Wind Engineering Challenges of the New Generation of Super-tall Building

ARTIC AR TICLE LE IN PR PRESS ESS J. Wind Eng. Ind. Aerodyn. Aerodyn. 97 (2009) 328–334

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Journal of Wind Engineering and Industrial Aerodynamics journal homepage: ww w.elsevier.com/locate/jwei w.elsevier.com/locate/jweia a

Wind engineering challenges of the new generation of super-tall buildings Peter A. Irwin RWDI, 650 Woodlawn Road West, Guelph, Ontario, Canada

a r t i c l e

i n f o

Article history: Accepted 18 May 2009 Available online 11 August 2009 Keywords: Tall buildings Wind effects Wind tunnel testing Wind statistics

a b s t r a c t

The new generation of tall buildings is going much higher than before. This poses new challenges for wind engineering. The boundary layer models in many building codes and standards have served well for building buildingss less less than than about about 300m but more realisti realisticc models models need to be used used above above 300 m. The statis statistic ticss of upper upper level level winds winds need need also also to be known known with better better certaint certainty. y. New tools tools such such as the archived global re-analysis data coming from weather forecast models can help shed more light on the upper level wind statistics. There are also questions to be answered about the effects on all tall buildings of non-synoptic wind profiles such as occur in thunderstorm downbursts and the Shamal winds of the Middle East. For the super-tall buildings wind tunnel testing is often commenced much earlier in the design than for lesser buildings. This permits the results to be used in a pro-active way to shape both the architectural design and structural design. The wind tunnel methods used include the force balance technique, aeroelastic modeling, high frequency pressure integration tests, as well as the traditional pressure model and pedestrian wind studies. A super-tall building pushes the limits of the force balance method due to difficulties in maintaining sufficient model stiffness and in accounting for the influence of higher modes of vibration. Since the impact of wind on people using terraces and balconies increases with building height, it is an issue needing particular attention for super-tall buildings. & 2009 Elsevier Ltd. All rights reserved.

1. Introduct Introduction ion

In the present day we are experiencing an unprecedented level of activity in the design and construction of super-tall buildings. It used to be that a 300 m high building was was a threshold threshold that only only a few buildings exceeded. The Sears Building, located in Chicago, Illinois, at 440 m, held the record for many years. Now, Now, numerous numerous buildings have either been constructed, are under construction, or, or, to use an old fashioned expression, are ‘‘on the drawing boards’’ in the height height ran range ge 400 m and up. The current current world’s world’s tallest tallest is Taipei Taipei 101 101 at 509 m. Several are under design with heights well over over that that of Taipei aipei 101. 01. There There are a number number of desig designs ns being being contempl contemplated ated in the 500–1500 500–1500 m range. range. Burj Dubai, Fig. Dubai, Fig. 1, 1, which is scheduled for completio completion n by 2009, will be well over 700 m tall. This new generation of towers poses new challenges for wind engine engineeri ering. ng. These These are discuss discussed ed in this this paper paper.. It is primar primarily ily based on the experience of the author and his colleagues and it is not intended intended to be a comprehe comprehensive nsive review review of the literature literature on the topic. topic. Nonetheles Nonetheless, s, it is important important to acknowle acknowledge dge that many other researchers have made major contributions, too numerous to cite comprehensively in a paper of this length. Some of these appear in the reference lists of the references cited in this paper.

E-mail address: [email protected]

0167-61 0167-6105/$ 05/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jweia.2009.05.001 doi:10.1016/j.jweia.2009.05.001

