Alternative Forms of Core Construction for Highrise Buildings - Coursework Example

Alternative forms of core construction technology in highrise buildings Alternative forms of core construction technology in highrise buildings Name Subject Table of Contents Alternative forms of core construction technology in highrise buildings 1 Alternative forms of core construction technology in highrise buildings 2 Table of Contents 3 Summary 5 The utility of core construction in highrise buildings has now long been understood to expedite the speed of construction, provide clarity to the work and attain the desired precision much needed in the construction of highrise buildings. Give the benefits core construction exerts on the highrise buildings, this paper deals with a number of its aspects that include the need for having cores constructed in the highrise buildings in the first place, significance of cores, how they speed up the construction process and what advantages do they offer. In light of the same the paper also discusses benefits construction industry derives by combining steel and concrete and how core construction and on-site installation thereafter prove a boon in erecting highrise buildings in a better and efficient manner. Furthermore the paper discusses the role of prestressed hollow-core concrete slabs and overall sustainability of alternative methods of core construction. 5 Introduction 6 Why traditional methods are passé? 10 What advantages do modern methods of core construction offer? 12 What is the faster, stronger and more efficient method of constructing building cores? 13 Combining the benefits of steel and concrete in a modern method of building core construction 14 Core construction and on-site installation 15 Slip-form application to Cconcrete structures; relevance today in high-rise buildings 16 Prestressed hollow-core concrete slabs 17 Sustainability of alternative methods of core construction 18 Comparison of different structural systems and construction methods 20 Evaluation of core construction with respect to given parameters 20 Conclusion and recommendations 21 References 22 Summary The utility of core construction in highrise buildings has now long been understood to expedite the speed of construction, provide clarity to the work and attain the desired precision much needed in the construction of highrise buildings. Give the benefits core construction exerts on the highrise buildings, this paper deals with a number of its aspects that include the need for having cores constructed in the highrise buildings in the first place, significance of cores, how they speed up the construction process and what advantages do they offer. In light of the same the paper also discusses benefits construction industry derives by combining steel and concrete and how core construction and on-site installation thereafter prove a boon in erecting highrise buildings in a better and efficient manner. Furthermore the paper discusses the role of prestressed hollow-core concrete slabs and overall sustainability of alternative methods of core construction. Introduction It is typical for urban landscapes to have large or highrise buildings. Highrise buildings are becoming increasingly popular driven by the need of the hour on account of expanding businesses and increased population of large cities. In other words these are gigantic structures of which core forms an important part. Core construction is considered as the efficient and fastest way of maintaining phased progress of the building's related parts. The type of core to be constructed depends on the type of building that is being developed. While using slip-forms to construct cores is a preferred method by some contractors, some are satisfied with the progresses they make by jumping gang forms from a particular floor to another (Hurd 1990). In all there are many formwork methods that are used to construct cores, which include slip-form, jump-form, climb-form, conventional forms and super-shafters (Jaafari et al, 1989). However, alternative methods of core construction are not supposed to compete with conventional forms when buildings less than 15 floors are to be constructed. But in taller buildings, which are more than 30 stories, alternative methods of core construction are capable to reduce the cost by up to 30-40 percent as against conventional methods. Jaafari et al have further stated that for buildings more than 20 stories, slip-forms offered a cost advantage, particularly on area around 600m squares. Of particular interest in the last two decades have been slip-forms. Slip forms are used to place concrete structures vertically and this technology has become an important feature of highrise buildings. In concrete silos this has become main method of core construction. . Even as slip-form construction does cost relatively greater than other construction methods, it reduced time of construction by around 3 months or more in case of highrise buildings. Cores are generally located centrally on the plan. It is because that provides ease of use of the buildings and overall stability to the structures linking with the core. All other spaces wrap around the core, which forms the central point of view. From the core radiate the main beams to the perimeter columns that support them. The core construction technology has, in a way, transformed the concept of highrise building construction as all services emanate from it to all the openings in the structure. Generally cores are formed in the reinforced concrete slips. In core construction technology, the top of building foundation is used for beginning slip-form work from. This is done by using a bracing system and a backup support so that the slip-form is able to maintain its required shape when the vertical movement takes place. The external and inner forms create the thickness of the wall, which, in