Tuesday, January 31, 2012

Uneven buildings turn heads

Leaning buildings are in the news. Members of the British Parliament are meeting with engineers to discuss ways to straighten St. Stephen’s tower – affectionately known as Big Ben – with a tilt now visible to the naked eye. In Italy, the Leaning Tower of Pisa has recently opened after a decade-long “straightening.” The technique used in Italy is being mentioned as a possibility for Big Ben. It is complicated engineering but in simplest terms boils down to removing soil from underneath the higher side of the building, allowing it to catch up with the sunken side.

Here in Mexico City we have no shortage of tilting and sinking buildings into which we can wander. The oldest is the first of the successive renditions of the Aztec Templo Mayor. Over a period of only 104 years the Aztec’s most important temple was rebuilt seven times. Each edifice was superimposed on the previous one in an effort to straighten out the tilting and sinking.

This was an inevitable consequence of building on the unstable soil of a swampy island. The Spaniards paid no heed to the fate of the Aztec temple and began to build their grand cathedral on almost the very same spot. Even during construction sinking and tilting were obvious and had to be accommodated. It remains a problem for contemporary Mexico City architects.

Driving to the Templo Mayor from Cuernavaca we can enter the city on Viaducto Tlalpan, a modern day expressway, originally the Aztec causeway from the island Tenochitlan to the southern shore of the Lake of Mexico.

Along the way we pass hundreds if not thousands of tilting and sinking buildings constructed during the last sixty years -- essentially built on mud. It should surprise no one they are sinking. There are many cross streets running through tunnels under the expressway. Even though not seen, you feel them. Every time you drive over a tunnel you get a bounce. It’s not that civil engineers can’t build a smooth ramp over the tunnel, it’s that the tunnel is hollow and trying to float. The roadway above is heavy and trying to sink.

In the 1950’s Mexican architect brothers Leonardo and Adolfo Zeevaert, with structural engineer Nathan Newmark, designed the Latin-American Tower and the technology that keeps it from sinking. A sophisticated system using three hundred pylons and hydraulic pumps that pump water in as needed keep the building straight and level. The building itself sits on a huge concrete plate. This design not only corrects for the fluid subsoil but also protects against seismic activity. It has had no sinking while just across the street the Palace of Fine Arts, only twenty years older, has sunk nearly one floor.

Famed Mexican architect Pedro Ramirez Vazquez also considered the fluid nature of the soil in Mexico City’s filled-in lakebed. He designed and built the new Basilica of Guadalupe only meters from the old and still sinking Basilica, and yet it remains level. Using pylons in combination with Archimedes’ Principle that maintains that an object will float if its displacement is greater than its weight, such as a plate floating on water, Ramirez Vazquez created an amazing circular space that allows worshippers unfettered views of Juan Diego’s cape as they essentially float safely in a basilica that will never sink.

Since beginning of the Templo Mayor archeological dig in 1978 with its necessary extraction of water to expose the prehispanic buildings, the pace of the sinking and tilting of the nearby Metropolitan Cathedral has accelerated. A graphic display under the central dome of the cathedral clearly documents this shift. From the dome hangs a giant plumb bob. On the floor is a diagram showing where the plumb bob would have pointed when construction began in 1573 and where it pointed in 1989 when the Cathedral was at its greatest inclination. A dotted line shows the progress made in straightening the building and the intended ‘path’ of the plumb bob as civil engineers attempt to eliminate the tilt. The method chosen is similar to that used on the Tower of Pisa – extraction of soil. It is successfully reducing the tilt and the results are impressive. However, it is doing nothing to halt the sinking of the cathedral. The source of the problem in Mexico City is different from that of Pisa.

Mexico City’s subsoil behaves as a fluid as seen in the design of the Latin-American Tower, the Basilica, and the hydraulic effect of the tunnels under Viaducto Tlalpan. Some years ago I asked an archeologist friend, Francisco Hinojosa, working on the Cathedral’s soil extraction, to present the civil engineers with my alternative idea. It was to take advantage of the large empty space in front of the cathedral and by using horizontal steel beams beneath street level ‘attach’ the Zocalo to the base of the Cathedral. The beams would disperse the weight of the Cathedral throughout the entire area covered by both the Cathedral and the Zocalo thus increasing its displacement and, according to Archimedes’ Principle, I theorize, allow the Cathedral to essentially float, be lifted, and straightened.

Francisco bravely advanced my theory to the engineers but returned to tell me “They laughed at you.” Meanwhile, the Cathedral continues to both lean and sink. “Watch out Cathedral!” The tilting National Palace might get first dibs to be attached to the Zocalo.

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