: mpa The Concrete Centre & Federation Internationale du Beton (fib)
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Tall buildings present unique challenges in terms of both design and construction. Their sheer scale demands that particular attention is paid simultaneously to strategic and detailed issues. Tall building design and construction requires an integrated approach, with the need for various engineering disciplines to coexist efficiently from the beginning of the project. This multi-disciplinary approach extends to consideration of how the building will be constructed, and thus ideally involves an integrated team (including construction and design professionals) at the earliest stage of the project. The definition of ‘tall’ for a building is not absolute. It is understood here as when the geometry of the building, for example overall height or height-to-minimum-plan dimension, significantly influences aspects of the design. These aspects are: structural lateral strength and stiffness vertical transportation fire escape services distribution vertical building movement (shortening) setting-out and verticality hoisting of materials. One definition is that if the building aspect ratio, height divided by lowest overall lateral dimension, is more than 5:1, then the building may be considered tall. For consistency, this document will refer to tall buildings in preference to other common terms including ‘skyscraper’, ‘high-rise’ or ‘tower’, with the exception of sections describing historical context. The term ‘tall’ may also be sub-divided as follows: Use (approx. storey height) Tall Super-tall Residential (3.0 m) Up to 100 storeys (300 m) Over 100 storeys (300 m) Office (4.0 m) Up to 75 storeys (300 m) Over 75 storeys (300 m) The following chapters provide guidance and insight into the design challenges and considerations relating to the design of ‘Tall’ buildings formed in concrete. Some guidance is provided for buildings in the ‘Super-tall’ range; however, it is recommended that readers interested in ‘Super-tall’ buildings research this subject further using the references provided throughout this document. Historic precedents The word ‘skyscraper’ originated as a naval reference to the tallest mast or main sail of a sailing ship. Tall buildings were in evidence around the globe long before the term was first applied in the late 19th century. Table 1.1 Definition of ‘Tall’ The highest of the Pyramids of Giza, built circa 2500 BC using rudimentary technology and manpower alone, still stands at 146.6 m and was not surpassed until the 14th century, with the construction of Lincoln Cathedral in England. The earliest known examples of urban living based on vertical or tall construction are the many (around 500) ‘tower houses’ built in the 16th century to protect the inhabitants of Shibam in Yemen from Bedouin invaders. Often called ‘the oldest skyscraper city in the world’, the mud towers range from five to 16 storeys, reaching heights of up to 40 m and accommodating one or two families on each floor. Examples are profuse across Europe, from masonry towers in Bologna dating from the 11th century onwards and reaching heights of 97 m, to the 11-storey, stone-built structures of Edinburgh constructed upwards in the late 17th century in response to the confines of the defensive stone walls of the Scottish city’s boundary. Post-Industrial Revolution advances in building technology saw the construction over 1884-1885 of the 10-storey Home Insurance Building in Chicago, generally considered to be the first modern skyscraper. Its design pioneered the first load-bearing structural frame, a construction type later known as the ‘Chicago Skeleton’. This revolutionary concept, whereby individual framing elements, rather than walls, carry the entire building load, is regarded as the antecedent to our current ability to conceive and construct buildings truly warranting the term ‘tall’ or ‘skyscraper’. Earlier in the century, Joseph Monier had invented reinforced concrete, using metals – originally iron but latterly steel – cast into fresh concrete. In 1867 it was patented and exhibited at the Paris Exposition. The devastating ‘Great Chicago Fire’ of 1871, meanwhile, not only prompted a rewriting of statutory fire regulations but revealed strong evidence of the inherent fire resistance of concrete as a structural material in tall buildings. By the early 20th century, the skyscraper was becoming the most prominent and progressive building type, aided by innovations such as mechanical lifts, the telephone and central heating systems. Urbanisation and increasing wealth had further boosted prospects for the proliferation of tall buildings. The Ingalls Building (1903) in Cincinnati, Ohio, with its 15-storey monolithic frame, standing at 64 m tall, was the first reinforced concrete skyscraper. Today, concrete is firmly established as one of the leading tall building construction materials. Enhanced construction techniques, dramatic increases in concrete and embedded steel strengths, and recognition of inherent properties such as natural damping, fire resistance and sound insulation have all contributed to longevity in its use. Indeed, today the tallest buildings are built almost exclusively with reinforced concrete. Tall building design involves all of the design interfaces present in low-rise construction but there are also a number of key additional factors which designers must consider. This is particularly relevant for structural engineers but equally so for clients, architects and building services engineers. In addition, the design development is likely to involve input and collaboration from other specialists, including: Façade engineers Wind specialists Geotechnical specialists Seismic specialists Fire consultants Lift specialists Construction advisors. For a design to be effective and economic, it is essential that all disciplines work holistically and gain a good understanding of the critical factors which have an impact on the associated disciplines. The following sections give an overview of the various elements structural engineers need to be aware of when embarking on the design of tall buildings. Further detail is provided in subsequent chapters. The reader will however need to research the various topics in more detail using the references provided throughout this document. Choice of structural system is fundamental to planning buildings and must be considered at the outset. One of the main factors in the design of tall buildings, and the key difference from the design of low-rise buildings, is the influence of lateral loading. For low-rise construction, measures to resist lateral loading are well understood by most designers and include well-positioned stiff vertical elements working in conjunction with horizontal diaphragms or braced panels. Such provisions, in conjunction with the provision of vertical and lateral ties for robustness, produce safe solutions which have stood the test of time. For tall buildings, the relative magnitude of lateral loadings to gravity loads generally increases significantly, just by virtue of building height. Wind loadings tend to increase with height from the ground which, combined with the large face area of a tall building and lever arm to the ground, serves to produce the dominant load case and hence govern the design and sizing of many of the main structural elements, particularly core walls and columns. Additionally, in tall buildings, lateral displacement or drift must also be calculated and may need to be limited. Excessive lateral displacement could potentially affect finish, internal partitions and external cladding, particularly if the inter-storey drift (lateral displacement over one storey) is too high. 4 2 Structural design strategies The dynamic performance of tall buildings must be considered in detail. Loading from wind and seismic actions occurs across a broad spectrum of frequencies and the response of the building will be influenced by its natural frequency and the degree of inherent damping. Where the natural frequencies of the building are close to the frequencies of applied loadings there is a risk that the response is amplified, resulting in increased loadings and movement. This mechanism requires detailed consideration by the structural engineer to investigate the performance of the structure across the full frequency spectrum of the applied loadings. If accelerations associated with any movement are excessive, building users could potentially experience motion sickness. In regions of the world subject to earthquakes, the response and performance of buildings during such events is also a critical design consideration. 2.1 The slenderness ratio At the initial planning stage, it is advisable to consider the basic proportions of the structure. The slenderness ratio (SR) can give a good initial indication of how hard the structural system will need to work. The SR is obtained by dividing total building height by the smaller base width. SRs of around H/6 or less can usually be accommodated whereas for H/8 or above the structural system will be working harder and the dynamic behaviour is likely to be dominant in the structural solution. The SR should, however, only be used as a guide to the potential behaviour of tall buildings. The following sections discuss the stability of tall buildings in more detail, and present a number of stability systems which can be used. As will be shown, the actual behaviour of the tall building is more closely related to the ratio of building height to the smaller dimension of the stability system.