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Ten questions concerning wind effects on supertall residential buildings
Journal Pre-proof Ten Questions Concerning Wind Effects on Supertall Residential Buildings Leighton S. Cochran PII:
S0360-1323(19)30790-5
DOI:
https://doi.org/10.1016/j.buildenv.2019.106578
Reference:
BAE 106578
To appear in:
Building and Environment
Received Date: 7 October 2019 Revised Date:
27 November 2019
Accepted Date: 27 November 2019
Please cite this article as: Cochran LS, Ten Questions Concerning Wind Effects on Supertall Residential Buildings, Building and Environment, https://doi.org/10.1016/j.buildenv.2019.106578. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Ten Questions Concerning Wind Effects on Supertall Residential Buildings Leighton S. Cochran MEL Consultants, 53 Saint Pauls Terrace, Suite 16, Ground Floor Spring Hill, 4000, Queensland, Australia. +61-(0)408-007-624, [email protected].
ABSTRACT Historically the taller buildings in our cities have been commercial or office buildings. In part, the massaging of national, corporate, or individual ego may have been a reason behind this trend for most of the twentieth century but, of course, the financial case was actually compelling too, as the market for CBD living was traditionally limited. However, more recently the trend to higher population densities close to the centre of many cities has resulted in new tall buildings having a dominant residential nature; mixed use, hotel/apartment, or pure apartment. From a wind-engineering perspective this move to homes in the sky introduces some new opportunities and sources of design angst, neither of which should be ignored. What follows is a discussion of issues noted over the last thirty years that may allow modern designers to learn from previous choices. Keywords: Supertall, residential buildings, super skinny, wind engineering, balconies, and top-floor accelerations. 1. Introduction A relatively recent trend in tall, supertall (>300 m) and megatall (>600 m) building design has been the move from commercial use to residential use. This introduces new or different wind-oriented design considerations for many of the project design consultants; some of whom may be unaware of the questions to ask [1, 2] and preferring a “business as usual” scenario as that has been professionally adequate in the past. Items as varied as the internal pressure changes generated by operable highelevation windows to the wave action present in rooftop swimming pools, all require a rethinking of standard design approaches. A related tall building trend is the appearance of “super-skinny” buildings that may, or may not, be very tall in an absolute sense but have a large aspect ratio (height to smaller width). This new subset of tall-building design shares many traditional tall-building attributes, while having some unique problems in its own right. 1.1 Residential trends Most of the design issues presented here have been a consequence of the supertall building population now containing many residential buildings. However, residential buildings shorter than this arbitrary 300-m height definition (CTBUH1) are not immune to the same design considerations. Hence, these thoughts may help residential building designers for the more common heights in the thirty to seventy storey range; depending on exposure, local wind climate, orientation, massing, facade sealing and structural asymmetry. Any of these attributes singularly has the potential to introduce unexpected wind-engineering issues for the design team to consider; several of them on the one project can result in a very real operational headache for the owners. 1.2 Approach The questions below all pertain to recent tall building projects (often seen on multiple projects) and the answers presented here are derived from a combination of physical wind-tunnel 1
Council on Tall Buildings and Urban Habitat
model studies, comprehensive data analysis, and simple experiential opinion. The last will be highlighted or be self-evident so that the reader does not confuse defensible research with opinion based on multiple project inputs from various design teams. It is hoped that the topics chosen will stimulate some in-house discussion in both the design and research communities. 2. Ten Questions (and Answers) Concerning Wind Effects on Tall Residential Buildings 2.1. What can be done in tall residential building design to limit perceived motion? Answer: Tall and supertall buildings frequently have the structural design guided by upper floor responses (accelerations). Unwanted motion perception in the upper third of the residential building should be avoided for reputable and legal reasons. As a New York architectural client once said to me, “if they can afford a penthouse then they can afford a competent lawyer”. Thoughtful architectural massing over the height can be key to a good design (orientation, varying the shape with height, corner design, elevator and shear wall location, twisting geometry, architectural penetrations, etc.) if one wishes to avoid supplemental damping systems (tuned mass dampers, tuned liquid column dampers, distributed elastomeric or friction dampers, etc.). The criteria available for designers to assess the acceptability and/or perception thresholds to tall building motion are discussed in some depth in two recent publications from ASCE [3] and ATC [4]. Some of the criteria for assessing upper-floor building accelerations are more stringent for residential buildings and less demanding for commercial or office buildings. The motivation for this distinction is not immediately clear, but some have written of owners preferring less perceptible motion in the home environment as opposed to the workplace. This seems as plausible a rationale as any. As such, the recent surge in tall residential building construction has focused the design team on this comfort condition for “homes in the sky”. One key attribute of the human perception of horizontal motion in tall buildings is the frequency dependency which has been long understood [5,6,7,8], with more recent research in [9,10]. Early work in the 1970s on building motion may be found in [11,12,13], while ASCE publication [3] provides an excellent historic overview. That said, the fundamental natural frequency of any tall building is generally below 1.5 Hz and typically well below 1 Hz; with supertall buildings often less than 0.1 Hz. At these frequencies the ability of humans to perceive horizontal accelerations generally diminishes as the frequency falls and, so, supertall and megatall buildings are somewhat more forgiving to design for motion perception. This can mean that the structural engineer may have more difficulty in designing for upper-floor accelerations of buildings in the 150-250 m range than, say, in the 400-700 m range. Consequently the use of auxiliary mechanical damping is apparent in buildings with a more modest height due to other architectural choices being fixed (shape, orientation, structure, etc.) prior to any wind-engineering influence. When this observation is combined with the mandatory nature of wind-tunnel studies for the taller designs (with consequent aerodynamic influence on the architecture; unless ignored) damping systems are not as common as one may expect in the supertall and megatall buildings. The architectural and structural engineering teams need to work together to avoid the additional acceleration consideration of a strong torsional response. Human perception of torsional motion can be enhanced by visual cues like relative mullion motion (facade verticals seen against outside distant structure). Depending on the modal complexity, twisting about the building’s vertical axis can add considerably to the perceived acceleration magnitudes. The cleanest way to fend off this problem is to keep the centre of stiffness (also called centre of rigidity) as close as possible to the centre of mass on typical floors up the building height. In many cases a symmetry about both horizontal axes with the structural design is an initial first step; although, obviously, not essential. The location of major structural components, like the lift core, stairwells and shearwalls, is key to controlling torsional response. Additionally, the aerodynamic shape will drive the aerodynamic torsional input; long rectangular or crescent shaped floor plates, for example, create strong torsioninducing pressure fields while circular shapes do not. A passing thought for tall and supertall building owners and designers to consider is the psychological preparation of the residents to the fact that their new home is designed to move and that some segments of the populations are more sensitive to that motion than others. By informing new owners that they may sense motion could prepare them and alleviate concern — of course it should
not be a frequent perception in a good design, but the rare motion in strong winds should not induce worry. Education within the population of a new building may be wise. 2.2 Why are high-aspect-ratio buildings (super skinny) becoming more common? Answer: In recent years the appearance of very slender buildings with aspect ratios in the range of 12:1 to 17:1 have become a new phenomenon (Figure 1). This evolution to more slender structures has resulted in the crosswind response gaining more importance in the structural design process as the dynamic resonant component of the wind load increases. For a given building shape the more slender design (with a lower natural frequency) has a design point higher on the crosswind response spectrum than its stockier brethren. In part these slender designs are a response to smaller plot sizes being available in city environs, but there are also some technical aspects to tall building design that have been necessary for this evolution. Improvements in structural analysis packages along with cost-effective supplemental damping systems have allowed the upper-level building accelerations to be understood and mitigated enough for successful residential slender building design. In the larger very slender buildings, such as New York City’s famous 432 Park Avenue (15:1), a traditional Tuned Mass Figure 1. High aspect ratio residential projects are becoming Damper (TMD) is used (as well as aerodynamic modifications in more common in cities with small the form of multiple open floors and additional floor mass at land parcels becoming the norm. upper levels). This TMD is a large mass at the top of the building This building in Flinders Street, which moves in a pendulum motion to counteract the building Melbourne, Australia is 90 m high motion. This concept has been an essential feature of the and 6 m wide (15:1). mechanical engineering profession for generations, but in a scaled-up form it is now occasionally applied to civil engineering structures. In some more modestly sized slender buildings with the same aspect ratio, such as the Phoenix Apartments (Figure 1), the use of less expensive Tuned Liquid Column Dampers (TLCD) are needed to control motion. These TLCDs are effectively giant rooftop U-tubes filled with water, allowing large masses of water to oscillate from one riser to another over several metres at the natural frequency of the building. However, even this technology has its limitations. The 90-m building in Figure 1 was originally designed to be 120 m tall (an aspect ratio of 20:1), but control of upper-floor motions contributed to the slightly shorter building shown here. The parking of residents’ cars in these very slender buildings becomes a problem when the width is insufficient for ramps and turnaround areas. The availability of modern robotic car stacking systems, that do not require drivable ramps, are a solution that allows such slender designs to be realistically pursued. Obviously this mechanical solution takes up less space, but it will be an ongoing maintenance cost for the building owners. 2.3. How do operable windows at elevation impact design of tall residential buildings? Answer: Open windows and balcony doors in tall residential buildings, during a strong wind event, will modify the internal pressure (positive or negative, depending on opening location relative to the wind) within a unit and are likely to result in a variation in pressure between units on a given floor. Thus, potentially large differential-pressure partition loads (commonly ignored, or assumed very small, in nominally “sealed” office building design) can be expected between apartments. Typically the lightweight walls within an apartment do not experience this type of substantial differential pressure loading as the gap (commonly in the 15 to 20 mm range) under the internal doors tends to equilibrate the pressure across walls within an apartment. However, between apartments the front door from the apartment to the lift lobby is usually better sealed, producing less pressure connectivity (in some designs local HVAC connections can mitigate this issue — depending on fire
