Design of Tall RC Buildings: Core Supported Structures and Outrigger Braced Systems

Transcription

1 Design of Tall RC Buildings: Core Supported Structures and Outrigger Braced Systems Example Writing Project from: CE 764 Advanced Design of Reinforced Concrete Structures Fall Semester of 2014

2 Introduction Gone are those days when the typical high self-weight of concrete used to impose a limitation on the height of buildings. Appropriate structural forms along with high-strength and lightweight concrete have conquered all economic constraints of a concrete high-rise. Rather, the significant dead loads from concrete is, in other words, its greater stiffness, which minimizes the sway deflection, floor vibration and local instability problems, and thereby make it more suitable for high-rises (Taranath, 1988). Selecting an efficient structural system in terms of both performance and economy is the key to have an effective high-rise concrete structure. In general, the higher the structure, the more significant is the lateral load. This demand also increases with increase in slenderness and flexibility. To identify a proper structural system, considerations should be made not only to select an appropriate lateral load resisting system, but also to deal with other non-structural issues functionality and aesthetics (from an architect s perspective), leasability (from an owner s perspective), constructability (from a contractor s perspective) and more. The outward appearance of tallness is relative and there is no hard and fast rule regarding how to categorize a building as tall ; it is not defined in terms of height, number of floors or proportion. According to some, it can be distinguished when dynamics plays a greater role in designing the structure than statics. However, a building can also be treated as tall when the effect of lateral loads has to be reflected in its structural analysis and design. Therefore, to evaluate the applicability of a structural system, all the factors that influence the lateral behavior building geometry, severity of wind exposure, seismicity of the region and the overall performance expected from the structure need to be considered. 1

3 To categorize all the structural systems into a number of distinct classes is a difficult, if not impossible, task as there may exist almost as many structural concepts as there are engineers. However, for presenting an opportunity for an engineer to compare or combine the characteristics of two or more systems, the most common systems have been classified core supported structure, outrigger braced structures, shear wall structures and tubular structures are frequently used for normally proportioned buildings of more than 30 stories. The scope of this paper is core supported structures and outrigger braced structures typically used for buildings 30 to 70 stories high and normally proportioned but there are examples when these structural systems, being combined with others, have been successfully used for buildings with greater heights or unusual proportions (Taranath, 1988). Core Supported Structures Rectangular box-shaped cores around stairs, elevators, and other shafts are one of the most regularly used forms of shear walls, which also makes an efficient use of vertical enclosures that are required around the cores. This system works quite well for commercial buildings, where maximum flexibility in layout is required, so that the open spaces can be divided by movable partitions. Other than the functional advantages, the structural benefits of this system is that being spatial, the walls around the core are capable of resisting all types of loads vertical forces, shear forces, and bending moments in all directions, as well as torsion - especially when adequate stiffness and strength are provided between flanges of open sections (Taranath, 1988). In other words, shear cores may be considered as members that resist lateral loads in a way similar to massive beams cantilevering out of the foundation; thus the shear and bending stresses observed in a core are analogous to those of a box girder. Unlike conventional beams, the core has to carry 2

4 a significant amount of axial load (gravity load of the structure), which induces compressive forces and as a result, the shear capacity of the structure is increased and the structure may not need to be designed for tensile stresses, especially in case of heavy concrete cores. Figure 1: Schematic response of core supported structure under vertical and lateral loading (Schueller, 1986) Figure 1 illustrates the response of a central core system to gravity and wind loads. The gravity loads increase progressively from almost zero at the top to maximum at the base, while the moment response due to lateral load is similar to that of a cantilever beam subjected to uniformly distributed load with maximum moment at the fixed end. To be more specific, this response of a core to lateral loading depends upon its shape, the degree of homogeneity and rigidity, amount of continuity provided by the spandrels, and the direction of the load (Schueller, 1986). There is no restriction regarding the shape and location of the core within the building form. To a large extent, the shape of the core is regulated by the elevator and stair layout. Variations in layout 3

