Advanced and Robust Analysis of Wind Effects
Advanced and Robust Analysis of Wind Effects
Williamsburg officially has a new tallest tower. The construction of Ten Grand Street also known as 260 Kent Avenue is visibly complete, with its impressive crystalline façade and two-legged structure proudly forming the first part of the redevelopment of the Domino Sugar Factory site. This mega project comprises a six-acre site which until now had been inaccessible to the community for over 160 years.
Ten Grand Street is a 45-story, 435-foot-tall tower designed by world-renowned architect COOKFOX for Two Trees Management. The tower features interlocking buildings with white precast concrete façade inspired by the molecular pattern of sugar crystals, giving recognition to the site’s history as a sugar factory.
This tower joins the SHoP Architects-designed and fully leased 325 Kent Avenue project which opened last year and is one extra piece in the puzzle in a bid to change the Brooklyn skyline and revitalise the riverside area into a thriving mixed-used community, connecting the neighbourhood’s industrial past with the present.
Windtech Consultants, headquartered in Sydney, have had the privilege of providing comprehensive wind engineering services on Ten Grand Street, with work being co-ordinated by their New York and London offices. Windtech Consultants conducted a Structural Loads and Building Motion Wind Tunnel Study using a modified Multi- High Frequency Pressure Integration (HFPI) Technique which Windtech’s technical team published in 2007. This technique enabled an accurate and definitive assessment of the wind loads on the structural frame as well as a set of equivalent-static load cases taking into consideration the effect of load transfer between dynamically independent substructures via the rigid link at Level 22. Key to the accuracy of the technique is the ability to provide a set of load cases for each interlocking building sub-structure using load transfer influence coefficients extracted from the FEA model developed by the New York, based structural consultants, RGCE. These influence coefficients provide clarity on how loads are transferred between the sub structures. Multi-substructure analysis is not new, however a key difference between the outputs provided by Windtech Consultants and other wind engineering firms, is the additional effort put into presenting only the critical load cases (i.e. in the case of Ten Grand Street, a total of 36 load cases across both sub-structures) governing the design rather than leave the structural consultant to sift through 300 load cases in the case where the less sophisticated technique is employed.
In addition, Windtech Consultants deployed a sophisticated directional method as part of the analysis for this tower which enables the highest possible accuracy and draws on the full set of directional extreme wind climate data. Other directional methods used with wind tunnel data can either under or overestimate the design loads. The building accelerations were also found to be within the prescribed comfort criteria which is generally expected for tall buildings that are structurally linked at high elevations. This is largely due to the lack of correlation of the peak wind actions on the sub-structures.
Windtech Consultants would like to congratulate the whole project team for delivering yet another fantastic project that will have a meaningful impact on the Brooklyn skyline and serve as one piece of the puzzle in this amazing redevelopment project for the lucky residents of Williamsburg
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Quality Assurance Manual for Wind Engineering Studies of Buildings
Dr Nicholas Truong to Present at 8th Annual Vertical Cities Conference
Extending the usability of outdoor spaces throughout the seasons is an essential consideration in any well-designed project. This is the case now more than ever as usable outdoor space promotes active and healthy lifestyles, improves the viability of outdoor businesses (i.e. restaurants) and improves the overall experience of users. The use of outdoor spaces for retail and restaurant use has recently become an important means of controlling the spread of contagions. It should also be noted that even moderate climates, neither hot or cold, can present challenges from a thermal comfort point of view which is a common misconception.
WINDTECH has undertaken detailed thermal comfort studies for a broad range of scenarios with experience on everything from small residential developments to large commercial developments (i.e. shopping malls). We use a range of metrics to assess thermal comfort including the Universal Thermal Comfort Index (UTCI) as indicated in Figure 1, which is one of the most widely recognised thermal comfort index globally. Our method of analysis accounts for solar load, humidity, wind, and the properties of building materials (i.e. glazing) throughout a development and results are presented in a way that enables accurate pinpointing of problem areas and effective identification of mitigation measures.
Figure 1. UTCI comfort criteria categories
The Universal Thermal Climate Index (UTCI) is a thermal comfort index used by COST to provide a measure of the perceived heat or cold stress of occupants. The UTCI is defined as the equivalent air temperature of an isothermal environment at a specific relative humidity, in which an occupant, wearing clothing to a quantitative level and at an activity level, experiences the same total heat loss from the skin in comparison to an occupant in the actual environment. Essentially, UTCI is a measurement of the perceived heat or cold stress of occupants and is calculated using the ambient temperature, mean radiant temperature, humidity, occupant activity, clothing level as well as airspeed, all measured at the occupant’s location. The model utilises heat balance principles to relate thermal comfort to ten key sensations.