engineering constraints). Additionally, an increase in the net facade pressures elsewhere on that floor can be expected with open windows and the resulting internal pressure augmentation (e.g., a negative internal adding to a positive external, or a positive internal adding to a negative external). This phenomenon may result in other pressure related problems such as door operation difficulty, elevator operational problems, fire stair concerns, and acoustic annoyance for residents. A lack of consideration of increased internal pressures from façade openings at high elevation has produced a variety of partition wall “failures” between residential apartments. The light-weight metal stud walls often used between apartments have been observed by the author to cause a range of problems for owners and developers. The failure may manifest itself as a separation of the partition wall from the ceiling; ranging from simple plaster cracking to more extreme wall movement and total separation or collapse. In one case the metal studs making up the partition wall were moved repeatedly by the dynamically changing differential pressures, resulting in metal-on-metal noise so annoying as to force the owners to move out of the apartments while the problem was rectified. Once the design issue is accepted by the design/construction team, the successful solutions typically include a mixture of stronger partition wall choices (only a modest cost impact), a variety of self-closing window and door ideas (e.g., positive pressure “snapshut” windows, inclined-track sliding balcony doors, etc.), to intrusive technologies demanding that the owner close the windows before leaving the apartment. One developer provided a system that would not allow the resident to lock the front door of the apartment, until all the windows were closed! This increased internal pressure phenomenon in tall building design may also contribute to other livability concerns in a residential building. When combined with other attributes, like orientation of the building relative to dominant winds and, say, badly sealed rooftop lift motor rooms, pressure differences via operable windows may yield strong flows in the elevator shaft. On rare occasions, when a variety of design attributes align, this may impact the operation of ground floor sliding entry doors, elevator doors, and/or generate strong flow induced noise through lift lobby areas (link to 2 Mb video). Commonly many of these problems have at least a partial cause in the increased internal pressure via some façade opening (apartment windows, undamped intake/exhaust louvres, natural-ventilation design features, porous garage doors, leaky lift motor rooms, etc.) that allows the pressure differential to adversely impact door operation or generate noise. 2.4. Where should kitchen and bathroom exhausts exit the building? Answer: While many designers prefer to install these exhausts locally at an adjacent external wall or balcony soffit the resulting system rarely has a fan of sufficient power to overcome the common external positive wind pressures (say, weekly or monthly recurrence interval winds), or have a back-flow damper, which results in pressure-related problems like apartment odours and noise. Additionally, there is often a substantial horizontal path for the tubing (commonly flexible ribbed piping that could accumulate kitchen grease, for example) from the extractor fan to the apartment wall, which has been observed to exceed the recommended length by the manufacturer. Such a design when exposed to a very common and modest breeze at elevation will experience mean and peak external wall pressures that far exceed any pressure magnitudes generated by the extractor fan within the exhaust tubing system. More importantly a long horizontal connection (greater than 5 m is not uncommon) in the ceiling space may accumulate kitchen greases/oils and become a fire hazard. Obviously, the motivation for the same-level exhaust approach is cost. The older proven approach of a common stack to a roof-top exhaust will consume some salable internal building space over the entire height of the building, but this is usually the better solution (opinion) since the rooftop extractor fan will never be competing with a positive pressure. The rooftop stack will always have a negative pressure over the exit when the wind blows and, in the worst case, will have no external pressure on a still day. Obviously this observation is not unique to tall residential buildings, but the motivation to
design kitchen and bathroom exhausts out the side of the apartment becomes more enticing in taller designs. 2.5. Are balconies an appropriate design feature for tall buildings? Answer: It is well understood that an increase in elevation will generally attract Figure 2. An example of balconies contained entirely behind the facade alignment and near the centre of the higher wind speeds around a tall building. That building face. They may be architecturally uninteresting, said, a good choice of balcony design, location but wind conditions are typically not a problem. on the building plan, combined with some selected local wind protection, can result in balconies that are useable spaces over great heights. Investigating the wind conditions with a physical model in a boundary-layer wind tunnel, using a hot-wire anemometer, indicates the influence of local geometry and suggests some general guidance for designers. For example, balconies at the building corner are generally much less usable than those centrally located on the building face. Balconies placed within the façade line (Figure 2) and centrally located on the face may be expected to be calmer than those protruding beyond the building face or those at a corner. An interesting recent trend with tall residential buildings is to have the balcony or terrace set back behind the façade line, but to have the opening smaller than the terrace edge in such a way that the façade protects part of the balcony (glazing above, to one or both sides, and the balustrade below the opening) with a modest rectangular opening to the atmosphere; often called a “letterbox balcony”. As a consequence the occupant is likely to be able to find a calm location within the larger balcony volume even at elevation on a windy day. It is interesting to note that with some curved building plans this small-opening-big-volume balcony design can have the unexpected attribute of a pulsating pressure within the volume when a strong wind blows close to parallel to the façade line (generated by a relatively stable rotating flow within the balcony volume). Initial consideration would suggest this could be a Helmholtz resonance, but wind-tunnel studies showed a frequency dependence of the pressure pulse with velocity (a Strouhal type response) and, conversely, no frequency dependence with facade opening area changes. The Helmholtz frequency has no velocity dependence [14] and should vary with balcony volume and/or facade opening size. The importance of understanding the mechanism is that the solution design choices are guided by the physics. For example since the response has a velocity dependence, if the apartment balcony glazing system can be stiffened sufficiently then the velocity to cause a resonant response can approach ultimate limit state (ULS) conditions; rather than serviceability limit state (SLS) conditions. Without mitigation this can severely rattle the doors and glazing of the apartment and be very unnerving for the owners; especially when the pressure pulse frequency closely matches the natural frequency of the glazing/frame structure. In the case of more exposed balconies or large terraces at elevation higher glazed balustrades and, perhaps, fixed or moveable porous/louvered screens will be needed to keep the outdoor space useable for medium to long-term activities. Without sensible design features like this not only will the balcony be of little value to the occupant, but there is a very real risk of furniture being blown off the balcony with potentially horrible consequences. Lightweight chairs and glass tabletops are common objects that are removed from these outdoor spaces by modest to strong winds. 2.6. How do winds altered by future tall buildings impact the existing proximate neighbours? Answer: As a general guide most city centres become aerodynamically taller, denser, and more complex over time and, as such, the expectation is that the addition of a major nearby building to the existing cityscape would typically yield altered wind loads on the existing, usually shorter, buildings that are similar in magnitude or reduced somewhat due to shielding (opinion). There are limited data on this as most owners do not want to know if their new building adversely impacts nearby buildings. Of course, there are exceptions to this generalization that come from a small number of wind-tunnel studies that have revealed deleterious impacts caused by the new building shape, the spacing between buildings, and/or the changed importance of local wind rose via wind flow variation with the new massing in place. The topic of interference effects on the structural wind loads in a simplified two-building environment is quite a well studied area going back to the 1980s [15], but
