TORRE MAYOR

Torre Mayor is a 57-story office tower built in Mexico City, Mexico. The $250-million project reaches a height of 225m above ground and is the tallest building in Latin America. The seismic design approach utilized in this project offers an innovative concept in absorption of seismic energy for tall buildings. Soil-structure interaction analysis and site-specific spectral analysis were performed to obtain realistic information with respect to seismicity and building response. A three-dimensional computer model using non-linear viscous supplemental damping elements was created to obtain structure response to time history ground excitation as well as spectral analysis.

Nine aboveground parking levels are provided in addition to four belowground parking levels. The tower is designed according to the Mexico City Building Code (MCBC), and its seismic provisions are among the most stringent requirements worldwide. It also complies with the Uniform Building Code-1994, and several of the latest FEMA-267 provisions proposed after the Northridge Earthquake in California.

Structural System
The building’s superstructure is primarily a steel structure. The columns at the interior and perimeter of the tower are encased in reinforced concrete for the lower half of the tower for added stiffness, strength, and economy.

The project has a four-story underground parking structure, placing the lowest level 15m below grade. A flat slab system with reinforced concrete and composite columns (steel columns encased in concrete) is utilized for the below-grade structure.

Foundation
The foundation for the tower is a combination caisson/mat system. The building is founded on caissons of up 1.2m in diameter reaching 40m down to the hard rock layer of “depositos profundos” existing below the soft deposit layers typically found in Mexico City.

The reinforced concrete mat system connects all the caissons and a 800mm foundation wall at the lowest basement level. The design incorporates a degree of redundancy to ensure uniform action under the most severe earthquake forces. The concrete mat thickness varies from 1.0m to 2.5m thick under the tower core columns where load concentration is the highest. Slurry foundation walls are specified for the project due to the poor soil condition and high water table. The 600mm slurry walls were placed prior to the site excavation and augmented by a 200mm concrete liner wall placed during the construction of the underground structure.

Lateral System
The lateral system selected for this project evolved from a series of studies of alternate structural concepts. More than 25 different structural systems were studied during the preliminary phase of the project in order to establish the merits of each structural system under the severe seismic conditions of Mexico City.

The selected structural system is based on a redundant multiple system, which is a further enhancement of the “Dual” concept recommended by seismic codes worldwide. This is accomplished by introducing a “Dual” conventional (deflection sensitive) lateral force resisting system in combination with a supplemental damping system (velocity sensitive). In effect, a “Trio” system is provided to respond to the seismic energy from an earthquake.

The “Trio” system is composed of a primary super braced frame at the perimeter of the tower coupled with a perimeter moment frame forming a tube system, and a trussed tube at the core of the building. The bracing connecting the composite core columns creates a structural spine in the building core. The perimeter frame and the powerful super-diagonal system create an efficient tube structure joining the spine in resisting the seismic forces. This system is augmented by a series of supplemental viscous dampers placed in North-South and East–West directions.

Various studies were performed for the selection of the dampers with respect to the type of damper as well as the capacity and location of the dampers. In the North-South direction, a total of 72 dampers are placed within the core truss system. A total of 24 dampers are placed as part of the perimeter bracing system. In the East-West direction dampers are placed at the North and South perimeter of the tower. Dampers are placed in such a configuration as to optimize their performance. This optimization attempts to improve the effectiveness of the dampers by increasing the dampers differential velocity for a given inter-story sway and velocity. This is accomplished by reversing the orientation of axial velocity of the columns adjacent to the dampers. This increases the net differential velocity of the damper. This could be physically achieved by modifying the placement of the dampers by placing them between two lateral systems comprised of truss system, frame system or wall system or any combination of them. This unique application resulted in a US Patent grant12.

The selected structural system incorporates supplemental damping devices that are highly effective in reducing the impact of seismic motion on the structure as well as on the non-structural elements (i.e. architectural and mechanical components). The supplemental damping reduces the overall and inter-story sway of the tower, as well as the vibration and the seismic forces of the structural elements.

The damping elements reduce the building response by absorbing and dissipating a significant portion of the seismic energy transmitted to the building and consequently reducing the ductility demand on the steel framing. They also add to occupants’ comfort level against sway perception, during either high wind or moderate levels of earthquake shaking.

The stiffness and load carrying capacity of the tower columns is enhanced by encasing them in concrete up to mid-height of the tower where demands on strength and stiffness are higher. The concrete encasement of core columns extends five floors above the perimeter columns in order not to create a sudden change in inter-story floor stiffness.

Supplemental Damping
During the schematic phase, the structure was studied with and without the supplemental damping system in order to ascertain quantitatively the advantages of the supplemental damping system with respect to building performance, under a seismic event. For example, designers studied the sway response of the tower under a seismic excitation with Richter magnitude of 8.2 for the structure with and without the supplemental damping system.

Viscous damping units made by Taylor Device, Inc. were selected after studying various damping systems for this project. The structure, using the supplemental viscous damping elements, produces equivalent damping ratios (as percentage of critical damping) of 8.5 percent in the North-South and 12 percent in the East-West direction for the fundamental modes of vibration.

Time–history analysis, using impulse excitation, was used to evaluate the equivalent damping of the system. Damping ratios were obtained by evaluating the decay function of the response time history, such as the response of the tower to an impulse loading in both primary directions. As a crosscheck, the damping calibration was verified by comparing time history responses of the structure with dampers with that of a system with equivalent modal damping.

Bracing of the structure follows a Super-X configuration at the East and West faces where the X covers the entire width of the tower. At North and South faces two sets of Super-X’s were introduced. No bracing is placed with in the two center bays, except at three locations where a set of diagonals forms a diamond shape connecting the Super-X systems. The dampers in the North and South faces are placed at these diamond-bracing locations. This in effect enhances the damping system’s performance by creating a damped link between the Super-X systems. Additional fine-tuning of the secondary link element was necessary to emphasize the basic concept of damped link element.

Site-Specific Soil-Structure Interaction Analysis
The building is located in seismic zone II, at the border between seismic zone II and seismic zone III, as defined by the Mexico City Building Code. Zone III is the MCBC’s most severe seismic zone.

Special Features
A special floor diaphragm system was designed at the 10th level where the structure’s footprint increases to include the low rise parking structure. The paths for the lateral force transfer between tower lateral system and additional low-rise lateral systems were studied and designed to accommodate the diaphragm action.

The floor plates below level 10 are set back to allow for an open space plaza and lobby entrance at the South side of the building. This is done in such a way as to form an arch with its apex at the 10th level. The free standings columns and beams in this zone were sized to maintain a similar stiffness and strength to the floors above and the frame at the north face of the tower. A set of detailed computer models was generated to provide a tool for the calibration of the sizes of the beams and columns. Column elements are comprised of two coupled circular composite columns providing sufficient strength and stiffness to span vertically between the bracing levels.

Seismic Study for Construction Phase
Dynamic studies were performed to verify the structural performance at various stages of construction under the design seismic event. Site-specific seismic studies were performed for building constructed to the 10th level and the 23rd level. Obviously, the period of the building at these stages is shorter than the final condition. Also effect of supplemental dampers was not considered for building reaching up to 23rd level. However, the partial mass associated with the constructed portion would compensate the impact from the change in period.

The engineering achievement in this project not only exemplifies the great potential in the collaboration of engineering practice and research but also highlights the importance of creative and constructive dialogue among design and construction team members. In one of the world’s worst seismic zones, the Torre Mayor is one of the world’s safest buildings achieved with an extremely cost-effective design