Commercial Building Structure - Essay Example

Running Header: Structural elements of China Central Television Building Student’s Name: Instructor’s Name: Course Code: Date of Submission: Table of Contents Table of Contents 2 Executive Summary 3 Introduction 4 Client and Construction Team 5 Structure 6 Spend and construction period 8 Construction difficulties and loads 9 Structure system 11 Materials and Structural members 11 Foundation 11 Soil condition and excavation 11 Foundation system analysis 12 Connections 14 Cores and Columns 17 Transfer Trusses 19 Bracing 21 Overhang 23 Construction methods 26 Conclusion 26 Executive Summary The new headquarters for CCTV is in now in the new 234m tall building. It is in the form of three-dimensional continuous cranked loop that is created by nine-storey structure that joins the two major towers. These towers are joined at the top by a 13-storey cantilevered overhang. This overhang is at 36 storeys above the ground. The structure and design of the building has innovative shape with a lot of safety consideration put in place towards providing the main support system of the structure. An extensive prepared foundation acts as a support for the two major towers within the building. An external skin of leaning columns, triangulated bracings, and horizontal beams that forms a network of diagrids has achieved the key support system of the structure or entire building. Steel cores and vertical columns reduce in size as the building moves up to support internal sections. The shape and angle of the structure of towers influences the columns loads and sizes. In collecting the column loads, transfer trusses are put at various sections. Introduction According to Richard (2009, p. 56), the new headquarters of China Central Television (CCTV) are in an iconic anti-skyscraper in Beijing. The work started in September 2004 after being reviewed of its design by Chinese experts. It contains entire television-making process within a single building. The 234m tall tower redefines the shape of skyscraper. It has continuous structural tube of columns, braces, and beams around the entire building. Figure 1. China Central Television Building Client and Construction Team The iconic building involves two L-shaped high-rise towers that are linked at the bottom and the top at an angle forming a loop referred to as ‘Z’ criss-cross. In 2002, CCTV organized AN international design competition. It was won by Rem Koolhaas’s practice Office for Metropolitan Architecture (OMA) working with Arup. The building was co-designed by Rem Koolhaas and Ole Scheeren of OMA (Office for Metropolitan Architecture). The civil engineering contractors of China Central Television building were Ove Arup & Partners (Arup) (East Asia and European divisions). They are responsible for mechanical and structural engineering of the building structure. Arup are carried out the risk analysis and design of security systems while providing security consultancy services of the building. OMA formed an alliance with the East China Architecture and Design Institute (LDI) of record for engineering and architecture. Figure 2. Construction Team Structure The CCTV building structure has become a great challenge to the Arup Contractors. They have had to design a plan to construct two 6° leaning towers bent at 90° at the top and bottom to meet a form of continuous loop. The building was constructed in two major towers parts that were joined to complete the loop of the structure. The iconic and innovative shape of the building is capitalized in order to provide the main structural support and system of stability. All the structural elements of support are of structural steel other than steel-reinforced concrete external columns because of amount of loads they are expected to carry. The floors are composite slabs that are on steel beams. The external primary structure consists of leaning columns, horizontal edge beams, and triangulated bracing. Cores and vertical steel columns support the internal structure. Figure 3. The structure of two towers Figure 4 Structure of the building According to Charles, (2009,p. 67), CCTV headquarter is a 234m tall building that is in form of a 3-Dimension continuous cranked loop that is made by a 9-storey podium structure that joins two 50-storey towers. These are linked at the top by a 13-storey cantilevered “overhang” structure 36 storey from the ground. The main support of the building due to seismic waves is through external skin of leaning columns, triangulated bracings, and horizontal beams. These form a network of diagrids a strong closed braced tube structure. In the building façade, there is a strong expression of external diagrid structure. This strengthens the transparency between architecture and structure. This is a central philosophy in the design of the structure. Steel cores and vertical columns are major supporters of internal structure. They reduce in size as they move up the height of the building in tune with leaning tower’s shape. The columns emerge and end the building’s height with the angled towers influence. At varying levels, they are transfer trusses for collecting the column loads into cores and external structure. Spend and construction period The total construction estimated at a total cost of $US1.2billion. The construction process started on September 2004, it took almost 4 years to be completed. The structure of the CCTV building was completed in May 2008. Figure 5. CCTV timeline Construction difficulties and loads Some of the loads that the building was expected to hold included huge forces generated by cranked and leaning form. Floor loads are some of the loads that were taken into consideration including roof, zone office floors among others. Wind loads in Beijing region for over 100 years are 0.50KN/m3. However, this depends on ground roughness and elevation. Snow loads were estimated to be equal to 0.45KN/m2. Other loads included earthquake loads. After the analysis by Arup, it was detected that inside faces of corner columns of tower