Buildings Disjunctive Sorts of Core Construction for Skyscrapers - Report Example

Report On Core Designs for High-Rise Buildings Name Professor Course Date Summary This study analyses the various types of core design and construction methods for high-rise buildings. It considers the available core designs and in this study, a description and the methodologies as well as the approaches for the best deigns. The report will base on some of the major high-rise buildings in Australia and the entire globe that have utilized these designs. An example of the considered buildings is the Eureka Tower Melbourne, Empire State Building, Aston Apartments and the World tower. The study will also describe the various characteristics dictating the construction method of the core, for instance, core location, their number, geometry, elements, functions, etc. The cores may also be aligned in various different placements, and this study will highlight them. The advantages and the disadvantages of the core designs will also be included and the various system types. Introduction High-rise buildings have been a major concern especially in urban centres. Due to increased population in urban areas, high-rise buildings have been a key to accommodating these changes. Before building of a high-rise building, it is, therefore, necessary to consider various elements. The core design is one of the crucial elements in any high-rise building design. A core can be defined as a central or arterial section of a high-rise building that incorporates functions and service requirements for the designated occupants. It is, however, necessary to have a fast, as well as efficient erection of the concrete core, to maintain phased progress for the other parts of the building. Construction designers have however established various core construction techniques. For instance, use of slip forms while others uses a method of jumping of gang forms between floors. Other formwork methods that can be used includes; climb-form, super-shafter, jump form and the conventional forms. Building cores also be classified on the basis of their location and includes; Central CoreOff- set Core, Exterior Core, Split Core the choice of core type is usually dependent on some factors. First, the forces that might be subjected on the building probably from wind or earthquakes. Secondly, the cost requirements as well as the availability of the required resources. The nature of the foundation ground may also influence the type of core design to be used. Characteristics to be analyzed Core shape Number of the cores Arrangements Location The form of building and core selection Core placements There are various core placements that have been developed. Each arrangement is chosen depending on design considerations favouring a particular installation. The commonly used placements are Off-centre Centred Split Attached Forms of core confiqurations The commonly used and modern core forms are 1. Slip and jump forms 2. Travelling forms 3. Colum system formwork 4. Pre-fabricated core forms Below are explanation of these core forms and illustrations using diagrams. Slip and jump forms The terms refers to a self-climbing formwork arrangements that are designed for the construction of lift and stair cores for high-rise buildings. It can as well be used for other structures such as silos, chimneys and stacks. The two terms may sometimes be collectively referred as climb forms. Slip and jump forms comprises of platforms/decks that can be used by the workers and may as well be fixed with trailing screens which are always suspended from the form. Since their design is dependent on the size and configuration of a permanent structure that’s to be formed, at no point will two slip or jump forms can be identical. The slip forms usually climb continuously and slowly during the pouring of concrete. Due to this, it can therefore be possible to achieve high and smooth structures such as chimneys without the obvious joints. Fig: showing how a slip form chimney are constricted. Case study Eureka tower Melbourne is an example of a structure constructed by slip form technique. It was constructed using reinforced concrete by slip form method. The tower was designed using a central core together with columns on the perimeter. Also, the construction used a Grocon-Lubeca jump form which was used for the core. This technique ensured that the lift mor room was able to be built and also become outlifted, Machinery The construction of this kind of building requires a concrete pump for pumping the concrete to various parts. For instance, in this case, a Putzmeister 35A 14000 HPD concrete pump was used. Moving of materials to the foundation place was done using Favco construction cranes. This building is however expensive when the machinery requirement is considered. In jump form construction the jump forms usually climbs in steps and follow each concrete pour. It is mostly suited for high-rise building cores in a case where there is regular floors thus joints are not seen. Fig: illustrating construction of a jump form building. Travelling forms Here, movement of the form occurs horizontally. This manner of movement allows for repeated construction of elements of a structure. The formwork is basically given support by the permanent structure as the completion progressively advances. Traveling forms are suited for cases where the capacity for constructing supporting false work is limited. Fig: Showing construction of a travelling formwork Colum system formwork This formwork type is usually modular and allows for quick assembly and creation on-site. This will minimize labour and crane time. Column system framework may be used with steel or cardboard. New techniques has emerged where the columns could be adjustable and reusable that could be simply clamped on-site so as to give