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By Angel L. Lazaro 3rd
TO avoid the disastrous consequences of a strong earthquake, all
major thoroughfares and high-rise buildings in the country should be
investigated for possible retrofitting.
It is strongly recommended that all major
structures—new or old, vertical or horizontal, with or without
signs of distress—should be analyzed and retrofitted as necessary.
Retrofitting is akin to a makeover—of major
structures such as bridges, buildings, airports, seaports, malls and
the like—where and whenever needed to prevent destruction during
earthquakes.
Based on theory and experience as well as
research and laboratory work, earthquakes vary from light to very
severe.
In a light earthquake, nothing happens to
properly designed structures. In moderate ones, the damage is
non-structural: broken mirrors, glasses, doors and the like. In
severe quakes, structures (like foundations and columns) are damaged
but they are not beyond repair. In very severe cases, structures may
be destroyed but should not collapse.
The indicators of a structure’s capacity to
withstand earthquakes are the capacity over demand (C/D) ratios of
its various elements. This is not as complicated as it looks. If a
structure’s capacity to carry a load is 100 kilograms, putting a
demand of 150 kilograms (like an earthquake) is an overload.
In such cases, when earthquakes occur,
structures may explode, shear, bend or pull every which way. For
example, some columns, if not all, that support the structure may
bend during an earthquake. If they bend too much, they break.
Major structures
This is why 11 major structures in Metro Manila
were retrofitted: at the interchanges of Nagtahan; EDSA (Epifanio de
los Santos Avenue)-Ortigas; EDSA-Magallanes; the flyovers on
EDSA-Kamias; EDSA-Santolan; EDSA-Gil Puyat Avenue; EDSA-Roxas
Boulevard; Roxas Boulevard-Gil Puyat Avenue; Barangka-Viaduct;
Tandang Sora-Commonwealth Avenue; and Manila North Diversion Road-Camachile.
Costs vary. The EDSA-Ortigas Interchange is a
1,900-meter pre-stressed concrete box girder structure constructed
in 1991 at about P144.15 million. The cost of retrofitting was
approximately P144 million.
The Nagtahan Interchange was built in 1992 for
P242.43 million; retrofitting: P56.7 million. The 185-meter EDSA-Gil
Puyat Avenue Flyover constructed in 1993 for P49.01 million required
P2 million of retrofitting.
The retrofitting of flyovers along major
thoroughfares in Metro Manila was carried out by Angel Lazaro and
Associates International (ALAI), consulting engineers and
architects, and the Department of Public Works and Highways as part
of the Sixth Asian Development Bank (ADB) Highway Project.
As a consulting firm, ALAI analyzed and planned
the retrofitting of 174 bridges in Metro Manila and Davao, many of
them small rural highway bridges. ALAI studied, analyzed, planned,
designed and supervised their retrofitting from 1994 to the present.
According to ALAI engineers, all columns in 11
major Metro Manila interchanges and flyovers—except that over the
Tandang Sora-Commonwealth and the Nagtahan intersections—needed
retrofitting. For example, 91 percent of the columns had problems
with flexure, or their potential to bend under pressure.
In case of a major earthquake, 10 of the 11
structures (91 percent) in Metro Manila will fail in column flexure,
seven (63 percent) of the 11 will fail in column shear and two (18
percent) in column confinement.
Cable restrainers were required for all 11
structures, steel jackets for 10 structures and concrete shear keys
for nine structures.
Cable restrainers prevent partial or total
collapse of the superstructure. Steel jackets for columns involves
the installation of a solid steel shell around the existing column
to improve its ability to withstand repeated stresses to the elastic
limit.
Shear keys were fitted for inadequate anchorages
where bearings may lose restraint and become misaligned and
displaced. The steel or concrete shear keys enhance restraint to
lateral movement in both directions and distribute seismic forces to
the piers.
All 11 major structures were recently
constructed and exhibited no damage or distress of any kind. They
are all engineered structures, designed using the latest codes and
techniques of their time. Their construction was carried out by
reputable contractors and supervised by qualified consultants.
Still, screening and evaluation indicated there
would be significant structural failures if subjected to strong
earthquakes.
These structures are no different from many of
the engineered structures that experienced major damage during the
1971 San Fernando earthquake in California; the 1987 Whittier
Narrows earthquake in Southern California; the 1990 and 1991 Costa
Rica earthquakes; the 1995 earthquake in Kobe, Japan; and the 1999
Chi-Chi earthquake in Central Taiwan.
All these do not mean that structures in Metro
Manila and the country are not sound. Except that the science has
advanced since they were built and tools that are available to
design now were not available then.
It’s nobody’s fault; it’s not a case of a
contractor reneging on the job nor a government official taking a
bribe cut. Our structural engineers and building contractors are
competent and the building code is adequate.
It’s just the way that structural engineering
is progressing. With even more advanced techniques, it is possible
that the retrofitting done now may not be considered adequate 20
years from now.
All large and major structures and thoroughfares
in the country should undergo engineering analysis to detect whether
retrofitting is required. Under the Sixth ADB Road Project, all 500
bridges built with ADB assistance will have undergone retrofitting.
Retrofitting for the major bridges started in
early 2000—after preliminary screenings, designs and planning.
(Dr. Angel L. Lazaro III, is a structural
engineer and Managing Partner of Angel Lazaro & Associates
International, Consulting Architects and Engineers. He is a member
of the National Academy of Science and Technology.)
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