HFFB studies are often conducted at the schematic-design (SD) stage and sometimes as early as the feasibility/concept stage, to allow detailed, shape-specific wind-induced response to be directly fed into the structural-design process.
SPI studies often follow during detailed design (DD) to refine the prediction made during SD. Aeroelastic studies are commonly used in tall building design (at DD stage) when the structural response is strongly governed by vortex-shedding approaching ‘resonance’ or when a direct measurement of the contribution to the total damping coming from the wind-structure interaction (‘aerodynamic damping’) is required.
9.4 Procuring a wind
The aim of this section is to help designers gather the information required by windengineering consultant
consultants to conduct wind-engineering studies, and ensure the outcome is sound and in line with best industry practice.9.4.1 Atmospheric boundary
The term ‘atmospheric boundary layer’ refers to the lowest portion of the troposphere-layer profile simulation
(usually 2-3 km above the surface of the Earth) where the effects of surface roughness as well as local topography control the vertical distribution/profile of both mean wind speed and turbulence intensity (see Figure 9.5).Free atmosphere
Atmospheric boundary layer Gradient height
Zg
Vg Figure 9.5
Schematic representation of the atmospheric boundary layer.
9 Wind engineering
A direct comparison over the entire height of the tall building between the selected characteristic wind exposures for the project site (identified using wind models such as the one proposed by Harris and Deaves(1981)[22]) and the mean wind speed, intensity of turbulence and appropriate gust wind speed profiles measured in the wind tunnel should be exhaustively documented by wind-engineering consultants.
Ahead of testing, the wind-tunnel laboratory should provide the design team with documentation showing that the measured mean wind speed/ turbulence intensity and turbulence length scale in the wind tunnel are respectively within 10 % and a factor of 2 from the prediction of the above theoretical model.
9.4.2 Wind-tunnel models
The geometrical scales typically employed for wind-tunnel studies ranges from 1:200 to 1:500 and the obstruction of the wind-tunnel-test section caused by the presence of the model and its surroundings should be kept to a minimum; ideally in the region of 5 % and no greater than 10 %.The area surrounding the project site – typically to a radius of 500 m – should normally be modelled (see Figure 9.6). Significant buildings outside this area may need to be included as part of the surrounds on selected wind angles. Different surrounding scenarios, such as existing site conditions, proposed/future/consented surrounds and masterplan/project phasing, should be considered as part of the studies. Should the tall building be part of a large masterplan of which no sufficient details about the phase of construction are available investigation of the structure in an ‘isolated’ condition is recommended.
Figure 9.6 Wind tunnel surround model.
Wind engineering 9
Electronic information necessary to construct surround models includes:
Survey maps (typically .dwg/.dxf format)
Building heights/number of storeys
Photos from site survey (when available)
Site plan/masterplan drawings.
To design and construct HFFB, pressure and aeroelastic models - floor plans, elevations and sections of the proposed building (typically .dwg/.dxf format) or full 3D surface models (typically 3D CAD/Rhino format) are additionally required. For aeroelastic models, a sufficiently settled set of structural properties is also necessary (see also Chapter 13).
With regard to wind-tunnel pressure models, pressure sensors designed by wind engineering consultants should be issued to designers for review and approval prior to wind tunnel testing. For aeroelastic wind-tunnel models, a direct comparison between the selected full-scale structural frequencies, mode shapes and inertial properties with those directly measured on the aeroelastic wind-tunnel model, should be exhaustively documented by wind-engineering consultants.
In general, in the design of HFFB, pressure and aeroelastic models, great care should be taken by wind-engineering consultants to minimise and control scale effects related to the geometry of the building, which could lead to results being sensitive to the wind- tunnel speed. The wind-tunnel laboratory should demonstrate that quantities such as mean aerodynamic force coefficients (drag and lift) as well as the Strouhal number are stable across a wide range of wind-tunnel speeds.
9.4.3 Wind tunnel studies
The measured frequency of HFFB, as well as the range of wind speed operated in the wind tunnel during the tests, should be documented by wind-engineering consultants.HFFB employed by wind engineering specialists needs to provide simultaneous measurements of wind base shears, wind base over-turning moments and torsion.
The measurements for wind loading studies (HFFB, SPI or aeroelastic) should be taken around the clock at a minimum of 10° intervals. Intermediate measurements at finer intervals should be taken to capture the wind-loading peak response.
