1 INTRODUCTION

1.1 Shamal winds

The Shamal wind, with its naming origins coming from the Arabic word for North, is a wind that blows over the Gulf Region and Iraq, predominantly during the summer months from late May to early July. These meso-scale events are a result of a number of climatic events and topographical fea-tures of the region. The heat low over Pakistan and Afghanistan, generated by monsoon circula-tion, creates a secondary low-pressure centre to the south of the Zagros Mountains in Iran. The interaction of this low pressure zone, featuring cyclonic (counter-clockwise) circulation, with the semi-permanent high pressure zone over the northern region of Saudi Arabia, which has non-cyclonic (clockwise) circulation, tends to result in an enhancement of the local flow fea-tures throughout the Arabian Gulf. This is further compounded by the topographical features of central Saudi Arabia, which focus this enhancement to low-level regions, particularly below 2,000m. The enhanced low level flow features are then subject to the local temperature effects within the region. One feature of the local temperature climate is that there is a significant difference in day and night temperatures. Typically, temperature decreases with height at the rate of ap-proximately 6.5oC per 1,000m during daylight hours. However, when the surface heating is es-sentially stopped, during hours of darkness, the air at the surface cools rapidly, resulting in a temperature inversion (where the temperature increases with height). This inversion inhibits the typical mixing of the troposphere that occurs during daylight hours, such that the overriding air layers become decoupled from the underlying air. This results in the forming of a nocturnal jet

The impact of Shamal winds on tall building design in the Gulf Region

L. Aurelius, V. Buttgereit & S. Cammelli BMT Fluid Mechanics, Teddington, Middlesex, United Kingdom

M. Zanina ENSICA (Ecole Nationale Superieure d'Ingenieurs de Constructions Aeronautiques), Toulouse, France

ABSTRACT: The nature and impact of wind loading on the built environment is a key consid-eration for the design of tall buildings. Until recently it has been typical practice that the vertical distribution of the full-scale wind profiles (wind shear and turbulence) modelled for boundary layer wind tunnel testing have focused on wind profiles that follow typical synoptic behaviour. However, the wind climate within the Gulf Region experiences an event known as the Shamal wind, where the vertical distribution of the wind profile significantly differs from typical synop-tic storm events.

The paper will discuss the differences between the Shamal winds and the standard synoptic wind profiles, and the impact these differences have on several critical design parameters, namely overall wind loads, wind pressure distributions and wind-induced building accelerations. As such, particular wind loading scenarios, where the impact of Shamal winds is considered critical, will be addressed. Keywords: Shamal Winds; Wind Tunnel Testing; Wind Climate; Wind Profiles; Wind Loads

Tall Buildings: Architectural and Structural Advances

Tall Buildings: Architectural and Structural Advances

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profile. It is the combination of this jet profile, with the enhanced low-level winds, that result in wind profiles that are significantly different to typical synoptic profiles. An example of a Shamal profile as recorded by Membery (1983) is shown in Figure 1.

Figure 1. Mean wind speed profile of a typical Shamal event recorded by on May 21st 1981, as detailed in Membery (1983) Of course, the atmospheric parameters described above can vary significantly depending on the time of occurrence. This was reported by Membery (1983) and the variation of Shamal pro-file shape and magnitude is shown in Figure 2.

Figure 2. Variations in the mean wind speed profiles of Shamal events as recorded in Membery (1983)

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1.2 Comparison between Shamal winds and synoptic wind profiles

At present, there is limited literature on Shamal wind profiles within the Arabian Gulf Region. Membery (1983) conducted an investigation into these events in Bahrain, using data measured by local pilots. The night-time occurrences of the Shamal winds were considered to be a form of nocturnal jet, with the height of the maximum wind speed being linked to the magnitude of the temperature inversion. The maximum recorded wind speed was approximately 30m/s at a height of approximately 350m, with the mode of the wind direction from where these events occurred being approximately north-north-west. Qiu et al (2005) examined surface wind records from Dubai and upper level balloon data from Abu Dhabi combined with numerical meso-scale modeling to study the wind profiles of various high speed wind events. The study indicated that for events that occurred at night-time the ratio of the mean wind speed at 600m height to that at 10m height (U600/U10) were in the range of 2.1 to 5.6, with an average of 3.1, indicating the nocturnal jet characteristics observed by Membery (1983). Day-time events indicated that the ratio U600/U10 was typically 1.6, and that these coincided with relatively high mean wind speeds at 10m height. However, the average ratio encompassing both night-time and day-time events was approximately 2.0. The mean wind profiles of typical synoptic events follow simple boundary layer theory. In general, information on the variation of mean wind speed with height, over particular terrain / topographic features, can be found within various wind codes, manuals and design guides. It is typical that for wind tunnel studies conducted by BMT Fluid Mechanics, this information is ob-tained through use of ESDU (Engineering Sciences Data Unit) Item 01008 (1993), which pro-vides wind profiles describing the variation of mean wind speed and turbulence intensity with height for a comprehensive range of terrain / topography. An example of a typical north-westerly boundary layer profile for a site located in the Gulf Region, is shown in Figure 3.