would draw a lot of dead load from overhang. Therefore, there was little spare capacity for seismic loads resistance. Column sizes increments were rejected, as they would become stiffer attracting even higher loads. Brace elements and corner column below the overhang were left out until construction was over. This forced dead loads to travel through diagonals down adjacent columns allowing the full capacity of the corner elements to be obtainable for seismic and wind loads in the as-built condition. At the podium and Towers intersection, key elements are fixed to full capacity in order to support the loads of two towers without tension building between the elements. Between Towers and the Base, delay joints are to let differential settlement between the foundations of two structures. Half of the expected settlements were to take place after full height construction of towers in order to reduce the loads effects before the construction is over. This is because of uneven effect of the overhang on forces in some columns. They are Late-cast strips at various positions around the basement in order to manage shrinkage because of tower forces (Green, 2005, p. 24). Structural forces are evenly distributed equally in the building due to its shape. This reduces the chances of forces concentrating at one part of the structure hence no build up of tensions or stresses in the structure elements. The leaning form and varied programme such as the need for accommodating large studio spaces, posed extra challenges for gravity structure resulting to introduction of large trusses in the entire tower. Erection and connection of two huge towers resulted to design and construction challenges to contractors ad engineers. Structure system Materials and Structural members Concrete and steel were the main materials that were used in the entire building. The 7m thick, reinforced concrete slabs with 39000m3 of concrete and 5000tonnes of reinforcement were used. A total of 133343m3 of concrete was used in the foundations of towers and podium. A total of 41882 steel members were used, these included connections that were put within 26 months. Foundation Soil condition and excavation In the building site, the soils were alluvial/diluvial sediments. After 130m depth exploration, subsoil was composed of silty clay, silt, sand (fine-medium), gravel, and cobble. Multi sedimentary rhythms were also found to have been vertically formed. In the 130m in-depth, the soil layers would be classified into man-made fill and quaternary deposits. The man-made fill was approximately 37 to 35m below. The ceremony of breaking the ground took place on22 September 2004 with excavation of 870000m3 of earth the month that followed. Up to 12000m3 of soil were removed every day, which made to the whole excavation process take 190 days (Andrew, 2008). As the groundwater level was above maximum excavation depth of 27.4m, they are dewatering wells that were put to remove excessive water from underground due to pressure. In the foundation, they are separate piled raft foundations that support two towers, this was because the subsoil around main towers is not enough to support the building load and remain within acceptable limits of settlement. Foundation system analysis Figure 6. The foundation system Foundation design will take into consideration the combination of dead and live loads, seismic, floatation, wind, and heave loads. Piles foundations are required in order to support the load requirements of superstructure. They are up to 370 reinforced concrete bored piles under each tower. They are 33m long and 1.2m in diameter with a total of 1242 piles installed. The two towers are set up on deep-piled rafts. Piles will be put at 5m centres under the raft’s full extent. An 8m depth of the raft will be required to provide strength and stiffness necessary for tower loads distribution into piles. More convectional discrete pile caps will be utilized under the podium and basement structures. The piles’ top (2m) was topped off by hand rather than the machine. The reinforced concrete slabs is 7m thick each having up to 39000m3 of concrete and 5000tonnes of reinforcement. They extended beyond towers’ footprint to distribute forces into the ground. It was estimated that total average settlement of the building was less than 100mm. pile grouting methods were under consideration, this is for increasing tip resistance and shaft friction of piles as well as reducing differential and building settlement. Figure 7. Cutting down piles by hand Foundation system is designed in such a way that centre of the raft is near centre of load at tower’s bottom with no permanent tension being allowed in the piles. The total volume of concrete is 133343m3 in laying down Tower’s and podium foundations (Carrol, 2006, p. 388). There are 9-storey Base and the 3-storey basement under the rest of the site. There are tension piles between columns locations of foundation; this is to resist uplift because of pressure from water on deep basement. In addition, there are 1200mm diameter piles under secondary columns and cores supporting large transfer trusses. The piled-raft analysis is highly iterative because foundation design requires redistribution of loads across pile-cap while soil properties are non-linear (Carrol, 2008, p. 45). Figure 8. Preparation of foundation raft Perimeter columns are embedded and their base plates 6m into rafts in order to improve anchorage. This is to reduce the tension on some columns and foundation in case of earthquake. Applied superstructure loads are redistributed on the foundation across the pile-cap (raft). This is towards engaging enough stiffness and strength (Carrol, 2006, p. 388). Connections The edge-beams and braces forces are transferred into and through sections of column with little disruption to stresses within the column. The connection is formed through the replacement of steel column flanges with large butterfly plates that pass through the column’s face. This is then connected with the edge-beams and braces. Figure 