varying column sizes. Its benefits include; increase is speed and efficiency during construction Reduced the need for skilled labour because of its simplicity in assembly and disassembly. The metal column forms unlike traditional metal forms, can be easily erected. Quality surface finish is obtained Fig: a plan view of a Colum system formwork that uses timber However, most high-rise buildings use metallic formwork systems. The formwork need a minimal use of power tools therefore it is economical Tunnel formwork This method is usually termed as modern construction method and it utilizes the speed accuracy and quality. It offers good flexibility as well as economy. Its layout provides an effective load bearing structure that can be used in a variety of ways. It is suited for construction of hotels, residential blocks etc . Advantages It is usually cost effective and offers high quality construction thus it saves time of construction as compared to other methods. Although its fitting is fast, the quality is maintained. Also a good surface finish is usually attained and in the case of decoration, it can be performed directly on the surface. Less machinery are required for laying the forms. Pre-fabricated core forms These are already constructed parts and can be erected by use of specific techniques. These forms allows the constructor to remove any forms at any point and replace them. It however makes the erection and stripping very simple. Prefabricated forms have developed to gang forms which in simple terms refers to grouping and moving of various forms as a one unit. Since these form have been already been precisely engineered, they give the best materials that are available and can be flexibly used in all jobs. This will therefore result to a lower cost. Advantages This technique is relatively cheap as it eliminates the need for dropping of concrete at a greater distance and it also maintains a constant placement rate. There are very little on site fabrication These forms can be re-used either as large or individual sections They can be suitable for both light and heavy applications The use of prefabricated forms to construct cores can make it easy to build any form of work from curved, cut-up to battered. These forms can also be flexible to be able to be set in any orientation Disadvantages They have a high cost factor The cost of transportation is high since they are heavy and huge cranes will be required It involves too many pieces to erect a core This type of formwork, usually leave a poor finish especially at the joints of each forming part. They may deflect during placements and this may cause stability effects Core as a supporting element of the outrigger In another case, outrigger, which is a rigid superstructure, is used. It is usually fitted on top of the core, and movable floors and columns connected to it. The outrigger mainly connects the columns to core. The fact that they are suspended from the outrigger, they are under tension, and this eliminates the possibility of buckling usually caused by pressure elements. Advantages of the modification There is a reduction in bending stresses at the base of the core thus preventing possible failure. A good example of such design is the Aston Apartments, Sydney Offset Outriggers. With a height of 90m and 13m width, the building has a considerably low height to base ratio of about 7:1. In order to solve the issue, offset outriggers are used. As stated earlier, the outrigger assists in reducing bending stresses on the core. In the design, the core and columns are directly coupled. In the apartment, the outriggers couple the columns and cores by creating shear stresses that are opposing in the floor diaphragms at both top and bottom of the outrigger. Case study 2: Eureka Place Tower Due to its slenderness, it could not have stability without further reinforcements. In this case, the techniques applied include; 1. Using of coupled lift cores 2. Use of a tube beam column and frame that incorporated two massive columns 3. Outrigger shear wall that links the central core to the perimeter frame The range of the shear wall outriggers are to be maintained at a minimum and its positioning is within the apartment layout of the wall in order to minimize loss of the floor area. In order to maximize their impact, the outriggers positioning is maintained vertically up in the building. Due to its cost effectiveness in Australia, concrete is used as the principle material for the construction of cores, columns and the vertical structure. Minimizing the core size alone may be a solution to obtaining a stable structure. For instance, the Eureka Place Tower used a relatively smaller sized center core and further modifications were necessary to make it stable. First, mobilization of the stiffness and strength of outer tube would be necessary for improved structure efficiency. It was then noticed that, the outer tube itself was relatively flexible and, therefore, it's necessary to examine schemes that could outrigger constructions that link the central core to the outer tube. These schemes include a provision of outraging frames that are located at the plant room levels and the refuge level. In addition, an outrigger shears walls that are situated at distinct levels above the lower height. In the design of Aston apartments, the offset outriggers comprise of two-storey high shear walls that are 200mm in thickness and located at the side elevations as described in the below figure. Fig: A frame view of the Aston Apartments Another advantage of such is the fact that it has no impact on the buildings planning and allows for a simple floor plate to be attained. This technique is, however, economical as well as efficient since it employs the axial strength of perimeter columns so as to resist the weight at the maximum lever arm. The outrigger serves