a sense, is void. It is this void that is used for tying reinforced steel together in order to reinforce the concrete wall. Hydraulic jacks are then used to connect the form to jack rods, thus enabling to move it vertically automatically. After this the concrete filling is begun until it reaches the structure's top. The significance of core construction The core construction technology attains a greater significance in the present highrise buildings' scenario since these have emerged as the latest trend in the United States of America; so much so that such buildings have begun to be known as "American Building Types". That is to give an idea that tallest of all buildings ever build have been built in the US, even as the trend has already fast caught up with the construction industry worldwide, particularly in nations like Japan, Korea, China, and Malaysia. The US was ahead of any other country in the world in the number of tallest buildings ever made. This was true specifically for North America. Traditionally the primary function of highrise buildings has been the commercial use of spaces with them. The recent trends have indicated their use also in private residences, hotel tower developments and even mixed-use. It is keeping in view the functionality of the highrise buildings that core construction attains greater significance. Highrise buildings involve several complex factors like aesthetics, economics, technology, politics and municipal regulations. Of all these factors economics is the most emphasized factor. It must be reiterated that building of such highrise constructions would not have been possible in the first place had not the new technologies like that of core construction worked as a supporting factor. The core construction technology has come as a boon since it has made it possible for highrise buildings to come up strongly on a core that has greater sustainability than what the traditional methods used to offer. Why its need has been felt? Construction industry has always come under severe criticism on account of time over-runs in projects. The time-runs have ranged anywhere from 50 to 80 percent of all projects in different locations of the world. These delays are responsible for multiple problems on all projects particularly in the wake of not only time but also loss of resources and money. The cost implications are of greater interest to businessmen or governments investing on such projects. The consequences can range from those that liquidate damages on both clients and contractors. It is keeping in view such problems that the construction industry has been continuously working on developing construction methods that reduce the time taken on completion of high-value projects like highrise buildings. Construction of highrise buildings have been working relentlessly on how to reduce the time-runs and manage cost implications of the buildings. In this direction Construction and Building Subcommittee of the National Science and technology Council in the US adopted certain changes known by the name of National Construction technology Goals so that it could improve competitiveness (CERF, 1997). Following changes as these the whole construction industry has realized that construction time performance is of critical value to how highrise buildings are constructed, what is the overall duration they are supposed to take, and what are the most appropriate methods of construction that can achieve these goals. Core construction technology has been found to have provided an impetus in this direction since it offers advantage not only on time but other factors as well. How does it help speed up the construction process? Highrise buildings are all about speed; lesser the speed of construction greater is the economical viability. Construction engineers and the contractors alike who are masters at erecting buildings faster are in extreme demand in the construction industry. It might be a boom time or a down time in the construction industry, getting the buildings up faster offer greater advantages than those that take too long to get completed. The most important question, however, is how to build buildings faster. At the core of this issue is building the core in the first place, because it is around the core only that rest of the things can align themselves with. Core construction sets the speed of the project. Getting the core right is fundamentally important and getting it up quickly all the more so. Cores dictate the frames of steel that weave themselves around it, then that is followed by cladding, which dictates several other parameters. Since core construction has come to stay as the quickest way of getting a building up, construction engineers and scientists alike are finding newer ways of core construction. The speed and quality of the building is so intertwined with core construction that not long ago one such way of speed-enabling core construction has been developed. This is called automatic self-climbing, which is a hydraulically-operated system. This system has replaced the traditional use of cranes. Another technique recently introduced is that of involving prefabricated steel sandwich panels, which are reinforced later with concrete after placing them in the core structure of the building. The very concept of speed in a highrise building is vested in the core plumb, which when gets straight and then joins with other interfaces, is considered to give the desired impetus that a highrise building needs to get completed faster. As soon as the embed plates, which are large steel plates tied to the core rebar, get linked to it the building starts taking shape instantly and around this shape it becomes easier and quite visible for rest of the frames to fall in place. The speed of construction is further increased when the in situ concrete cores are made using either