Strategies to Reduce Lateral Forces on High-rise Buildings that Use Diagrid Structural System Binh K. Nguyen and Hasim Altan School of Architecture The University of Sheffield Sheffield, United Kingdom Email:
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Abstract — The study focuses on strategies to enhance the performance of a building against lateral forces and earthquake. The strategies range from aerodynamic modifications to structure reinforcement to adopting auxiliary damping devices/systems. The study’s direction was toward Diagrid structural systems – the latest trend in high-rise pr ojects. Keywords-component; lateral forces; tall-buildings; high-rise; diagrid, damping strategies.
I.
I NTRODUCTION
The evolution of tall-building structural systems, based on new structural concepts with newly adopted high-strength materials and construction methods, has been towards ‘stiffness’ and ‘lightness’. Structural systems are becoming 1 stiffer and lighter . Diagrid , acknowledged worldwide as a very light structure and one of the best when it comes to withstanding lateral forces, has been leading the trend. The lighter a structure is, the higher it can rise. On the other hand, it is also easier to blow away a light subject than a heavy one. Diagrid can save from 20% to 30% the amount of structural steel in a high-rise building. Moreover, high-strength material technology has come a long way since the invention of modern high-rise building in 1930s. Materials themselves are stronger and lighter. It is common knowledge that, rather than directly standing the forces, it is better to reduce them and dissipate the magnitude of vibrations. A high-rise structure needs both stiffness and damping characteristics. The strategies to enhance tall-buildings’ lateral performance can be divided into 3 categories: (1) Aerodynamic Modifications; Modifications; (2) Structural Reinforcements Reinforcements;; and (3) Using Auxiliary Damping Devices/Systems Devices/Systems.. The strategies presented below below are the the ones that can be used for that use Diagrid structure. However, most of the strategies can be applied to other types of tall-building structures.
II.
AERODYNAMIC MODIFICATIONS
A. Aerodynamic Shapes The form of a tall-building is usually limited to rectangular prisms. From geometrical point of view, this form is rather susceptible to lateral drift. Other building shape such as cylindrical, elliptical, crescent, triangular and like, offer better lateral performance due to inherent strength in their geometrical form. They provide higher structural efficiency and allow greater building height at lower cost. Building codes permit a reduction of the wind pressure design loads for circular or elliptical buildings by 20%-40% of the usual values for comparably sized rectangular building [1]. B.
Corner Modification
Investigations have established that corner modifications such as chamfered corners, horizontal slots, a nd slotted corners can significantly reduce the along wind and across wind responses when compared to a basic perpendicular building shape [2]. Fig. 1 shows some types of modification to building corner. Chamfers of the order of 10% of the building width makes 40% reduction in the along wind response and 30% reduction in the across wind response [3]. Excessive rounding of corners of the cross section, approaching a circular shape in the cross section, significantly improves the response against wind. With a building of roughly 70 stories, peak deflection of the model in circular cross section was about half of the one with square cross section [3].
Fin
Vented Fin
Slotted Corners
Chamfered Corners
Stair-step Corners
Figure 1. Corner modification types
C. Tapering and Setbacks 1
Diagrid (or diagonal (or diagonal grid ) is a design for constructing large buildings with steel that creates triangular structures with diagonal support beams. It requires less structural steel than a conventional steel frame. It also obviates the need for large corner columns and provides a better distribution of load in the case of a compromised building.
Reducing floor areas gradually toward the top is a good strategy to enhance lateral performance of a building. This way, the mass of the building is concentrated in the lower floors. More importantly, when hitting a building using
tapering and setbacks, the vortices will try to shed at different frequencies at different heights. In other words, the winds become “confused” and incoherent, which can dramatically reduce the associated fluctuating forces [4]. Examples of “tapering” strategy are: John Hancock Center (Chicago), Millennium Tower (Tokyo) (see Fig. 2), and Transamerica Pyramid (San Francisco). The Jin Mao Building (Shanghai) (see Fig. 3) and the Petronas Tower (Kuala Lumpur) are good examples of using “setbacks” to slightly taper the building shape. While “setback” is rather tricky to apply to a Diagrid building because of the continuity of the structure, “tapering” is highly suitable.
Figure 2. Millennium Tower
G. “Spoilers” Vortex shedding can also be reduced by adding “spoilers” to the outside of the building. The most well-known form of spoilers is the spiral Scruton strake which is widely used on circular chimney stacks [4]. It is very convenient to use this strategy on Diagrid structures due to the geometry benefit of the grid. Example: 30 St Mary building (London) (see Fig. 4). H. “Through-building” Openings In sustainable design of tall buildings, opening areas are fundamental. This can be exploited to provide areas which wind can blow all the way through, thus reduce wind loads. Diagrid, with its unique structure characteristic, can provide random openings without affecting the structure at all. Multiopenings is also a good strategy. Example: Shanghai World Financial Center (Shanghai) (see Fig. 5)
Figure 3. Model of Jin Mao Building
D. Sculptured Building Tops The sculptured building tops not only highlight the height of the structure, but also serve the practical aerodynamic purposes such as reduction in the wind response of the building. The more sculptured a building’s top, the better it can the along wind and across wind responses. Examples: Jin Mao Building, Petronas Towers, etc. E.
Varying Cross-section Shape
Varying cross-section shape with height, e.g. going from square to round, can force the wind to behave differently, preventing in becoming “organised”. This results in “confused” vortices, which is a similar effect to using tapering and setback [4]. Examples include Buji Khalifa (Dubai) and the Sears Tower (Chicago).