9. Connection analysis Figure 10. Connection analysis Connection section No connection is made the column’s web in order to simplify the detailing and construction of concrete around steel section. The joint acted the same way as beams, braces, and columns and as a strong joint/weak component. The connections resist maximum amounts of load possible delivered to them from the braces with low yielding and a comparatively degree of stress concentration. A brittle fracture may result at the welds because of high stress concentrations under loading of cyclic seismic loading hence two connections were modeled from original drawings of AutoCAD with use of MSC/NASTRAN (Green, 2005, p. 27). The first column component was positioned on February 2006. There were 41882 steel elements erected and a total of 125000tonnes weight connections included. Steel used came from china with steel sections being fabricated at the yards of Grand Tower in Shanghai and Huning in Jiangsu. However, minor fabrication work was carried out at the site. Two tower cranes (Favco M1280D from Australia) operating inside every tower lifted the elements into place. Locating temporary ground-level platform for lift was put within operating radius in ensuring elements are delivered to all areas of sloping towers. The perimeter elements are adjusted to correct angle of installation of 60 slopes of tower. The vertical core structure is erected three storeys before the perimeter frame. The perimeter columns are bolted in place and braced in columns of the core using temporary stays. It would then be released from tower crane before final positioning and surveying. A full-penetration butt welds were put by welders at every connection it was required. The thickness of plate columns is 110mm at maximum with volume of weld is as much as 15 percent of total connection weight. Few plates of connection near the tower base are a site splice of 15m long and 100mm thick plate. Figure 11. The seven initial connection elements Figure 12. Installation of first connection element In the figure above, it is the initial connection of the two major towers by the overhang was constructed using the pins that carried the thermal loads while the joints were welded fully. Cores and Columns The first column element was placed on 13 February 2006. A total of 41882 steel elements together with 125000 tonnes of combined weight including connections were put in next 26 months. Vertical steel columns and cores support the internal structure. There is a special floor configuration of each floor created through the combination of vertical internal elements and sloping external tube. The floors span between internal columns or cores are different on every floor. Figure 13. Internal columns starting from pile cap level The spans of the building increases on two adjacent sides of the building as it move up as well as decreasing on the other two opposite sides. There are extra columns in the increasing floor spans on the outward leaning sides of the building. There were initial seven connections elements in the inside corner of overhang as illustrated below. Figure 14. Introduced internal columns supported on transfer structures Transfer Trusses According to Carrol, (2006, p. 388), the building has transfer trusses at strategic locations in the building. There were introduced because of looping form of the building, combined with sloping external faces as well as the need for large open internal spaces. Transfer trusses are for collecting the required columns at tower’s intermediate heights in order to cope with increasing floor spans. The transfer trusses span between external tube structure and internal core. The transfer trusses large member sizes act as outriggers in linking internal steel cores with external tube. The transfer trusses are connected to internal cores and external columns to avoid outrigger effect. The connection is at a single pin-joint location with detailed analysis carried out to verify no outrigger effects from geometry of transfer truss. Figure 15. Transfer trusses Above the ground at 36 storeys, the two leaning towers crank horizontally and cantilever 75m outwards in the air. They join forming a loop that is continuous that clearly defines the shape of building. The 75m cantilever structure has 13 storeys and is referred as overhang with its floor being supported by columns that lands on transfer trusses. These trusses span the bottom two storey of the overhang in two directions connecting the external tube’s structure back hence making the overall structure be supported off. Other major transfer trusses are found in the base part of the building. Braces are crucial members in the structural system, connection of braces to the columns needs cautious design as well as carrying extra of the load in bending like cantilever. Figure 16. Sections through the building showing vertical internal structure and transfer trusses The overhang corner is 300mm under the dead weight of the building. This is to take into account the deflections and built-in forces of completed building. The structure is preset upwards and backwards to compensate in order to be no overall downward deflection under the load. They are longer columns on one side of every tower in order to shorten to the right geometry under the load. Floor plate concreting progress was controlled to fit the assumptions prepared in the presetting estimation. The overhang construction movements are more crucial than in the earlier stages due to lean of tower. The key focus of analysis was on critical stage of overhang construction because of high number of variables required for calculation presetting such as final construction sequence, foundation settlement, variable axial stiffness, and thermal movements among others. Bracing The tube is formed by entirely bracing in all façade sides. The bracing planes are continuous all through the building. This is for reinforcing and stiffening the corners. Braces within the structure play a key role in distribution of the forces