jump-form or slip-form methods. Both, to some extent, are similar. Their placement is at the top of the core; following which casting of walls is done. The same formwork moves up as many levels as high the building is intended to be. This happens with an unimaginable speed, which can be practically calculated by the amount of time each core takes being completed. This completes the main structure into which landings and stairs go in. As other work gets stalled until the core is finished, recent advances in construction engineering have devised the method of prefabricating the cores as well. That means the cores are to brought to the site of construction, installed quickly, and allow interior workers to let the landings and stairs soon thereafter. The advantage of this technique is that almost all parts - cores and core walls, landings and stairs can be built simultaneously; thus making it possible for the building to get completed even faster (Lane, 2008). Why traditional methods are passé? The highrise buildings that are being seen now are a long extension of modern architecture that began in Europe in 19th century and which coincides with the industrial revolution. This revolution had propelled the economy by leaps and bounds and of course accelerated industrialization. Around the same time there was a drastic change in the demands for buildings and the focus was more on industrial units and factories from the economical, functional and efficiency perspectives. Use of concrete and steel along with glass became a steady affair. More often than not traditional methods still got to be used in constructing these buildings. But as the demand for space and height increased a new dimension of speed evolved with regard to these highrise buildings. This made old traditional methods of brick and mortar not as relevant as they were earlier. Once buildings over 200 metres high were constructed only with steel-frame structures. This posed a limitation because it was not possible to take the height above this limit only with the use of steel frames. Hence reinforced concrete was found as an alternative option which when used with steel frames offered greater durability. Over a period even concrete began to be thought about as a poor choice to offer greater strength, so high-strength concrete technology was used to provide buildings with greater height greater durability and reliability. Furthermore since highrise buildings are more prone to natural calamities like earthquakes, not much can be expected from the traditional methods to safeguard these buildings from the same. These buildings need earthquake resistant engineering. Besides this it is important for these buildings to employ vibration damping structures such as reaction-engineering pendulums and braces so that an external force can be set off due to wind pressure or seismic action. The traditional methods of countering a seismic shock was to build buildings strong enough to resist the same. That means rigidity of the building was the key to withstand the shock. This was, to some extent, enough to withstand the jolts but did not provide any resistance to the material inside such a building; they would get devastated by the tremors. The highrise buildings of now are so designed that they either control or mitigate the seismic forces hitting them. The new technologies are able to help these buildings reduce vibration and thus protect both buildings and as well as what is inside the same. Most of the highrise buildings now rely on the vibration-controlling structures embedded in them which give seismic shocks a counter force to reduce their impact. These include both active and passive vibration damping measures. Today's highrises have vibration damping measures incorporated in them in an indispensable manner. These buildings are fitted with seismic isolation structures which are an intelligent method of rubber-fitting the isolation layers. Rubber or sliding bearings are used in these layers so that the seismic force transmission is reduced to mitigate the transmission of vibration. What advantages do modern methods of core construction offer? Alternative methods of core construction offer many advantages. These include that of faster construction, lower costs, more flexible and open architecture. These advantages are over the moment frames which have been used traditionally in highrise buildings. The schedule and cost savings become very widely apparent when the modern highrise buildings are compared with traditionally-built buildings at the times of natural calamities like earthquakes. The core construction has enabled engineers reduce the framing depth as the need for moment frames has been done away with. In place of these flat slabs of smaller framing members have begun to be used. While the seismic advantages have been outlined above, what is of significance is that one can locate in the core itself other aspects of the building like restrooms, stairs and other mechanical features. As per Code Acceptance of Non-Prescriptive Designs there is no restriction on the use of alternate design, materials, equipment and methods of construction (ICC, 2006). The code does not limit the use of innovativeness, prohibition of any design or prevention of installation of any materials not previously used or specifically prescribed by the code. If all these alternate methods are found to be satisfactory by the building officials, there is a provision of incorporating the same in the design and construction elements. Another advantage that it offers is that highrise building core walls can be built in a two-stage process that assesses seismic performance and follows capacity-design approach (Paulay and Priestley, 1992). What is the faster, stronger and more efficient method of constructing building cores? Core construction embraces innovation with every new