The post-processing of the measured time-histories of wind base loads (HFFB) or external wind pressures (SPI) requires information on the structural properties of the building. Provided by structural engineers, it should include mass, its in-plane eccentricity and mass moment-of-inertia distribution along the height of the tall building, structural frequencies and associated mode shapes for the fundamental and – if required – higher modes of vibration, as well as assumptions on structural damping levels to be considered.
9 Wind engineering
The directional variation of wind base loads including shears, bending moments and torsion for foundations design purposes – typically calculated for 50-year-return period design wind speeds or higher – should be presented both in terms of mean, peak static (including the ‘broad-band’ component) and peak dynamic (including the
‘narrow-band’ component).
To assist the design of the super-structure, wind loads should be presented in a floor-by-floor format. The directional variation of wind-induced peak accelerations is typically calculated for 1- and 10-year-return-period design wind speeds. The results need to be compared against preferably frequency-dependent criteria such as the ISO 10137-2007 (ISO 2007) (see also Chapter 13).
When required, wind-engineering consultants, in agreement with designers, can provide a range of sensitivities analyses to cover uncertainties of structural parameters such as natural frequencies, structural damping and inertial properties.
When HFFB, SPI and aeroelastic studies are performed on tall buildings, direct comparison between the different techniques should be provided by wind-engineering consultants.
For additional information on the subject of wind-tunnel testing, please refer to ASCE Manual of Practice No.67 for Wind Tunnel Studies (ASCE 1998)[23] and the AWES-QAM-1-2001 Quality Assurance Manual (AWES, 2001)[24].
Table 9.1 shows indicative budget fees and timescales for wind-engineering studies on a typical single-tower tall-building project.
Type of study Indicative budget fees1 in
USD Timescales in weeks
Wind microclimate 15-20k 2-3
Wind loading 20-25k 3-4
Cladding pressure 20-30k 3-4
Aeroelastic 50k+ 5-6
9.4.4 Computational
Computational fluid dynamics (CFD) is used in engineering applications such asfluid dynamics
aeronautics and investigation of internal flows but external flows around buildings in complex urban environments are far from being streamlined and are dominated by separation phenomena.The correct mathematical modelling of turbulence driving the dynamic response of a given structure of this type of flow regime is still subject to debate among the research community, even for fairly simple geometrical configurations. It is likely to remain
Note
Wind engineering 9
impractical to model turbulence at all the length-scales necessary to model separated wind-flow behaviour reliably for some decades.
Although in some cases apparently good results may be achieved, in others it is clear that they have not. Large eddy-simulation-modelling developments have more potential than other CFD methods at present but require more computing power than is typically available on a commercial basis.
The use of CFD is not recommended without full-scale or model-scale validation of key results.
Wind-tunnel testing does not suffer from the above-mentioned drawbacks and is a practical (with time-scale advantages) and well-proven approach for which the methodology has reached consensus among both the scientific and engineering community.
10 Seismic engineering
10. Seismic engineering
Seismic engineering encompasses the concept, analysis, design and detailing of structures, structural elements and non-structural building elements to withstand seismic events of varying intensity and frequency.
Characteristics of a tall building that make its response to seismic movements unique include:
A fundamental, translational period of vibration significantly in excess of two seconds.
Significant mass participation and lateral response in higher modes of vibration.
A seismic-force-resistance system with a slender aspect ratio such that significant portions of the lateral drift are due to the axial deformation of the walls and/or columns as compared to shearing deformation of the frames and walls.
Traditional codes which use an elastic response analysis with force reduction factor R are not suitable for the seismic design of high-rise buildings. They do not deal with the non-linear behaviour of tall buildings, whereby several modes of vibration contribute significantly to the seismic response of the structure.
The aim of this chapter is to provide guidance for designers of tall buildings in seismically active regions.
10.1 Risk and code derivation
Allowance for potential intensity of seismic events used in building design is determined from a statistical assessment of historical data and accepted levels of risk.These statistical results are used to produce tables and maps of design ground accelerations, known as seismic hazard maps, for use in seismic analysis. In the case of tall buildings, it is usual to carry out specific risk/hazard analysis, including the characterisation of specific seismic input, such as accelerograms and seismic displacements which come from different sources.
For each separate, associated event-return period, there is a correspondingly different expectation for the performance of the structure and this forms the basis of a performance-based design approach appropriate to tall buildings.
Eurocode 8 (EC8), as well as the Council on Tall Buildings and Urban Habitat (CTBUH) [1]
in its Recommendations for the Seismic Design of High-rise Buildings, promote such an approach with their requirement to meet two or more separate performance criteria.