ELASTIC CONCRETE AND EARTHQUAKE-RESISTANT COLUMNS
In Mexico City, public surveys cite seismic risk as the most worrying threat to daily life for residents, and for good reason: it’s one of the most seismically active regions on earth. The nation of Mexico sits atop the intersection of five large tectonic plates, with Mexico City located in the centre of the country, directly above the five intersecting faults.
The danger posed by seismic activity in Mexico City was never clearer than in September 1985, when a massive earthquake collapsed over 400 buildings and severely damaged 3,100 more. More than 9,500 people were killed, with another 30,000 injured and more than 100,000 left homeless. The overall economic losses were an estimated $4 billion US. The site of over 340 recorded earthquakes, seismic tremors poses an ever-present danger for residents of Mexico City, with geologists expecting an earthquake as severe as the 1985 quake will inevitably strike again.
In the decades since, Mexico City has taken significant steps toward addressing earthquake risk. One area of focus has been the city’s buildings, with heightened structural resistance measures mandated in step with the latest advances in engineering. Zoning regulations, building codes, and laws focusing on new construction projects are very demanding and strictly enforced.
For father-and-son team Yoram and Sholem Cimet—the designers and developers of the 25-story Torre Glorieta office building in the heart of the city’s downtown core—there was no shortage of challenges ensuring their building is earthquake-proof. The tower’s vertical structure was to be made in large part from concrete, a material not favoured in
fault zones because it exhibits less flexibility than, for instance, wood or steel. Advisors for the Faultline project at San Francisco
’s Exploratorium science and research center elaborate on the fact that, with
tower projects, much of the focus must be on flexibility: “skyscrapers everywhere must be reinforced to withstand strong forces from high
but in quake zones, there are additional considerations. Engineers must design structur
es that can absorb the energy of the waves throughout the height of the building. Floors and walls can be constructed to transfer the shaking energy downward through the building and back to the ground. The joints between supportive parts of a building [must] be reinforced to tolerate being bent or misshapen by earthquake forces.”
The Torre Glorieta project being a tall tower, the columns of the building became a focal point. Yoram, Sholem and their team had to prove that the building’s vertical structure could withstand the stress and strain of being rocked by a major seismic event without toppling. In engineering terms, the concrete used in the columns must have a suitable modulus of elasticity—the flexible property of a material being able to absorb impact energy and return to its original size and shape—to meet Mexico City’s stringent regulations. Carlos Tapia was a consulting engineer on the Torre Glorieta project. He explains, “for a structure of this size, city regulations require us to use concrete known as Class One. That means the highest dimensional weight and modulus of elasticity that can be obtained.”
Sourcing concrete of such high specifications in the very center of Mexico City, the largest metropolitan area in the western hemisphere, was a massive challenge. Sholem explains: “in our investigation we were told that it is difficult to obtain that elastic modulus due to the travel time of the trucks from the plant to the building site, because much of the time these mixer trucks get stuck in traffic under hot conditions. The concrete arrives and we know we have to add some more water to the mix. That little bit more water, the travel time, and the other special additives in the mix cause us not to be able to obtain that elastic modulus.”
In the end, no supplier or contractor made offers on the job, as none could assure that concrete could be supplied within the city’s tight standards. When Yoram and Sholem went looking for a solution, they found a perfect fit with ProAll Reimer Mixers, purchasing a mobile mixer and supplying their own concrete on site. One Reimer Mixer allowed the crew to take total control of their delivery logistics, and eliminated the possibility of a poor mix design by only mixing the concrete materials when needed. “To have the volumetric mixer here at the construction site has allowed us to reach those elastic modulus values”, says Tapia, “in other places it is not possible because the concrete, from the moment it is mixed to the time when it is put in its place, it can be up to 3 hours of time.” Sholem concurs, “we, by having this Reimer Mixer on site, could guarantee that we will have the elastic modulus that the structural engineers wanted the building to have, to guarantee good columns”.
To ensure all specifications were being met, an on-site lab performed over 800 tests throughout the construction process, testing the concrete for elasticity, dimensional weight, and resistance. There wasn’t a single failed test. “The ProAll Reimer volumetric mixer is the only way we could achieve the adequate elastic modulus during the process without any problems”, explains Sholem, “it gave us the peace of mind that we have a very well-built structure, even with the heightened risk of earthquake activity here”.