Most Most buildi building ng codes codes still still use ‘‘tra ‘‘tradit dition ional’ al’’’ models models of the planetary boundary layer, layer, developed developed in the 1960s, 1960s, that assume the boundary boundary layer tops out between about 250 and 500 m, depending on exposure exposure.. The validity validity of these models models is questiona questionable ble when dealing with building building heights above about 300 in. The The stat statis isti tics cs of wind wind spee speed d and and dire direct ctio ion n used used in wind wind engineering engineering have tradition traditionally ally been almost almost entirely entirely based on records records from ground ground based meteorolo meteorological gical stations, stations, taken at about 10 m height. It is a long extrapolation extrapolation to develop roof height wind statistics statistics for supersuper-tall tall buildings buildings from the ground-b ground-based ased data alone, alone, especi especiall ally y whe when n their their respo response nse is so sen sensit sitive ive to wind wind speed, speed, and sometimes sometimes,, direction direction.. More reliable reliable informatio information n on upper level wind statistics is needed. The aerodynamics of tall buildings can have a huge impact on their cost. The main structural system is a large part of the cost and for super-tall super-tall buildings buildings wind is the governing governing lateral lateral load. Wind affects not only the structural integrity of the tower but also its servic serviceab eabili ility ty.. Keepi Keeping ng the motion motionss of the tower tower within within comfor comfortab table le limits limits is often often a bigge biggerr challe challenge nge than than meetin meeting g structural strength requirements. Therefore, the aerodynamics of the towe tower’s r’s sha shape pe needs needs to be consid consider ered ed as a critic critical al design design parameter from the very outset. The response response of the tower tower to wind depends depends not only only on its shape but also its stiffness distribution, mass distribution and damping. For optimal design the interplay between these variables needs to be carefully examined. Increasingly the structural designers of very

ARTICLE IN PRESS P.A. Irwin / J. Wind Eng. Ind. Aerodyn. 97 (2009) 328–334

329

Fig. 1. Burj Dubai—future view, aeroelastic model and recent construction photo.

tall towers are prepared to extend their thinking beyond the traditional structural variables of stiffness and mass, and to treat the damping as a third controllable structural parameter. Supplementary damping systems allow them to do this and open up a whole new range of possibilities for optimizing the design. To date their use has been targeted primarily towards satisfying serviceability criteria. However, they also have the potential to mitigate ultimate design wind loads, in a similar manner to their use in earthquake design. The wind tunnel techniques used for super-tall buildings are largely the same as for lesser towers but their extreme height can pose challenges. Typically smaller model scales become necessary, and, because of the importance of aerodynamics, more iterations of shape may well be needed during the design optimization process. Because of the time required to build super-tall towers the design of the upper portions is often still underway during construction. However, wind tunnel tests to establish base loads may well have to be done while there is still uncertainty as to what the top part of the tower will finally look like. Therefore, initial testing to supply foundation loads must allow for the possible range of shapes that the final design might take. Another issue that arises for super-tall towers is that wind speeds on terraces high up on the tower can be expected to be much higher than on normal buildings. Yet there is often a desire to have these terraces as usable space.

2. Wind statistics and wind profiles

In North America the ASCE 7-05 standard sets the standard for wind design in the USA and in Canada the National Building Code serves this purpose. The boundary layer models in these documents are very similar to each other and were developed empirically in the 1960s. They will be referred to here as ‘‘traditional models’’. They have boundary layer depths ranging from about 210 m in very flat open terrain to 460 m in dense urban terrain. These models appear to have served well for the vast majority of buildings. However, they are purely empirical and not based on much consideration of atmospheric physics. The vast majority of buildings on which our experience is based come nowhere near high enough to test the assumptions concerning boundary layer depth in these traditional models. However, the new generation of super-tall towers certainly does. The Harris and Deaves (1981) model, which was adopted in the 1980s by ESDU (1993), is based on more fundamental physical considerations than the traditional model and at high wind speeds indicates considerably deeper boundary layers in synoptic

Predicted Wind Speeds by height from Balloon Upper Air ) 60.0 s / m ( d 50.0 e e p S 40.0 d n i W 30.0 d e t c 20.0 i d e r P 10.0 1

10

100

1000

Return Period (years) 100 m

200 m

400 m

600 m

Fig. 2. Estimated extreme wind speeds for Las Vegas at various heights from balloon data.

type winds, more in the 2000–3000 m range. Not only does the wind speed in this model continue to increase with height all the way to the tops of super-tall buildings (and beyond) but, just as important, the flow is turbulent up there. The traditional model would have the tops of these buildings in smooth uniform flow. Turbulence can have important influences on vibration phenomena and aerodynamic instabilities such as vortex shedding and galloping. The continuation of the boundary layer to much greater heights than predicted by the traditional models is also supported by balloon measurements and weather forecasting computer models. Figs. 2 and 3 show estimated extreme mean hourly wind speeds at various heights for the Las Vegas area based, respectively, on 12 years of twice daily upper air balloon soundings and 20 years of global re-analysis data. The results were obtained using extreme value analysis methods on the monthly extremes. The 20 years (1987–2006) of re-analysis data were obtained from the National Center for Atmospheric Research/National Centers for Environmental Prediction (NCAR/NCEP). The NCAR/NCEP datasets are based on a worldwide meteorological observation network, including surface and upper air balloon measurements, satellite and radar measurements, etc. The data were available at 3 h intervals on a three-dimensional grid and were derived by meteorological modeling software similar to that used for weather forecasting.