5 of shear walls could occur from a single rectangular box core to a complicated arrangement of multiple shear walls. The core can be located at the middle, at the perimeter, or even at the external part of the building, leaving the arrangement to be either symmetrical or asymmetrical. Unless the other vertical structural elements within the building consists of relatively closely spaced columns and deep spandrel, it is more than likely that the stiffness of the core will overwhelm that of other vertical elements; therefore it is justifiable to ignore the resistance of other vertical elements and design the core system for the entire lateral load (Schueller, 1986). Case Study 1: Burj Khalifa Burj Khalifa, the 828m tall reinforced concrete tower structure, is an example of core supported structures. Unlike typical core supported structures, this building was designed utilizing all of the vertical members to support both gravity and lateral loads. The basic structural system consists of a six-sided central hub with three wings clustered around it. The system can be described as a buttressed core, where each of the wings support the others via the central hexagonal core as shown in Figure 2. This central core, consisting of high performance concrete wall, provides the torsional resistance of the structure. The corridor walls are extended from the central core to near the end of each wing and are terminated in thickened hammer head walls. While resisting the shear and moment demands induced from wind loads, these corridor walls behave similar to the webs and hammer head walls behave like flanges. In addition to providing high structural performance and enhancing the views from the building, the spiraling Y shaped design helps to reduce the wind forces on the tower along with fostering construction by keeping the structure simple (Baker et al., 2007). 4

6 Central hexagonal core Corridor walls Hammer head walls Figure 2: 3D view of a single story model of Burj Khalifa (Baker et al., 2007). The rest of the structural system is the perimeter columns with a flat plate floor system. At mechanical floors, 3-story high outrigger walls are used. These walls link the perimeter columns to the interior corridor walls and thereby, force them to be a part of the lateral load resisting system. With that, not only the gravity load resisting system is almost fully utilized to resist lateral loads, but also the structure is extremely stiff, both laterally and torsionally (Baker et al., 2007). As the tower rises, the setback of the wings one wing at each tier in an upward helical pattern - provides many different floor plates and a decreasing cross section as the tower reaches toward the sky in 26 spiral levels, giving a visual impression of a series of towers of differing heights shown in Figure 3. To avoid the presence of any structural transfers so that a direct downward load path 5

7 can be ensured, the setbacks are structured with the tower s grids. The stepping of the building is accomplished by simply aligning columns above with walls below. As these setbacks provide different widths for each differing floor plate, the wind has to encounter a different building shape at each new tier over the height of the structure; and as a result wind vortices never get organized. Thus, this upward spiraling pattern of setbacks along with the Y shaped floor plans, determined based on extensive wind tunnel tests using the concept of confusing the wind, help reduce the wind force significantly (Baker et al., 2007). Figure 3: Floors of Burj Khalifa showing setback at different level ( 6

8 Outrigger Braced Structures An outrigger is a part of a boat's rigging which is rigid and extends beyond the side or gunwale of a boat to stabilize an inherently unstable main hull. The outrigger is positioned rigidly and parallel to the main hull so that the main hull is less likely to capsize. Originating from this concept, an outrigger-braced high-rise structure consists of a central core and flexurally stiff horizontal cantilever outrigger girders connecting the core to the exterior columns. Under lateral loads, the windward column-restrained outriggers restrain the vertical plane rotations through tension and the outriggers in the leeward columns through compression. This reduces both lateral deflection and bending moments in the core from that if the free-standing core were the only resisting system. In other words, by connecting some of the perimeter columns to the central core, the outriggers ensure a partly composite cantilever type behavior of the whole structure. This efficient structural system increases the effective structural depth of the building when it flexes as a vertical cantilever, which thereby enhances the lateral stiffness of the building and reduces the lateral deflections as well as the moments in the core, as illustrated in Figure 4. To utilize the other peripheral columns, which are not directly connected to the ends of the outriggers, a horizontal deep spandrel girder joins all the perimeter columns around the face of the building at the outrigger level (Smith and Coull, 1991). The extent of the composite behavior of an outrigger braced structure depends upon the stiffness and the number of outrigger levels. Though multilevel outrigger structures show a significant increase in their lateral moment resisting capacity over single outrigger structures, the degree of this increase reduces with each additional level of outriggers. Using up to four outrigger levels has been shown to be economical in very tall buildings. Though the outrigger system is very effective 7

9 in increasing the structure s flexural stiffness, shear resistance of the structure is almost entirely provided by the core (Smith and Coull, 1991). Figure 4: Typical Deflection and moment pattern of outrigger braced structures. (Smith and Coull, 1991) Unlike the approximate methods of analysis for other types of high-rise structures, where advantage can be taken of the repetitive story-to-story arrangement, the outrigger braced structure has to be considered in its discreet arrangement. Using compatibility method, the rotations of the core at the outrigger levels are matched with that of the corresponding outriggers. The assumptions that are the basis of this method are the behavior of the structure is in the linear elastic range, axial forces are the only forces induced in the columns, both the outrigger-core connection and the core-foundation connection are sufficiently rigid, and the cross-sectional properties of all the structural elements core, columns, and outriggers are uniform throughout their height. These assumptions may appear to be impractical because in a real life structure the dimensions of the 8

