Aerodynamic Optimization of Building Shapes
Published on Jan 10, 2017
The development of high strength concrete, higher grade steel, new construction techniques and advanced computational technique has resulted in the emergence of a new generation oftall structures that are flexible, low in damping, slender and light in weight. These tall structures are sensitive to dynamic wind loads. Due to high slenderness, low natural frequencies, low inherent damping levels and high wind speeds at upper level, super-tall buildings are susceptible to wind excitations, particularly to vortex-induced oscillations. From design point of view, not only the wind loads, the wind-induced building motions are also within the scope of design to ensure building’s serviceability. It is well known that the behavior of wind response is largely determined by building shapes.
Considerations regarding aerodynamic optimization of building shapes in early architectural design stage is proved to be the most efficient way to achieve in windresistant design. Wind-resistant design and aerodynamic optimization are the modern topics in building design community. However, its practice and successful example can be traced back a long time ago. In ancient China, tall buildings appear to be those of traditional pagodas. Some of them even meet the modern definition of slenderness for super-talls Fig. 1.1 shows the renowned ancient pagodas in China.
These three pagodas located in ChongSheng Temple, Dali, Yunnan Province, were built 1180 years ago (824–859AD). The tallest one is 69.13 m in height with a square base of 9.9m in width, the slenderness (height/width ratio) being 7. The two identical shorter pagodas have a height of 42.19m. After the completion of these pagodas, a monastery was built. Over the long period of extreme climates and natural disasters, the original monastery was completely destroyed by natural forces but the pagodas have miraculously survived. In addition to extremely strong earthquakes in 1514 and 1925 the pagodas also experienced strong winds in history.These surviving ancient structures at least reveal two important facts which are helpful even for modern design practice.
AEORODYNAMIC FORCES ON BUILDINGS
A structure which is immersed in a given flow field is subjected to aerodynamic forces. For typical tall buildings, aerodynamic forces includes are drag (along-wind) forces, lift (across-wind) forces and torsional moments. The alongwind forces act in the direction of the mean flow. The alongwind motion primarily result from pressure fluctuations on windward and leeward faces and generally follows fluctuations in the approaching flow. The crosswind forces act perpendicular to the direction of mean wind flow. The common source of crosswind motion is associated with ‘vortex shedding’.
The torsional motion is developed due to imbalance in the instantaneous pressure distribution on each face of the building. In other words, if the distance between elastic center of the structure and aerodynamic center is large, the structure is subjected to torsional moments that may significantly affect the structural design. It has been recognized that for many high-rise buildings, the crosswind and torsional responses may exceed the along wind response in terms of both limit state and serviceability designs.
ALONG-WIND AND ACROSS-WIND RESPONSES
For wind-resistant design of buildings, it is important to identify the type of wind response that governs the design. For most super-tall buildings, it is often found that acrosswind dynamic response dominates the design wind loads and some-times causes excessive motions in terms of building’s serviceability criterion. Fig. 2.1 presents a typical azimuth plot of overall wind-induced shear force in y-direction for a building. The plot indicates that for along-winds (i.e., 25degree and 205degree), the mean loads are 3.5*10^7 N and -3.3 *10^7 N, and the peak absolute loads are 6.7*10^7 N and 6.6*10^7 N, respectively. For across-wind directions (i.e., 115degree and 295degree), the mean loads are almost zero but the peak absolute loads reach to 1.4*10^8 N and 1.0*10^8 N. The main reason that the across-wind loads can dominate the design of super-tall buildings is explained in Fig. 2.2. Fig. 2.2 shows a typical across-wind force spectrum in comparison with an along-wind force spectrum.
While the along-wind force spectrum mainly reflects the approaching wind turbulence properties, the across-wind force spectrum is largely determined by flow separation and vortex formation, so called “signature turbulence”. Compared with alongwind response, across-wind response is more sensitive to wind speed. At lower wind speeds, the along-wind loads normally dominate but with increase of wind speed the acrosswind loads take over. Due to relatively lower natural frequencies of super-tall buildings (or longer natural periods),in addition to higher wind speeds at upper levels of the boundary layer, the reduced frequency of a super-tall building at design wind speed can be very close to the reduced frequency where the peak of the across-wind force spectrum occurs.