330

Predicted Wind Speeds by height from NAM\NARR 60.0 ) s / m ( 50.0 d e e p S 40.0 d n i W 30.0 d e t c i d 20.0 e r P 10.0 1

10

100

1000


200 m

400 m

600 m

Fig. 3. Estimated extreme wind speeds for Las Vegas at various heights from archived global re-analysis data.

3. Optimization of shape

One of the critical phenomena that effect tall slender towers is vortex excitation. The well-known expression of Strouhal gives the frequency N at which vortices are shed from the side of the building, causing oscillatory across-wind forces at this frequency.

Predicted Wind Speeds by height from Surface Observations

) 60.0 s / m ( d 50.0 e e p S 40.0 d n i W 30.0 d e t c i d 20.0 e r P 10.0

While the Harris and Deaves (1981) model is an improvement on the traditional model, it does assume that the winds of interest are created by synoptic, i.e. large-scale wind systems such as anticyclones, where the wind blows in a roughly constant direction over large distances. However, there is evidence that a good proportion of the strong wind events experienced at ground level in many parts of the world are due to small-scale phenomena such as thunderstorms and the downburst phenomenon associated with them. The wind profiles in downbursts are very different from the equilibrium profiles depicted in traditional or the Harris and Deaves boundary layer models, taking the form of a jet of high-speed air near the ground as depicted in Fig. 7. Jets at somewhat higher levels can occur in the Shamal winds that are found in the Arabian Gulf area (Qiu et al., 2005) as also illustrated in Fig. 7. The impact of these ‘‘non-standard’’ wind profiles on tall buildings needs further research.

N ¼ S

U

(1)

b

where S ¼ Strouhal number; U ¼ wind speed; and B ¼ building width. The Strouhal number is a constant with a value typically in the range 0.1–0.3. For a square cross-section it is around 0.14 and for a rough circular cylinder it is about 0.20. When N matches one of the natural frequencies N of the building, resonance occurs which results in amplified across-wind response. From Eq. (1) this will happen when the wind speed is given by r

1

10

100

1000


100 m

200 m

400 m

600 m

Fig. 4. Estimated extreme wind speeds for Las Vegas at various heights extrapolated from 10 m height data.

What is clear in both Figs. 2 and 3 is that when estimating extreme wind speeds at the tops of very tall buildings, the wind speed continues to increase significantly above 400 m. Fig. 4 shows the predictions of extreme wind speeds based on the traditional boundary layer model, i.e. a power law with exponent 0.14, but assuming the power law extends to at least 600 m height. Comparing with 600 m results in the 50–100 year return period range in Figs. 2 and 3, the traditional boundary layer model, Fig. 4, gives slightly higher wind speeds. It is noteworthy also that at high return periods the slope of the wind speed versus return period is lower in all cases than implied in the commentary of ASCE 7 (2005). The ratio of wind speed at 600 m to that at 10 m as a function of wind speed as derived from the NCAR/NCEP re-analysis data shows interesting trends. Figs. 5 and 6 show this ratio as function of wind speed at 10 and 600 m, respectively, for the Chicago area. At high 10 m level speeds the ratio asymptotes very well towards the value 1.77 that is derived by assuming a 0.14 power law for the mean velocity. However, at high 600 m level speeds the ratio is higher indicating that 10 m wind speeds are not always a good indicator of high winds up at 600m. This emphasizes the importance of direct measurements of winds at upper levels rather than relying on extrapolations from ground-based data.

