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Raising the Roof in a Fault Zone

Massive, forged shock-absorbing dampers lend earthquake protection to Latin America's tallest tower.

Until recently, soaring skyscrapers and earthquake zones were considered mutually exclusive, but the use of revolutionary shock-absorbing dampers are now safely raising skylines in cities that are situated in fault zones. The new forged damper technology was employed to incorporate 17 additional stories into Mexico City's Torre Mayor Building while keeping the same load per square foot that it would have had with 38 stories, the former local building code limit for seismic protection. As a result, the 55-story Torre Mayor, meaning "Big Tower," is the tallest building in Latin America.

A High Profile Project

The Torre Mayor is a world class corporate complex and a premier Mexico City landmark, standing 738 feet tall. It is the ambitious development project of Paul Reichmann of Canada, with Canadian architects Ziedler Roberts Partnership providing an exquisite design that blends the light, airy feel of glass with a state-of-the-art tubular steel framework. It has 800,000 square feet of office space, as well as 32,000 square feet of commercial space housed within a two-level retail concourse that surrounds the lobby.

Due to the tower's innovative dampers, provided by Taylor Devices (Buffalo, NY) with forged components supplied by Scot Forge (Spring Grove, IL), the tower represents an exception to the 38-story building code restriction that protects Southwestern Mexico's buildings from the area's volatile seismic activity. An earthquake measuring 8.1 on the Richter scale struck Mexico City in September 1985, and further quake activity occurred in 1999 due to the city's situation on top of the Cocos Plate, a highly active subduction thrust fault. "The 38-story code restriction was calculated to limit the amount of load per square inch that can be located safely upon this fault zone and its soft, sand-based soil," says Taylor Devices' president, Doug Taylor. "So, Torre Mayor project engineers approached us with the question: can the building be taller than 38 stories if we use dampers to lighten the load per square inch? A programming analysis showed the answer was yes. Consequently, with the Torre Mayor's seismic loads dramatically reduced by the dampers, the tower meets the load-per-square-inch restriction even with 17 extra stories."

In fact, the dampers from Taylor Devices, who develops products for seismic protection in partnership with the headquarters for U.S. seismic research at the State University of New York at Buffalo, ensured structural reliability for the tower in earthquakes measuring up to 8.5.

The Dampers that are Making the Difference

To understand damper technology, "think of the shock absorbers in your car," says Taylor. "A damper is a very large shock absorber with a cylinder-and-piston design, in which the piston forces oil through orifices to exert force. Now, here's the main difference between the shocks in your car and a damper in a building: a car's shock absorber exerts 400 pounds of force, while each damper in the Torre Mayor building exerts well over a million pounds of force." He notes that the Torre Mayor damper design evolved from a product formerly used in military applications to protect missile launch sites against nuclear attacks.

In architectural applications, the dampers can be built into the steel bracing. The Torre Mayor design incorporates 24 dampers within the bracing on each of the building's two long walls, mounted with hidden bolts. It is the first tall building to use mega brace damping elements, where a single damper spans multiple floors. The dampers are plainly visible through the windows. The 24 large dampers are 6' long x 24" O.D., with a 16" bore, and are rated at 1,260,000 lbs. damping force each, while 74 smaller dampers used in the short walls of the structure are rated at 600,000 lbs.

High Strength Construction

The dampers for the Torre Mayor were constructed using the open die forging process. Open die forging is ideally suited for damper production because of the large dimensional requirements of the cylinders, as well as the ability to forge the inside diameter around a pin, rather than drilling it out. By forging a hollow rather than drilling out a solid cylinder, considerable material savings were realized. Scot Forge placed a hot ring preform over a mandrel pin, then elongated the workpiece to form the cylinder. Forged end caps were later threaded onto the end of each cylinder to close the bore. The forging process also provides superior strength due to continuous directional grain flow-i.e., steel grains are deliberately oriented in a direction that improves mechanical properties and metallurgical soundness. Forging provides unmatched structural strength and integrity, because internal voids and porosity are eliminated as cylinder walls are consolidated during the forging process. The pistons also benefit from the extra strength and integrity provided by forging.

Taylor sought a reliable forging supplier for the Torre Mayor project, stressing that "it's important to choose an experienced forging company with the right equipment, and one that takes responsibility for their work." Taylor turned to Scot Forge after a previous forging supplier's product cracked, due to a failure to ensure uniform cooling. "Scot Forge has the know-how and the machinery to produce high performance forgings efficiently," Taylor says.

Scot Forge produced a total of 504 components to make up the 96 dampers. These components included rough machined cylinders, cylinder caps, cap nuts, mounting flanges, and piston heads. All were forged from 4140 normalized, quenched and tempered steel.

The pistons were produced using Scot Forge's unique Tartan BarÒ process. In the process, the round bars that would become pistons were initially forged to produce sound centers for internal structural integrity. Each bar was then rolled to a smooth surface in under five minutes, in the company's state-of-the-art bar planishing mill. The process allowed the bars to be produced efficiently, while providing the improved soundness, integrity, and high strength that is required of the pistons when in service and under high stress.

Recent Quake

On January 21, 2003, the coastal region of the State of Colima, Mexico experienced a 7.6 magnitude earthquake. When the quake reached Mexico City it was amplified by the soft soils in the area. This resulted in a relatively strong response with some 30 seconds of shaking. At the time of the quake, thirty-one floors of the recently opened Torre Mayor were occupied, the balance still undergoing final interior finishing. An occupant reported that he saw hanging light fixtures beginning to sway and heard a slight noise, then turned toward the noise and saw that the large damper outside his office was stroking. This, of course, signified that an earthquake was occurring.

Occupants also reported that from inside the building the quake felt far less severe. This may well be due to the extensive use of fluid dampers as a primary element of the building's seismic protection and earthquake resistance capability.

A Government required post-earthquake inspection was performed with no damage of any kind noted. The Torre Mayor has received several American Construction Industry awards and was one of the four finalists for the U.S. Civil Engineer Research Foundation's 2003 Charles J. Pankow Award for Innovation.

Conclusion

Forged dampers are poised to improve the future of building in earthquake zones. In addition to their incorporation into the Torre Mayor Building, they are appearing in other high profile earthquake-zone projects including the San Francisco Bay Bridge. And forged dampers helped keep the new Seattle Mariners stadium intact during the 6.7-magnitude quake that hit in February 2001- more proof that the art of earthquake protection has reached a new level.