Curtain Wall Failure of Hancock Building, Boston - Research Paper Example

Curtain wall buildings: Introduction: Curtain wall, as the implies, is a non-structural system of external finishing of a building, that is not meant to support the structural load, but contributes in minimizing the overall load on the foundations by replacing the weight of non-load bearing external walls with its light weight and provides the structure with a good in-pour of natural light, wide natural views and elegant looks while fighting the adverse weather conditions and obviating the need to paint the structure. The use of curtain wall concept in the exterior of multi storey buildings can be traced back to the beginning of the nineteenth century, when it was first introduced in the construction industry. “The earliest curtain wall appeared in 1918 when new steel frame and fenestration technology afforded this liberation of skin from structure.” (Weber and Thompson, 2003, para. 3). However, the same report by (Weber and Thompson, 2003, para. 6) mentions that the concept of curtain walls was not widely employed in the construction of high rise buildings until 1950, when sufficient insulating and air treatment methods, necessary to counteract the high speed winds the curtain-walled building was subjected to, had been unveiled. In the beginning, the curtain wall system was made up of steel frames and other materials. Nowadays, there is a wide range of materials which are used in the formation of the curtain walls that include but are not limited to glass, marble, stone and other metals. Today, glass is used more commonly in combination with the aluminium frames, which offers additional thermal benefits for the structure. Unlike the conventional system in which load of the structure is fundamentally carried by the perimetric walls of a building, the curtain walled buildings rely upon the structural members of the framed structure that carry an additional weight of the curtain wall. Benefits of the curtain wall buildings: Reduction in structural load: Use of curtain wall in the structures causes considerable reduction in the total load of the structure. Its self weight is much lesser than the conventional non-load bearing walls that were used in place of curtain walls, once the concept was not very common. Besides, any wind pressure that the curtain wall is subjected to is transferred to the structural members, namely the concrete slabs and columns that the curtain wall is connected to and taken down to the foundations. Shielding effect: The curtain wall essentially acts like a shield against the weather. It serves to protect the building against the wind pressure and other environmental factors that can be detrimental to the internal environment of a building. Aesthetic significance: Glass is one of the most common materials used in curtain wall systems. Once the whole exterior face of the building is made up of glass, it lends the structure a lustrous, shiny and elegant appearance which is undoubtedly one of the major concerns about a building in the modern age. Light effects: The screen of glass does not prevent sunshine from pouring into the building. As a result, the internal environment remains lit with natural sunlight which may serve to cause considerable reduction in the electricity bills while keeping the environment natural. Thermal effects: The internal temperature of a building can be regulated by the selection of appropriate material for the curtain wall. Use of fibre glass can considerably lower the temperature, while ordinary reflecting mirror can be used in winters to benefit from the natural heat of sunlight. Water barrier: The materials used in the curtain wall system serve as a barrier for the water and disallow the water to penetrate inside the building, thus keeping the structure safe against water intrusion as a result of rains and cyclones. Failures in the curtain wall system: As mentioned above, the curtain wall system is preferred over the conventional non-load bearing wall structure for a number of reasons. Yet, the delicate nature of materials employed in the curtain wall system generates additional risks of unique nature for the structure. This paper aims at evaluating the causes of failure of curtain wall structures with special reference to the Hancock building in Boston as a case study. Case study: A brief introduction to the John Hancock Tower: The glass John Hancock Tower - a 60 storey building constructed in 1976 in Copley Square, Boston, US is the tallest structure of Boston and compliments the innumerable historical buildings in the vicinity of the structure. (www.aviewoncities.com, 2010). Height of the John Hancock Tower is 800 ft. (Knowledgerush, 2009). Through its height and elegance, it has defined Boston in a different way. (www.aviewoncities.com, 2010). John Hancock Tower is a rectangular multi storey building in Boston. Exterior of the building is made up of curtain wall which employs reflective glass, that serves to reflect the image of the local buildings in the surroundings as well as the weather of Boston. It is a very beautiful rectangle shaped building with v-notches in the shorter sides of the rectangle, that extend from ground to the top of the building. Since it is a very high building, it is known as the John Hancock Tower. Significance of the study of John Hancock Tower: John Hancock Tower is of great importance in the Civil Engineering studies, especially when discussing the curtain wall systems because of the huge volume of information revealed about the pros and cons of the curtain wall systems as various issues were encountered during the execution of this tower. The construction of John Hancock Tower uncovered extensive knowledge for the designers that would help them in deciding the tests needed to be performed prior to and during the execution phase and the materials required in the curtain wall systems. In particular, a study of the construction of the John Hancock Tower provides guidance on the ways in which massive glass panels are to be installed in a tower and also the ways to lessen the structural sways in towers as huge as the John Hancock Tower. The John Hancock Tower is known for several flaws in its design which are elaborated in the following text. Damages experienced in the execution phase: Settlement of the ground: The construction of John Hancock Tower commenced in 1968. (Schweinberg, 2007). It was the summer season. The immense height of the John Hancock Tower called for the need of pile foundation to take the load to firm strata underground. 