CASE STUDY 1: Kings Walk Mall, Jeddah, KSA
A hybrid wind tunnel and computational fluid dynamics (CFD) approach was undertaken to assess the outdoor conditions within the mall. This method leverages the accuracy of wind tunnel testing and the additional physics capabilities of CFD, enabling a level of detail and accuracy which otherwise would not be possible.
Figure 2. Kings Walk Mall, Jeddah, KSA – Architectural Impression, Wind Tunnel Model, 3D CFD outdoor wind and thermal comfort model with glazing and wood boundary conditions highlighted and Outdoor wind environment CFD ground level contour of UTCI (Universal Thermal Climate Index) accounting for the solar load and glazing/material properties throughout the proposed development.
Initially, a wind tunnel model was fabricated and tested at Windtech’s boundary layer wind tunnel facilities to determine the approaching flow conditions to the site. This data was then inputted as boundary conditions and used to validate the CFD model, which ensured the accuracy of the computational domain. The computational simulation was then run incorporating buoyancy effects, thermal models, glazing properties (including solar transfer through the proposed transparent canopies), as well as humidity and solar properties to assess the occupant thermal comfort within the development. This assessment was undertaken for various scenarios including summer day, summer night, shoulder season day and shoulder season night. The results of the study accurately modelled the thermal comfort of the outdoor regions within the development, illustrating the thermal comfort of the occupants with respect to Wind, Occupant clothing, Occupant Activity, Humidity and Universal Thermal Comfort Index (UTCI). The results indicated higher levels of thermal stress levels were present under the transparent canopies during the summer season. Where heat build-up was present due to air flow stagnation. Through the introduction of mitigation options such as adjustment of the height the canopies to be above the roof level of the retail pods to act as passive vents, flushing the hot air as well as changes to their material properties. This enabled the end thermal comfort conditions be deemed appropriate for the occupants. It is through detailed CFD modelling of this level that appropriate mitigation measures can be formulated without compromising the architectural design intent.
CASE STUDY 2: Jakarta International Stadium, Indonesia
Figure 3. Jakarta International Stadium, Indonesia – 3D CFD outdoor/indoor wind and thermal comfort model with porous façade, ETFE transparent roof, seating and pitch materials and Internal Thermal Load from occupants for music event.
The previously described hybrid wind tunnel and computational fluid dynamics (CFD) approach was also applied to the Jakarta International Stadium in Indonesia. A detailed analysis of the solar transfer through the semi-transparent ETFE roof was carried out for each of the ambient wind and environmental conditions. Furthermore, an additional simulation was carried out to replicate the heating load from the attendees of a sold-out music event.
Figure 4. Jakarta International Stadium, Indonesia – External Wind Streamlines, plan view of interior occupied height Peak wind and UTCI and Peak Summer external solar irradiance.
WINDTECH’s unique approach of providing measurements from physical testing in the wind tunnel to form the accurate boundary conditions for further analysis using CFD is the main driver in our ability to provide a highly accurate multi-faceted solution. This then provides the appropriate tools during the design stage to identify problem areas and tailor solutions to mitigate adverse thermal comfort issues that could be experienced by end users, thereby avoiding the potential for expensive remedial measures post construction.
The results within the stadium indicated evaluated levels of thermal stress levels during the music event were present, but with the inclusion of slight adjustment to the transparent roofing system and additional façade porosity the end thermal comfort conditions were deemed appropriate for the occupants.
Hong Kong Buildings Department has published a new wind code on 30 September 2019 – the Code of Practice on Wind Effects in Hong Kong 2019 (COP-HK:2019), includes the following new features:
– Calculation of torsional and across-wind forces
– Load combination factors
– Consideration of sheltering effects
– Wind directionality factors
– Additional guidance on requirements for wind tunnel testing
There is a 12-month grace period for the transition from the 2004 edition to the COP-HK:2019.
Windtech Consultants is grateful to Henderson Land for the opportunity to facilitate the first successful submission and approval by the Hong Kong Buildings Department of the structural design based on a wind tunnel study adopting COP-HK:2019. This significant milestone was achieved for a proposed residential development at No. 2 Tat Cheong Street in Hong Kong. Windtech Consultants wishes to thank Henderson Land’s project and engineering team for their collaboration.