U ¼

N r b S

(2)

Thus for a building 50 m wide, and with Strouhal number 0.20 say, and with a fundamental natural frequency of 0.06 Hz, which is possible for a super-tall structure of 150 or more stories, the resonant condition of the fundamental mode will happen when the mean speed at the top of the building is only U ¼ 15m/s. This is a very common speed. A representative second mode frequency of such a building would be around 0.2 Hz, which is similar to the first mode of a typical 50-story building. The resonant condition for the second mode will occur at 50m/s, which is likely to still affect the response at the upper end of the design speed range. Fig. 8 illustrates the estimated across-wind peak acceleration response, due to both buffeting and vortex shedding, of a 600 m tall tower with the modes similar to those just described. It can be seen that there is hump in the response around 17 m/s due the excitation by vortex shedding in the first mode. Then at high speed around 50–60 m/s there is a second hump. Traditional criteria for maximum acceptable accelerations in buildings, typically based on events with return period in the 1–10 year range, need to be re-assessed for super-tall buildings with very low first mode frequency. Perceptible motions could be felt in these buildings on a very frequent basis unless care is taken to avoid vortex excitation or to supplement the damping to reduce vortex-shedding amplitudes. While there are measures that can be taken in the structural design such as stiffening, adding mass or introducing supplementary damping systems, these do not attack the vortex shedding at its source. The source is the building shape and it is possible to virtually eliminate the vortex shedding forces through selection


331

Fig. 5. Ratio of mean wind speeds at 600–10 m as a function of mean wind speed at 10 m for the Chicago area. Line at 1.77 represents value obtained from 0.14 power law.

Fig. 6. Ratio of mean wind speeds at 600–10 m as a function of mean wind speed at 600 m for Chicago area.

800

Modes 1 and 2

100

Shamal 600

g i l l i m , n o i t a r e l e c c A

Synoptic

m , t h 400 g i e h

Thunderstorm

200

80

Mode 1 only

f(1) = 0.058Hz f(2) = 0.200Hz CL' (1) = 0.170 CL' (2) = 0.120 dCLda (1) = 0.700 dCLda (2) = 0.700 zeta (1) = 0.010 zeta (2) = 0.010

60 40 20 0

0 0

10

20

30

40

50

0

10

20

30

40

50

60

70

80

90

Mean wind speed at top, m/s

wind velocity, m/s Fig. 7. Typical mean velocity profiles in synoptic, thunderstorm and Shamal winds.

and refinement of the building shape. There are several directions that one can go in developing an aerodynamically favorable shape.

 Softened corners: Square or rectangular shapes are very common for buildings and experience relatively strong vortex shedding forces. However, it is found that if the corners can be ‘‘softened’’ through chamfering, rounding or stepping them inwards, the excitation forces can be substantially reduced. The softening should extend about 10% of the building width in from the corner. The corners on Taipei 101 were stepped in

Fig. 8. Example of estimated across-wind response of 150-story tower with first and second harmonic frequencies of 0.06 and 0.20 Hz.

order to reduce across-wind respond and drag, resulting in a 25% reduction in base moment (Irwin, 2005).  Tapering and setbacks: As indicated in Eq. (1), at a given wind speed, the vortex shedding frequency varies depending on the Strouhal number S and width b . If the width b can be varied up the height of the building, through tapering or setbacks, then the vortices will try to shed at different frequencies at different heights. They become ‘‘confused’’ and incoherent, which can dramatically reduce the associated fluctuating forces.  Varying cross-section shape : A similar effect can be achieved by varying the cross-section shape with height, e.g. going from


332

square to round. In this case the Strouhal number S varies with height, which again, in accordance with Eq. (1) causes the shedding frequency to be different at different heights.  Spoilers: One can also reduce vortex shedding by adding spoilers to the outside of the building. The most well known form of spoilers are the spiral Scruton strakes used on circular chimneystacks (Scruton, 1963). Architecturally and practically, the Scruton strake leaves something to be desired for circular buildings, but other types of spoiler could be used that might be more acceptable, such as vertical fins at intervals up the height.  Porosity or openings: Another approach is to allow air to bleed through the building via openings or porous sections. The formation of the vortices becomes weakened and disrupted by the flow of air through the structure. While vortex shedding is the principal culprit causing undesirably high across-wind motions, another cause is buffeting by turbulence cast off from upstream buildings. This is less easy to deal with through the building shape since the origin of the turbulence is not the building itself. However, some cross-sectional shapes, e.g. a lens shape, are more prone to across-wind buffeting because their streamlined shape causes them to act somewhat like a vertical aerofoil, generating high across-wind force variations for relatively small changes in angle of attack of the wind caused by turbulence. Shape changes that make them less like an aerofoil can help in this situation. Irwin et al. (1998) describe some results with different shapes.

