19jun30-Risky rebars used in high-rise condos
POSTSCRIPT / June 30, 2019 / Sunday
Risky rebars used in high-rise condos
WITH everybody talking of Filipino fishermen being harassed by Chinese intruders in Philippine waters, many other urgent matters of life and death have been pushed away from our notoriously short span of attention.
One such subject is the ever-present danger posed by the “Big One,” the killer earthquake of at least magnitude-7.2 that experts warn may hit Metro Manila any day soon based on monitoring of seismic pressures waiting to be released in the Marikina valley fault system.
A risk analysis in 2014 by the Philippine Institute of Volcanology and Seismology projected as many as 41,000+ people may get killed in the heavily built-up area in the national capital where 12.9-million people now live and work.
Concerns resurfaced when a magnitude-6.1 quake jolted Central Luzon and rattled Metro Manila last April 22. The temblor damaged a heritage church and flattened a four-story supermarket in Porac, Pampanga, killing at least eight. Train systems and several government offices suspended operations.
In 1990, a magnitude-7.8 quake originating in the Digdig fault in Nueva Ecija crushed an upscale hotel in Baguio City, burying hundreds in the rubble. It damaged 25 percent of houses in Metro Manila and left at least 3,000 people dead and P10 billion in damage to infrastructure.
The collapse in April of the Porac supermarket and the damaging of several Metro Manila structures raised a clamor for a review of building standards and enforcement, especially in high-rise structures. But soon interest waned – as usual.
One focus in the enforcement of building standards is the quality of design and the kind of materials used, especially of reinforcement steel bars (rebars) in high-rise structures.
Rebars, usually stranded together with wires and welded, are embedded in concrete structural parts of tall buildings, providing a steel skeleton for multi-story office buildings and condominiums.
To cut costs and boost profits, however, some contractors use cheaper substandard rebars, or deviate from the approved specifications. The cheating puts in serious jeopardy everyone, especially the occupants of the building.
It may be useful for laymen to know that steel is different from iron (“bakal”). Steel is an alloy, or a combination of iron (the basic metal) and carbon in varying proportions. Carbon gives steel its hardness or toughness, but more of it makes the alloy brittle.
• The difference between MA and QT bars
TWELVE years ago, makers of steel products changed their manufacturing process without notifying the government. They replaced micro-alloyed (MA) steel bars with quenched tempered (QT) steel bars without the knowledge of contractors, developers and end-users.
In metallurgy tests conducted abroad, QT steel bars compared with MA bars failed during the first cycle of an earthquake. The steel of the QT bars was found to be strong only in its outer layer due to the quenching process.
QT steel is made by spraying cold water on a red-hot steel bar. This alters the steel’s metallurgy — the outer layer becomes very strong and brittle, but this is relatively very thin, about one or two millimeters in thickness. The inner core remains as the weaker component.
The QT steel could kill, as its thin brittle outer layer could be easily damaged during a strong earthquake. Imagine if an entire building used QT rebars…. (Also subject to quality check are other elements, including the concrete, which is the mix of cement, aggregates [gravel and sand], and water in correct proportion.)
A high-rise built with substandard materials cannot withstand a magnitude-7.2 earthquake. The Philippines, as well as Taiwan, New Zealand, and Japan, lies near the Pacific Ring of seismic faults, so it experiences frequent seismic and volcanic activity.
Some steel-makers reportedly refuse to have their products undergo cyclic loading tests to measure their behavior while encased in concrete during an earthquake. The test involves the continuous and repeated application of load on a material, or on a structural component, that causes its degradation and ultimately leads to fatigue.
Subjected to multiple cycles under pressures mimicking high-magnitude shakes, a high-quality rebar can take at least 5-7 cycles before showing signs of fatigue. A QT rebar breaks during the first cycle, while MA steel can take 7-8 cycles before showing signs of fatigue.
But the standards of testing steel in the Philippines are not as thorough as desired. A product made with grade 40 steel reportedly could pass as grade 60 because of QT coating.
During a high-magnitude earthquake, grade 40 steel bars could cause a building’s foundation to crumble. They cannot weather the pressure, because grade 40 bars are only for mid-rise, not high-rise buildings.
Engineer Emilio Morales, former chair of the Association of Structural Engineers of the Philippines, said that in the face of recent quakes in the region, the stability of locally made steel bars for high-rise buildings is something to worry about.
He revealed that substandard materials, particularly steel rebars, are still being used in high-rise commercial buildings and residential condominiums.
The recent earthquakes that jolted northwestern Japan, southern China, the East Timor in Indonesia and several Mindanao provinces, have raised fear that thousands of high-rise buildings in the country — built in the last 10 years using substandard, locally made steel bars — are in peril.
China has started banning QT steels bars, with Taiwan banning these outright two years ago, despite inconclusive metallurgy test results undertaken on QT rebars. Taiwan government testers decided that QT steel is strong only on its outer layer due to the quenching process.
More than a decade ago, Morales was commissioned by a big steel maker to conduct a study on the dangers of QT process. He found the use of QT rebars too risky.
The steel-maker that funded the study ignored Morales’ research and reportedly opted to go into producing steel bars using QT process because of its lower cost and higher profit compared with MA.
(First published in the Philippine STAR of June 30, 2019. Follow the author on Twitter as @FDPascual.)
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