Figure 4. 30 St Mary building uses “spoilers” to defect wind loads
III.
STRUCTURAL R EINFORCEMENTS
Without going too deep into the technical aspects, basically, structural reinforcement strategies that can be applied to highrise Diagrid structures include: -
Super-columns addition: Adding super columns or sets of columns to strategic locations of a Diagrid structure – Fig. 6a.
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Coupled buildings: 2 or more buildings are linked together for better structural performance - Fig. 6b. The Petronas Towers are a typical example of this approach.
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Core and Outrigger support: Adding a core and outriggers to the Diagrid structural shell – Fig. 6c.
F. Rotated and Twisted Forms Rotated building forms can minimize loads from prevailing directions, while twisted forms can avoid simultaneous vortex shedding along building’s height.
Figure 5. Shanghai World Financial Center
1) Energy-dissipating-material-based damping systems are generally installed as integral parts of primary structural systems at strategic points, reducing the dynamic motion of tall buildings. The damping force in a viscous damper or visco-elastic damper is dependent upon the time rate of change of the deformation [5]. Damping is accomplished through the phase shift between the force and displacement.
a) Super-columns addition
b) Coupled buildings
c) Core & Outriggers support
Figure 6. Structural reinforcement strategies
IV.
An example of viscous dampers, installed as an integral part of the bracing members, can be found in the 55-story Torre Mayor in Mexico City – the tallest building in Latin America at present, and visco-elastic dampers were installed in the destroyed World Trade Center Towers in New York. Other types of damping systems in which the damping mechanism is through direct dissipation of energy from the system include hysteretic damping and friction damping. Fig. 8 presents an example of a viscous dampers system applied to a Diagrid high-rise building.
AUXILIARY DAMPING DEVICES/SYSTEMS
Damping systems for high-rise buildings can be divided into two categories: (1) Passive systems and (2) Active systems – Fig. 7. Passive systems have fixed properties, and, in order for them to perform as intended, they do not require energy, while active systems do need an “actuator” or “active control” mechanism relying on an energy source to modify the system properties against ever-changing loads. Thus, active systems are more effective in general. However, due to their economy and reliability, passive systems are more commonly used.
Source: [5]
Figure 8. Example of a viscous dampers system applied to a Diagrid structure Source: [5]
Figure 7. Types of Auxiliary Damping Devices/Systems
A. Passive Systems Passive damping systems can be divided further into two sub-categories: -
Energy-dissipating-material-based damping systems such as viscous dampers and viscous-elastic dampers
-
Auxiliary mass systems to generate counteracting inertia forces such as Tuned Mass Dampers (TMD) and Tuned Liquid Dampers (TLD).
2) A TMD is composed of a counteracting-inertia-forcegenerating huge mass accompanying relatively complicated mechanical devices that allow and support the intended performance of the mass. The frequency of the TMD mass is generally tuned to the fundamental frequency of the primary structure. Thus, when the fundamental mode of the primary structure is excited, the TMD mass oscillates out of phase with the primary structure, generating counteracting inertia force [5]. Fig. 9 shows different types of TMD. A TMD system, located near the top of the building for its best performance, is installed in a room that is usually not accessible to the public, as in the cases of the sliding type TMDs installed in the John Hancock Building in Boston and
the Citicorp Building in New York. However, the pendulumtype TMD installed in the Taipei 101 Tower is used as a decorative element in the building interior as well, attracting interest of visitors – Fig. 10.
Source: [5]
Figure 9. Different types of TMD
B. Active Systems An active structural control system is the one that has the ability to determine the present state of the structure, decide on a set of actions that will change this state to a more desirable one, and carry out these actions in a controlled manner and in a short period time [7]. While some passive systems, such as TMDs or TSDs, are effective only for a narrow range of loading conditions, active systems can perform effectively over a much wider range and they are a more advanced form of functional performance-driven technologies in architecture. Examples are active mass dampers (AMD) and active variable stiffness devices (AVSD). The AMDs resemble the TMDs in appearance, although the vibration of a building is picked up by a sensor, the optimum vibration control power calculated by a computer, and the movement of the building is reduced by shifting a moveable mass with an actuator [5]. The AVSDs continuously alter the building’s stiffness to keep the frequency of the building away from that of external forces, such as earthquakes, to avoid a resonance condition. Although their cost-intensiveness and reliability issues are limiting the use of active systems at present, with more research, they would have great potential for future applications. V.
CONCLUSION
Among various types of structures for tall buildings, Diagrid is the one which has many advantages such as: aesthetic, structural performance, material saving potential, freedom of space arrangement thanks to the lack of structural columns, etc. Diagrid is becoming the predominant choice for high-rise projects. Keeping in mind these features, this paper summarises the strategies to mitigate lateral forces which are suitable for high-rise Diagrid buildings, but also applicable to other types of structures. The strategies range from architectural design techniques such as modifying building form, to structural reinforcement tactics, and using damping systems/devices. R EFERENCES Figure 10. Taipei 101 Tower’s TMD system
3) TLD, such as tuned sloshing dampers (TSD), use waving water mass as a counteracting inertia force generator. Thus, this system can be designed using the existing water source in tall buildings, such as a pool or water tank located near the top of a building. In a TSD, sloshing frequencies are tuned by adjusting the dimensions of the water container and the depth of water. Another type of TLD is tuned liquid column dampers (TLCD), which uses a U-shaped vessel [6]. Fig. 11 shows different types of TLD.
Source: [5]
Figure 11. Different types of TLD
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