the complexity within a many-building cityscape is less well understood without multiple “what-if” wind-tunnel studies and those are unlikely to be performed in a litigious real estate environment. However, the addition of new buildings to an existing cityscape is, in at least one jurisdiction, a required area of investigation for peak cladding pressures. In the downtown of Singapore, when a wind-tunnel study is performed for design cladding pressures the wind-engineering laboratory is required to take some sample pressure measurements on the nearby existing buildings with, and without, the new building in place. Having gone through this type of investigation for several Singaporean tall buildings the trend for cladding pressures is that a new tall building in the mix very rarely makes the proximate peak cladding pressures larger in magnitude — no change or a reduction is by far the most common result. This reinforces the intuitive expectation of increased shielding or wind protection due to CBD growth; especially away from the edges of the downtown cityscape. It should be noted that structural design wind loads and cladding pressures are extreme events (ultimate limit state, ULS, in engineering parlance) and the apparent lack of structural or cladding failures as cities grow and change aerodynamically may well be due to extreme-event statistics rather than any fluid mechanics or turbulence principles. We may actually be seeing acceptable wind-driven design results in our growing cities for the wrong reasons. 2.7 Do tall building dynamics impact the design of rooftop pools? Answer: For some pool shapes (with similar building and wave-action natural frequencies) experience has shown that relatively modest return period winds can induce wave action within a rooftop pool via building motion; transferring a significant portion of the water out of the pool and causing substantial damage and expense to the floors below. In one case CCTV footage during a wind storm showed about one-third of a rooftop pool leaving the pool volume via wave action over a few minutes. If this phenomenon can be explored prior to construction then suitable mitigation measures may be developed. These range from changing the pool shape to designing for a suitable capacity drainage Figure 3. An example of a 1:25 pool model on a system around the pool. Small-scale, physical, pendulum apparatus used to simulate rooftop dynamic studies using Froude Number modeling can building motion and check for wave action (after aid in predicting when a pool shape is likely to be a MEL Consultants). concern (Figure 3). With taller residential buildings the potential for a coincidence of building fundamental natural frequency and pool sloshing frequency to create seiching problems becomes more likely. Tall and, perhaps, supertall residential buildings are increasingly having the lifestyle choice of a pool on the roof and this was never a design feature for commercial tall buildings. The designers simply need to be aware of this issue (many are not) and include it in the design topics to be investigated. 2.8. Do mid-level cooling towers have different dispersion needs from those on the roof? Answer: Tall residential buildings often contain one or more mid-level plant rooms containing, amongst other items, some evaporative-based cooling systems for the HVAC system. Shorter buildings typically have rooftop cooling towers which have a well-understood dispersion mechanism of a fan assisted rising, largely unobstructed, water vapour plume. However, in a taller building there is typically a need for one or more evaporative cooling systems at, say, mid or third heights up the building. Additionally, many tall residential buildings are sold and occupied in segments as the construction process occurs; a developer’s cash-flow methodology to enhance financial success. Thus, a segmented HVAC system works well with these huge projects. An apartment building will commonly have operable windows and a mid-building cooling tower exhausting out the side of the facade may create health concerns for the nearby apartments. Even with suitable bacteria-fighting chemicals in the cooling water the odours and humidity may be unacceptable for proximate open windows. The design team could elect to delete operable apartment
windows in the design near the horizontal cooling-tower exhausts, but without a detailed knowledge of the plume path for key statistical wind azimuths it is very difficult to judge the needed extent (above, below, and around the building shape) of the sealed apartments. Obviously in the traditional tall commercial building design, with a nominally sealed facade, this is not a consideration. One solution adopted by some mechanical designers has been to use, so-called, dry cooling towers which have the appearance of larger versions of a car radiator. Air passing over a massive collection of fine fins keeps the atmosphere and heat-containing liquid separated; a sealed system. Such sealed heat-exchange systems, sadly, require some increased plantroom area and installation expense, but they make mid-building cooling systems less problematic.
2.9. Why have some external sunshades and photovoltaics failed on tall buildings? Answer: More energy efficient design of tall buildings has led to the popularity of external sunshades (vertical or horizontal) and photovoltaics (PV) attached as facade appurtenances; particularly in sub-tropical and tropical locales. The power generating PVs are typically installed on the roof, although some have been proposed to serve as both facade sunshades and power generation features. That said, several attachment Figure 4. An example of prototype sunshade and failures of facade-mounted sunshades in the connections to the facade in a large wind tunnel. The support last decade have occurred. Typically these frame allows both azimuth and tilt angles to be explored as failures are due to low-return-period winds the approach flow in a complex city environment is rarely (i.e., very modest wind speeds; not ULS horizontal (after MEL Consultants). design events) causing turbulent buffeting and fatigue failure on non-edge sunshades away from the first sunshade impacted by the wind. The relatively less turbulent approach flow arriving at the first sunshade is likely to have a strong mean flow with smaller gusts, but the complex turbulent flow (smaller mean flow velocity with large superimposed gusts) experienced by sunshades further within the field buffet the protruding elements and cyclically load the connections to failure. Failure of these elements is a serious danger to the public that is not receiving the attention needed. International wind codes and standards generally do not present useful design guidance for these items that protrude off the facade line and, so, designers are left to make various assumptions to estimate the wind load which often ignore the potential for a dynamic resonant response. Full-scale studies (Figure 4) in the wind tunnel allow designers to understand both fatigue and acoustic issues. Rooftop arrays of PVs are similarly difficult to design from a wind-load perspective. The major international codes and standards do not currently address this deficiency well. For the case of the common sloped house roof, or similar, with parallel PVs mounted just off the surface the SEAOC [16] document provides competent advice, but that still leaves many geometries (building height, flat roofs, PV inclination, PV spacing, parapets, influence of neighbours, lift overruns, etc.) where there are little useful data. This topic of wind loads on PV systems (both in the open field and building roof mounted) has been studied in wind tunnels since the 1970s [17], but the data are not well collated into a simplified code format that covers many rooftop designs. Unfortunately, some structural engineers are tempted to use the wind loads presented in national codes for sloped carport roofs for the planar inclined PV surfaces, but this typically underestimates the wind loads substantially due to the smaller size of an inclined PV resulting in more correlated peak pressures than a large carport roof. 2.10. Do tall buildings always adversely impact ground-level pedestrian conditions?
Answer: There are two principal mechanisms that attract strong, and usually unacceptable, winds to the public spaces at the base of tall buildings: (i) downwash and (ii) horizontally accelerated flow. In a general sense the former mechanism of downwash is usually associated with tall buildings having a rectilinear plan shape; often with little or no variation in floor size over the height. With this geometry the faster winds at high elevation are drawn to ground level on the windward face via the secondary pressure gradient between the positive stagnation pressure (typically at about the two-thirds height of the broad flat building face) and the lesser magnitude positive pressure closer to the ground; that is, a secondary flow down the rectangular building face. In a complex cityscape nearby buildings will alter this phenomenon, mitigate or make more severe, but as a guide a rectangular tall building face oriented towards a dominant (i.e., strong and statistically common) wind direction is a good candidate for unpleasant to dangerous conditions at street level. The latter mechanism of horizontally accelerated flow is less dependent on the building shapes and more on the spacing between buildings, upwind approach conditions, and the orientation relative to the dominant local winds. Both mechanisms commonly result in degraded wind conditions around new tall buildings and are discussed in more detail in [18] while some ameliorative architectural features are noted in [19]. A Progressive Architecture article published in the 1980s [20] has some useful illustrations of the flow physics and possible solutions too. As a general guide if one considers the architectural plan massing varying from rectilinear to circular (i.e., rectangle, to triangle with large radius corners, to hexagon, to circle, etc.) the tendency for downwash diminishes. However, all these shapes can experience horizontally accelerated flows where pressure differences from the windward to leeward sides of adjacent buildings “squeeze” the streamlines between buildings; yielding undesirable conditions between two or more buildings. With these thoughts in mind, a new tall building will always alter the ambient conditions at ground level and it will almost always be an adverse change; at best not substantially degraded. For example, a new 300-m highly curved or even circular building placed within a substantial existing, but shorter, cityscape will not attract much downwash and accelerated horizontal flows may be modest or blocked. Thus, one could have a new tall building with very little impact on the pedestrian-wind conditions. Conversely, a four or five storey building in an exposed position on the edge of a CBD facing a common and strong wind azimuth (perhaps on a river edge, say) may produce horrendous conditions for pedestrians nearby. It is for this reason that the tendency of local Councils, and other responsible authorities, to use an arbitrary number of storeys to determine the need for a wind study may seem desirable as a simple “box to tick” but, in fact, is rarely justifiable. The same flow mechanisms that cause unpleasant conditions in cities can, in some climates, be a useful way to disperse automobile fumes [21] or provide a breeze in a hot humid summer. Of course, on a small scale these ideas of tall elements drawing down wind to ground level or orientation choices have an ancient history in India and the Middle East [22] to make life more comfortable. So, at a larger city scale what would yield unacceptable wind conditions in a temperate city may well be desirable in a tropical city. For example, one tall building in the CBD of Singapore (Hitachi House) is elevated on columns with a full-sized plaza under the tower. Such a design in a cooler climate would be an unpleasant and windy choice, but in humid Singapore the result is a pleasant cooling wind under the tower. On my last visit I noticed a temporary food market established in this shady, breezy place. 3. Summary and Conclusions It is hoped that this discussion of a selection of wind-engineering considerations in tall and supertall residential building design may stimulate some thought, debate and, perhaps, new research [23]. The standard approaches used for tall and supertall commercial buildings needs to be modified or augmented for the residential case due to the clearly different use of the building within the CBD. Consulting with architects, engineers and developers on tall residential building design has resulted in the observations written about herein and the points raised are not always welcome in the first instance, but ultimately the result is a better product as discussed in an ASCE document entitled “Wind Issues in the Design of Buildings” [24]. With any expansion of old methodologies (design and construction of tall buildings in this case) one must be aware of new pitfalls that would adversely impact the end product as assumptions are tested and expanded upon during the process.