a lot of support functions and prevents drifting of the building. It also reduces the need for a thicker core. Offset core In other designs, the core is offset and this may be chosen for other design requirements or probably to suit a given spacing requirement. Its application is much similar to that of Centre core. The major component of the core is concrete reinforced with steel and at some cases it consists of steel having reinforcing shear walls. Case study A good example of this design is observed in Henry Cobb Electicite de France Paris, France. The core is fitted just as in the central core but slightly moved to give a better view and bear the weight depending on the buildings shapes. Split core A split core is usually located in a building such that it leaves a central space in the building. All the core’s components can, therefore, be accessed from this point. Split cores may, however, have varying configurations basing on the nature of the building. Just like the Centre core, the split core can be attached to more supporting elements such as the outrigger. It can also be divided in various different ways as described in the figure below. Fig: Describing different possible split core arrangements. Case study: Murphy /Jahn Deutsche Post The building comprises of a unique form of a tower with two elliptical segments. It consists of concrete reinforced frame, core walls and circular columns. The structures depth is kept to a minimum due to the close column grid. This, therefore, aids in reducing the height, as well as the overall weight of the building. The shafts in between either side of stair and lift and between the central sections are coupled with steel out ridges in each of the 9th floor. Exterior core design In this core configuration, the core is located to either the side or edge of the building. The location of the core is dependent on design factors as well as the expected use of the building. For instance, the building is designed to offer good views such as in the case of party walls, and then a core on one side will not be suitable. Another design may however, consist of two cores on either side of the building on the exterior side. Michael Graves Humana Building is an example of building employing an exterior core design. The core is located on one side of the building, and this single core is coupled together with the other parts of the building to provide full support. Machineries The commonly used machinery for high-rise building is the cranes for lifting concrete and other materials to the required part of the building. For waste crushing purposes, a magnetite cradle machine is usually used. It is cheap and consumes less energy. Earthmovers are used especially at the foundation stage. The earthmover will remove the loose soil to allow the setting up of the foundation. A good foundation is vital for a stable building. Advantages and disadvantages of the different placements Central core Advantages It permits utilization of all the window spaces and, therefore, the building can allow for offices of verging depths to get natural light. It makes area division simple as well as in some way, provide for flexibility of tenant Very convenient in case of accessing the building and in some cases iy=t may be equidistant for all the sides Horizontal utility runs can be equidistant from the core. When combined with a square plan, accommodating exterior and core walls, allows for a floor plan that is free of columns being totally flexible for an office layout, Disadvantages Due to the central location of at the interior, the office’s depth at the middle is limited. When a Centre core is used, an access corridor at around the perimeter will be required. Off Centre Advantages It offers all window or perimeter space that can be used for offices. Where the cores are close to the interior walls, small scheduled space can still be developed in those spaces. The technique offers more flexibility in the maximum depth and space arrangements Disadvantages This core arrangement tends to create access problems There is less flexibility in terms of flexibility of tenant distribution, The core is remote in terms of its location, and this creates inconvenience when accessing the further sides and the building’s corners. Types of system Core in frame is a model comprises of an inner core and applicable where land is expensive and extensively used and this results in a vertical development. This space is mainly focused on for transport systems, and it experiences a high concentration of the population during the daytime. However, the outer frame and core have a lesser land value and thus less intensively developed. There are however other systems that include, Cantilever core, Core cable and Composite system. Conclusion It can, however, be noted from this study that, for the design of any building, the core is the backbone feature and should be critically selected. It is also clear that, there are various different types of cores in high-rise building constructions, and each of them can be chosen depending on its suitability. The core of in a building acts as the supporting element of the entire structure and can, therefore, be linked with other parts of the buildings to provide more strength. It is also evident that most buildings in Australia, for instance, are incorporated with outriggers and together with the core, provide sufficient strength to withstand the commonly experienced winds. As far as a more stable and efficient building is needed, the cost is an issue in all the design processes, and, therefore, proper selection of materials is important. This study has however described all the design considerations during the design of a high-rise building. Reference WELLS, M. (2005). Skyscrapers: structure and design. New Haven, Conn, Yale University Press. Read More

