Standing on seismic Mexico City's dry central lake bed or "bowl of jello," where many buildings collapsed in 1985's magnitude 7.3 earthquake, Latin America's tallest building might be mistaken for a giant sitting duck waiting for the next Big One. But on closer inspection, it becomes apparent that the $250-million-plus Torre Mayor, which officially opened last week, is no quack. The 225-meter-tall office tower sparkles with damper-studded, diamond-shaped superdiagonal bracing, architecturally expressed on its perimeter moment frame. It is a giant billboard for seismic strength. The diamonds and dampers offer structural efficiencies and other advantages inside and outall the way to slashed insurance premiums.
|(Photo courtesy of Reichmann International)|
The damped steel diamonds that adorn each broad face of the tower have already proven they are more than window dressing. Torre Mayor's lower floors had just been occupied when, on Jan. 21, a magnitude 7.6 quake rocked the city. Almost all of Torre Mayor's occupants were oblivious to the temblor. The tower survived without a scratch (ENR 2/3 p. 16).
The secret of the seismic success is "the way we engage the dampers," says Ahmad Rahimian, executive vice president of The Cantor Seinuk Group Inc., a WSP Group Co. The New York City-based firm is the project's structural consultant in association with Enrique Martinez Romero S.A., the local engineer of record.
The engineer used proven technologyfluid viscous dampers and superdiagonal bracing within a perimeter structural tubeto raise the bar on seismic engineering and provide a 55-story building that resists earthquake forces nearly four times as efficiently as a conventionally damped building.
The brilliance of the scheme is in the configuration of structure, say sources. All four perimeter walls contain superdiagonals configured as diamonds, rather than Xs. Broad south and north faces contain dampers, which resist seismic loads in the east-west direction. Each elevation has four steel diamonds, with 42-m legs. The diamonds overlap each other vertically at their peaks and valleys to form three smaller diamonds. Each small diamond has four, 1,200-kip-capacity dampers, one on each leg near the apex or valley.
The structure behaves as if there is a line of dampers sandwiched between two vertical megatrusses with undamped "zigzag" webs. Dampers between megatrusses rather than on their diagonals "makes the dampers work harder," says the engineer.
A damped building is analogous to a human body, where muscles, or dampers, protect bones, or the skeleton, from breaking. Torre Mayor has extra muscle. In a quake, its top moves 0.6 m less than a conventionally designed building, says Rahimian.
By dissipating the earthquake's energy through damping, "you keep the structure's ductility as a second line of defense," explains Martinez-Romero. That allows the building to go through that many more cycles of shaking without using up reserve strength.
It's a "really elegant" design, says Douglas P. Taylor, president of Taylor Devices Inc., North Tonawanda, N.Y., the damper maker. If not for the geometric arrangement, the braces would be larger and the dampers would be three times their diameterso big they would not fit into the building.
The primary reason for spending $4 million on a system of 98 dampers, including 74 in the core, was to provide a safer-than-standard building, says Gerald W. Ricker, director general in the local office of owner-developer-builder, Reichmann International, Toronto. The goal was to have a building that would offer greater creature comfort, reduce panic attacks in a temblor and barely skip an operational beat after a major quake.
But the dampers nearly paid for themselves, says the engineer, thanks to resultant economies in the steel and concrete structure. For example, the tower used 18,000 metric tons of steel instead of 23,000. Typical 1.5-m-dia caissons, supporting a reinforced concrete mat, are roughly 20% shallower, or 55 m long.
Caisson length was optimized because the seismic analysis looked at both code requirements and site spectra response for the specific location, says Raymond J. Poletto, senior associate for New York City-based Mueser Rutledge Consulting Engineers, project geotechnical engineer. "We did field load tests," he adds.
Reduced movement allowed a 1�2-in. tolerance in curtain wall joints, rather than 2 to 3 in., and reduced tolerances in ductwork and piping runs. The approach also limits quake-induced damage to hung ceilings, fire sprinklers, partitions, mechanical systems and cladding. That, in turn, limits a tenant's down time and economic losses relating to computers, filing cabinets and furniture. "The whole building is...put in a safer range," says Dalibor Vokac, senior associate partner with Zeidler Grinnell Partnership, which is associated with architect-of-record Adamson Associates, both Toronto.
|MUSCLE Dampers cost $4 million but nearly paid for themselves in reduced structure alone. ((Photo courtesy of Reichmann International)|
Safety has it rewards. The original insurance premium has been reduced by 33%, according to Reichmann. A further reduction to 50% of the original is expected once soil mechanics and foundation studies are complete.
The damper-structure configuration is so unusual that Rahimian patented it. "Ahmad is the only structural engineer with a patent on a steel arrangement on a building," claims Taylor. "I never thought it could be patented," adds Martinez-Romero.
U.S. Patent No. 6,397,528 B1 applies to the configuration of dampers and diamonds, not to 600-kip-capacity core dampers, which resist seismic loads in the north-south direction. Core dampers are located conventionally on diagonals of the vertical trusses that transverse the core, two in the end walls and two in between.
Advantages of the engineering propagate beyond the building. To fulfill a pledge to city officials, Reichmann spent $175,000 on a building movement monitoring system. Workers are completing the wiring of foundation sensors installed during construction to a computer control room in the lowest of the four basement levels. Data compiled will be shared with engineers, locally and internationally, to increase knowledge about the behavior of tall buildings in seismic zones, says Ricker.
Torre Mayor marks the first use of dampers for seismic resistance in a perimeter frame but is likely not the last. Martinez-Romero already plans to use the system.