Figure 3. Typical boundary layer mean wind speed profile, referenced to a height of 600m From the current literature and research available, it appears that the mean wind speeds in Shamal events reaches a maximum at heights varying between 200m and 600m above the local ground level during evening or night-time events, whilst day-time events tend to resemble large-scale synoptic events. It should be noted that because of the large difference between ground and upper level wind speeds, particularly during the night-time events, the detection of strong upper level winds from ground recordings (typically from anemometers at 10m height) may be difficult.

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The other key parameter, on which the open literature is quite limited, is the turbulence inten-sity throughout the boundary layer under the impact of Shamal winds. Measurements of the tur-bulence length scales within convective downdrafts (thunderstorm downbursts), which have a similar mean velocity profile to Shamal winds, indicate that the level of turbulence is compara-ble to synoptic winds (Holmes (2006), Holmes et al (2007)). Thus a level of turbulence similar to what would occur in a synoptic profile was assumed for the tests. This is considered a fair as-sumption, as the turbulence at the lower levels is likely to be driven by the surface friction and the localised building morphology. In addition, very little research is currently available for the effect of the local terrain rough-ness on the boundary layer properties (mean wind speed and turbulence intensity) within the upper levels of the boundary layer for the Shamal wind climate.

2 EXPERIMENTAL SETUP

2.1 Modeling of Shamal wind profile characteristics

To study and investigate the impact of Shamal events on the response of tall buildings within the Gulf Region, a series of wind tunnel tests were conducted. These tests examined key loading responses that are considered significant to the design of tall buildings, namely, the overall base bending moments, the loading distributions, cladding pressures and wind-induced building ac-celerations. The building chosen to examine for the study was selected as it was considered typical, with regards to overall height and size of floor plan of tall building design within the Gulf Region. That is, with a height of the order of, or exceeding approximately 250m, and generally rectangu-lar in shape. The overall height of the structure is 290m, whilst the two main plan dimensions are approximately 50m and 26m. As reported by Membery (1983), depending on the time of occurrence and local temperature, the shape of the Shamal profile can vary significantly. Thus, the selection of the Shamal profile to model was left to what could best be considered a typical event.

2.1.1 Shamal Profile Rig Design

To achieve the jet-like characteristics within a boundary layer wind tunnel a number of novel approaches have been used in the past, such as flaps attached to either the underside of wind tunnel roof or placed in the free-stream flow, or additional fans placed on the wind tunnel floor. Though these designs have, in some respects, achieved what they have been designed to do, a number of drawbacks were highlighted, predominantly the difficulty in predicting the level of turbulence generated prior to the installation of the device. Thus it was proposed to look at historical cases where the control of velocity and turbulence profiles was paramount. The use of angled or partial mesh-like devices placed within wind tun-nels to generate inclined velocity profiles in laminar flow, along with the use of coarse grids to produce particular turbulence intensities in flow with a uniform velocity gradient, have both been extensively researched (Whitbread (1963, 1967), Vickery (1965)). A review of these stud-ies led to the decision to combine these two methods of controlling profile characteristics. It was also considered that the overall design of the rig selected for the simulation of the Shamal profile should also be versatile such as that alternate profiles could be achieved. As such a variable grid design was selected for installation in the wind tunnel, as shown in Figure 4.

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Figure 4. Setup of the rig for replicating the Shamal wind profile in the wind tunnel The span-wise slats were cut with square holes to form a grid, with characteristic dimensions such that a turbulence intensity of approximately 10~15% could be achieved at the top of the tower. The slats, being either 300mm wide or 400mm wide, could then be varied in their height above the wind tunnel floor as to adjust the profile such that a representative Shamal profile could be achieved. A series of tests were conducted to “tune” the arrangement of the slats, roughness elements along the tunnel floor and the two-dimensional barrier at the entrance to the test section, to achieve what was considered a typical Shamal profile. The comparison between the measured wind tunnel profile and a full-scale Shamal mean wind speed profile is shown in Figure 5.

Figure 5. Comparison between the simulated and full-scale Shamal mean wind speed profiles With regards to the synoptic wind climate, the modeling of typical boundary layer wind pro-files was achieved using an arrangement of roughness elements distributed over the floor of the wind tunnel and a two dimensional barrier placed at the entrance to the test section.