For example of a 600 m tall square building that has a width of 60 m and the first sway period of 9 s, the vortex-induced resonance can occur at about 56 m/s winds at upper level, or 31 m/s at a standard 10 m height. The approaches of aerodynamic optimization would be different when dealing with along-wind or across-wind responses. Although some approaches that can benefit both, many approaches focus on one type or other. For optimization of along-wind response, a basic approach is to modifying building’s corners, such as making round or chamfering.
Opening is sometimes within the list of feasible options. The optimization of across-wind response, two basic approaches can be taken: (1) to reduce the magnitude of vortex excitation by modifying the building’s cross-section such as corner recession or opening; and (2) to reduce the synchronization and correlation of fluctuating forces by varying building’s shape with height such as tapering or twisting.
Since across-wind dynamic response is normally the main source that causes excessive wind loading and discomforting motion for super-tall buildings, the following discussion will focus on the aerodynamic optimizations for across-wind dynamic response.
AERODYNAMIC MODIFICATIONS TO BUILDING SHAPE AND CORNER
The various aerodynamic modifications applied to the tall buildings to mitigate the wind excitations maybe classified in two groups, Minor modifications: Aerodynamic modifications having almost negligible effects on the structural and architectural concept, for examples corner modifications like fitting of fins, fitting of vented fins, slotted corners, chamfered corners, corner recession, roundness of corners and orientation of building in relation to the most frequent strong wind direction. Major modifications: Aerodynamic modifications having considerable effects on the structural and architectural concept, for examples setbacks along the height, tapering effects, opening at top, sculptured building tops, varying the shape of buildings, setbacks, twisting of building etc.
MODIFICATION OF CROSS-SECTION
A few examples of aerodynamic modifications for building cross-sections are shown in Fig 3.1 to 3.3. The corner of Taipei 101 Tower, shown in Fig3.2, was designed during wind tunnel testing, which effectively reduces the overall design wind loads by about 25% compared with the original design of square section.
During the wind tunnel tests of Taipei 101, other type of corner modifications are also investigated, shown in Fig3.2. With the dimension of modifications being about 0.1B (10% of building width), the effects of these modifications are similar. The final selection of the corner modification was basically the choice of architects. Helical strike is a traditional device to suppress vortex excitation to chimney stacks. However, few of these devices are used for buildings due to aesthetical concern. However, it has been found that commonly designed corner balconies can perform similar roles in suppressing vortex shedding.
The concept was used again in a recent tall building in a very strong typhoon area, shown in Fig3.3, and proved to be able to reduce design wind loads by about 20%. The overhang eaves shown in ancient pagodas, Fig1.1, are also the examples, Opening is not commonly used in design practice due to potential impact on useable spaces. However in some cases the corner slot can not only significantly reduce the across-wind excitation, but also make internal space design more logical
EFFECT OF FINS AND VENTED FINS
The aerodynamic modifications to basic square cross-sectional shape of buildings by using small fins or vented fins have significant effects on the alongwind and crosswind response characteristics. Small fins/vanes fitted to the corners of a prismatic building with a gap between the vanes and the corner can help to alleviate negative pressures under the separated shear layers on the side faces. However, the added drag introduced by these vanes increases the along wind responses. The fitting of fins served only to increase the critical wind speed without any noticeable disruption to vortex shedding process.
