Tall buildings in the Gulf Region are exposed to a wind climate that cannot always be described by conventional atmospheric boundary-layer wind profiles. One important regional phenomenon is the Shamal — a predominantly north to north-westerly wind that occurs across the Arabian Gulf and Iraq, particularly during the summer months.
For tall building design, the significance of a Shamal is not simply its wind speed. Of particular interest is the vertical distribution of wind speed through the atmosphere.
Why is a Shamal wind profile different?
Conventional synoptic wind profiles generally exhibit increasing mean wind speed with height in a manner broadly consistent with atmospheric boundary-layer theory.
Shamal events can behave quite differently.
During night-time conditions, rapid surface cooling can create a temperature inversion that inhibits normal atmospheric mixing. Combined with enhanced low-level winds, this can produce a nocturnal jet, with maximum wind speeds occurring well above ground rather than continually increasing with height.
Available observations examined in the study suggested that maximum Shamal wind speeds can occur at heights of approximately 200–600 m above ground level. Earlier measurements reported by Membery included a maximum of approximately 30 m/s at around 350 m height.
For very tall buildings, this distinction is important: the building may extend directly through the region of maximum wind speed.
Replicating Shamal winds in a wind tunnel
The study investigated the effect using a 290 m tall building, approximately 50 m × 26 m in plan. Wind tunnel testing considered base bending moments, load distributions, façade pressures and wind-induced building accelerations.
A conventional boundary-layer simulation was not sufficient to reproduce the jet-shaped Shamal profile. A purpose-designed variable grid system was therefore developed to control the velocity distribution and turbulence characteristics within the wind tunnel.
The rig used adjustable slatted grids together with floor roughness and a barrier at the entrance to the test section. The resulting wind-tunnel profile was tuned against a representative full-scale Shamal profile.
This would be an excellent place to use Figure 5 from the paper. The chart on page 5 gives a very clear visual comparison between the measured full-scale Shamal profile and the wind-tunnel simulation.
Does the Shamal govern structural wind loads?
Interestingly, the study found that the unusual Shamal profile did not necessarily make it the governing wind condition.
For the design conditions considered, conventional synoptic winds produced substantially greater façade pressures. Peak one-second gust pressures and suctions were approximately three times greater under the synoptic design event than under the Shamal event.
The difference in overall structural loading was even more pronounced. Base bending moments were significantly greater under the synoptic event — in some cases approaching ten times those produced by the Shamal case.
Floor-by-floor loading similarly remained greater for the synoptic design event.
What about occupant comfort?
The acceleration results revealed a more interesting distinction.
For the Shamal simulation, the peak acceleration at the highest occupied level was 6.2 milli-g. The corresponding synoptic results were:
0.1-year: 3.2 milli-g
1-year: 5.7 milli-g
5-year: 9.9 milli-g
10-year: 12.4 milli-g.
Consequently, the Shamal produced greater predicted accelerations than the synoptic wind at the lower return periods examined, although the study concluded that this was not particularly significant because the absolute accelerations remained relatively low. At return periods greater than one year, the synoptic climate again became more significant.
What does this mean for tall building design?
The study ultimately concluded that synoptic winds remained the more critical design condition for the tall building examined, particularly for façade pressures, structural loads and higher-return-period accelerations.
However, the research also highlighted an important limitation in the available understanding of Shamal winds. At the time of the study, significant uncertainties remained regarding their return periods, the influence of terrain roughness and the distribution of turbulence intensity through the atmospheric boundary layer.
For tall and super-tall buildings in the Gulf, this reinforces a broader principle: regional wind phenomena should be understood before assuming that conventional synoptic profiles adequately represent every relevant design condition.
Publication:
Leighton Aurelius, V. Buttgereit, S. Cammelli and M. Zanina.
Related Research
A Detailed Review of Site-Specific Wind Speeds for Tall Building Design in Dubai
Understanding Shamal winds forms part of the wider challenge of defining an appropriate design wind climate for tall buildings in the Gulf Region.