3000 piles were required to be drilled into the ground, given the exaggerated height of the building. In order to achieve this, 500 million lbs of earth had to be excavated. (Schweinberg, 2007). The volume of earth removed was so large that it disturbed the underground load distribution of the structures located in the vicinity of John Hancock Tower. Damage caused to the structures nearby: Because of the loss of underground shear resistance as a result of substantial removal of earth, the nearby structures began to undergo settlement. Among the buildings affected the most are the Sheraton Copley Plaza Hotel and the Trinity Church. Besides, a number of road and footpaths including some utility lines were also damaged. The transept walls of the Trinity Church located near the construction site were fractured and dislocated in such a manner that their ability to hold the roof was marred. As a result of this, the John Hancock Tower was much criticized even when it was not built and the owners were also sued for the structural damages caused to the Trinity Church as a result of the interference with the underground load equilibrium caused by the construction of the pile foundations for the tower. Shattering of the windows: According to (Schweinberg, 2007), in 1972, four years after the commencement of the construction, the structure of John Hancock Tower was sufficiently erected to allow for the windows to be installed. The size of windows installed in the John Hancock Tower was no less than 5 ft by 11 ft, which made them thus far, the largest of all windows used in any tower. The activity of the installation of windows in the tower was started in 1972 and also completed in August of the same year. A total of 10348 windows were installed in the tower. All this while, the structure had been subjected to some of the most severe cyclones in Boston. The weather continued to remain the same throughout the year. The frequently occurring storms had had a significant impact on the building and resulted in the shattering of up to 2472 windows till July of the following year. The frequent shattering of the windows of John Hancock Tower was a matter of big concern for the owners, the Civil Engineers and the scientists of that age. They severely felt the need of carrying out a detailed structural analysis of the tower. It was mutually decided to uninstall all windows and replace them with the reflective tempered glass with a thickness equal to 0.5 inches. But in order to escape from such problems in the future, it is imperative that the ways in which the storms interact with such tall towers are identified. The interaction between wind-storms and the windows of a tower can be best explained through scientific theories and principles. One such principle that explains the phenomenon is the Bernoulli’s principle which is described in the text below: The Bernoulli’s principle: The Bernoulli’s principle states that, “For fluids in an ideal state, pressure and density are inversely related: in other words, a slow-moving fluid exerts more pressure than a fast-moving fluid.” (Science Clarified, 2010). The Bernoulli’s principle is equally applicable both to the liquids and the gases (air) since both are considered as fluids. Let’s analyze the breakage of the glass windows of the John Hancock Tower with respect to the Bernoulli’s principle. As the lightly blowing wind comes into contact with the curtain wall of the tower, it exerts a pressure on the wall while catching up with the wind flowing towards the sides of the tower simultaneously. This causes the wind to speed up. The pressure of the air instantly surrounding the external sides of the tower drops because of the high velocity of wind blowing in those regions while pressure inside the building and accordingly on the inner side of the curtain wall remains constant. Thus, a difference of pressures is created between the air instantly outside the curtain wall and that on its inner side. This difference of pressures causes the delicate glass windows to burst towards the region of lower air pressure. “Wind tunnel studies displayed on all four sides of the John Hancock Tower contradict the previously held notion that wind pressure simply increased with building height.”. (Schwartz, 2001). However, this was not the only reason of the shattering and falling of the windows of John Hancock Tower, some other factors were also involved in the matter which were revealed as the designers investigated the subsurface conditions. Unequal settlement of the John Hancock Tower: The designers had anticipated a uniform settlement in the case of John Hancock Tower while designing its foundations. Later, it was realized that the underlying soil had not been able to get sufficient time for settlement prior to the construction of the tower on it. Moreover, the structure actually underwent some settlement, but unfortunately, the settlement was not uniform in nature. Instead, the tower had experienced differential settlement. This induced stresses in the glass curtain wall of the tower which was not principally meant to take any structural loads. In other words, the delicate glass panels were loaded to their ultimate capacity even when they were not exposed to any harsh weather conditions or storms. This condition caused the glass panels to detach from their frames on their own and fall on the ground, especially when the matter was further exaggerated by the creation of a difference of pressure between the inside and the outside of the building. The low outside pressure of air created a suction effect on the stressed glass panels and as a result, they chipped off their frames. Other possible reasons: It is an established fact that things expand when heated and contract when cooled. Portions of a glass-curtain walled tower exposed to the direct heat of sun expand, while other portions which are never exposed to the sun remain cool. This creates a difference in the level of expansion between the former and the latter. The uneven expansion results in cracking. This is a general principle applicable to all structures including the John Hancock Tower. Sways experienced in the structure: The John Hancock Tower had experienced large sways, especially in the higher storeys. This was probably happening because of the inadequate stiffness of the