Almas Tower Atkins

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Description of the design philosophy for the design of Almas Tower
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   S  T  R  U C T   UR E   S   65 Abstract Atkins Middle East Senior Structural Engineer Farshad Berahman Atkins Middle East Senior Structural Engineer and Associate Ranjith Chandunni Introduction The Dubai Multi Commodity Centre’s Almas Tower is a 360m high slender office tower located in the Jumeirah Lake Towers development in Dubai, United Arab Emirates (UAE) (Figure 1). The building consists of five basements, two podium levels, 60 storeys of offices and three mechanical floors. It has a total floor area of approximately 85,000m².A typical tower floor plan is in the form of two diagonally offset ellipses, with a floor plate approximately 64m long and 42m wide (Figure 2). The floor plan from level 53 to level 64 consists of only one of the two ellipses. An 81m slender spire peaks at 360m, forming the highest point in the development.The building was completed in September 2008 and according to the Council for Tall Buildings and Urban Habitat (CTBUH), was the world’s second tallest building completed that year. Structural system The following constraints had a significant impact on the structural design of the tower and were considered at concept stage designthe office floors to have an ã efficiencyof not less than 80%flexible column-free and ã wall-free office spacefinal sellable area to be within ã ±2.5% of the area sold by the client to ultimate office ownerseach office floor to be capable ã of supporting a 2.5t safe placed anywhere within an office space.The principal structural framing consists essentially of a tube-in-tube system. This is made up of a reinforced concrete peripheral frame and a central core wall, which are connected to each other by central spine beams on each floor and outrigger walls at service floor levels.A parametric study of the effectiveness of different arrangements of the external frame, belt walls and outrigger walls was carried out and the findings are shown in Table 1.The peripheral frame consists of 1000mm deep, 500mm wide beams supporting precast units which span onto peripheral columns. The columns are at a maximum spacing of 5m and form part of the lateral load resisting system. The columns are designed compositely in the lower half of the building to keep the column sizes small compared to what would be needed for a reinforced concrete column alone (Figure 3).A typical floor slab consists of 320mm thick hollow-core precast panels with 80mm thick structural topping. It ties the external frame to the central reinforced concrete core walls or central spine beam. The floor is also designed to act as a diaphragm, transferring lateral wind and The Almas Tower is a 360m high office tower in Dubai, UAE. The design comprises two intersecting elliptical towers located on a sculpted three-storey podium. The architectural form and client’s requirement for floor efficiencies of 80% resulted in significant challenges for the structural design team. This paper discusses the structural framing adopted, wind-tunnel studies undertaken – including building acceleration, lateral movements and column-shortening effects – and mitigation measures introduced. It also describes the design of the tower’s spire, which features tuned mass dampers. The structural design of Almas Tower, Dubai, UAE89 ParametersCore wallCore wall + peripheral frameCore wall + peripheral frame + belt walls + outriggers Natural period: s14.612.29.650y wind sway: mm17851258771Table 1 - Parametric analysis to study the effectiveness of the structural system        S      T      R      U      C      T      U      R      E      S 66 89The structural design of Almas Tower, Dubai, UAE seismic forces to the central core and external frame. The precast slab option was chosen because of programme benefits: they are comparatively lightweight and provide uninterrupted space for services.The plant floors at levels 42, 121, 212 and 279m above ground are 450mm thick solid reinforced concrete slabs, with the roof to the plant floors being a 400mm thick solid reinforced concrete slab (except the top plant floor) to provide an acoustic barrier to the floor immediately above.The building was designed to British standards, while UBC-97 7  was used for seismic load assessment in accordance with local authority requirements. The concrete grades range from 45 to 70MPa cube strength with a reinforcement grade 460 (  f   y  = 460 MPa) and structural steelwork S355 (  f   y  = 355 MPa). Finite-element modelling A three-dimensional finite-element model of the tower and podium was generated in Etabs 5 , which included the raft slab on spring supports to simulate the piles – although the raft weight was not considered for the purpose of assessing the seismic base shear. An allowance was made in the section properties for cracking under ultimate limit state according to UBC-97 and it was assumed that all loads would be transferred to the ground through the piles. The spring stiffness for the piles was based on the pile working load capacity and the theoretical settlement of the pile under that load, which was taken from the geotechnical assessment. The effect of the podium on the