28-11-2012, 02:53 PM
AERODYNAMIC MODIFICATIONS TO THE SHAPE OF THE BUILDINGS: A REVIEW OF THE STATE-OF-THE-ART
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ABSTRACT
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 of
tall structures that are flexible, low in damping, slender and light in weight. These types of
flexible structures are very sensitive to dynamic wind loads and adversely affect the
serviceability and occupant comfort. For a typical tall building, oscillations have been
observed in the alongwind and crosswind directions as well as in the torsional mode. To
ensure the functional performance of tall flexible structures and to control the wind induced
motion of the tall buildings, generally different design methods, various types of passive as
well as active control devices and various types of aerodynamic modifications to the
shape/geometry of the buildings are possible. This review paper presents an overview and a
summary of past/recent work on various aerodynamic modifications to the shape of the
buildings like corner cuts, chamfering of corners, rounding of corners, horizontal and
vertical slots, dropping of corners, tapering etc. to reduce the wind excitation of tall flexible
buildings and its application in some of the tall buildings across the world.
INTRODUCTION
The advancements in the development of high strength materials, better understanding of
structural behavior coupled with more advanced analytical tools and structural design
procedures have led to a new generation of tall buildings which are slender and light as
compared to their predecessors. This types of buildings, in addition to gravity loads, are
subjected to time-varying loads arising from winds, earthquake etc. These loads are
dominant over a certain frequency ranges. These types of tall flexible buildings are very
sensitive to the wind excitation, which could be the important design criteria determining
the structural system of tall buildings [1].
AERODYNAMIC FORCES ON BUILDINGS
A structure 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’. Tall buildings are
bluff as opposed to streamlined bodies that cause the flow to separate from the surface of
structure, rather than follow the body contours. For a particular building, the shed vortices
have a dominant periodicity defined by the Strouhal number. Hence, the building is
subjected to periodic cross pressure loading which results in an alternating crosswind forces.
The wind tunnel test on the model of 420 m high Jin Mao Building, Shangai showed that its
maximum acceleration in acrosswind direction at its design wind speed is about 1.2 times of
that in alongwind direction. (Gu and Quan [11]).
AERODYNAMIC MODIFICATIONS TO THE SHAPE OF THE BUILDINGS... 435
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 alongwind
response in terms of both limit state and serviceability designs (Holmes [12]).
SERVICEABILITY REQUIREMENTS
The design of typical structure requires the engineering of system that efficiently and
effectively carries the anticipated lifetime loads. The increase in height, often accompanied
with increased flexibility and even low damping, caused the structure becomes even more
susceptible to the action of the wind, which governs the design of the lateral system. While a
given design may satisfactorily carry all the loads, the structure may still suffer from levels
of motion causing significant discomfort to its occupants. Wind-induced serviceability
issues are of concern in two areas; (1) building envelope performance under wind-induced
deformations, and (2) occupant discomfort due to building motion. Thus many design
modifications are explicitly incorporated, be they aerodynamic or structural, to improve the
performance of structure to meet the serviceability or perception requirements. Before
discussing the various aerodynamic techniques to reduce the wind-induced responses,
serviceability requirements are briefly discussed in subsequent paragraph.
For the performance of the building envelope to be adequate, the peak interstorey drift
must not exceed 1/300 to 1/500 of the storey height under unfactored loads, although this
criterion may vary depending on type of cladding or glazing and cladding attachment
details. In absolute terms, interstory drift should not exceed 10 mm unless special details
allow nonstructural partitions, cladding, or glazing to accommodate larger drift. However
this criterion must also be qualified, depending on specific building features (Simiu and
Miyata [13]).
Occupant comfort is affected by the visual perception of building oscillations. Windinduced
motions have various categories like the sway motion of the first two bending
modes termed along and acrosswind motions, a higher mode of torsional motion about the
vertical axis, or for buildings with stiffness and mass irregularities, complex bending and
torsion in the lower modes. Any of these motions can be quite unnerving and unsettling to
the occupants and symptoms may range from concern, anxiety, fear to headaches. It is
hypothesized that occupant comfort is affected by rapid changes of acceleration, but
unfortunately, no criteria based on such changes have been developed so far. The occupant
perception of accelerations is highly uncertain and complex, therefore criteria on acceptable
accelerations vary among codes and practioners. For example, in typical North American
practice the allowable peak ground acceleration with 10-year MRIs is taken as 10-15 milli-g
(0.1-0.15 m/s2) at the top floor for residential buildings and 20-25 milli-g (0.2-0.25 m/s2) for
office buildings. However, it has been determined that acceptable acceleration levels
decrease as the oscillation frequency increases, so it has been suggested that these limits be
reduced for higher frequencies of vibration, from the values stated above, which are
assumed to be valid for frequencies of 0.1 Hz, to about half of those values for frequencies
of 1 Hz (Simiu and Miyata [13]). British standard defines the comfort criterion as complaint
by more than 2% of people in the upper floors of the building during the worst 10 minutes
of a storm with a return period of 1 in 5 years.
4. AERODYNAMIC MODIFICATIONS TO BUILDING SHAPE AND
CORNER
Geometry
Wind-induced motion of a tall building can be controlled either by reduction at the source or
by reducing the response. An appropriate choice of building shape and aerodynamic
modifications can result in the reduction of motion by altering the flow pattern around a
building. The aerodynamic concern for wind-induced responses has prompted many
researchers to study the relationship between the aerodynamic characteristics of a structure
and the resulting wind–induced excitation level (Kwok and Bailey [3]; Kwok, [4];
Melbourne [14]; Melbourne and Cheung [15]; Dutton and Isyumov [16]; Hayashida and
Iwasa [17]; Miyashita et al. [18]; Karim and Tamura [19]; Kawai [20]; Kim and You [21]).
The aerodynamic modifications of a building’s cross-sectional shape, variation of its crosssection
along the height, or even its size, can significantly reduce building response in
alongwind as well as acrosswind direction by altering the wind flow pattern around the
building. Aerodynamically efficient plan shapes are shown to be an effective means of
suppressing wind-induced loads, and hence construction cost, but may come at the cost of
reducing both the size and value of saleable/rentable floor area (Tse et al. [22]). The various
aerodynamic modifications applied to the tall buildings to mitigate the wind excitations may
be 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.
Effects 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 (Karim [2]).