The 70 x 33-m tower has a structural concrete basement that fills the 80-m-sq site. Floors one to 10 in the tower have structural steel columns encased in reinforced concrete, which limits the size of the steel members. Damper clusters begin at the eleventh floor. Floor diaphragms, which connect the perimeter frame to a 27.6 x 15-m structural steel core, provide the stiffness that ensures that all the elements, whether dampers, bracing or columns, respond simultaneously and uniformly to a seismic event, says the engineer.
Maintenance workers will periodically inspect dampers and check their oil content. They are expected to be engaged a maximum seven or eight times every 50 years. They are tested, however, for hundreds of cycles.
The engineering team also was challenged in convincing local building officials that the performance-based design met the intent of the code. "We needed to demonstrate that with energy dissipation we were able to provide the same level of protection" as with other systems, says Martinez-Romero.
After many meetings and a thorough engineering review by an independent team, building officials decided the system had merit. "The basic structural system, together with the supplemental damping, were found to be very efficient for resisting earthquake loading...," says Roberto Meli, professor emeritus at National University of Mexico, Mexico City, and a member of the technical committee that reviewed the design. "It is expected the building will show an excellent seismic performance," he says.
The expressed superstructure might be super-strong but, from the point of construction, the foundations were the most formidable. Not only was the site hemmed in by roads or existing buildings, it was strewn with timber piles from previous structures, about 150 of which were in the path of new caissons. The strategy was to use top-down construction, divide the site into three sections and complete one at a time.
Workers from ICA Solum, the local substructure and concrete contractor, first built an 80-m-sq perimeter slurry wall, 22 m deep. Then, they started extracting piles. Crews inserted a steel pipe fitted with a cutting edge, as long as the pile. The pipe had a sleeve over it that contained a gripping device. The tool cut through the soil, gripped the bottom of the pile and, through vibration, pulled it up.
The contractor then used a slurry trenching method to install two precast concrete divider walls, breaking the site into three sections. The first section tackled was the 22-m-wide area directly under the tower's core. The strategy allowed the core superstructure to get under way when the foundation was only one- third complete.
Workers started drilling caissons in the core area before excavating the 5-m layer of fill, 3 m of silty sand and 25 m of soft clay. Once caissons were drilled, crews began excavating 3 m of soil at a time and installing horizontal bracing perpendicular to the dividers. Crews then cut off the tops of exposed timber piles using chain saws. Eventually, workers removed a total of 300 old piles.
Work started at the center of each section and moved toward the sides, in a V-shaped excavation. Once four levels of bracing were installed, workers cut the upper section of the caissons and prepared caisson tops to receive the 2.5-m-thick mat. In a leapfrog fashion also from the center out, workers installed reinforcing steel and cast the mat in 10-m-wide strips.
The strip and leapfrogging approach prevented base heave in the soft clay, says Poletto. It also solved a logistical problem. In Mexico City, because of chronic traffic congestion, it is not wise to bring in more than 500 cu yd of concrete within an eight-hour work day, says Poletto.
Moving from the bottom up, workers prepared and cast each concrete basement floor, while removing braces to be reused in the other sections. The rhythm was to "install, dismantle, move and reuse," says Poletto.
As soon as the center was complete, other workers brought in the tower crane and began erecting the core steel. The foundation operation then moved sequentially to the two other strips, beginning with the south section so that deliveries and staging could continue from the north.
|(Image courtesy of Cantor Seinuk)|
Before removing divider walls, workers cut holes in them to connect the horizontal bracing from section to section. The bracing worked like a strut to support the slurry wall perimeter until the basement floor diaphragms were finished. "It was like threading the needle," says Poletto.
The total operation was "very labor intensive," he adds, saying no one in the U.S. "would contemplate" this approach because labor costs so much more.
Foundation work began in May 1998 and was completed in 20 months. Core superstructure started in January 2001. Core steel was 30 stories up before the basement was finished.
The approach incorporated traditional Mexican methods of labor-intensive foundation construction and the American tradition of getting a jump-start on the tower steel. "It's a Mexican quickstep but not in American time," says Poletto.
He calls the tower an "emerging building." Unlike most Mexico City buildings that sink 10 to 20 centimeters per year because of groundwater extraction, Torre Mayor was designed to allow outside soils to settle around it. After 10 to 15 years, the sidewalk will slope away from the building and will have to be rebuilt, says Poletto.
Reichmann's Ricker says handling superstructure steel was no picnic, either, because there was no room for storage, staging or to maneuver. Steel would arrive by truck the night before it was scheduled to be erected.
Steel topped out last August. "Being able to do that in 18 months in Mexico City was pretty good," says Ricker, who reports that the project finished on time and on budget, with no deaths or major injuries. The owner imposed U.S. safety standards, though there are no equivalent standards in Mexico, he says.
The Sept. 11, 2001, terrorist attacks on the World Trade Center resonated all the way to Torre Mayor. At the time, Reichmann was negotiating its first lease on the building with Marsh & McLennan Cos. The firm lost 295 employees in the 9/11 attack. After hiring its own engineers to review the building, Marsh & McLennan ended up leasing even more space than originally intended, says Ricker.
Nothing about Torre Mayor was changed after 9/11. "We were already building everything to such a high standard," says Ricker, because of pre-existing security concerns in Mexico. Ricker adds that dampers don't differentiatethey provide increased resistance to any kind of impulse, whether an earthquake or a blast.
Torre Mayor raises the bar. "We are not as proud of the height as we are of the engineering," says Martinez-Romero. "It was not a matter of building the tallest building in South America," he says. "The merit is building the safest building."
|(Image courtesy of Cantor Seinuk)|