lateral movement was considered by modelling lateral springs at various levels based on the stiffness of the podium structure.The belt walls and outrigger walls include large service openings to allow for air intake and discharge as well as to allow for ductwork and piping routing (Figure 4).The outrigger walls, if constructed along with the floors, would have transferred significant dead loads from the peripheral columns onto the core, in addition to which the outriggers would have attracted forces due to differential axial shortening between the core and the peripheral frame. To Figure 1 - The 360m high Almas Tower in Dubai, UAE was completed in 2008   S  T  R  U C T   UR E   S   67 89The structural design of Almas Tower, Dubai, UAE overcome this, the outrigger walls were disconnected from the floor slab above and on one vertical side until all of the floor slabs were cast, which significantly reduced the load transfer due to dead load and minimised differential shortening between the core and the frame (Figure 5). Wind engineering Wind tunnel testing was carried out by Rowan Williams Davies & Irwin Inc. 9  using a high-frequency force-balance model (Figure 6) with wind loads based on a 3s gust wind speed of 37.7m/s for open terrain at 10m height, in accordance with measurements at Dubai International airport between 1983 and 1997. The proximity model was based on a 575m radius.The model was placed on a turntable and was rotated at 15° intervals to determine wind loads for 24 directions. Structural properties such as mass, mass distribution, mode shapes and frequencies were obtained from the structural analysis model and input to assess overall structural loads, building acceleration and cladding pressures. A damping value of 2% was assumed for the calculations. The tests provided overall structural loads using 24 load combinations taking into account directional effects for each sector.The expected deflections under 50 year winds were greater than H/500. As a result, soft joints are provided between the blockwork walls and the structure – in accordance with BS 8110-2 2  - to allow for racking movement between adjacent storeys under wind loads.The expected building accelerations at the top floor for a 10 year return period were 18.7mg, which is within the commonly accepted threshold of 23.4mg (Table 2) as per ISO criteria. A sensitivity check for a 1.5% damping resulted in an acceleration of 21.6mg – or an increase by a factor of √ (2/1.5) – which was still within acceptable limits.The overall cladding pressure results gave a maximum value of 4.5MPa in certain local areas. Figure 2 - Typical structural floor plan up to level +232m showing hollow core slab supported on external beams to core walls and internal beamsFigure 3 - Cross section through a typical composite column (dimensions in mm)        S      T      R      U      C      T      U      R      E      S 68 Seismic design Seismic loads used were based on UBC-97 zone 2A in accordance with local authority requirements. A response spectrum analysis based on UBC-97 was carried out with appropriate scale factors used to obtain member forces and associated drifts.Section modifiers as per UBC-97 were applied to the design, that is, 0.7 for uncracked walls and columns, 0.35 for cracked walls and 0.35 for beams. Ductile detailing for the coupling beams using diagonal reinforcement was specified according to UBC-97, although this is not required for zone 2A. Foundation system The foundation for the tower is a 3m thick piled raft supported on 1200mm diameter friction piles, which are approximately 40m long. To mitigate the effect of the heat of hydration, 50% of the raft cement was replaced with ground granulated blastfurnace slag (ggbs). Appropriate concrete cover was provided for the foundation and perimeter retaining walls to achieve the intended building design life.The columns and walls in the podium area are supported by slabs spanning between pile caps and, to reduce the slab thickness, tension piles are designed to resist uplift in the podium basement caused by the high water table. Vertical asymmetry The tower has an inbuilt vertical asymmetry due to one part of the tower extending 12 floors above the other while connected to one core throughout the height of the building. It was realised early on in the design that there could be lateral movement in the building that would be in excess of sways in a conventional, symmetrically loaded building.Building movement monitoring was included in the specification to allow the structural designers to compare actual movements with those estimated. This required survey points at each floor, which were monitored by laser surveying instruments (Leica TPS700) for lateral drift against a fixed benchmark located at ground level outside the building. Further Figure 4 - Extract of three-dimensional finite-element model of a plant floor showing core walls, outriggers with openings and external belt wallsFigure 5 - Elevation of a typical outrigger wall with service openings – gaps in the wall adjacent to slab and belt wall were provided to disengage outrigger during construction to minimise differential shortening effects (dimensions in mm) The structural design of Almas Tower, Dubai, UAE89
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