structure which had caused it to be increasingly flexible. “As a rule, the top of skyscraper should never drift more than 1/400 of its height at a wind velocity of 150 km/h.” (Customessaymeister.com, 2010). Diagonal struts weighing 1650 tonnes were planned to be added to the design of the structure which incurred the owners a cost of $ 5 million in addition to the total cost of construction borne thus far. (Schweinberg, 2007). Consequences of the failures: Construction delays: The project of the construction of the John Hancock Tower was delayed by 1 year in the start because the constructors were not able to get the permit for the breaking of the ground any sooner as this would cause a violation of the zoning. After a continued struggle of about a year, they managed to get the permit. However, because of the failures experienced both in the subsurface construction of pile foundations and the shattering of a huge amount of windows, and the time elapsed in deciding the alternatives, the original schedule designed for the completion of the building in 1971 suffered and no more remained practicable and accordingly the process of construction was delayed by about 5 years. In September, 1976, the building was finally completed. (Schweinberg, 2007). Additional costs: Due to the losses and additional expenditures necessitated by the failure of the building in various stages of construction, the owners of John Hancock Tower had to incur a total amount of $ 100 million in addition to the pre-estimated cost of construction. (Schweinberg, 2007). This includes the $ 5 million incurred because of the replacement of all windows with the reflective glass as they shattered. The additional costs were magnified by the indirect costs associated with the issues encountered during the course of construction. Response of the society: John Hancock Tower was expected to be the tallest building with the strongest structure. It was anticipated to be very sound structurally. Later, people began to think of it the other way round as its windows began to fall. It was ridiculed and called as the “Plywood Palace” because of the massive use of plywood in place of the broken windows. The tower lost its integrity because of its failures and the people feared to walk past the tower on the roads. It was guarded by a team of armed personnel who used to figure out the possibility of breakage of any window and take necessary actions. However, no such defects have been encountered since its completion and the tower has regained its value and is now known to represent Boston. Remedial measures: Soil consolidation tests: Before the construction of any tower irrespective of the fact whether it is to be equipped with a curtain wall or otherwise, it is imperative that soil consolidation tests are carried out in order to learn the degree of settlement of the structure and the possible limits of the settlement of the ground under the load of the structure once it is constructed on it. Pressure adjustment mechanisms: In order to prevent the development of a difference between the internal and external air pressure of a high rise building, the internal air pressure should be mechanically controlled and adjusted to the external air pressure. The technique was successfully implemented by the Mechanical Engineers in the case of the John Hancock Tower and the structure is considered reliable today. Thus by lowering the internal air pressure of the building in the moments of occurrence of wind-storms and hurricanes, the suction effect produced on the windows by the high-velocity outside air can be nullified. Increased stiffness of the structure: High rise buildings are common to experience sway especially in the higher storeys when subjected to wind. The amount of sway a tower experiences can be significantly reduced by designing special diagonal bracing to link the structural members and provide the structure with additional stiffness and safety. This technique was also implemented to control the sways in John Hancock Tower and the results were remarkably better and appreciable. Increased cautiousness: Despite all necessary measures, some chance of falling of glass from the high rise buildings is inevitable. Hence, people should be careful and watchful while walking past a high rise building with glass panels on the exterior of the building. Present condition of the John Hancock Tower: The John Hancock Tower was principally constructed to serve the official purposes and has successfully achieved that. Maintenance involves regular cleaning of windows. To limit the excessive sway, the tower is well-equipped with the damper system. Overall, the structure is well-conditioned and the buildings in the vicinity of the tower, including the Trinity Church have undergone thorough rehabilitation and are no more unstable. Conclusion: The study of construction failures of the John Hancock Tower provides very useful information for the designing and construction of curtain walled structures. The owners incurred huge financial losses because of insufficient testing and inadequate designing of the tower. However, a thorough analysis of the failures of the John Hancock Tower reveals that before constructing a curtain walled tower, the soil should be tested for consolidation, an air-pressure control mechanism should be provided in the tower and also, the tower should be designed for increased stiffness to withstand the high speed winds without swaying. In addition to that, strength of the glass used in the curtain wall should be more than usual. Diagonal steel struts were introduced in the design of John Hancock Tower to increase its stiffness and the air-pressure control mechanism was employed to balance the lowering of outside pressure by high velocity winds. The technique worked. Since then, these factors are established as fundamental requirements of a curtain wall system. Works cited: “Bernoulli’s Principle”. Science Clarified. Advameg, Inc. 2010. 7 July 2010. . “Essay, Research Paper: Skyscrapers”. Custom Essay Meister. 2010. 7 July 2010. . “John Hancock Tower”. A view on cities. 2010. 6 July 2010. . “John Hancock Tower”. Knowledgerush. 2009. 6 July 2010. . Schwartz, Thomas, A. “When Bad Things Happen to Good Buildings”. 4 April 2001. 7 July 2010. . Schweinberg, Lindsey. “John Hancock Tower”. 19 May 2007. 7 July 2010. . Weber, Blaine J. and Thompson. “Unwrapping modern building envelopes”. 20 Nov. 2003. 6 July 2010. . Read More