4. Stiffness, mass and damping

Based on Eq. (2), one approach to tackling the problems caused by vortex shedding is to stiffen the building sufficiently (i.e. to increase N enough) to force the resonant speed above the top end of the design range. However, this can be extremely expensive and can become impractical on a super-tall tower. Another approach is to increase the building’s mass. The amplitude of motion caused by wind excitation tends to vary inversely with Scruton number 2md=ðrB2 Þ, where m ¼ generalized mass per unit height, d ¼ logarithmic decrement of damping and r ¼ air density (Zdravkovich, 1982). From this relationship it can be seen that increasing the mass helps. However, adding mass also results in extra cost. From the Scruton number relationship just described it is clear also that another way to reduce the amplitude of wind-excited motion is to supplement the damping, i.e. increase d. This leaves the vortex resonance within the design speed range but, with sufficient damping, suppresses the resulting motions to an acceptable level. The supplementary damping system bleeds energy out of the motion sufficiently fast to counter the energy being put in by the vortex shedding. There are a variety of damping systems that have been used. In recent years tuned mass dampers or tuned liquid dampers of various types have been increasingly implemented as the most cost beneficial approach. It is not intended here to discuss the various types of damping system that have been used in detail. Irwin and Breukelman (2001) describe several projects where they were used. Fig. 6 illustrates several different damper configurations. The advantage of a damping system is that to a large extent it frees the design team from the need to constrain the building shape because of aerodynamics or to spend large amounts on increasing stiffness and/or mass. With the trend towards taller and taller buildings the day of the supplementary damping system has truly arrived. r

5. Wind tunnel testing 5.1. Its use as design tool

How has the advent of the massively tall building in the last few years altered wind tunnel testing? Since shape is so critical for these buildings, one of the trends has been for the wind tunnel to be used more proactively to optimize the aerodynamic shape in an iterative process starting early on in the design process. This is in contrast to the more conventional and passive approach of simply testing at a point near the end of design and then presenting results when the shape is already fixed with little or no consideration of aerodynamics. To quote the structural engineer for Burj Dubai, Bill Baker of Skidmore Owings Merrill: ‘‘We practically designed the tower in the wind tunnel’’ (ENR, 2006). The shape of Burj Dubai was tailored to optimize the aerodynamics through a series of iterative tests starting at the earliest stages of design (Irwin and Baker, 2005). Through this approach a building shape was developed that was extremely efficient from a wind loading point of view to the point where the tower has no need for a supplementary damping system. The same philosophy was extended right to the top pinnacle structure, a slender steel tube. A combination of tapering and aerodynamic fins was developed through wind tunnel tests on the pinnacle to suppress vortex shedding. 5.2. High frequency force balance method

One of the most useful tools in wind tunnel testing of high-rise buildings has been the high frequency force balance (HFFB) (Tschanz, 1982; Xie and Irwin, 1998). It is still useful for super-tall buildings, especially for early determination of design loads at the base of the structure, but its limitations must be understood. One important limitation is that the HFFB method only works well for the fundamental mode of vibration. For super-tall buildings the excitation of higher order modes of vibration can become significant, particularly for the upper portions of the structure. Another challenge for the traditional HFFB method, where the model is mounted on a fast response force balance at its base, is that a very stiff model is needed. For extremely slender and tall towers, achieving sufficient stiffness in the model is difficult. In order to avoid encroachment of the model resonance frequency into the frequency range of interest for wind loading, it becomes necessary to test at lower wind speeds than normal. This in turn can cause the Reynolds number of the test to stray into a low enough range that Reynolds number effects become significant. A recent development that promises to make the HFFB method even more useful as part of a wind tunnel test program has been the move from frequency domain analysis to time domain analysis (Xie et al., 2007). The HFFB method has traditionally been applied in the frequency domain only. However, by going to the time domain more precise information can be gathered on the correlations between peak responses in two different directions, and on the peak factors relating peak responses to RMS responses. When used with simplified aeroelastic model tests, as described by Xie et al. (2007) the time domain analysis of HFFB data can be particularly powerful. For buildings with supplementary damping systems the time domain has further advantages in allowing the non-linear characteristics of the damping system to be incorporated in the analysis. 5.3. High frequency pressure integration method

A way around the limitations of the HFFB method in handling higher mode effects is to use what RWDI has called the HFPI method, HFPI standing for high frequency pressure integration. This approach, first published by Irwin and Kochanski (1995), is in


333

Fig. 9. China Central Television Headquarters.

