 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.
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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.
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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.
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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.
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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.
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