Civil Engineering Reference
In-Depth Information
(1985)
The Pino Suarez was constructed in Mexico City in the early 1970s and consisted of
five high- rise steel buildings supported on a two-level reinforced subway station that
acted as a rigid foundation common to all five buildings. One of the functions of the
complex was to act as a counterweight against uplift forces caused by the expansive
soils which occurred as the consequence of the large excavation for the subway
station. In the middle of the complex were three identical 21-story buildings each two
bays wide and four bays deep, aligned and parallel, with their narrow sides facing
east/west. Directly adjacent to, but north and south of the 21 story buildings were two
identical 14 story towers. These two buildings were positioned with their narrow
faces directly north and south respectively relative to the longitudinal sides of the 21
story buildings. The structural framing of the 21-story buildings consisted of truss
beams connected to hollow box columns with moment resisting connections. In
addition a bracing system was employed that consisted of two cross-braced frames in
the transverse direction and one V-braced frame in the exterior longitudinal plane.
As a result of the September 19, 1985 Mexico City earthquake, the complex
suffered severe structural failure. One of the three 21-story steel towers collapsed
onto an adjacent 14 story tower, destroying it also. The performance of the other two
21-story towers that were severely damaged but did not collapse provided invaluable
information with respect to the progression of failure. Computer simulations
confirmed the progression of failure to a reasonable degree of engineering certainty.
It was inferred that plastic hinges first developed at the girder ends leading to yielding
and plate buckling of the two fourth story columns located on the south side (Figure
5-15). Columns failing in this way lost most of their gravity load carrying capacity
and suffered shortening. This led to buckling of the X-brace framing and the
redistribution of forces throughout the structure that eventually caused the failure of
the other four story columns and the collapse of the tower.
Lessons Learned
The Pino Suarez complex failure was caused by a design flaw that was not recognized
in the then current seismic design codes. After the failure, Mexico updated its seismic
design code recognizing the need to concentrate energy dissipation in less critical
“fuse” elements, and protect the main columns and other key structural elements.
Although member yielding provides a great energy dissipation capability, its effects
should be explicitly considered in design. Sufficient connection strength is necessary
and yielding should be confined primarily to beams and bracing not to columns and
joints. Finally, the importance of structural redundancy was reemphasized by the
failure of the Pino Suarez complex.