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2.1.2 Wind speed considerations It is standard practice that wind tunnel tests combine boundary layer profiles with extreme wind speeds from synoptic events, either specified by codes/local regulations or determined by means of extreme value analysis of reliable wind records. This is a relatively straightforward and well-established procedure currently employed by the majority of wind tunnel laboratories. However, when testing for a Shamal profile, the shape of the mean wind speed profile at any one time is very difficult to predict due to the large number of factors affecting this shape. This is clearly indicated by Membery (1983) where the shape of the mean velocity profile of the Shamal events recorded at different times of the year varies considerably (See Figure 2). The two key factors characterizing the shape of the Shamal profile are the height at which the mean wind speed peaks, and its magnitude. Thus, it was considered sensible to apply the considered typical mean wind speed profile (See Figure 1) as the mean wind speed profile for a range of typical return periods considered when examining the impact of wind loading. As there is limited data available on full-scale measure-ments of Shamal profiles to support meaningful statistical analysis, this was considered a fair assumption in the current study. A comparison of the mean wind speed profile of the typical Shamal event considered and synoptic events to be considered is shown in Figure 6. These are also compared to the strongest Shamal event recorded by Membery (1983) to illustrate that the synoptic profiles continue to “envelope” even the strongest Shamal events. Another parameter that is generally considered in the post-processing of wind tunnel meas-urements is the directionality of wind speed. The current research examined for the present paper indicates that the Shamal wind events typically occur for wind angles spanning from West to North. This presents a similar distribu-tion of synoptic events for the Gulf Region, where the winds are predominantly westerly to northerly. However, as there is currently limited data available on the directionality of the Shamal Cli-mate to support a robust statistical analysis, for the purposes of comparison between the two type of events, the analysis of the wind tunnel measured data was conducted without taking any wind directionality into account. It should be noted that for this research the synoptic wind speed selected for the loading de-sign (50-year return period wind speed) complies with the requirements of the Dubai Municipal-ity (38m/s for 3-second gust wind speed, 10m height over open terrain), and is similar to those stipulated by the regulatory authorities of the neighbouring countries.

Figure 6. Comparison between the full-scale Shamal and considered synoptic mean wind speed profiles for different return periods

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3 RESULTS

3.1 Cladding pressures The cladding pressures over the facades of the building are typically based on a 50-year return period wind speed. As discussed earlier, for the Shamal event this is simply the recorded typical Shamal profile as shown in Figure 1. With regards to the synoptic event, the 50-year return pe-riod basic wind speed is 22.7m/s (mean-hourly at 10m height over open country terrain, z0 = 0.03m). The peak 1-sec gust cladding pressures, for both the Shamal and synoptic events, have been presented as a ratio of the synoptic to the Shamal peak external 1-sec gust pressures. For a typi-cal façade this is presented in Figure 7.

3.2 Base Moments

The overall wind loads are based on the 50-year return period wind speeds as described above. Such as to compare the response between the two events (Shamal and synoptic) the results have been normalised by the maximum response. These values for base bending moments about the critical axis of tower are shown in Figure 8.

Figure 7. Peak-positive and peak-negative 1-second gust pressures ratios (synoptic pressure/shamal pres-sure)

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Figure 8. Variation of normalized base-bending moments with wind direction for the Shamal and synoptic events

Figure 9. Normalised floor-by-floor load distributions (Shear Force) for the Shamal and synoptic events

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3.3 Floor-by-floor loading distributions A comparison of the floor-by-floor loading distributions based on the 50-year return period wind speeds, for each of the two events is shown in Figure 9. Again these have been normalised by the maximum response between Shamal and synoptic.

3.4 Wind induced building accelerations The peak-combined accelerations at the highest occupied level of the tower have been deter-mined for a number of return periods, namely, 0.1-, 1-, 5- and 10-years. It should be noted that the accelerations vary with return period for the synoptic winds examined, due to the variation of wind speed, whilst for the Shamal event these remain constant as it has been assumed that the wind speed will not vary with return period. Thus the peak acceleration for the Shamal event was determined to be 6.2milli-g. For the synoptic events, the peak accelerations for the 0.1-, 1-, 5- and 10-year return periods were 3.2, 5.7, 9.9 and 12.4milli-g respectively.

4 DISCUSSION

4.1 Cladding Pressures As detailed in Figure 6, the cladding pressures are significantly higher when the building is ex-posed to the synoptic event, with the 1-sec gust pressures and suctions being approximately three times greater than design pressures measured during the Shamal event. This is due to the design wind speeds being significantly greater for the synoptic event, as clearly shown in Figure 6.