10 vertical members reduce with the increase in height. However, the key factors in a preliminary analysis (such as the drift at the top, the moment at the base of the core, the axial forces in the columns) are mostly affected by the properties of the lower level of the structure. Therefore, an analysis of a hypothetical uniform structure with properties taken from lower level of the actual structure can result in a sufficiently accurate preliminary design (Smith and Coull, 1991). Case Study 2: Aston Apartments, Sydney Aston Apartments is a residential building accommodating 145 serviced and owner occupied units. This reinforced concrete framed structure is a slender 30-story tower having a height-to-base ratio of 7:1 as the 90m high building has a width of only 13 meters in the north-south direction. The structure called for an exclusive design solution due to this slender form and difficult positioning (Dean et al., 2001). An innovative wind resisting system using offset outriggers has been developed for this building, which significantly reduces building deflections and core bending stresses. To overcome the drawbacks of typical outrigger systems, where outrigger arms commonly obstruct valuable serviceable floor space, the offset system allows the outrigger arms to be placed across the width of the building at locations away from the central core as shown in Figure 5. Detailed structural analysis for Aston Apartments have shown that for typical floor slab plan dimensions, the effectiveness of the conventional outrigger system, which requires the core and external columns to be directly coupled, is not significantly higher than that of the offset outriggers if they are further from the center core. In this structure, the floor diaphragms transfer opposing shear forces at the top and bottom levels of the outriggers to activate the perimeter columns of the building by introducing a coupling action between the columns and the core. As shown in Figure 9

11 5, the 200mm thick, two story high shear walls of Aston Apartments are located on the side elevations at mid-height and the top of the building, ensuring essentially no influence on the interior planning of the building and maximizing flexibility for the architect (Dean et al., 2001). Figure 5: Frame Perspective - Aston Apartments (Dean et al., 2001) Moreover, in conventional outrigger structures, where the core and columns are rigidly connected by outriggers, there always remains a possibility of differential shortening between them. In case of the offset outriggers of Aston Apartments, the outrigger walls effectively link perimeter 10

12 columns of similar load, so that the deflection can be expected to be similar as well. These offset outriggers utilize the axial strength of the perimeter columns to resist the load at maximum lever arm, and thus are very economical and efficient. The presence of these outriggers not only limit the drift of the building, but also reduce the bending actions in the core resulting in a reduction in the core wall thickness as well as the steel reinforcement. This system is so effective in reducing the requirement of reinforcement of the central core that it can be designed for resisting only 20% of the total base bending moment. To be more precise, as an advantage of the optimal location of the outriggers in Aston Apartments, the thickness of the core wall is 200mm over the entire 30 stories, and this exceptional absence in the variation of thickness made the core very simple and quickly constructible. (Dean et al., 2001) Conclusion Core supported systems and outrigger braced systems are two of the most frequently used lateral load resisting systems for concrete high-rises. The innovation of these systems brought an end to an era when the tallest of the buildings were inevitably steel buildings. The art of designing tall buildings is to bestow them with enough strength to resist forces generated by windstorms and earthquakes and enough stiffness or energy dissipation so that safety is never at stake. Concrete high-rises have been successfully designed using these concepts and have been proven to exhibit such satisfactory structural behavior having minimized lateral deflection, being adequately strong and dissipating as much energy as required to ensure life safety. These established structural systems, despite of being proved as effective and efficient, sometimes are just not enough for a specific condition, or even if they are, the scope of improvement has no bounds. Much engineering judgment supported by applicable experiments are required to reach a 11

13 sound conclusion. It s not possible if being limited by only the existing theories; the key is rather to explore ideas and concepts, to blend the brilliance of the past with imagination for the future. The two case studies presented here were examples of surpassing the precincts of basic structural concepts and emerging with an exclusive solution most suitable for the specific circumstance. 12

14 References Baker, W. F., Korista, D. S. and Novak, L. C. (2007). Burj Dubai: Engineering the World s Tallest Building. Struct. Design Tall Spec. Build. 16, Burj Khalifa. (2014). (Dec. 05, 2014). Dean, B., Martin, O., Emery, D., and Chancellor, P. (2001). Tall building design innovations in Australia. Proc, CTBUH 6 th World Congress, Melbourne. Schueller, W. (1986). High-rise Building Structures. Robert E. Kreiger Publishing Company, Malabar, Florida. Smith, B. S., and Coull, A. (1991). Tall Building Structures: Analysis and Design. John Wiley and Sons, Inc. Taranath, B. S. (1988). Structural Analysis & Design of Tall Buildings. McGraw-Hill Book Co., Singapore.