development as construction technologies keep suggesting newer methods of efficiency and speed particularly in case of highrise structures. It is not the core construction only that can be said to be responsible for the overall speed of construction. Many more things go along with the core construction that tend to increase speed of construction. It is a joint effort on all fronts. In technologically-driven highrise buildings most of the different components of a building are produced off-site in a vast factory, transported to the site of construction and neatly erected to create mindboggling highrise structures. That is not to say no construction work activity goes on at the site of construction; it does, but with less clutter. To make this easier further recently highrise constructions have used 3D volumetric construction techniques, which means creating three-dimensional units of the proposed buildings in controlled factory settings before the materials are transported to the site of construction. Similarly other things that add to the speed of construction is use of tunnel form, which is a scientific way of building monolithic walls in one operation along with slabs on a daily cycle. This has revolutionized the construction industry because it combines the quality, speed and accuracy of of-site produced components with great economy and flexibility. When there are repetitive cellular projects or highrise buildings which need the repetition of the same materials across all floors, this method offers tremendous economy and flexibility (ConcreteCentre, nd). Combining the benefits of steel and concrete in a modern method of building core construction The modular construction technologies have gone a long way in simplifying the making of highrise buildings. Most of the jobs have been made easier by off-site manufacturing and the credit to such manufacturing precision must, in part, go to the impregnable combination of steel and concrete. This combination is considered as the mainstay of the strength the highrise buildings get. This offers a superior alternative to the traditional concrete reinforcements. Structures made of this combination are precision-oriented while keeping the structural thickness under control. Steel-concrete composites offer high performance and structural rigidity. The overall benefits of the structures from this combination include the following (Corus, nd): Faster to build: the speed can get multiplied by up to six times than a traditional concrete core High flexibility: Flexibility can be determined on the basis of the purpose and site of use Improved efficiency: This becomes possible because of reduced site congestion and better build sequence Site safety enhancement: not much formwork is needed, which means workers do not need unnecessary hours working at heights Greater accuracy: Provides improved interface with the steelwork that is adjoining Increased capital values: walls can be made slimmer without compromising on quality thus making further room for increased lettable floor space By combining steel with concrete, engineers have shifted from the traditional designs that used either steel or concrete alone for structure-designing. Irrefutably each material offers its own sets of advantages. But when these are used in combination, results are remarkable. Now the composite material is used to construct in a way that uses structured beams and steel columns for floor construction and concrete for metal deck floors. Concrete core structure is used for emergency stairwells and elevators. Not long ago only structured steel was used to build highrises buildings, like the one in Chicago - Sears Tower, considered to be the tallest building in the US. Both steel and concrete industry has undergone much advancement in the recent years and the current trend of using composite materials for construction has attained a new dimension. And both, as mentioned above, have their own sets of advantages to offer. Concrete, for example, offers reduced inter-unit noise, less wind shear, and increased fire safety. Stiffness and mass of concrete is supposed to dampen movement, which is one reason why designers, owners, engineers and contractors prefer using it (Nasvik, 2009). Core construction and on-site installation In the first place this type of construction provides a potential for quicker construction delivery times. Highrise building market throughout the world has embraced this type of construction for quicker turnaround times; even though it cannot be refuted that highrise construction vis-a-vis use of this technology does pose some challenges too, but those challenges are offset when huge cost savings are made by use of core construction and on-site installation methods. Alternative methods of core construction and even better methods of on-site installation come at the right time as the construction industry in the last three decades has seen diminishing operating and development margins. This is also seen in the wake of excess profits having been squeezed from highrise construction due to higher acquisition and land prices (Cassidy, 2008). So that way off-site core construction and on-site installation comes a viable alternative to the traditional, and often more challenging, construction methods done on-site. On-site installation is essentially the second stage of off-site development of construction materials, which have the potential to stand alone and are transported to the site of construction. This has revolutionized the construction industry. As part of the highrise building construction, core construction should not be seen in isolation, because cores are just one component of these buildings to which 60 to 90 percent components get attached to later and are developed off-site. These are known as modular constructions and are manufactured in a controlled environment. The