Civil Engineering Reference
In-Depth Information
FIGURE 4.2
Close-up view of the tilted tower shown in Fig. 4.1. The tilting of the tower was caused by
the washing away of soil due to a tsunami generated during the Prince William Sound earthquake in Alaska
on March 27, 1964. (
Photograph from the Steinbrugge Collection, EERC, University of California,
Berkeley.
)
4.3 TORSION
Torsional problems develop when the center of mass of the structure is not located at the
center of its lateral resistance, which is also known as the center of rigidity. A common
example is a tall building that has a first-floor area consisting of a space that is open and
supports the upper floors by the use of isolated columns, while the remainder of the first-
floor area contains solid load-bearing walls that are interconnected. The open area having
isolated columns will typically have much less lateral resistance than that part of the floor
containing the interconnected load-bearing walls. While the center of mass of the building
may be located at the midpoint of the first-floor area, the center of rigidity is offset toward
the area containing the interconnected load-bearing walls. During the earthquake, the cen-
ter of mass will twist about the center of rigidity, causing torsional forces to be induced into
the building frame.
An example is shown in Figs. 4.3 and 4.4. The two views are inside the Hotel Terminal
and show the collapse of the second story due to torsional shear failure of the second-floor
columns during the Gualan earthquake in Guatemala on February 4, 1976. This torsional
failure has been described as follows (EERC 2000):
Figure 4.3 is a view inside Hotel Terminal showing the collapse of the second story due to
shear failure of the second-floor columns. Note the significant lateral displacement (interstory
drift to the right) due to the torsional rotation of the upper part of the building.
Figure 4.4 is a close-up of one of the collapsed columns of Hotel Terminal. Note that the
upper floor has displaced to the right and dropped, and the top and bottom sections of the col-
umn are now side-by-side. Although the columns had lateral reinforcement (ties), these were
