CHAPTER 5: STRUCTURAL ANALYSIS
5.2 Theoretical analysis
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CHAPTER 5: STRUCTURAL ANALYSIS
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The building was constructed in the year 1997 from reinforced concrete and has a total roof area of approximately 2500 m2.
5.2.1.2 107 Cheapside
The building located at 107 Cheapside, is a mixed used development currently being utilised as office and retail space. The structure was constructed in the 1950s from reinforced concrete. Developers decided to construct two types of green roofs on the structure i.e. an extensive and intensive system on the 1187 m2 roof area. The combined green roof system represented a 20% coverage of the total roof area with the construction of the 44.5 m2 extensive and 188.81 m2 intensive green roof systems.
5.2.1.3 10 Queen Street Place
The 10 Queen Street Place structure is currently utilised as a mixed used development type utilised as office, retail and an underground car park. The building was constructed from reinforced concrete in the year 1991 with a total roof area of approximately 4905 m2. The green roof system was reported to cover approximately 50% of the total roof area. The intensive system comprised of 302 m2 soft landscaping and 2170 m2 hard landscaping.
5.2.1.4 150 Cheapside
The 150 Cheapside building is reported to be a mixed used development structure utilised as office and retail space. The building was constructed in 2009 with reinforced concrete and has a total roof area of approximately 1990 m2. The extensive green roof system upon the structure represents a 79% roof coverage with 1025 m2 of soft landscaping and 556 m2 of hard landscaping.
The study carried out on the existing green roof retrofit projects in London provide evidence to support the fact that retrofitting can be conducted on existing structures. An emerging trend from the study suggests that reinforced concrete structures are typically the structural type onto which a green roof system is retrofitted with all four retrofits being carried out upon reinforced concrete structures. Another noticeable trend from the study shows that of the four retrofits, three of the green roofs were intensive systems and they sharing a common factor, all the supporting structures were constructed the 1900s.
This suggests that structures that are much older have the heavier (Intensive) green roof systems fitted.
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In addition to the London case studies, major green roof systems in the United States of America were investigated.
5.2.2 American green roof studies
Green roof systems within the United States of America were investigated to determine the structural properties of structures that current support green roof systems (Waterproof Magazine, 2010).
5.2.2.1 Chicago City Hall
The Chicago City Hall is a concrete structure with a intensive green roof system that is comprised of a range of lightweight soils that extend 102-458 mm in depth. The weight of the system is reported to, during heavy rainfall, weigh as much as 2.87 KN/m2. It was also reported that the overdesigning for during the building’s construction in the late 19th century has attributed to the structure having the necessary capacity to carry the large additional load.
5.2.2.2 Atlanta City Hall
The Atlanta City Hall was reported to have been constructed 50 years after the construction of the Chicago City Hall. The structure is a predominately reinforced concrete structure that now accommodates a 280 m2 green roof system. During preliminary investigations engineers determined that the design of the building allowed the structure to carry an additional load of approximately 8.91 KN/m2. This made the structure more than capable of carrying the total green roof system of approximately 2.68 KN/m2.
5.2.2.3 Bronx J Building
The Bronx J building was constructed around the 1920s from reinforced concrete and given its age, was regarded as a historic building. This was the contributing factor to the preservation of the structure under its historic designation implying that the structure could not be demolished. However, the building had undergone a complete transformation when it was identified as one of the structures as part of a number within a redevelopment plan. The transformation included a green roof system in excess of 1000 m2. The study suggests that due to the age of the structure, the structure’s carrying capacity exceeded the minimum required load to sustain a green roof system.
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As part of the Schwab Hospital’s plan to assist in the rehabilitation of patients, a designated area to carry out horticultural therapy was proposed. In order to achieve this, the executives at the hospital decided to implement a green roof system. The structure was constructed in 1998 from reinforced concrete and was designed to accommodate an additional floor as part of a proposed future development plan. As a result, the structure had the necessary structural capacity to carry the proposed additional loading imposed by the green roof structure.
5.2.2.5 A&P Lofts
In the case of the A&P lofts, the structure, without any structural upgrades, did not meet the required additional load carrying capacity to sustain a green roof. The structure was constructed in the year 1930 utilising red brick as the primary construction material. In order to sustain a green roof system, structural engineers utilised a steel support frame system that allowed the green roof to be raised and constructed approximately 150 mm off the existing roof structure.
5.2.2.6 Ballard library
The Ballard library forms a subsidiary of the City of Seattle’s public libraries. The library was constructed in 2005 and comprises of a combination of a wooden framed roof structure supported by slender structural steel columns. The roof structure was designed to be inaccessible and is estimated to be 1900 m2. The structure now accommodates a 20500 sq ft extensive green roof system at a 25% slope utilised for test and research purposes.
The studies of the American green roof systems provide further evidence of the potential of existing structures to be retrofitted with green roofs and in particular, the retrofitting potential of concrete structures. The cases provide evidence that the retrofitting potential and the ability of a structure to carry an additional load is largely dependent on the considerations taken during the design process in terms of structural members. This is evident when considering the Ballard library green roof system, the structure is a both a relatively new structure and makes use of structural steel and timber construction materials deviating from the common reinforced concrete structures.
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5.2.3 Results and discussion of the theoretical analysis
In an attempt to present an objective analysis, green roof systems from different regions of the world were assessed to determine the type of structure that is most commonly retrofitted with a green roof and how the age of the structure has influenced the carrying capacity of the structure and hence, its ability to be retrofitted with a green roof.
The findings of the study show that the most common form of green roof retrofits take place upon concrete structures and in particular, structures that are much older (attributed to the potential construction utilising ‘good building practices’). However, the research further highlights that green roof retrofits are not limited to concrete structures but is entirely dependent on the structure’s carrying capacity which, in turn, is dependent on the structural elements of the associated structure. From the study of the literature it has also highlighted that in certain cases, concrete structures or any other structure for that matter, may not necessarily be able to sustain an additional load and in such cases, a structural upgrade would be required.
A greater understanding of the relationship between the elements of a structure and the structural carrying capacity and the basis of the findings may provide further insight into the potential of retrofitting existing structures with green roofs.