scope and magnitude of a project determines how much of work is to be done on-site and how much off-site (Modular building Institute, 2011) Slip-form application to Cconcrete structures; relevance today in high-rise buildings Slip-forming technique enables the highrise buildings to be built quickly, in which the extrudition of wet concrete takes place by retaining in forms until it hardens. A predetermined rate is used in slip-forms atop travelling form, emerging from the bottom in a hardened state. Slip-forms provide a uniform cross-section and is most economical. Jacking or slipping has a fixed rate, that means the form will leave the concrete only after it has hardened enough to sustain on its own, retain the desired shape and bear new load. Even as there is high initial investment in using slip-forms, these are outweighed by cost savings made at the end of the project by saving time and labour. They offer numerous advantages like high operational speed, accurate operations, better economics, high quality and finished surfaces, and a monolithic structure that is continually moving. Highrise buildings benefit by using slip-forms but there is no dearth of challenges too that they pose. High-level management is needed while using slip-forms in the highrise building construction. These are further affected by labor union and weather restrictions. There is a great investment of time in assembling these forms and then disassembling at the end. The equipment is too cumbersome to handle. Prestressed hollow-core concrete slabs Precast or prestressed hollow core slabs have come as a boon to the construction industry. In case of highrise buildings they offer multiple advantages that include heavy weight capacity, lower self-weight, exceptional fire resistance, superior acoustic insulation and thermal properties, better designing flexibility to builders, cost-effective construction solution, moderate use of raw material, rapid speed of erection, highly effective methods for circulating fresh and warm air, and requires few construction site workers, and they offers preformed site services and more. Prestressed concrete is used to make hollow core slabs. These are preferred over traditional methods of building in highrise and multi-storey apartments. They make a common choice among engineers in low seismic zones. More importantly they are low cost, easy to transport and assemble, use raw material and have lower self weight. These are widely used in countries in USA, Europe and Canada. Highly automated hollow core machines are used to manufacture prestressed slabs and are considered as the most viable option for the current construction market trends related to modern buildings. Supposed to consume lesser raw material, they are regarded as highly environmental friendly, are structurally efficient, possess possibility of recycling and reuse and have reduced thickness. In case of highrise buildings these are considered as the most useful elements, providing considerable benefit to contractors, engineers and builders. Quick erection time and low cost is the two most luring features they posses and thus they are an ideal choice among builders to use not only in highrise buildings but also small residential dwellings. Considerate as versatile precast elements their other benefits include robust structure requirement of less concrete in their making, extended length without central supports, provision of a secure working platform, can be easily altered to enable heating and cooling of a building even without burning fossil fuels; efficient span/depth ratio resulting in decreased structure heights; can be easily changed to include electrical plumbing, wiring, and sprinkler facilities within the building; factory produced based on strict quality standards and safety principles, broad array of applications; appropriate for healthcare, residential, education, commercial and industrial segments etc. On account of their longitudinal cores the weight on the floor is reduced. Given the span and loading requirements of a building these slabs can be prestressed in various depths. In other words they are able to fulfil diverse requirements in highrise construction industry, thereby assuring durable, safe and healthy constructions (Valdes, 2012). Sustainability of alternative methods of core construction The UNEP (2006) has propounded a vision for sustainability in construction industry, which state: Buildings are routinely maintained and designed so that they can be optimized for entire life span; Building standards and legislation should be such that they include sustainability requirements and considerations; Building should consider environmental aspects from the very start and adhere to them till the end; Government should provide such support and policies that encourage sustainability in buildings All stakeholders in the construction industry, like investors, engineers, insurance companies and buyers/ tenants must be aware of the considerations pertaining to sustainability In order for a highrise building to be talked in the context of sustainability, it is important for it to meet a common framework, which must include: general principles as mentioned above; common use of terminology; building meeting sustainable indicators; monitoring and assessment methods for design, production, construction and maintenance methods. Sustainable construction methods are supposed to meet three requirements, which include economic, environmental and community benefits. The first one includes reduced operational costs, second includes proper inflow of quality air and proper waste disposal methods and the third one includes increased health and comfort for the occupants after they move in (Andrews et al, 2006). The core construction in highrise buildings in fact aid in meeting these sustainable standards of the buildings, It is