use at a number of wind tunnel laboratories, and as the name implies, involves integration of point pressure measurements on an instantaneous basis to obtain time histories of overall modal loads in each mode of vibration. Its advantage is that it is not limited to the fundamental mode and the test speed is not constrained by model frequency issues, as is the case for the HFFB method. However, the obvious limitation of the HFPI method is that it is only as good as the density with which the model can be instrumented with pressure taps. For some designs, particularly those with many intricate features it may simply be impractical to install a sufficient density of pressure taps to be confident that the overall integration will be accurate. A useful approach to is to cross check the HFPI method results for the fundamental mode with those of the more direct HFFB method. The HFPI method is particularly well suited to buildings with complex shapes such as the China Central Television Headquarters in Beijing, Fig. 9 (Xie and To, 2005). In fact it is difficult to conceive of an alternative approach for this building other than a full aeroelastic model which would be extremely intricate and costly to construct. For the Burj Dubai the wind tunnel test program included special high Reynolds number studies using a 1:50 scale model in a large wind tunnel capable of wind speeds up to 55 m/s (Irwin and Baker, 2005), see Fig. 10. Since it was not possible to model the entire tower at this scale, only the top portion was modeled. The measurement technique used was the HFPI method, applied to six rings of pressure taps at different levels, and this permitted direct comparison with identical measurements made on the 1:500 scale model at lower speeds. The same remarks that were made above with respect to use of time domain analysis in HFFB studies applies also to HFPI studies, only of course with HFPI studies higher order modes can be analysed as well as the fundamental mode. 5.4. Aeroelastic models

To check the final design of a super-tall building there are many benefits to undertaking a full aeroelastic model test. The full

Fig. 10. 1:50 scale model of top portion of Burj Dubai for high Reynolds number testing.

response can be measured, including higher order modes and including aeroelastic feedbacks effects such as aerodynamic damping. The tests can also be run at higher Reynolds number than the HFFB tests. For very tall slender towers the design of the aeroelastic model can often be simplified since these towers usually respond in a very similar manner to a simple cantilever. Thus the model stiffness can be incorporated into a single metal spine member, its cross-section varying with height so as to achieve the desired deflection shapes for the modes of vibration that need to be included in the study. The central photograph in Fig.1 shows the aeroelastic model of Burj Dubai, which was of this type, with some of the outer shell segments removed to show the spine inside. Measurement of bending moments at various heights can be made in a very straightforward manner using strain gauges on the spine. Fig. 11 illustrates the power spectrum of bending moment at the base of the aeroelastic model of Burj Dubai. It can be seen that the first three modes were modeled well and even the fourth mode response can be identified. Higher mode responses can contribute significantly to the wind loading and accelerations in the upper parts of super-tall buildings. Aeroelastic model testing is the most accurate type of test but is also more intricate than HFFB or HFPI tests and, for buildings with significant torsional response can become very complex indeed. For these reasons it is usually best to reserve aeroelastic model testing until the design has evolved to close to its final form.

5.5. Winds at ground and higher levels

It is normal to test for pedestrian level comfort around tall buildings and to assess the results against comfort criteria such as described in ASCE state of the art report on outdoor human comfort (ASCE, 2004). There are a number of massing features

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6. Concluding remarks

The age of the super-tall building, combined with a move towards increasingly novel shapes, has brought interesting new challenges for wind engineers. This paper has concentrated primarily on the tasks of determining and controlling the structural response to wind action. However, we are also in an age when interest in ‘‘green’’ buildings is rapidly increasing, brought on by concerns about global warming. One way a building can improve its ‘‘greenness’’ is to use less material in its structural systems and the techniques discussed here for refining knowledge of wind loads, and methods of reducing them, certainly help achieve this. However, the use of natural ventilation, double layered wall systems, and even integrated wind turbines are beginning to present yet further exciting challenges for wind engineers. Fig. 11. Response spectrum of base moment from an aeroelastic model of Burj Dubai.

