrophe. Primary consideration was given to the main structural framing sys-
tem(s) and to energy-absorbing/dissipating effects of interior partitions, exterior cladding,
different types of materials, damping, and drift.
After their publication in 1978, the ATC-3 provisions were subjected to an extensive
several-year evaluation process by the Building Seismic Safety Council (BSSC) under con-
19
tract to the Federal Emergency Management Agency (FEMA). The ATC-3 provisions
were then modified to better apply to all regions of the United States. The process culmi-
nated in 1985 with publications by BSSC of NEHRP Recommended Provisions for the
20
Development of Seismic Regulations for Buildings, 1985 edition. NEHRP stands for the
National Earthquake Hazard Reduction Program, which is managed by FEMA.
The 1982 edition of ANSI A58.1 and the 1988 edition of ASCE 7 contained seismic pro-
visions based upon those in the 1985 Uniform Building Code. The UBC provisions for seis-
mic safety were based upon recommendations of SEAOC and predecessor organizations.
The 1955 and 1972 editions of A58.1 contained seismic provisions based upon much earlier
versions of SEAOC and UBC recommendations.
In 1993, the ASCE 7 seismic provisions were substantially changed from prior editions.
The 1993 provisions were adopted from the NEHRP Recommended Provisions for the