To avoid disastrous consequences during 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, such as broken mirrors, glass, 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.

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 R144.15 million. The cost of retrofitting was approximately R144 million. The Nagtahan Interchange was built in 1992 for R242.43 million; cost of retrofitting was R56.7 million. The 185-meter EDSA-Gil Puyat Avenue Flyover constructed in 1993 for R49.01 million required R2 million of retrofitting.

The retrofitting of flyovers along major thorough fares in Metro Manila was carried out by Angel Lazaro and Associates International (ALAI), consulting engineers and architects, as part of the Sixth Asian Development Bank Highway Project with the Department of Public Works and Highways. 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 those over the Tandang Sora-Commonwealth and the Nagtahan intersections – needed retrofitting. Ninety-one percent of the columns, for example, 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 would fail in column flexure, seven (63 percent) of the 11 would fail in column shear, and two (18 percent) would fail in column confinement.

Cable restrainers were required for all 11 structures (100 percent), steel jackets for 10 structures (91 percent), and concrete shear keys for nine structures (82 percent). Cable restrainers prevent partial or total collapse of a 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 might 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.

It should be pointed out that all 11 major structures were recently constructed and they exhibited no damage or distress of any kind. They are all engineered structures, and they were 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 they were 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.

This does not mean that structures in Metro Manila and the country are not sound. It is simply that the science has advanced since they were built; tools that are available to design now were not available then.

It’s nobody’s fault; it’s not a case of a contractor cheating 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 by the end of this year. Retrofitting for the major bridges started in early 2000 – after preliminary screenings, designs, and planning.
A structure may not require retrofitting if it is not large or important enough or if it is technically and structurally sound. If retrofitting is required, a detailed evaluation is conducted. If the provided capacity or strength is less than the demand or load, retrofitting is required. A financial analysis determines whether the retrofitting costs will be excessive, in which case replacement of the bridge may be the better solution.

With the development and availability of more advanced analysis and design procedures and facilities, deficiencies in older structures can be detected and corrected.

(Academician Angel L. Lazaro III, is a structural engineer and Managing Partner of Angel Lazaro & Associates International, Consulting Architects and Engineers, and is a member of the National Academy of Science and Technology.)