4.2 Base Moments

The base moments, as shown in Figure 8, are significantly higher (in some cases almost ten times as high) for the synoptic event as opposed to the Shamal event. Again, this is driven by the design wind speeds being significantly higher for the synoptic event. It is interesting to note that due to the higher design wind speeds, significant dynamic effects, such as vortex shedding (indicated by the “spikes” in the response), were shown as being prominent for the synoptic event, whereas these events do not occur for the Shamal event as the reduced frequency of the building is now outside of the range of this phenomena.

4.3 Floor-by-floor loading distributions

Figure 9 shows that, as expected, the distribution of loads over the height of the building is sig-nificantly higher for the synoptic event as opposed to the Shamal event. Again, this is driven by the design wind speeds being significantly higher for the synoptic event.

4.4 Building Accelerations

The peak-combined building accelerations at the highest occupied floor of the tower are gener-ally higher when the building is exposed to the synoptic events. However, as the wind speed of the Shamal event has been assumed to be constant over the range of return periods examined, for the lower duration return periods (0.1- and 1-year) the building accelerations are greater when the tower is exposed to the Shamal event. Though for the 0.1-year return period the accel-erations for the Shamal event are almost double those of the synoptic event, this is not consid-ered that significant as the magnitude of the accelerations at these return periods are inherently

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low due to the low velocities associated with them. Thus this effect is not considered significant to the design of the tower.

5 CONCLUSION

The impact of the Shamal wind profile on a tall building within the Gulf region has been exam-ined through wind tunnel modeling techniques. This required the adjustment of typical synoptic wind profile modeling. As expected, design parameters that are dependant on larger return periods, such as cladding pressures, base moments and floor-by-floor loading distributions, were significantly greater when assessed upon synoptic wind profiles as opposed to the Shamal wind profiles. This was driven by the significantly larger design wind speeds associated with synoptic events when compared to the Shamal events. However, it was interesting to note that for design parameters dependant on lower return pe-riods, namely building accelerations at low return periods (0.1- and 1-year), the peak combined accelerations at the top of the tower examined were greater for the Shamal events. This is essen-tially due to the assumption that the strength of the Shamal events is not-dependant on return pe-riod, unlike synoptic events. That aside, the building accelerations for return periods greater than 1-year were considerably higher for the synoptic events, and as such this climate is consid-ered of greater significance. This aside, one key outcome of the present research is that the current depth of knowledge on the Shamal climate is relatively limited, particularly with regards to the return period associated to these events, the effect of terrain surface roughness, and the distribution of turbulence inten-sity throughout the boundary layer. As such the authors expect to continue their research in these directions. In conclusion, the study has led the authors to agree that for the design of tall buildings within the Gulf region it is vital to examine the synoptic events as these are considered more critical to the design process with regards to wind loading.

6 ACKNOWLEDGEMENTS

The authors gratefully acknowledge Prof. Mike Graham of Imperial College, London, UK, for his advice and opinions during the course of the research.

7 REFERENCES

Engineering Sciences Data Unit, Item 01008, Computer program for wind speeds and turbulence proper-ties: flat or hilly sites in terrain with roughness changes, 1993

Holmes, J.D, Review of Extreme Thunderstorm and Shamal winds in the Arabian Gulf Region, Report prepared for BMT Fluid Mechanics, September, 2006

Holmes, J.D, Hangan, H.M, Schroeder, J.L, Letchford, C.W, Orwigc K.D, A forensic study of the Lub-bock-Reese downdraft of 2002, Part 1: Storm Characteristics, 12th International Conference on Wind Engineering, Cairns, Australia, 1-6 July 2007

Holmes, J.D, Hangan, H.M, Schroeder, J.L, Letchford, C.W, A forensic study of the Lubbock-Reese downdraft of 2002, Part 2: Running Mean Wind Speeds and Turbulence, 12th International Conference on Wind Engineering, Cairns, Australia, 1-6 July 2007

Membery, D.A 1983. Low-Level winds during the Gulf Shamal, Weather Vol 38: pp18-24 Qiu, X., Lepage, M., Sifton, V., Tang,V. & Irwin, P.A., Extreme Wind Profiles in the Persian Gulf Re-

gion, 6th Asia Pacific Conference on Wind Engineering, Seoul, Korea, 12-14 September, 2005 Vickery, B.J, On the flow behind a coarse grid and its use as a model of atmospheric turbulence in studies

related to wind loads in buildings, National Physical Laboratory Aero Report 1143, 1965 Whitbread, R.E, Model Simulation of Wind Effects on Structures, NPL International Conference on Wind

Effects on Buildings and Structures, Teddington, England, 1963

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Whitbread, R.E, An Investigation of the Aerodynamic Stability of a Model of the Proposed Tower Blocks for the World Trade Centre, New York, Part II – Further wind-tunnel studies relating mainly to the re-sponse of the tower blocks to turbulent winds, National Physical Laboratory, Teddington England, 1967

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