At the high range of reduced velocities, there was an apparent reduction in galloping response when vented fins were fitted. The fitting of fins or vented fins is acceptable for general usage only for certain range of reduced wind velocities. The aerodynamic modifications to buildings like fitting of fins and vented fins causes noticeable increase in the alongwind response due to an increase in the projected area normal to wind direction. The aerodynamic modifications, which in general increase the projected area or the effective width of a building, would not be beneficial
EFFECT OF SLOTTED CORNERS, CHAMFERED CORNERS, CORNER RECESSION
The double step corner recession modifications had been applied to the cross section of 508 m high, 101 storey, Taipei101 building, Taiwan. A corner modification Investigations have established that corner modifications such as slotted corners, chamfered corners/corner cut, corner recession are in general effective in causing significant reductions in both the alongwind and crosswind responses compared to basic building plan shape. The modification of windward corners is very effective to reduce the drag and fluctuating lift through changing the characteristics of the separated shear layers to promote their reattachment and narrow the width of wake. This type of modifications is also effective to suppress the aero elastic instability.
The effects of slotted corners and chamfered corners were investigated by Kwok and Bailey through wind tunnel tests on aero elastic square and rectangular models of dimension 60mm 60mm 540mm and 112.5mm 75mm 450mmrespectively with and without slotted and chamfered corners. The modifications to the building corners ranged from 9% to 16% of building breadth. It is concluded that, slotted corners and chamfered corners were causing noticeable reductions in both the Dynamic alongwind and crosswind responses as compared to plain rectangular shape building. Venting through the slotted corners appears to be effective in reducing the drag force without undesirable effect of using vented fins. For building heights ranging from 240 to 280m the recessed corners are more effective than the chamfered corners in reducing both along wind and crosswind moments due to buffeting and vortex shedding excitations respectively.
In general, for the range of building forms and heights tested, the construction cost was reduced with the introduction of chamfered and recessed corners even though the building height was increased to maintain the total usable floor area for the entire building. This type of corner modifications (corner recession) had been applied to the 150 m high Mitsubishi Heavy Industries Yokohama Building as shown in Fig 3.5, which was located in a water front area in the wake of peripheral tall buildings. To reduce the wind-induce responses, all the four corners were chamfered, which consequently reapplied to the Taipei101 building reduces the base moment by 25% as compared to building of basic square section.
Aerodynamic optimization of building shapes is an important portion of super-tall building design. Two categories of optimization are discussed in the paper: aerodynamic modifications that are normally considered as remedial measures; and aerodynamic designs that integrate architectural design with aerodynamic study in the early design stage. While aerodynamic modifications mostly involve building corner treatments, aerodynamic designs have more options in building shapes, including overall elevation optimizations such as tapering, twisting, stepping, opening, top sculpturing, etc.
A few examples of aerodynamic optimization schemes that have been implemented with success in building designs are illustrated in this paper. The main challenges in building aerodynamic optimization are to compromise aerodynamic solutions with other architectural design aspects and to compromise between benefits and costs. Therefore, it is important to have a reasonable assessment of effectiveness of various aerodynamic options in the early design stage so that the potential pros and cons can be evaluated in the decision-making process. The method proposed in this paper can serve this purpose. With this method, the assessment of aerodynamic effectiveness for tapering, stepping and twisting can be conducted with minimum wind tunnel tests at low cost. The results of the case study reveal some important phenomena:
1. In general, tapering and stepping can reduce across-wind responses. However, for a low-return period response such as a building’s performance in common winds where the corre-sponding reduced velocity is low, it is possible that tapering or stepping may actually increase accelerations affecting occupant comfort.
2. While twisting can considerably decrease the maximum across-wind responses, it can also lead to equalized response over wind directions. Therefore, a potential mutualeffect between the two common optimization options, optimization through twisting and optimization through building orientation, should be considered.
3.Although aerodynamic effectiveness of twisting generally increases with the increase of twisting level, the increment of effectiveness tends to decrease after a limit.
4.The corner roundness is the most effective to suppress the aeroelastic instability for a square building. The amplitude of the wind-induced vibration reduces as the extent of the corner roundness increases.
5.Tapering effect has a more significant effect in acrosswind direction than that in alongwind direction.
6.The through building opening along the alongwind and crosswind direction, particularly at top significantly reduces the wind excitation of the building.
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