When super cyclone Amphan hit the vibrant city of Kolkata last week with wind speeds gusting up to 130km per hour (80mph), few could have imagined its destructive power. Just over a decade ago, the city was rocked by cyclone Aila and more recently by cyclone Bulbul, however most had suggested that Amphan was on another scale not seen in a lifetime. Some had likened the experience to that of an earthquake where buildings were swaying back and forth. This feeling was more pronounced for those living in high-rise developments.
“We felt as if the entire building was swaying. Initially, we thought that an earthquake was taking place even as the cyclone was raging outside. But then, we found out that it was because of the storm. It was really scary,” said Arpita Pal, who resides on the 10th floor of a high-rise building in east Kolkata.
As the city is left to pick up the pieces, many have reflected on how things could have been done better to brace for such a significant event. One of the talking points up until recently has been the effect that extreme winds such as the ones bought on by Amphan have on high-rise buildings both from a structural and cladding performance point. It is not uncommon for building owners and operators to study such impacts during the design stage in a bid to ensure the safety and/or comfort of end users and the general public. This is because wind impacts are generally seen as one of the fundamental inputs into the design of tall buildings, which when ignored can potentially result in a poor performing building or in the worst-case scenario a catastrophic failure leading to capital loss or even loss of life.
One such example of a well-engineered building is The 42, the brain child of a consortia of Mani Group, Sattva & Salarpuria , Alcove Realty and Diamond Group. Being the tallest and one of the most high-profile developments in Kolkata, the consortia with the guidance of J+W Consultants (Structural Engineers) was intent on creating a structural and cladding system that would withstand a 1 in 50 year extreme wind event with the usual margins of safety. During the design stage, a wind tunnel study was undertaken by Windtech Consultants using a scale model of the tower equipped with over 400 surface pressure sensors capable of measuring pressure at each location more than 2000 times a second. From these tests, the building designer was able to obtain an accurate distribution of pressure over the building envelope to help design the cladding elements and the fixtures that tie them back to the building. Factors taken into consideration included the regional wind climate and the wind flow interferences effects caused surrounding buildings/structures and the terrain. In addition to this, the loads on the super structure and the tower’s responses to these loads were also obtained to assist the structural engineer when tuning their structural system.
Due to the height, extremely slender form and the architectural constraints dictating the structural design, it was found that under serviceability wind conditions, the tower’s movement was predicted to be uncomfortable for its occupants. As this issue was identified at the design stage using accurate wind tunnel testing, it created an opportunity to investigate a range of possible solutions to mitigate this effect. Solutions such as building corner modification, removal of floor cluster, building tapering to confuse synchronous loads from increasing the magnitude of oscillation and/or significant modifications to the structural system were not well suited to this project. The solution chosen was to install an auxiliary damper. Building motion dampers work in much the same way as the shock absorbers on a car – however, instead of absorbing of impact of a pothole for the comfort of those driving the car, the building damper reduces the impact of wind vortices thereby improving the comfort of tenants in the building. Windtech designed and tested a prototype of the tuned liquid damper (TLD), then provided a detailed design, tuning and commissioning the liquid damper installation.
The developer of The 42 has commented that their tower was relatively unscathed in the aftermath of cyclone Amphan, which was sadly not the case for a number of low to mid-rise buildings and structures surrounding the development. As The 42 is by far the tallest and most slender building in the city. This just highlights the significant importance of designing for wind.
CTBUH Best Tall Building in Asia and Australasia Award CTBUH Urban Habitat Award
Tony Rofail presents at the 10th International Urban Design Conference
Creating outdoor spaces that are comfortable and fit for purpose is more important now than it has ever been. For us this means finding wind mitigation strategies that fit with the aesthetics and intent of a development and this can be one of the most critical aspects of the studies we undertake.
For this reason, we felt it would be useful to highlight some of the options for wind mitigation and perhaps in the process give you some ideas for your next project.
We often employ a combination of the following strategies to reduce the impact of wind effects and if you’d like to get our take on your project, we’d welcome you to reach out – we’re always happy to assist.
Natural Landscaping
As well as often being aesthetically pleasing, hedges and trees can mitigate horizontal wind acceleration at ground and elevated levels. It is also possible to mitigate downdraught from facades as well as wind acceleration around building corners.
Trees along the boundary of a space are good for mitigating direct exposure to wind as well as wind funnelling between buildings. They are unlikely to be suitable for downdraught from buildings.