because the core provides ease of use of other elements in the building which increases performance. The building sector is currently laying greater emphasis on performance as it is said to be directly linked with its sustainability. This is a relative shift from the early stand in which construction industry studied environmental impact on the buildings; now they talk more about making buildings in sync with the environment so as to make them more compatible. Performance-based building is considered to be complementary to sustainable construction. There is another, and apparently stronger statement going on at the moment and that is concerns for sustainability are going to change building sector such that they start following strategies that are performance-based. Several scholars have remarked that "the success in the field of sustainable construction requires the inputs from performance based building" (Riley et al, 2005). Comparison of different structural systems and construction methods The number of storeys to be reached is what determines the type of structural systems that must be used. Rigid frame is for up to 3o storeys, frame shear truss between 30 to 40, belt truss up to 50, framed tube up to 70, truss tube with interior columns up to 80, bundled tube up to 90 and truss tube without interior column up to 100 storeys. Each system has it advantages and disadvantages. For example, shear frame system resists lateral deformation by joint rotation; it needs high bending stiffness of beams and columns, requires rigid joints for stability and is not recommended for heights more than 30 stories. In core structure system gravity and lateral loads are supported by the central core, it eliminates bracing and column elements, is insufficient since it is not deep with regard to bending, and moment supported floors are inefficient. Evaluation of core construction with respect to given parameters Much of this has already been discussed above in this paper but the following points must be able to provide a recap: 1. Cost As the construction of the core facilities other movements in the buildings as speedily as it come up, there is a resource and a labour cost that is saved. 2. Time As this paper has demonstrated elsewhere, several highrise projects have been seen getting completed ahead of the determined schedule, so there is considerable time-saving factor involved. 3. Environmental impact Use of cores in highrise buildings helps it adhere to a number of environmental regulations, apart from providing the buildings with that much needed clarity and ‘de-cluttering’ ambience that each attempt which goes into its making is environmentally-friendly. That is on one side; on the other due to advancements in the construction technology the raw materials used now are more environmentally-friendly than before. 4. Logistic constraints While every construction activity has logistic constraints, use of core construction would also depend on certain project-specific conditions. But it can be said that there are not any major ones that might prove of impediment in the smooth flow of construction. 5. Flexibility for design/use It offers tremendous flexibility of design and use. Since the whole building revolves around the central core, designers are at liberty to develop the envelope around it in whichever creative manner they desire. Conclusion and recommendations Urban landscapes are dotted with highrise buildings. It is because they are the need of the hour. These are mammoth structures, exquisite examples of engineering accomplishment, of which the core is an important and basic element. each building has its own type of core, depending on what functions is the building expected to perform. Several methods of core construction have been used, of which slip-form method is widely preferred one. Core construction techniques vary on the basis the storeys that are intended to be constructed. Whichever the plan, core construction is considered as a sure -hot method of reducing time-runs on highrise buildings, reducing cost incurred on the traditional building techniques and increasing financial viability of the overall project. Since highrise buildings are normally erected with an aim of making them commercial, the central cores attain greater significance because they offer freedom of creativity with the interior of the building which is, in reality, exterior to them. Notwithstanding the advances that have been made in construction engineering, there are also several challenges faced in their construction. In the wake of such challenges it is highly recommended for highrise buildings to be compliant with local, federal, regional codes of conduct. References Andrews, A., Rankin, J.H. & Lloyd, M.W. (2006). A framework to identify opportunities for ICT support when implementing sustainable design standards. ITcon, 11, p. 17 - 33. 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Accessed May 20, 2014. ICC, (2006). International Building Code 2006, International Code Council, Falls Church Virginia. Nasvik, J. (2009). Concrete Superstructures. Available: http://www.concreteconstruction.net/concrete-construction/concrete-superstructures.aspx. Accessed May 20, 2014. Paulay, T. and Priestley, M.J.N. (1992). Seismic Design of Reinforced Concrete and Masonry Buildings, John Wiley and Sons, New York. Riley, R.D., Sanvido, V., Horman, J.M., McLaughlin, M and Kerr, D. (2005). Lean and green: the role of designbuild mechanical competencies in the design and construction of green buildings, Proceedings of the Construction Research Congress 2005, San Diego, California Valdes, A. (2012). Advantages of Precast/Prestressed Hollow Core Slabs Available: http://www.ultraspan.ca/index.php?option=com_content&view=article&id=121:advantages-of-precast-prestressed-hollow-core-slabs&catid=83&Itemid=566. Accessed May 20, 2014. Read More