The choice of the ten questions presented is entirely mine and, in fact, may not be the best topics to consider for some readers. Others may have chosen different wind-engineering topics in tall building design, but since this format is in part experiential these are the queries that have routinely appeared in my observations of the design process. Acknowledgements This discussion of ten questions relating to the wind issues seen in tall and supertall building design could not have happened without the experience provided by the key design professionals engaged by the developer community and their preparedness to consider alternative choices and “what if” scenarios. I would like to give thanks to the many architects and engineers who have discussed and considered these topics at numerous design meetings. Additionally, I would like to thank Professor Blocken for inviting me to put pen to paper on these topics that have been central to my work over some decades. Lastly, this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] L.S.Cochran, R.G. Derickson, R. Meroney and H. Sharp, On What a New Building Project Manager Needs to Know About Wind Engineering, Proceedings of the 17th Australasian Wind Engineering Society Workshop, Wellington, New Zealand, February, 2015. [2] L.S. Cochran and R.G Derickson, On the Complementary Nature of Resilient Building Design and Wind Engineering, Proceedings of the 16th Australasian Wind Engineering Society Workshop, Brisbane, Australia, July, 2013. [3] K.C.S. Kwok, M.D. Burton and A.K. Abdelrazaq, Wind Induced Motion of Tall Buildings – Designing for Habitability, American Society of Civil Engineers, Monograph, 2014. [4] P. Irwin, L. Griffis, W. Baker and C. Letchford, ATC Design Guide 3, Serviceability Design of Tall Buildings Under Wind Load, Applied Technology Council. Redwood City, California, 2019. [5] ISO 6897-1984(E), Guidelines for the Evaluation of the Response of Occupants of Fixed Structures, Especially Buildings and Off-Shore Structures, to Low Frequency Horizontal Motion (0.063 to 1 Hz), International Organization for Standardization, Geneva, 1984. [6] ISO 10137, Bases for Design of Structures - Serviceability of Buildings and Pedestrian Walkways Against Vibration, International Standard ISO 10137, 2004. [7] W. H. Melbourne, Comfort Criteria for Wind-Induced Motion in Structures, Structural Engineering International, Volume 1, pages 40-44, 1998. [8] N. Isyumov, Criteria for Acceptable Wind-Induced Motions of Tall Buildings, Proceedings of the CTBUH International Conference on Tall Buildings, pages 401-411, Rio de Janeiro, 1993. [9] M.N Michaels, K.C.S. Kwok and P.A. Hitchcock, Human Body Response to Low Frequency Narrow-band Random Building Motions, Proceedings of 7th Asia-Pacific Conference on Wind Engineering, Taipei, 2009. [10] M.D. Burton, R.O. Denoon, R.D. Roberts, K.C. Kwok and P.A. Hitchcock, A Motion Simulator to Investigate Wind-Induced Building Motion, Proceedings of the Eleventh International Conference on Wind Engineering, pages 1341-1348, Lubbock, Texas, 2003. [11] A.W. Irwin, Human Response to Dynamic Motions of Structures, The Structural Engineer, Volume 56A, Number 9, pages 237-244, 1978. [12] A.W. Irwin, Perception, Comfort and Performance Criteria for Human Beings Exposed to Whole Body Pure Yaw Vibration Containing Yaw and Translation, Journal of Sound and Vibration, Volume 74(4), pages 481-497, 1981. [13] R.J. Hansen, J.W. Reed and E.H. Vanmarcke, Human response to Wind Induced of Buildings, ASCE Journal of the Structural Division, Volume 99, Number ST7, pages 1598-1605, 1973. [14] J.D. Holmes, Wind Loading of Structures, Third Edition, CRC Press, 2015. [15] P.A. Bailey and K.C.S Kwok, Interference Excitation of Twin Tall Buildings, Journal of Wind Engineering and Industrial Aerodynamics, Volume 21, pages 323-338, 1985. [16] R. LaPlante and J. Maffei (Editors), Wind Design for Low-Profile Solar Photovoltaic Arrays on Flat Roofs, Structural Engineers Association of California Report SEAOC PV2-2012, 38 pages. 2012. [17] L.S. Cochran, On Wind Load Reduction: Solar Arrays in the Field and Building Roofs, Proceedings of the SENG 2015 National Conference, Adelaide, Australia, September 2015. [18] L.S. Cochran, Causes of Unpleasant Pedestrian Wind Conditions, Proceedings of the Eleventh Australasian Wind Engineering Society Workshop, Darwin, 2004. [19] L.S. Cochran, Solutions to Unpleasant Pedestrian Wind Conditions”, Proceedings of the Eleventh Australasian Wind Engineering Society Workshop, Darwin, 2004.
[20] R. Rush, Structure and Circumstance, Progressive Architecture, 8 pages, 1980. [21] L.S. Cochran and K.F. Howell, Wind Tunnel Studies for the Aerodynamic Shape of Sydney, Australia, Journal of Wind Engineering and Industrial Aerodynamics, Volume 36, pages 801-810, October 1990. [22] R.M., Aynsley, W.H. Melbourne, W. and B.J. Vickery, Architectural Aerodynamics, Applied Sciences Publishers Ltd., London, 1977. [23] L.S. Cochran, Contemporary Architectural Aerodynamics and the Future of Wind Engineering, Proceedings of The International Workshop on Wind Engineering Research and Practice, Chapel Hill, North Carolina, May 28-29, 2010. [24] L.S. Cochran (editor), Wind Issues in the Design of Buildings, American Society of Civil Engineers Publications, Technical Council on Wind Engineering, Structural Wind Engineering Committee, 99 pages, 2012.
