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Seismic proportions

Mexico Torre Mayor

The resurrection of Mexico as a world economy has begun with the construction of a landmark tower in the centre of Mexico City. Darrell Morcom reports .

The superstructure of an 225m high tower rising at the centre of Mexico City is destined to be the highest building in Latin America, a focal point of the skyline and a symbol of the city as a place to conduct business.

Its location at one end of a major economic corridor, the Paseo de la Reforma built by Emperor Maximilian in the 19th century, places the 55 storey Torre Mayor among international companies, embassies, world famous museums and auditoria. These will all help ensure its commercial success after scheduled March 2003 completion, justifying the confidence of developer Paul Reichman since the initial conception of the project in 1992.

Reichman International's joint venture partner is ICA, the largest and most experienced construction company in Latin America.

In the design, curved green glass is juxtaposed with burnished granite with a rectangular floor plate. The 10 storey arched entrance is echoed at the top by a sloping glazed 10 storey facet which will glow like a beacon at night.

A two storey retail concourse surrounds the entrance plaza and lobby.

The tower will have 800,000sqft of column free office area, achieving considerable efficiencies of space, while four underground levels and nine above ground with 2,000 parking spaces, allow ample provision for Mexico's car based society.

At 2,250m above sea level, Mexico City provides complex environmental challenges on energy efficiency and internal air quality. The tower is effectively airtight, with a rain-screen cavity wall system providing a two-line defence against the city's dramatic storms and dust laden air, creating an enclosure that will comply with US energy efficiency standards.

In addition, the use of special glass minimises reflectance, provides better vision from inside the building and allows 60% more natural light into the office space.

The fabulous views of the surrounding soaring mountains including the active Popocatapetyl volcano 60km to the southeast, also hint at the problems for the internationally funded development in one of the world's worst seismic zones.

The Canadian design architect Zeidler Roberts Partnership has worked with consultants including IDEA Asociados de Mexico and Adamson Associates Architects from Toronto.

Structural problems are compounded by the building's site over the old Aztec lake of Texcoco which was gradually drained by the Spanish colonisers to gain more building land around the site of the ancient Aztec city. Their legacy is a founding of poor alluvial soils and high water tables, exacerbated by pumping of the water from under the city to sustain its 20M plus inhabitants.

Evidence of major ground settlements over many years can be seen all around the centre of the city with massive building subsidence and cracked pavements.

The removal of around 15,0000m 3of soil for the construction of the four storey deep sub-basement and foundations started in 1998 and was completed early this year.

Foundations are a combination of 1.2m diameter caissons reaching down more than 50m to the hard rock which underlies the soft deposits, and a connecting reinforced concrete mat from 1m to 2.5m thick giving a degree of redundancy to ensure uniform action under severe earthquakes.

Outer retaining walls were designed as 600mm diaphragm walls augmented by a 200mm concrete liner placed during the construction of the underground structures.

Mexico City building codes are based on seismic studies of the ground conditions around this central square. For Torre Mayor, ICA Reichman commissioned seismic tests and designed a structural system to meet the site specific response spectra and the Mexico City Building Code MCBC.

More than 25 different structural systems were studied by Mexican structural engineer Enrique Martinez Romero SA, working alongside the Cantor Seinuk Group (now part of WSP) during the preliminary phase of the project to assess the merits of each under the severe seismic conditions.

The performance based criterion adopted are now becoming the standard of advanced seismic design, embracing both modern understanding of earthquakes with technology from the defence industry to tame and control the flow of seismic forces and energy.

The 'trio-system' involves primary six storey module super-braced frames at the perimeter coupled with a perimeter moment frame. This forms an outer tube system with a trussed tube at the core of the building formed from composite steel and concrete columns. The perimeter frame, supplemented by a powerful super-diagonal bracing system, creates an efficient tube with the central spine connected by rigid floor diaphragms.

The system is augmented by highly innovative large viscous dampers placed in both directions in the perimeter cross bracing. These dampers, originally developed for the NASA space programme, provide an extraordinary cushioning effect from forces produced by seismic pressures or wind. This not only protects the building frame and non-structural elements, but reduces vibration, increasing the comfort level for occupants in conditions that would otherwise produce excessive sway.

The trio-system vastly increases the building's structural performance and reliability, and at the same time allows for large and efficient, columnfree floor spans. This alone is enough to set the Torre Mayor apart from other real estate in Mexico.

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