Introduction - Design of Reinforced Concrete - page 33

complex, the work can be greatly simplified with the tables presented in the speci-
fication. The reader is cautioned, however, that the tables presented are for build-
ings of regular shapes. If a building having an irregular or unusual geometry is
being considered, wind tunnel studies may be necessary.
The basic form of the equation presented in the specification is:

p qCG

In this equation p is the estimated wind load (in psf) acting on the structure. This
wind load will vary with height above the ground and with the location on the
structure. The quantity q is the reference velocity pressure. It varies with height
and with exposure to the wind. The aerodynamic shape factor C is dependent
upon the shape and orientation of the building with respect to the direction from
which the wind is blowing. Lastly, the gust response factor G is dependent upon
the nature of the wind and the location of the building.
4. Seismic loads. Many areas of the world are in “earthquake territory,” and in those
areas it is necessary to consider seismic forces in design for all types of structures.
Through the centuries there have been catastrophic failures of buildings, bridges,
and other structures during earthquakes. It has been estimated that as many as
50,000 people lost their lives in the 1988 earthquake in Armenia. 22 The 1989
Loma Prieta and 1994 Northridge earthquakes in California caused many billions
of dollars of property damage as well as considerable loss of life.

Recent earthquakes have clearly shown that the average building or bridge that has
not been designed for earthquake forces can be destroyed by an earthquake that is not
particularly severe. Most structures can be economically designed and constructed to
withstand the forces caused during most earthquakes. On the other hand, the cost of pro-
viding seismic resistance to existing structures (called retrofitting ) can be extremely high.
Some engineers seem to think that the seismic loads to be used in design are merely
percentage increases of the wind loads. This assumption is incorrect, however, as seismic
loads are different in their action and are not proportional to the exposed area of the build-
ing, but rather are proportional to the distribution of the mass of the building above the
particular level being considered.
Another factor to be considered in seismic design is the soil condition. Almost all of
the structural damage and loss of life in the Loma Prieta earthquake occurred in areas that
have soft clay soils. Apparently these soils amplified the motions of the underlying rock. 23
It is well to understand that earthquakes load structures in an indirect fashion. The
ground is displaced, and because the structures are connected to the ground, they are also
displaced and vibrated. As a result, various deformations and stresses are caused through-
out the structures.
From the above information you can understand that no external forces are applied
above ground by earthquakes to structures. Procedures for estimating seismic forces such
as the ones presented in Section 9 of ASCE 7-02 are very complicated. As a result, they
usually are addressed in advanced structural analysis courses such as structural dynamics
or earthquake resistance design courses.

22 Fairweather, V., 1990, “The Next Earthquake,” Civil Engineering (New York: ASCE, March), pp. 54-57.
23 Ibid.