Dr Cochran has forty years of experience in project management (site, design studio, and consulting), civil and structural engineering, wind-engineering applications, and research. He has evaluated over 600 buildings and structures for wind loads (local cladding pressures and/or frames loads) and responses (accelerations) through wind-tunnel testing; evaluated pedestrian-wind climate for many of these buildings; measured forces on other structures like stacks, outdoor monuments/art, and solar collectors; defined snow loads for structures via wind-tunnel data; investigated pollutant dispersion from laboratory buildings, stacks, and ships. Leighton is now a director at MEL Consultants with a focus on northern Australia/Asia and is based in Brisbane. He has published 25 papers in peerreviewed journals (plus another 50 in conference proceedings) and gives the occasional invited lecture to fourth year students at Griffith University and University of Queensland, as well as professional groups of architects, engineers and developers via CTBUH/ASCE. Professional volunteer roles include: past secretary of the American Society of Civil Engineers (ASCE) Environmental Wind Engineering Committee, 2000-2006; past chair of the ASCE Structural Wind Engineering Committee, 2005-2011; past president of the American Association for Wind Engineering (AAWE), 2007-2008; past board member for the Applied Technology Council, 2011-2017; past Americas Regional Coordinator for the International Association for Wind Engineering, 2009-2017; and current board member of the Australasian Wind Engineering Society. He is the recipient of the AAWE Michael Gaus Award (2013), and the coauthored the AAWE Best Journal Paper in 2013. Number of Words = 6,622 (<8,000)
HIGHLIGHTS
• • • • •
Wind engineering considerations for tall and supertall residential buildings Upper floor motion perception and wave action in rooftop pools Open windows, increased internal pressures and partition failures Sunshade and photovoltaic panel fatigue wind failure Pedestrian wind conditions at base of tall buildings
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Dr Leighton Cochran 07 October 2019
S0360-1323(19)30790-5
DOI:
https://doi.org/10.1016/j.buildenv.2019.106578
Reference:
BAE 106578
To appear in:
Building and Environment
Received Date: 7 October 2019 Revised Date:
27 November 2019
Accepted Date: 27 November 2019
Please cite this article as: Cochran LS, Ten Questions Concerning Wind Effects on Supertall Residential Buildings, Building and Environment, https://doi.org/10.1016/j.buildenv.2019.106578. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Ten Questions Concerning Wind Effects on Supertall Residential Buildings Leighton S. Cochran MEL Consultants, 53 Saint Pauls Terrace, Suite 16, Ground Floor Spring Hill, 4000, Queensland, Australia. +61-(0)408-007-624, [email protected].
ABSTRACT Historically the taller buildings in our cities have been commercial or office buildings. In part, the massaging of national, corporate, or individual ego may have been a reason behind this trend for most of the twentieth century but, of course, the financial case was actually compelling too, as the market for CBD living was traditionally limited. However, more recently the trend to higher population densities close to the centre of many cities has resulted in new tall buildings having a dominant residential nature; mixed use, hotel/apartment, or pure apartment. From a wind-engineering perspective this move to homes in the sky introduces some new opportunities and sources of design angst, neither of which should be ignored. What follows is a discussion of issues noted over the last thirty years that may allow modern designers to learn from previous choices. Keywords: Supertall, residential buildings, super skinny, wind engineering, balconies, and top-floor accelerations. 1. Introduction A relatively recent trend in tall, supertall (>300 m) and megatall (>600 m) building design has been the move from commercial use to residential use. This introduces new or different wind-oriented design considerations for many of the project design consultants; some of whom may be unaware of the questions to ask [1, 2] and preferring a “business as usual” scenario as that has been professionally adequate in the past. Items as varied as the internal pressure changes generated by operable highelevation windows to the wave action present in rooftop swimming pools, all require a rethinking of standard design approaches. A related tall building trend is the appearance of “super-skinny” buildings that may, or may not, be very tall in an absolute sense but have a large aspect ratio (height to smaller width). This new subset of tall-building design shares many traditional tall-building attributes, while having some unique problems in its own right. 1.1 Residential trends Most of the design issues presented here have been a consequence of the supertall building population now containing many residential buildings. However, residential buildings shorter than this arbitrary 300-m height definition (CTBUH1) are not immune to the same design considerations. Hence, these thoughts may help residential building designers for the more common heights in the thirty to seventy storey range; depending on exposure, local wind climate, orientation, massing, facade sealing and structural asymmetry. Any of these attributes singularly has the potential to introduce unexpected wind-engineering issues for the design team to consider; several of them on the one project can result in a very real operational headache for the owners. 1.2 Approach The questions below all pertain to recent tall building projects (often seen on multiple projects) and the answers presented here are derived from a combination of physical wind-tunnel 1
Council on Tall Buildings and Urban Habitat
model studies, comprehensive data analysis, and simple experiential opinion. The last will be highlighted or be self-evident so that the reader does not confuse defensible research with opinion based on multiple project inputs from various design teams. It is hoped that the topics chosen will stimulate some in-house discussion in both the design and research communities. 2. Ten Questions (and Answers) Concerning Wind Effects on Tall Residential Buildings 2.1. What can be done in tall residential building design to limit perceived motion? Answer: Tall and supertall buildings frequently have the structural design guided by upper floor responses (accelerations). Unwanted motion perception in the upper third of the residential building should be avoided for reputable and legal reasons. As a New York architectural client once said to me, “if they can afford a penthouse then they can afford a competent lawyer”. Thoughtful architectural massing over the height can be key to a good design (orientation, varying the shape with height, corner design, elevator and shear wall location, twisting geometry, architectural penetrations, etc.) if one wishes to avoid supplemental damping systems (tuned mass dampers, tuned liquid column dampers, distributed elastomeric or friction dampers, etc.). The criteria available for designers to assess the acceptability and/or perception thresholds to tall building motion are discussed in some depth in two recent publications from ASCE [3] and ATC [4]. Some of the criteria for assessing upper-floor building accelerations are more stringent for residential buildings and less demanding for commercial or office buildings. The motivation for this distinction is not immediately clear, but some have written of owners preferring less perceptible motion in the home environment as opposed to the workplace. This seems as plausible a rationale as any. As such, the recent surge in tall residential building construction has focused the design team on this comfort condition for “homes in the sky”. One key attribute of the human perception of horizontal motion in tall buildings is the frequency dependency which has been long understood [5,6,7,8], with more recent research in [9,10]. Early work in the 1970s on building motion may be found in [11,12,13], while ASCE publication [3] provides an excellent historic overview. That said, the fundamental natural frequency of any tall building is generally below 1.5 Hz and typically well below 1 Hz; with supertall buildings often less than 0.1 Hz. At these frequencies the ability of humans to perceive horizontal accelerations generally diminishes as the frequency falls and, so, supertall and megatall buildings are somewhat more forgiving to design for motion perception. This can mean that the structural engineer may have more difficulty in designing for upper-floor accelerations of buildings in the 150-250 m range than, say, in the 400-700 m range. Consequently the use of auxiliary mechanical damping is apparent in buildings with a more modest height due to other architectural choices being fixed (shape, orientation, structure, etc.) prior to any wind-engineering influence. When this observation is combined with the mandatory nature of wind-tunnel studies for the taller designs (with consequent aerodynamic influence on the architecture; unless ignored) damping systems are not as common as one may expect in the supertall and megatall buildings. The architectural and structural engineering teams need to work together to avoid the additional acceleration consideration of a strong torsional response. Human perception of torsional motion can be enhanced by visual cues like relative mullion motion (facade verticals seen against outside distant structure). Depending on the modal complexity, twisting about the building’s vertical axis can add considerably to the perceived acceleration magnitudes. The cleanest way to fend off this problem is to keep the centre of stiffness (also called centre of rigidity) as close as possible to the centre of mass on typical floors up the building height. In many cases a symmetry about both horizontal axes with the structural design is an initial first step; although, obviously, not essential. The location of major structural components, like the lift core, stairwells and shearwalls, is key to controlling torsional response. Additionally, the aerodynamic shape will drive the aerodynamic torsional input; long rectangular or crescent shaped floor plates, for example, create strong torsioninducing pressure fields while circular shapes do not. A passing thought for tall and supertall building owners and designers to consider is the psychological preparation of the residents to the fact that their new home is designed to move and that some segments of the populations are more sensitive to that motion than others. By informing new owners that they may sense motion could prepare them and alleviate concern — of course it should
