Building Design and Construction Handbook - Merritt - Ventech!

PROTECTION AGAINST HAZARDS 3.11
as columns to transmit floor and roof loads to the ground. Figure 3.6g shows how
multistory X bracing has been used to create a braced tube for a skyscraper.
See also Arts. 3.3.5, 5.18–19, and Secs. 7 through 10.
(Council on Tall Buildings and Urban Habitat, ‘‘Planning and Design of Tall
Buildings,’’ Vols. SC, SB, and CB, American Society of Civil Engineers, New York;
E. Simiu and R. H. Scanlon, ‘‘Wind Effects on Structures,’’ John Wiley & Sons,
Inc., New York; Minimum Design Loads for Tall Buildings and Other Structures
ANSI/ASCE 7-98, American Society of Civil Engineers, New York.)
3.3 PROTECTION AGAINST EARTHQUAKES
Buildings should be designed to withstand minor earthquakes without damage, because
they may occur almost everywhere. For major earthquakes, it may not be
economical to prevent all damage but collapse should be precluded.
Because an earthquake and a high wind are not likely to occur simultaneously,
building codes usually do not require that buildings be designed for a combination
of large seismic and wind loads. Thus, designers may assume that the full strength
of wind bracing is also available to resist drift caused by earthquakes.
The methods of protecting against high winds described in Art. 3.2.4 may also
be used for protecting against earthquakes. Shaking of buildings produced by temblors,
however, is likely to be much severer than that caused by winds. Consequently,
additional precautions must be taken to protect against earthquakes. Because
such protective measures will also be useful in resisting unexpectedly high
winds, such as those from tornadoes, however, it is advisable to apply aseismic
design principles to all buildings.
These principles require that collapse be avoided, oscillations of buildings
damped, and damage to both structural and nonstructural components minimized.
Nonstructural components are especially liable to damage from large drift. For
example, walls are likely to be stiffer than structural framing and therefore subject
to greater seismic forces. The walls, as a result, may crack or collapse. Also, they
may interfere with planned actions of structural components and cause additional
damage. Consequently, aseismic design of buildings should make allowance for
large drift, for example, by providing gaps between adjoining buildings and between
adjoining building components not required to be rigidly connected together and
by permitting sliding of such components. Thus, partitions and windows should be
free to move in their frames so that no damage will occur when an earthquake
wracks the frames. Heavy elements in buildings, such as water tanks, should be
firmly anchored to prevent them from damaging critical structural components.
Displacement of gas hot water heaters is a common cause of gas fires following
earthquakes.
3.3.1 Earthquake Characteristics
Earthquakes are produced by sudden release of tremendous amounts of energy
within the earth by a sudden movement at a point called the hypocenter. (The point
on the surface of the earth directly above the hypocenter is called the epicenter.)
The resulting shock sends out longitudinal, vertical, and transverse vibrations in all