Building Design and Construction Handbook - Merritt - Ventech!

3.2 SECTION THREE
tall and narrow, are low but very large, have irregular or unusual shapes, house
hazardous material or critical functions, or are of novel construction. Furthermore,
building codes may not contain provisions for some hazards against which building
designers nevertheless should provide protection. Examples of such hazards are
vandalism, trespass, and burglary. In addition, designers should anticipate conditions
that may exist in buildings in emergencies and provide refuge for occupants or safe
evacuation routes.
Building designers also should use judgment in determining. the degree of protection
to be provided against specific hazards. Costs of protection should be commensurate
with probable losses from an incident. In many cases, for example, it is
uneconomical to construct a building that will be immune to extreme earthquakes,
high winds of tornadoes, arson, bombs, burst dams, or professional burglars. Full
protection, however, should always be provided against hazards with a high probability
of occurrence accompanied by personal injuries or high property losses. Such
hazards include hurricanes and gales, fire, and vandals.
Structures containing extremely valuable contents or critical equipment justifying
design for even the most extreme events may require special hardened rooms
or areas.
3.1.1 Design Life of Buildings
For natural phenomena, design criteria may be based on the probability of occurrence
of extreme conditions, as determined from statistical studies of events in
specific localities. These probabilities are often expressed as mean recurrence intervals.
A mean recurrence interval of an extreme condition is the average time, in
years, between occurrences of a condition equal to or worse than the specified
extreme condition. For example, the mean recurrence interval of a wind of 60 mi/
hr or more may be recorded for Los Angeles as 50 years. Thus, after a building
has been erected in Los Angeles, chances are that in the next 50 years it will be
subjected only once to a wind of 60 mi/hr or more. Consequently, if the building
was assumed to have a 50-year life, designers might logically design it basically
for a 60-mi/hr wind, with a safety factor included in the design to protect against
low-probability faster winds. Mean recurrence intervals are the basis for minimum
design loads for high winds, snowfall, and earthquake in many building codes.
3.1.2 Safety Factors
Design of buildings for both normal and emergency conditions should always incorporate
a safety factor against failure. The magnitude of the safety factor should
be selected in accordance with the importance of a building, the extent of personal
injury or property loss that may result if a failure occurs, and the degree of uncertainty
as to the magnitude or nature of loads and the properties and behavior of
building components.
As usually incorporated in building codes, a safety factor for quantifiable system
variables is a number greater than unity. The factor may be applied in either of two
ways.
One way is to relate the maximum permissible load, or demand, on a system
under service conditions to design capacity. This system property is calculated by