A combination of large planters that break up wind flow at ground level with dense planting may be enough to reduce wind speeds for a given activity. This approach can be effective to reduce the impact of funnelling, for which a combination of shrubs and small trees should be used.
Trees and hedges do not always have to go in straight lines. Here is an example of a green tunnel, which could make for an extremely effective mitigation strategy for combatting downwash. Pergolas or green canopies could be equally effective, although care should be taken to make sure that foliage is dense and effective during the right season.
Balconies and Raised Terraces
Commonly found on balconies, but also outdoor terraces, balustrades are most effective for direct exposure to winds. For larger outdoor terraces such as this one, a combination of a balustrade and natural landscaping can be effective as balustrades are not suitable for reducing downwash.
Balcony edge screens can be used when an elevated terrace is relatively deep and subjected to strong wind being upwashed from the façade below. This does not work if the wind conditions are generated by the tower above the terrace.
End screens or architectural features such as this one can be incorporated to negate the impact of wind acceleration around the corner of a building. If done correctly it can create an interesting feature too.
Excellent for creating, sheltered, usable spaces inset balconies are often the most effective method for ensuring private balconies meet comfort criteria. By combining screens such as that shown in the image below with an inset balcony a space is created that is versatile and protected all year round.
Screening can be positioned between different apartment blocks to reduce the impact of side-streaming at elevated levels as well as reduce the impact of direct exposure to the wind. Positioning these midway screens correctly is key to ensuring the most comfortable space.
This treatment is very effective for situations where there is a venturi effect between two tall buildings or a street canyon effect. This idea can also be used to design an airlock for the main entrance to a building.
For balconies that are north facing and exposed, it is often worth considering where winter gardens or a combination of winter gardens and balconies might be a preferred alternative to standalone balconies.
Where screening might be unsightly and natural landscaping impossible due to other restraints, it might be worth considering a piece of artwork or sculpture. By picking something that has a porosity appropriate for wind mitigation, you could have a beautiful addition to your space that is functional too! This might be used to reduce wind funnelling between buildings.
Perhaps the most famous canopy in London is The Leadenhall Building. Canopies such as this may be required when flat facades create downdraughts that impact at ground level.
The location and size of the building will have a significant impact on the size and position of the canopy required. They are usually not suitable for direct exposure to wind, wind funnelling between buildings or wind acceleration around building corners.
Similar to canopies, but on a much smaller, more localised scale, overhang shading is often porous and has the combined benefit of sun shading. Useful for mitigating downdraught from building façades, it is less suitable for direct exposure.
This is a great solution for open spaces where seating is required, but the opportunity for natural landscaping is limited.
Screens such as this can mitigate direct exposure to wind and wind funnelling between buildings but are not suitable for downdraught from buildings.
Porous screening is good for mitigating direct exposure to wind and wind funnelling between buildings. By making the screens porous, it has been proven that the distance for which shelter is provided is increased (when compared to solid screens). However, screening is unlikely to be suitable for downdraught from buildings.
Building Shaping and Building Form
Getting the overall building massing right at an early stage can save significant headache later in design.
As a rule of thumb, a cluster of buildings can offer shelter to one another and push the windy areas to the edge of the cluster. Also, orienting the narrower side of a tall building to the prevailing wind direction helps to reduce downdraught forming.
Masterplan Studies using Computational Fluid Dynamics (CFD) are particularly effective at informing early stage design as it is relatively easy to move buildings around in the simulation and re-test.
Podiums are often effective at mitigating downdraught off a building façade. However, if the podium itself is then designated as a usable space, such as a garden or a seating area, care must be taken to ensure that this meets comfort criteria. Podiums are not suitable for reducing the impact of direct exposure or wind funnelling between buildings.
By recessing the corner of a building, it is possible to reduce the wind acceleration around the corner. This mitigation strategy is not suitable for direct exposure to wind, wind funnelling between buildings or downdraught.
Sharp corners create more acceleration than rounded ones. By rounding off corners, it may be possible to bring comfort levels within comfort criteria if corner acceleration is leading to exceedances.
CTBUH Best Tall Building in Asia and Australasia Award CTBUH Urban Habitat Award
Tony Rofail presents at the 10th International Urban Design Conference
Advanced Modelling for Comfort to Enhance the Experience of Shopping Centre Customers
Pedestrian Microclimate Testing in London
Wind Environment Effects Case Study
