In the exposed anatomy of fallen buildings in the devastated areas of Mexico City, engineers are finding clues to this riddle: Why did some seemingly sound buildings fall during the recent earthquakes and others, right alongside, remain standing?
Complete answers could take weeks or even months to obtain, the experts say. But important early signs are coming to light, even while some of the evidence is being swept away as rubble is cleared by authorities and by residents who, in the words of one foreign diplomat, “are eager to get back in business.”
The demolition of some ruined buildings has proceeded rapidly. Vacant lots have appeared where rubble heaps of concrete and steel stood but a few days earlier, and plans are being made to bring down larger buildings--damaged and partially standing--with explosives.
But Mexican and foreign experts are studying many buildings to learn why they collapsed, a discipline known as forensic engineering.
In some cases, materials were used that were too weak for the size of the structure, U.S. engineers said. In others, supports between columns were missing or too weak to absorb the lateral movement of the buildings caused by the disastrous Sept. 19 earthquake and the aftershocks that ensued. In such instances, the columns took the full burden of shifting weight and crumbled.
Still other buildings went up before successive advances were made in quake-resistant construction techniques. Some engineers speculated that the already soft ground beneath some buildings--much of Mexico City rests on an ancient lake bed--was weakened by the construction of this capital city’s extensive subway system.
“The damage depended a lot on what kind of ground the buildings were founded on,” said engineer William Stone of the U.S. Bureau of Standards. “Of course, on soft ground, precautions should have been taken about what kind of building to construct.”
Stone said that many of the damaged buildings he has inspected were 25 to 40 years old and lacked advanced quake-resistant design. In addition, many were taller than eight stories and therefore tended to sway sharply in the powerful Sept. 19 temblor, which measured 8.1 on the Richter scale.
U.S. and Mexican engineers cited other problems.
At the collapsed Bancomer bank building on the 20th of November Street, an engineer pointed to a spaghetti-like tangle of steel rods that reinforced the concrete columns. “They’re tiny,” he said. “Much too weak for the size of the building, which is eight or nine floors.”
Each floor of the Bancomer building collapsed in a pattern of a stack of pancakes, a phenomenon that was common in the cases of many other wrecked buildings.
‘All the Columns Failed’
At Benito Juarez Hospital, where perhaps 700 people were trapped in the closely packed rubble, engineers noticed destruction of joints where floor slabs met supporting columns.
“All the columns failed,” a U.S. engineer said. “The stories fell on one another.”
The Pino Suarez complex, a group of five high-rise buildings housing courts and legal offices, suffered from multiple problems, according to Hector Vizcarra, a government engineer.
At Pino Suarez, a 20-story building fell onto a neighboring 14-story structure. Its visible columns and floor-support trestles were bent like giant ribs. At least one other 20-floor structure now lists to one side. This and two other buildings still standing in the complex will have to be destroyed.
The ground at Pino Suarez is notoriously soft. The complex had already been reinforced at its base, Vizcarra said. “Troubles had been noticed before,” he added. “The buildings had swayed.”
Made With Steel Beams
In addition, Pino Suarez was not constructed with reinforced concrete but with steel beams, Vizcarra pointed out. “Mexico does not have the technology to build a steel building,” he said.
A few buildings that were reinforced against possible earthquake damage after being built also fared badly. Among the so-called retrofitted buildings were the Pino Suarez complex, the Nuevo Leon apartment building in the huge Tlatelolco housing development and a group of apartments called Multifamiliar Juarez.
The possible role of subway excavation is highly speculative. The location of some of the damaged buildings coincides with the location of important underground stations and lines. However, Vizcarra and other engineers point out that the greatest damage occurred in sectors near downtown Mexico City, built over part of what centuries ago was a large lake.
A Mile to Firm Ground
The soft, dry lake bed is at its deepest beneath the central area, and firm, rocky ground may be as far as a mile below the surface.
Full inspection of collapsed and damaged buildings requires that they be left in place for a month, a U.S. structural engineer said. During that time, samples of reinforcing rods and concrete can be analyzed to see if the quality was adequate and whether the materials used corresponded to the standards called for in construction specifications.
Multiple inspections at the site are needed to check on construction material, to count and measure reinforcement rods and to measure dimensions of columns.
The Engineering Institute of the National Autonomous University here is one of the main official entities deploying experts in search of answers for future construction among today’s ruined buildings.
No New Survivors Found
Meanwhile, as of late Sunday, no new survivors had been found in the devastated areas for more than 36 hours, and chances grew slim that more could be brought out alive.
Physicians at the Nuevo Leon apartment building, where hundreds of residents were trapped in 13 stories of wreckage, said that rescuers may have detected “movement” and hoped to turn up more survivors.
The search continued at Benito Juarez Hospital and at General Hospital, the scenes last week of several dramatic rescues of days-old babies. But as one American volunteer at the Juarez location, Joaquin del Cueto of the Dade County, Fla., Fire Department, said: “Lets face it. There’s not much chance of finding more alive.”