Fig. 12. Model testing of solutions to improve wind comfort on terraces.

that can help reduce ground level speeds, see for example (Williams et al., 1999). For super-tall buildings the ground level wind problems can be amplified due to their height but many of the same solutions apply at ground level. A more challenging issue is the increasing desire to have balconies and terraces high up on the towers that can be used by occupants. In many cases it is difficult to satisfy the normal comfort criteria at these higher levels. For example, in a leisure area it would normally be desirable to have wind conditions suitable for sitting over 80% of the time. At upper level terraces this may simply not be achievable but perhaps 50% is achievable and may be satisfactory as far as the occupants are concerned. It is important for the owner and occupants that they be informed about how frequently these areas will be usable and be warned if unsafe conditions exist. Improved conditions on terraces and balconies can be achieved through testing of screens and different parapet designs such as illustrated in Fig. 12. These devices represent a compromise between having the open feeling that makes a terrace or balcony attractive and closing them in which, if carried too far defeats, the original purpose of these design features.

References ASCE, 2004. Outdoor Human Comfort and Its Assessment. In: Irwin, P.A. (Ed.), ASCE State of the Art Report. Prepared by a task group of the Aerodynamics Committee. American Society of Civil Engineers. ASCE 7, 2005. Minimum Design Loads on Buildings and Other Structures. American Society of Civil Engineers ASCE 7-05 Standard. ESDU, 1993. Strong Winds in the Atmospheric Boundary Layer. Part 2: Discrete Gust Speeds, Item 83045, Issued November 1983 with Amendments A and B April 1993. Engineering Sciences Data Unit, ESDU International, 27 Corsham Street, London N16UA. ENR, 2006. Engineering News Record. Harris, R.I., Deaves, D.M., 1981. The structure of strong winds, paper no. 4. In: Proceedings of the CIRIA Conference, London, 12–13 November, 1980, Construction Industry Research and Information Association, 6 Storey’s Gate, London SW1P 3AU. Irwin, P.A., Kochanski, W.W., 1995. Measurement of structural wind loads using the high frequency pressure integration method. In: Proceedings of ASCE Structures Congress, Boston, USA. Irwin, P.A., Breukelman, B., Williams, C.J., Hunter, M.A., 1998. Shaping and Orienting Tall Buildings for Wind. ASCE Structures Congress, San Francisco. Irwin, P.A., Breukelman, B., 2001. Recent applications of damping systems for wind response. In: Proceedings of the Council on Tall Buildings and Urban Habitat, World Congress, Melbourne, Australia. Irwin, P.A., Baker, W.F., 2005. The wind engineering of the Burj Dubai Tower. In: Proceedings of the Council on Tall Buildings and Urban Habitat Seventh World Congress, Renewing the Urban Landscape, New York, October 16–19. Irwin, P.A., 2005. Developing wind engineering techniques to optimize design and reduce risk. In: Biennial Scruton Lecture. UK Wind Engineering Society, The Institution of Civil Engineers, London UK, November 2. Written version is available at the UK Wind Engineering website at / www.ukwes.bham.ac.uk S. Qiu, X., Lepage, M., Sifton, V., Tang, V., Irwin, P., 2005. Extreme wind profiles in Persian Gulf region. In: Proceedings of the Sixth Asia-Pacific Conference on Wind Engineering, Seoul, Korea. Scruton, C., 1963. On the wind-excited oscillations of stacks, towers and masts. In: Proceedings of the First International Conference on Wind Effects on Buildings and Structures, Teddington, Middlesex, UK, June 26–28, pp. 798–819. Tschanz, T., 1982. The base balance technique and applications to dynamic wind loading of structures. Ph.D. Thesis, University of Western Ontario, London, Ontario, Canada. Williams, C.J., Wu, H., Waechter, W.F., Baker, H.A., 1999. Experiences with remedial solutions to control pedestrian wind problems. In: Proceedings of the 10th International Conference on Wind Engineering. Copenhagen, Balkema, 21–24 June, pp. 813–818. Xie, J., Irwin, P.A., 1998. Application of the force balance technique to a building complex. Journal of Wind Engineering and Industrial Aerodynamics 77&78, 579–590. Xie, J., To, A., 2005. Design-oriented wind engineering studies for CCTV new building. In: Proceedings of the Sixth Asia-Pacific Conference on Wind Engineering, Seoul, Korea. Xie, J., Haskett, T., Kala, S., Irwin, P., 2007. Review of rigid building model studies and their further improvements. In: Proceedings of the 12th International Conference on Wind Engineering, Cairns, Australia. Zdravkovich, M.M., 1982. Scruton number: a proposal. Journal of Wind Engineering and Industrial Aerodynamics 10, 263–265.

Wind Engineering Challenges of the New Generation of Super-tall Building

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