Since their design is dependent on the size and configuration of a permanent structure that’s to be formed, at no point will two slip or jump forms can be identical. The slip forms usually climb continuously and slowly during the pouring of concrete. Due to this, it can therefore be possible to achieve high and smooth structures such as chimneys without the obvious joints. Fig: showing how a slip form chimney are constricted. Case study Eureka tower Melbourne is an example of a structure constructed by slip form technique.

It was constructed using reinforced concrete by slip form method. The tower was designed using a central core together with columns on the perimeter. Also, the construction used a Grocon-Lubeca jump form which was used for the core. This technique ensured that the lift mor room was able to be built and also become outlifted, Machinery The construction of this kind of building requires a concrete pump for pumping the concrete to various parts. For instance, in this case, a Putzmeister 35A 14000 HPD concrete pump was used.

Moving of materials to the foundation place was done using Favco construction cranes. This building is however expensive when the machinery requirement is considered. In jump form construction the jump forms usually climbs in steps and follow each concrete pour. It is mostly suited for high-rise building cores in a case where there is regular floors thus joints are not seen. Fig: illustrating construction of a jump form building. Travelling forms Here, movement of the form occurs horizontally.

This manner of movement allows for repeated construction of elements of a structure. The formwork is basically given support by the permanent structure as the completion progressively advances. Traveling forms are suited for cases where the capacity for constructing supporting false work is limited. Fig: Showing construction of a travelling formwork Colum system formwork This formwork type is usually modular and allows for quick assembly and creation on-site. This will minimize labour and crane time.

Column system framework may be used with steel or cardboard. New techniques has emerged where the columns could be adjustable and reusable that could be simply clamped on-site so as to give varying column sizes. Its benefits include; increase is speed and efficiency during construction Reduced the need for skilled labour because of its simplicity in assembly and disassembly. The metal column forms unlike traditional metal forms, can be easily erected. Quality surface finish is obtained Fig: a plan view of a Colum system formwork that uses timber However, most high-rise buildings use metallic formwork systems.

The formwork need a minimal use of power tools therefore it is economical Tunnel formwork This method is usually termed as modern construction method and it utilizes the speed accuracy and quality. It offers good flexibility as well as economy. Its layout provides an effective load bearing structure that can be used in a variety of ways. It is suited for construction of hotels, residential blocks etc . Advantages It is usually cost effective and offers high quality construction thus it saves time of construction as compared to other methods.

Although its fitting is fast, the quality is maintained. Also a good surface finish is usually attained and in the case of decoration, it can be performed directly on the surface. Less machinery are required for laying the forms. Pre-fabricated core forms These are already constructed parts and can be erected by use of specific techniques. These forms allows the constructor to remove any forms at any point and replace them. It however makes the erection and stripping very simple. Prefabricated forms have developed to gang forms which in simple terms refers to grouping and moving of various forms as a one unit.

Since these form have been already been precisely engineered, they give the best materials that are available and can be flexibly used in all jobs.

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