Raise High the Roofbeam!
By all rights, Mexico City's Torre Mayor should only be 38 stories in height. Through the use of special shock-absorbing dampers, the Building Team was able to gain an additional 17 stories, nearly half again the height that would have been allowed under the city's seismic codes.
"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," said Doug Taylor, president of Taylor Devices, Buffalo, N.Y., who developed the building's dampers. "Torre Mayor's engineers asked us: Can we use dampers to lighten the load per square inch? A programming analysis showed the answer was yes." With the dampers, the tower meets the load-per-square-inch restriction even with 17 extra stories.
The building's 24 large dampers, each rated at 1,260,000 pounds of damping force, and 74 smaller ones, rated at 600,000 pounds of force, were developed to ensure reliability for the tower in earthquakes measuring up to 8.5.
The dampers are large shock absorbers with a cylinder-and-piston design. During an earthquake, the piston forces oil through orifices to exert force and, ideally, the dampers absorb the motion instead of the building.
The dampers had a chance to prove their reliability last January 21, when a magnitude 7.6 quake shook Mexico City. Torre Mayor came through unscathed.
At 55 stories, Mexico City’s Torre Mayor building exceeds the city’s height limit by 17 levels. The Building Team received approval from city officials to go higher after proposing a novel seismic structural system.
Toronto-based Zeidler Partnership employed a tube-in-tube construction technique when building Torre Mayor. The two tubes are linked horizontally and vertically. On the outer tube, horizontal braces link the columns and beams together. Every six stories the braces come together on the front and back of the building, creating the diamond pattern visible from the building's exterior. The braces are absorbed by the dampers, which flex when the building flexes. Forces are directed naturally along the structural lines and are dissipated when they hit the dampers.
"The structural engineer told us the kinds of things we were going to have to do to deal with the seismic forces," said architect Rob Eley of Zeidler Partnership, Toronto. "Eventually, it did influence how the building looked and worked architecturally.
"We actually wanted to expose the fact that there was technology used to make the building safe," Eley said. "So that pattern of the diagonals was something that we talked about during the early design phases. We wanted a design that we could make visible and that fit with the architecture."
To conserve energy and achieve the ASHRAE 90.1 standard without using extremely heavy glazing tints, Zeidler chose sealed, double-glazed glass units with low-e coating and a light tint to achieve a shading coefficient of 0.35 for the building's curtain wall.
Cut it out
The front face of the building, which is curved and cut out on the bottom, in front of the lobby, and at the top, faces Mexico City's famous Paseo de la Reforma.
Columns and beams, necessary for the seismic design, fall from 10 stories high and hit the street, where the glass is cut out. The cut spans the lobby, all the way back to the building's 20 high-speed elevator cars. Eley said the lobby's glass is the lightest glass available, so that it appears transparent. At a glance, you just see steel beams and the lobby itself.
The front face of the building, which is curved and cut out on the bottom, in front of the lobby, and at the top, faces Mexico City’s famous Paseo de la Reforma.
"We wanted to make sure there was as much space as possible in front of the building so there was a big space for a big street," said Eley, referring to the 330-foot-wide paseo. "The building being cut away helps increase the volume of that space."
The cutout at the top of the building spans six stories and is a reflection of the one at the lobby. Behind it is a four-story atrium topped by a roof terrace and a helicopter pad.
"Part of the reason we made the cutouts was to reflect the arch shape on the building from a distance as a theme for the building," said Eley.