not be a frequent perception in a good design, but the rare motion in strong winds should not induce worry. Education within the population of a new building may be wise. 2.2 Why are high-aspect-ratio buildings (super skinny) becoming more common? Answer: In recent years the appearance of very slender buildings with aspect ratios in the range of 12:1 to 17:1 have become a new phenomenon (Figure 1). This evolution to more slender structures has resulted in the crosswind response gaining more importance in the structural design process as the dynamic resonant component of the wind load increases. For a given building shape the more slender design (with a lower natural frequency) has a design point higher on the crosswind response spectrum than its stockier brethren. In part these slender designs are a response to smaller plot sizes being available in city environs, but there are also some technical aspects to tall building design that have been necessary for this evolution. Improvements in structural analysis packages along with cost-effective supplemental damping systems have allowed the upper-level building accelerations to be understood and mitigated enough for successful residential slender building design. In the larger very slender buildings, such as New York City’s famous 432 Park Avenue (15:1), a traditional Tuned Mass Figure 1. High aspect ratio residential projects are becoming Damper (TMD) is used (as well as aerodynamic modifications in more common in cities with small the form of multiple open floors and additional floor mass at land parcels becoming the norm. upper levels). This TMD is a large mass at the top of the building This building in Flinders Street, which moves in a pendulum motion to counteract the building Melbourne, Australia is 90 m high motion. This concept has been an essential feature of the and 6 m wide (15:1). mechanical engineering profession for generations, but in a scaled-up form it is now occasionally applied to civil engineering structures. In some more modestly sized slender buildings with the same aspect ratio, such as the Phoenix Apartments (Figure 1), the use of less expensive Tuned Liquid Column Dampers (TLCD) are needed to control motion. These TLCDs are effectively giant rooftop U-tubes filled with water, allowing large masses of water to oscillate from one riser to another over several metres at the natural frequency of the building. However, even this technology has its limitations. The 90-m building in Figure 1 was originally designed to be 120 m tall (an aspect ratio of 20:1), but control of upper-floor motions contributed to the slightly shorter building shown here. The parking of residents’ cars in these very slender buildings becomes a problem when the width is insufficient for ramps and turnaround areas. The availability of modern robotic car stacking systems, that do not require drivable ramps, are a solution that allows such slender designs to be realistically pursued. Obviously this mechanical solution takes up less space, but it will be an ongoing maintenance cost for the building owners. 2.3. How do operable windows at elevation impact design of tall residential buildings? Answer: Open windows and balcony doors in tall residential buildings, during a strong wind event, will modify the internal pressure (positive or negative, depending on opening location relative to the wind) within a unit and are likely to result in a variation in pressure between units on a given floor. Thus, potentially large differential-pressure partition loads (commonly ignored, or assumed very small, in nominally “sealed” office building design) can be expected between apartments. Typically the lightweight walls within an apartment do not experience this type of substantial differential pressure loading as the gap (commonly in the 15 to 20 mm range) under the internal doors tends to equilibrate the pressure across walls within an apartment. However, between apartments the front door from the apartment to the lift lobby is usually better sealed, producing less pressure connectivity (in some designs local HVAC connections can mitigate this issue — depending on fire
engineering constraints). Additionally, an increase in the net facade pressures elsewhere on that floor can be expected with open windows and the resulting internal pressure augmentation (e.g., a negative internal adding to a positive external, or a positive internal adding to a negative external). This phenomenon may result in other pressure related problems such as door operation difficulty, elevator operational problems, fire stair concerns, and acoustic annoyance for residents. A lack of consideration of increased internal pressures from façade openings at high elevation has produced a variety of partition wall “failures” between residential apartments. The light-weight metal stud walls often used between apartments have been observed by the author to cause a range of problems for owners and developers. The failure may manifest itself as a separation of the partition wall from the ceiling; ranging from simple plaster cracking to more extreme wall movement and total separation or collapse. In one case the metal studs making up the partition wall were moved repeatedly by the dynamically changing differential pressures, resulting in metal-on-metal noise so annoying as to force the owners to move out of the apartments while the problem was rectified. Once the design issue is accepted by the design/construction team, the successful solutions typically include a mixture of stronger partition wall choices (only a modest cost impact), a variety of self-closing window and door ideas (e.g., positive pressure “snapshut” windows, inclined-track sliding balcony doors, etc.), to intrusive technologies demanding that the owner close the windows before leaving the apartment. One developer provided a system that would not allow the resident to lock the front door of the apartment, until all the windows were closed! This increased internal pressure phenomenon in tall building design may also contribute to other livability concerns in a residential building. When combined with other attributes, like orientation of the building relative to dominant winds and, say, badly sealed rooftop lift motor rooms, pressure differences via operable windows may yield strong flows in the elevator shaft. On rare occasions, when a variety of design attributes align, this may impact the operation of ground floor sliding entry doors, elevator doors, and/or generate strong flow induced noise through lift lobby areas (link to 2 Mb video). Commonly many of these problems have at least a partial cause in the increased internal pressure via some façade opening (apartment windows, undamped intake/exhaust louvres, natural-ventilation design features, porous garage doors, leaky lift motor rooms, etc.) that allows the pressure differential to adversely impact door operation or generate noise. 2.4. Where should kitchen and bathroom exhausts exit the building? Answer: While many designers prefer to install these exhausts locally at an adjacent external wall or balcony soffit the resulting system rarely has a fan of sufficient power to overcome the common external positive wind pressures (say, weekly or monthly recurrence interval winds), or have a back-flow damper, which results in pressure-related problems like apartment odours and noise. Additionally, there is often a substantial horizontal path for the tubing (commonly flexible ribbed piping that could accumulate kitchen grease, for example) from the extractor fan to the apartment wall, which has been observed to exceed the recommended length by the manufacturer. Such a design when exposed to a very common and modest breeze at elevation will experience mean and peak external wall pressures that far exceed any pressure magnitudes generated by the extractor fan within the exhaust tubing system. More importantly a long horizontal connection (greater than 5 m is not uncommon) in the ceiling space may accumulate kitchen greases/oils and become a fire hazard. Obviously, the motivation for the same-level exhaust approach is cost. The older proven approach of a common stack to a roof-top exhaust will consume some salable internal building space over the entire height of the building, but this is usually the better solution (opinion) since the rooftop extractor fan will never be competing with a positive pressure. The rooftop stack will always have a negative pressure over the exit when the wind blows and, in the worst case, will have no external pressure on a still day. Obviously this observation is not unique to tall residential buildings, but the motivation to
design kitchen and bathroom exhausts out the side of the apartment becomes more enticing in taller designs. 2.5. Are balconies an appropriate design feature for tall buildings? Answer: It is well understood that an increase in elevation will generally attract Figure 2. An example of balconies contained entirely behind the facade alignment and near the centre of the higher wind speeds around a tall building. That building face. They may be architecturally uninteresting, said, a good choice of balcony design, location but wind conditions are typically not a problem. on the building plan, combined with some selected local wind protection, can result in balconies that are useable spaces over great heights. Investigating the wind conditions with a physical model in a boundary-layer wind tunnel, using a hot-wire anemometer, indicates the influence of local geometry and suggests some general guidance for designers. For example, balconies at the building corner are generally much less usable than those centrally located on the building face. Balconies placed within the façade line (Figure 2) and centrally located on the face may be expected to be calmer than those protruding beyond the building face or those at a corner. An interesting recent trend with tall residential buildings is to have the balcony or terrace set back behind the façade line, but to have the opening smaller than the terrace edge in such a way that the façade protects part of the balcony (glazing above, to one or both sides, and the balustrade below the opening) with a modest rectangular opening to the atmosphere; often called a “letterbox balcony”. As a consequence the occupant is likely to be able to find a calm location within the larger balcony volume even at elevation on a windy day. It is interesting to note that with some curved building plans this small-opening-big-volume balcony design can have the unexpected attribute of a pulsating pressure within the volume when a strong wind blows close to parallel to the façade line (generated by a relatively stable rotating flow within the balcony volume). Initial consideration would suggest this could be a Helmholtz resonance, but wind-tunnel studies showed a frequency dependence of the pressure pulse with velocity (a Strouhal type response) and, conversely, no frequency dependence with facade opening area changes. The Helmholtz frequency has no velocity dependence [14] and should vary with balcony volume and/or facade opening size. The importance of understanding the mechanism is that the solution design choices are guided by the physics. For example since the response has a velocity dependence, if the apartment balcony glazing system can be stiffened sufficiently then the velocity to cause a resonant response can approach ultimate limit state (ULS) conditions; rather than serviceability limit state (SLS) conditions. Without mitigation this can severely rattle the doors and glazing of the apartment and be very unnerving for the owners; especially when the pressure pulse frequency closely matches the natural frequency of the glazing/frame structure. In the case of more exposed balconies or large terraces at elevation higher glazed balustrades and, perhaps, fixed or moveable porous/louvered screens will be needed to keep the outdoor space useable for medium to long-term activities. Without sensible design features like this not only will the balcony be of little value to the occupant, but there is a very real risk of furniture being blown off the balcony with potentially horrible consequences. Lightweight chairs and glass tabletops are common objects that are removed from these outdoor spaces by modest to strong winds. 