Building Design and Construction Handbook - Merritt - Ventech!

PROTECTION AGAINST HAZARDS 3.3
dividing by the safety factor the ultimate capacity, or capacity at failure, for sustaining
that type of load. For example, suppose a structural member assigned a
safety factor of 2 can carry 1000 lb before failure occurs. The service load then is
1000/2 � 500 lb.
The second way in which codes apply safety factors is to relate the ultimate
capacity of a system, to a design load. This load is calculated by multiplying the
maximum load under service conditions by a safety factor, often referred to as a
load factor. For example, suppose a structural member assigned a load factor of 2
is required to carry a service load of 500 lb. Then, the member should be designed
to have a capacity for sustaining a design load of 500 � 2 � 1000 lb, without
failing.
While both methods achieve the objective of providing reserve capacity against
unexpected conditions, use of load factors offers the advantage of greater flexibility
in design of a system for a combination of different loadings, because a different
load factor can be assigned to each type of loading in accordance with probability
of occurrence and effects of other uncertainties.
Safety factors for various building systems are discussed in following sections
of the book. This section presents general design principles for protection of buildings
and occupants against high winds, earthquakes, water, fire, lightning, and intruders.
3.2 WIND PROTECTION
For practical design, wind and earthquakes may be treated as horizontal, or lateral,
loads. Although wind and seismic loads may have vertical components, these generally
are small and readily resisted by columns and bearing walls. Vertical earthquake
components can be important in the design of connections as in precast
concrete structures. Wind often generates significant uplift forces that require special
attention to vertical restraint and lateral support for members in reverse bending.
The variation with height of the magnitude of a wind load for a multistory
building differs from that of a seismic load. Nevertheless, provisions for resisting
either type of load are similar.
In areas where the probability of either a strong earthquake or a high wind is
small, it is nevertheless advisable to provide in buildings considerable resistance to
both types of load. In many cases, such resistance can be incorporated with little
or no increase in costs over designs that ignore either high wind or seismic resistance.
3.2.1 Wind Characteristics
Because wind loads are considered horizontal forces, wind pressure, for design
purposes, should be assumed to be applied to the gross area of the vertical projection
of that portion of the building above the average level of the adjoining ground.
Although the loads are assumed to be horizontal, they may nevertheless apply either
inward pressures or suctions to inclined and horizontal surfaces. In any case, wind
loads should be considered to act normal to the exposed building surfaces. Furthermore,
wind should be considered to be likely to come from any direction unless