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A simple architectural failure can result in a massive tragedy, as history has shown. However, because we all learn from our mistakes, even a single misstep may be a great lesson for everyone. These failures can teach engineers how to improve future designs by learning from the highlighted flaws in previous designs.

The John Hancock tower

Figure 1 John Hancock tower (Janberg. N, 2021)

The I.M Pei Partners and the architectural company were the designers of the John Hancock tower, a 60-story, 240.8 meters tall skyscraper in Boston. The company completed the building in the year 1976. Although the architectural world praised it for its stunning, minimalist appearance, the building was infamous for its struggles.

Shattering glass windows of the John Hancock tower

One of the biggest issues faced by the building was that its 500-pound windows were dropping to the sidewalk, hundreds of feet below. The reason for this issue was identified as to unplanned repetitive heat strains on the panels, as per the book “Why buildings fall down”, written by Matthys Levy and Mario Salvadori in the year 1992.

Figure 2 John Hancock Tower’s windows

Henry Cobb, the John Hancock tower’s designer, had a proposal to cover all 10,344 windows with a form of insulated window known as Vari-Tran. This was a new product introduced in 1968 that was quite similar to the previous model. These alternate units consisted of two deeply sealed plated glasses varying in thickness from 0.63 cm to 0.95 cm, separated by 1.27cm thick lead separators. At the time, the Vari-Tran was both innovative and effective. Its reflective coating worked well to reduce glare and head movement. With its mirror-like surface, it also allowed privacy inside the building while maintaining visual transparency.

Although the tower was scheduled to open in 1971, it was postponed for another five years due to a new issue. The windows have started to crack. Until the late 1970s, this problem was thought to be due to construction flaws, and re-installations were carried out using the same insulated window units. Windows continued to fracture, and by 1973, over 2,472 windows were smashed and rebuilt with plywood panels painted with flame-resistant paint. Many criticized the project, which was nicknamed “the world’s highest plywood tower” and “the plywood palace.”

The swinging of the tower

Skyscrapers are built to swing to absorb heavy wind gusts, however, the swing is normally undetectable by the building’s residents. The John Hancock Tower swung so fiercely that those on the upper floors of the skyscraper experienced motion sickness. This was another major issue identified at the John Hancock tower.

The building’s swinging was thought to have put stress on the tower’s thick walls, breaking the windows. The tower’s difficulties were investigated by the head structural engineer of the Citigroup Center in New York, who installed the Tuned Mass Damper (TMD) at City Group Center to reduce the building’s swinging. On the 58th story of this 600-ton structure, two steal TDMs were placed. Springs and shock absorbers were used to link the steel masses to the steel frame. As a result, the TDMs remained in position as the tower rocked. The steel plate floor underneath the tower was able to lessen the tower’s motion. Unfortunately, this did not prevent the glasses from shattering, but it did make life easier for the occupants of the higher floors of the building.

Solutions found

As a consequence of expensive and time-consuming investigations, the causes of the glass window failure were eventually established. Cobb did not have a good consideration before picking these units, considering the timetable between the Vari-Tran unit’s market release date and his selection.

In contrast to the original insulated units, Vari-Tran with the reflective coating produced in bigger sizes did not have better records. It did not, however, give adequate certainty of success. The 5000 Vari-trans that were not damaged were sold for a hundred dollars apiece. All 10, 344 windows were replaced with completely tempered monolithic glasses with a thickness of 1.27 millimeters. These could withstand 732 kilometers per square meter of force.

As a result of the disagreement with the Hancock insurance company, with compensation up to USD 7.5 million for the glass repair, a ‘gag order’ was issued. Clear films were applied to all internal surfaces of the windows to prevent shattered pieces from falling in the event of possible glass breakage.

Furthermore, 1500 tons of lateral bracings were installed around the core center of the tower to strengthen the structure due to the potential for the tower to collapse in the future. The project’s total cost was $175 million, more than doubling the original estimate. However, there were a few windows that cracked after this, the tower has not faced any other potential structural damages to this day.

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