2.6. How do winds altered by future tall buildings impact the existing proximate neighbours? Answer: As a general guide most city centres become aerodynamically taller, denser, and more complex over time and, as such, the expectation is that the addition of a major nearby building to the existing cityscape would typically yield altered wind loads on the existing, usually shorter, buildings that are similar in magnitude or reduced somewhat due to shielding (opinion). There are limited data on this as most owners do not want to know if their new building adversely impacts nearby buildings. Of course, there are exceptions to this generalization that come from a small number of wind-tunnel studies that have revealed deleterious impacts caused by the new building shape, the spacing between buildings, and/or the changed importance of local wind rose via wind flow variation with the new massing in place. The topic of interference effects on the structural wind loads in a simplified two-building environment is quite a well studied area going back to the 1980s [15], but
the complexity within a many-building cityscape is less well understood without multiple “what-if” wind-tunnel studies and those are unlikely to be performed in a litigious real estate environment. However, the addition of new buildings to an existing cityscape is, in at least one jurisdiction, a required area of investigation for peak cladding pressures. In the downtown of Singapore, when a wind-tunnel study is performed for design cladding pressures the wind-engineering laboratory is required to take some sample pressure measurements on the nearby existing buildings with, and without, the new building in place. Having gone through this type of investigation for several Singaporean tall buildings the trend for cladding pressures is that a new tall building in the mix very rarely makes the proximate peak cladding pressures larger in magnitude — no change or a reduction is by far the most common result. This reinforces the intuitive expectation of increased shielding or wind protection due to CBD growth; especially away from the edges of the downtown cityscape. It should be noted that structural design wind loads and cladding pressures are extreme events (ultimate limit state, ULS, in engineering parlance) and the apparent lack of structural or cladding failures as cities grow and change aerodynamically may well be due to extreme-event statistics rather than any fluid mechanics or turbulence principles. We may actually be seeing acceptable wind-driven design results in our growing cities for the wrong reasons. 2.7 Do tall building dynamics impact the design of rooftop pools? Answer: For some pool shapes (with similar building and wave-action natural frequencies) experience has shown that relatively modest return period winds can induce wave action within a rooftop pool via building motion; transferring a significant portion of the water out of the pool and causing substantial damage and expense to the floors below. In one case CCTV footage during a wind storm showed about one-third of a rooftop pool leaving the pool volume via wave action over a few minutes. If this phenomenon can be explored prior to construction then suitable mitigation measures may be developed. These range from changing the pool shape to designing for a suitable capacity drainage Figure 3. An example of a 1:25 pool model on a system around the pool. Small-scale, physical, pendulum apparatus used to simulate rooftop dynamic studies using Froude Number modeling can building motion and check for wave action (after aid in predicting when a pool shape is likely to be a MEL Consultants). concern (Figure 3). With taller residential buildings the potential for a coincidence of building fundamental natural frequency and pool sloshing frequency to create seiching problems becomes more likely. Tall and, perhaps, supertall residential buildings are increasingly having the lifestyle choice of a pool on the roof and this was never a design feature for commercial tall buildings. The designers simply need to be aware of this issue (many are not) and include it in the design topics to be investigated. 2.8. Do mid-level cooling towers have different dispersion needs from those on the roof? Answer: Tall residential buildings often contain one or more mid-level plant rooms containing, amongst other items, some evaporative-based cooling systems for the HVAC system. Shorter buildings typically have rooftop cooling towers which have a well-understood dispersion mechanism of a fan assisted rising, largely unobstructed, water vapour plume. However, in a taller building there is typically a need for one or more evaporative cooling systems at, say, mid or third heights up the building. Additionally, many tall residential buildings are sold and occupied in segments as the construction process occurs; a developer’s cash-flow methodology to enhance financial success. Thus, a segmented HVAC system works well with these huge projects. An apartment building will commonly have operable windows and a mid-building cooling tower exhausting out the side of the facade may create health concerns for the nearby apartments. Even with suitable bacteria-fighting chemicals in the cooling water the odours and humidity may be unacceptable for proximate open windows. The design team could elect to delete operable apartment
windows in the design near the horizontal cooling-tower exhausts, but without a detailed knowledge of the plume path for key statistical wind azimuths it is very difficult to judge the needed extent (above, below, and around the building shape) of the sealed apartments. Obviously in the traditional tall commercial building design, with a nominally sealed facade, this is not a consideration. One solution adopted by some mechanical designers has been to use, so-called, dry cooling towers which have the appearance of larger versions of a car radiator. Air passing over a massive collection of fine fins keeps the atmosphere and heat-containing liquid separated; a sealed system. Such sealed heat-exchange systems, sadly, require some increased plantroom area and installation expense, but they make mid-building cooling systems less problematic.
2.9. Why have some external sunshades and photovoltaics failed on tall buildings? Answer: More energy efficient design of tall buildings has led to the popularity of external sunshades (vertical or horizontal) and photovoltaics (PV) attached as facade appurtenances; particularly in sub-tropical and tropical locales. The power generating PVs are typically installed on the roof, although some have been proposed to serve as both facade sunshades and power generation features. That said, several attachment Figure 4. An example of prototype sunshade and failures of facade-mounted sunshades in the connections to the facade in a large wind tunnel. The support last decade have occurred. Typically these frame allows both azimuth and tilt angles to be explored as failures are due to low-return-period winds the approach flow in a complex city environment is rarely (i.e., very modest wind speeds; not ULS horizontal (after MEL Consultants). design events) causing turbulent buffeting and fatigue failure on non-edge sunshades away from the first sunshade impacted by the wind. The relatively less turbulent approach flow arriving at the first sunshade is likely to have a strong mean flow with smaller gusts, but the complex turbulent flow (smaller mean flow velocity with large superimposed gusts) experienced by sunshades further within the field buffet the protruding elements and cyclically load the connections to failure. Failure of these elements is a serious danger to the public that is not receiving the attention needed. International wind codes and standards generally do not present useful design guidance for these items that protrude off the facade line and, so, designers are left to make various assumptions to estimate the wind load which often ignore the potential for a dynamic resonant response. Full-scale studies (Figure 4) in the wind tunnel allow designers to understand both fatigue and acoustic issues. Rooftop arrays of PVs are similarly difficult to design from a wind-load perspective. The major international codes and standards do not currently address this deficiency well. For the case of the common sloped house roof, or similar, with parallel PVs mounted just off the surface the SEAOC [16] document provides competent advice, but that still leaves many geometries (building height, flat roofs, PV inclination, PV spacing, parapets, influence of neighbours, lift overruns, etc.) where there are little useful data. This topic of wind loads on PV systems (both in the open field and building roof mounted) has been studied in wind tunnels since the 1970s [17], but the data are not well collated into a simplified code format that covers many rooftop designs. Unfortunately, some structural engineers are tempted to use the wind loads presented in national codes for sloped carport roofs for the planar inclined PV surfaces, but this typically underestimates the wind loads substantially due to the smaller size of an inclined PV resulting in more correlated peak pressures than a large carport roof. 2.10. Do tall buildings always adversely impact ground-level pedestrian conditions?