within the structure of the building. Figure 17. Uniform bracing pattern Figure 18. Unfolded view of final bracing pattern Overhang After the steelwork construction of two towers to roof level, overhang construction started. The tower 2 overhang began first in August 2007. The structure was cantilevered out one piece after the other from every tower within five months. This was vital stage of construction because the way it was build would alter the parts of Tower already constructed. The two halves of partially constructed overhang forces were concentrated in towers until the two halves were linked to become a single continuous loop. This could change the behavior of already constructed parts as well as temporary stability. Figure 19. The continuous loop structure of CCTV The loads of overhang are shared between permanent structures because the loop is a continuous or a single continuous form. The two bottom levels of overhang have 15 transfer trusses that support the internal columns. This transfers their loads into the external tube. In the overhang corner, the trusses are two way leading to 3-D nodes that are complex with 13 connecting elements with each having roughly 33tonnes. In order to reduce the number of adjustments at the high height, trial assembly of trusses was carried out at the fabrication site. Accuracy of fabrication was extremely crucial for overhang because it was 160m above the ground. Before connection, the two towers would move separately of each other because of conditions of environment like thermal expansion and contraction and wind. After joining, the elements at link would be capable of resisting the stresses by movements. Therefore, the strategy of connection needed a delay joint in order to allow enough number of elements to be connected loosely between the towers. They were then locked off fast in order to let them all carry the forces safely before any relative movement occurs. According to Arup’s specifications, one week of monitoring global and relative movements was required before connection in order to predict correct dimensions of linking elements. The towers’ relative movements at the day time was around ±10mm. Contractor made the ultimate measurements of gap 24 hours ahead of time in order for the final adjustments to be made to linking elements length while still on ground before installation. The contractor decided to connect seven link elements at the overhang’s inside corner at the initial phase of connection (figure 1). They were lifted into position to less than 10mm tolerance. It was fixed temporarily with pins in the space before relative movement of towers to each other. The thermal loads were carried by towers as pins allowed them to carry them. Initially, the connections were required to take place when temperatures were 12-280 C. However, the connection took place during winter when temperatures were as low as 00C hence further analysis of structure was necessary in checking increased design thermal range impact (Green, 2005, p. 24). After making initial connection, the remaining overhang steelwork was installed. The overhang propped, stabilized the two towers, and continued attracting locked-in stresses as more weight was applied. There is a continuous steel plate deck up to 20mm thick on the overhang’s lowest floors in addition to the primary steelwork elements. This is for resisting the high in-plane forces of propping action. The steel plate is not continuous; there were 3m diameter circles punched into deck for providing glass-viewing platforms for public view from bottom level of overhang. After the entire primary structure was completed, the concrete floor slabs were added. This was to ensure the loads were reduced during the stages of partial construction. Figure 20. The completed Overhang structure with three 3m diameter circles punched in the deck Construction methods The building of the structure was made in s tube like structure with fully bracing throughout the sides. The tube is a regular pattern with steel as the perimeter. The steel-reinforced concrete columns and perimeter beams played a crucial role in the construction process. The construction method was to involve the building of four elements that includes the nine-storey base, the two leaning towers sloping at 60 in every direction, 9 to 13 storey overhang, and suspended 36 storeys in the air. The construction method of every section was to determine the construction and connection of other sections or parts. This resulted to the completion of the entire structure or building. Conclusion CCTV structural design had many technical challenges to international team. However, they were able to overcome them successfully. The global and wealth of experience by Arup made the entire construction process possible. Steel columns and cores are the main supporters of the building. They have played a great role towards giving required strength to the building. Foundation layout has the required. The desirable features such as overhang and leaning towers are areas that leave a lot to be desired. Arup ensured that all the resources including physical, human resources were utilized towards success of the project, and ensuring it was through in scheduled time. All the loads and tensions that may occur within the building were looked through proper installation of connections, braces, beams and other necessary columns. References Andrew, C 2008, Seismic design for architects: outwitting the quake, Architectural Press, Michigan. Carrol, C, et al 2008, ‘CCTV Headquarters, Beijing, China: Building the structure’, The Arup Journal, Vol. 43, No. 2, pp. 40-51. Carrol, C., et al 2006, ‘China Central Television Headquarters-structural Design’, Journal of steel structures, Vol. 6, No. 2, pp. 387-391. Charles, H 2009, Rising China and its postmodern fate, University of Georgia Press, Georgia. Green, G, et al 2005, ‘CCTV Headquarters, Beijing, China: Building the structure’, The Arup Journal, Vol. 40, No. 3, pp. 22-29. Richard, F 2009, Skyplane, UNSW Press, New York. Read More