The John Hancock Toweralso colloquially called just “Hancock”, is the tallest skyscraper in the city of Boston (Massachusetts, USA) and is located at 200 Claredon Street. With its 60 floorstall 241 metersis a giant that stands out on the panorama of the city. Finished building in 1971, it is remembered, among other things, for its “sagging” windows…let’s take a closer look at this interesting project.
The John Hancock Tower in Boston: the design and the parallelogram plan
Designed by the architect Henry N. Cobb of the study IM Pei & Partners (now Pei Cobb Freed & Partners), the John Hancock Tower in the downtown Back Bay neighborhood is a skyscraper featuring one modernist and minimalist stylewith simple lines and characterized by a profound symmetry. The plant is shaped parallelogramwhich gives the structure an elegant and tapered line. The stained glass windows, not surprisingly, they were made in a slightly blue color, so that, on the clearest and brightest days, the tower does not “detach” too much from the horizon. As a final modernist touch, the short sides of the parallelogram are marked with a deep vertical notchwhich breaks up the mass of the tower and emphasizes its verticality. Late in the evening, the vertical notch in the northwest captures the last of the sky’s light, while the larger portions of glass reflect the darkening sky.
What happened to Boston’s tallest skyscraper: glass defects
The John Hancock Tower has presented, since its inauguration (scheduled for 1971, but which took place in 1976) a whole series of problems from a purely engineering point of view.
A first problem arose during the construction phase you dig, for the construction of the foundations of the structure. Gods were made retaining walls in steel which soon gave way under the horizontal thrust of the enormous quantities of clay which characterizes the subsoil of the place, the Back Bay of Boston. This very important failure resulted in damage to structures near the construction site. Among the most important structures damaged by this collapse was the Trinity Church on St. James Avenue. He asked for it and subsequently obtained compensation of a good amount 11 million dollars.
Another important problem concerned i glasses with which the skyscraper is covered. Let’s talk about windows 1.2m × 3.4m and the weight of 230kg and which they often are falleneven from the highest floors, crashing disastrously to the ground, with the not too remote risk of killing someone. Police often closed surrounding streets when winds exceeded 72 km/h.
Furthermore, complications affecting the skyscraper are closely related to structural stability of the same. The large oscillations of the skyscraper, due to horizontal loads due to the wind, caused motion sickness among the occupants of the highest floors. In order to reduce these oscillations two were installed mass dampers agree to 58th floor.
To understand exactly how these devices manage to stop the oscillations, we rely on the description of Robert Campbellarchitect and architecture critic for the Boston Globe. Imagine two huge steel boxes filled with lead, weighing well 300 tons eachlocated at opposite ends of the 58th floor.
These blocks are anchored to the structure via a system of springs and shock absorbers and slide freely on a lubricated steel plate. When the wind makes the skyscraper sway, the weights tend to remain stationary due to inertia while the floor moves beneath them. At that point, the springs come into action and “pull” the building backwards, slowing its movement.
The effect is similar to that of a gyroscope which stabilizes the tower. The choice to install two counterweights is not random but also serves to counteract torsions, pulling in opposite directions when the building tends to rotate on itself. An ingenious corrective intervention which, at the time, cost approximately 3 million dollars for damper.
