John hancock tower
The John Hancock Tower: Failures
Carlos Nazario UAB REU Site 2000
In the historical area of Copley Square in downtown Boston, Massachusetts, surrounded by small masonry structures built more than 100 years ago, stands the John Hancock Tower. It is 790 feet tall and is supported by conventional steel framing. But it is the unusual rhomboidal shape and the reflective glass curtain wall that makes it one of the most admired structures of the world. It is hard to imagine that multiple controversies and structural failures plagued the building since design stage, back in the late 1960's.
At first, the development of the high-rise building encountered much opposition by defenders of the historical scene of Copley Square, but after endless discussions and defense of its architectural design the owners received the city's consent. Construction started, and the building was topped on 1972. That same year glass panes from the curtain wall started to break and fall, which is not unusual on structures of that height. But early in 1973 hard winds hit Boston and one third of the glass panes failed turning it into a dangerous situation for pedestrians and nearby structures. The broken glass panes were replaced with plywood sheets and many safety measures were taken, including the use of two security guards to closely monitor the glass behavior. The excessive glass breakage was considered unacceptable since the building had been designed to resist much higher wind loads. After studying the failure, the architect ordered the replacement of the 10, 344, double pane (separated by ½ in.) glass units by single panes of glass, ½ in. thick. Some attributed the failure to the building's movement; others to the large spans inherent in the sheets of glass and to the curtain wall detail. It was discovered that the lead edge assemblies of the glass panes developed fatigue, which at the same time created the cracks on the outside glass pane and made them fail.
During the investigation of the causes of the glass breakage, it was discovered that the building had excessive swaying motion due to the wind loads. After analytical studies, it was concluded that the building had an unacceptable dynamic response in the wind. The building’s acceleration produced by strong winds could have caused discomfort to the inhabitants. To solve the problem, two giant tuned mass dampers were added to the building’s structure. But in-depth studies, including wind tunnel tests, showed that the building’s swaying problem was grave and required immediate attention. It was discovered that the building wasn’t stiff enough in the long direction and represented a real hazard because a phenomenon known as “second order effect” or “P-delta” (incremental force when gravity acts on a building that has already being pushed by the wind). Steel stiffeners were installed along the longitudinal dimension of the frame; doubling its stiffness on this direction. This action provided additional structural safety.
The excavation and foundation construction caused damage to adjacent historic structures supported on timber piles. For example the H.H. Richardson’s Trinity Church, built on 1877, suffered from significant settlement and structural distortion. The collapse of the excavation support system also caused serious damage to the underground utility lines.
After correcting all of its structural flaws the building was finally occupied in 1976, three years behind schedule. At the same time legal battles between all the parties were taking place. In 1981 the owner reached a settlement with all the parties to the lawsuit. The settlement included an oath of secrecy about its terms and forbade all technical consultants from discussing the results of their investigations.
The John Hancock Tower suffered many structural and procedural troubles and caused serious court litigation; that is why it is considered one of the most controversial construction projects of modern history, as well as one of the most beautiful buildings ever built.
Lessons Learned:
The Hancock's structural problem led to increased use of wind tunnel testing for models of new buildings and to the modification of building codes. It also highlighted the challenges that can be encountered in the design of tall buildings with lightweight, flexible high-strength steel structural frames, especially when large aspect ratios are involved.
Due to the nature of the legal settlement reached by the building's owner all the technical information and conclusions known about the building failures are based on assertions made by people related to the project who were not included on the settlement agreement. This is perhaps the biggest failure concerning the John Hancock Tower case.
In order to institute more effective and safer construction methods it is absolutely necessary to learn from previous mistakes. The secrecy utilized in the Hancock's case prevented the construction industry from learning how to avoid similar mistakes in the future.