Answer: There are two principal mechanisms that attract strong, and usually unacceptable, winds to the public spaces at the base of tall buildings: (i) downwash and (ii) horizontally accelerated flow. In a general sense the former mechanism of downwash is usually associated with tall buildings having a rectilinear plan shape; often with little or no variation in floor size over the height. With this geometry the faster winds at high elevation are drawn to ground level on the windward face via the secondary pressure gradient between the positive stagnation pressure (typically at about the two-thirds height of the broad flat building face) and the lesser magnitude positive pressure closer to the ground; that is, a secondary flow down the rectangular building face. In a complex cityscape nearby buildings will alter this phenomenon, mitigate or make more severe, but as a guide a rectangular tall building face oriented towards a dominant (i.e., strong and statistically common) wind direction is a good candidate for unpleasant to dangerous conditions at street level. The latter mechanism of horizontally accelerated flow is less dependent on the building shapes and more on the spacing between buildings, upwind approach conditions, and the orientation relative to the dominant local winds. Both mechanisms commonly result in degraded wind conditions around new tall buildings and are discussed in more detail in [18] while some ameliorative architectural features are noted in [19]. A Progressive Architecture article published in the 1980s [20] has some useful illustrations of the flow physics and possible solutions too. As a general guide if one considers the architectural plan massing varying from rectilinear to circular (i.e., rectangle, to triangle with large radius corners, to hexagon, to circle, etc.) the tendency for downwash diminishes. However, all these shapes can experience horizontally accelerated flows where pressure differences from the windward to leeward sides of adjacent buildings “squeeze” the streamlines between buildings; yielding undesirable conditions between two or more buildings. With these thoughts in mind, a new tall building will always alter the ambient conditions at ground level and it will almost always be an adverse change; at best not substantially degraded. For example, a new 300-m highly curved or even circular building placed within a substantial existing, but shorter, cityscape will not attract much downwash and accelerated horizontal flows may be modest or blocked. Thus, one could have a new tall building with very little impact on the pedestrian-wind conditions. Conversely, a four or five storey building in an exposed position on the edge of a CBD facing a common and strong wind azimuth (perhaps on a river edge, say) may produce horrendous conditions for pedestrians nearby. It is for this reason that the tendency of local Councils, and other responsible authorities, to use an arbitrary number of storeys to determine the need for a wind study may seem desirable as a simple “box to tick” but, in fact, is rarely justifiable. The same flow mechanisms that cause unpleasant conditions in cities can, in some climates, be a useful way to disperse automobile fumes [21] or provide a breeze in a hot humid summer. Of course, on a small scale these ideas of tall elements drawing down wind to ground level or orientation choices have an ancient history in India and the Middle East [22] to make life more comfortable. So, at a larger city scale what would yield unacceptable wind conditions in a temperate city may well be desirable in a tropical city. For example, one tall building in the CBD of Singapore (Hitachi House) is elevated on columns with a full-sized plaza under the tower. Such a design in a cooler climate would be an unpleasant and windy choice, but in humid Singapore the result is a pleasant cooling wind under the tower. On my last visit I noticed a temporary food market established in this shady, breezy place. 3. Summary and Conclusions It is hoped that this discussion of a selection of wind-engineering considerations in tall and supertall residential building design may stimulate some thought, debate and, perhaps, new research [23]. The standard approaches used for tall and supertall commercial buildings needs to be modified or augmented for the residential case due to the clearly different use of the building within the CBD. Consulting with architects, engineers and developers on tall residential building design has resulted in the observations written about herein and the points raised are not always welcome in the first instance, but ultimately the result is a better product as discussed in an ASCE document entitled “Wind Issues in the Design of Buildings” [24]. With any expansion of old methodologies (design and construction of tall buildings in this case) one must be aware of new pitfalls that would adversely impact the end product as assumptions are tested and expanded upon during the process.
The choice of the ten questions presented is entirely mine and, in fact, may not be the best topics to consider for some readers. Others may have chosen different wind-engineering topics in tall building design, but since this format is in part experiential these are the queries that have routinely appeared in my observations of the design process. Acknowledgements This discussion of ten questions relating to the wind issues seen in tall and supertall building design could not have happened without the experience provided by the key design professionals engaged by the developer community and their preparedness to consider alternative choices and “what if” scenarios. I would like to give thanks to the many architects and engineers who have discussed and considered these topics at numerous design meetings. Additionally, I would like to thank Professor Blocken for inviting me to put pen to paper on these topics that have been central to my work over some decades. Lastly, this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] L.S.Cochran, R.G. Derickson, R. Meroney and H. Sharp, On What a New Building Project Manager Needs to Know About Wind Engineering, Proceedings of the 17th Australasian Wind Engineering Society Workshop, Wellington, New Zealand, February, 2015. [2] L.S. Cochran and R.G Derickson, On the Complementary Nature of Resilient Building Design and Wind Engineering, Proceedings of the 16th Australasian Wind Engineering Society Workshop, Brisbane, Australia, July, 2013. [3] K.C.S. Kwok, M.D. Burton and A.K. Abdelrazaq, Wind Induced Motion of Tall Buildings – Designing for Habitability, American Society of Civil Engineers, Monograph, 2014. [4] P. Irwin, L. Griffis, W. Baker and C. Letchford, ATC Design Guide 3, Serviceability Design of Tall Buildings Under Wind Load, Applied Technology Council. Redwood City, California, 2019. [5] ISO 6897-1984(E), Guidelines for the Evaluation of the Response of Occupants of Fixed Structures, Especially Buildings and Off-Shore Structures, to Low Frequency Horizontal Motion (0.063 to 1 Hz), International Organization for Standardization, Geneva, 1984. [6] ISO 10137, Bases for Design of Structures - Serviceability of Buildings and Pedestrian Walkways Against Vibration, International Standard ISO 10137, 2004. [7] W. H. Melbourne, Comfort Criteria for Wind-Induced Motion in Structures, Structural Engineering International, Volume 1, pages 40-44, 1998. [8] N. Isyumov, Criteria for Acceptable Wind-Induced Motions of Tall Buildings, Proceedings of the CTBUH International Conference on Tall Buildings, pages 401-411, Rio de Janeiro, 1993. [9] M.N Michaels, K.C.S. Kwok and P.A. Hitchcock, Human Body Response to Low Frequency Narrow-band Random Building Motions, Proceedings of 7th Asia-Pacific Conference on Wind Engineering, Taipei, 2009. [10] M.D. Burton, R.O. Denoon, R.D. Roberts, K.C. Kwok and P.A. Hitchcock, A Motion Simulator to Investigate Wind-Induced Building Motion, Proceedings of the Eleventh International Conference on Wind Engineering, pages 1341-1348, Lubbock, Texas, 2003. [11] A.W. Irwin, Human Response to Dynamic Motions of Structures, The Structural Engineer, Volume 56A, Number 9, pages 237-244, 1978. [12] A.W. Irwin, Perception, Comfort and Performance Criteria for Human Beings Exposed to Whole Body Pure Yaw Vibration Containing Yaw and Translation, Journal of Sound and Vibration, Volume 74(4), pages 481-497, 1981. [13] R.J. Hansen, J.W. Reed and E.H. Vanmarcke, Human response to Wind Induced of Buildings, ASCE Journal of the Structural Division, Volume 99, Number ST7, pages 1598-1605, 1973. [14] J.D. Holmes, Wind Loading of Structures, Third Edition, CRC Press, 2015. [15] P.A. Bailey and K.C.S Kwok, Interference Excitation of Twin Tall Buildings, Journal of Wind Engineering and Industrial Aerodynamics, Volume 21, pages 323-338, 1985. [16] R. LaPlante and J. Maffei (Editors), Wind Design for Low-Profile Solar Photovoltaic Arrays on Flat Roofs, Structural Engineers Association of California Report SEAOC PV2-2012, 38 pages. 2012. [17] L.S. Cochran, On Wind Load Reduction: Solar Arrays in the Field and Building Roofs, Proceedings of the SENG 2015 National Conference, Adelaide, Australia, September 2015. [18] L.S. Cochran, Causes of Unpleasant Pedestrian Wind Conditions, Proceedings of the Eleventh Australasian Wind Engineering Society Workshop, Darwin, 2004. [19] L.S. Cochran, Solutions to Unpleasant Pedestrian Wind Conditions”, Proceedings of the Eleventh Australasian Wind Engineering Society Workshop, Darwin, 2004.
[20] R. Rush, Structure and Circumstance, Progressive Architecture, 8 pages, 1980. [21] L.S. Cochran and K.F. Howell, Wind Tunnel Studies for the Aerodynamic Shape of Sydney, Australia, Journal of Wind Engineering and Industrial Aerodynamics, Volume 36, pages 801-810, October 1990. [22] R.M., Aynsley, W.H. Melbourne, W. and B.J. Vickery, Architectural Aerodynamics, Applied Sciences Publishers Ltd., London, 1977. [23] L.S. Cochran, Contemporary Architectural Aerodynamics and the Future of Wind Engineering, Proceedings of The International Workshop on Wind Engineering Research and Practice, Chapel Hill, North Carolina, May 28-29, 2010. [24] L.S. Cochran (editor), Wind Issues in the Design of Buildings, American Society of Civil Engineers Publications, Technical Council on Wind Engineering, Structural Wind Engineering Committee, 99 pages, 2012.
Dr Cochran has forty years of experience in project management (site, design studio, and consulting), civil and structural engineering, wind-engineering applications, and research. He has evaluated over 600 buildings and structures for wind loads (local cladding pressures and/or frames loads) and responses (accelerations) through wind-tunnel testing; evaluated pedestrian-wind climate for many of these buildings; measured forces on other structures like stacks, outdoor monuments/art, and solar collectors; defined snow loads for structures via wind-tunnel data; investigated pollutant dispersion from laboratory buildings, stacks, and ships. Leighton is now a director at MEL Consultants with a focus on northern Australia/Asia and is based in Brisbane. He has published 25 papers in peerreviewed journals (plus another 50 in conference proceedings) and gives the occasional invited lecture to fourth year students at Griffith University and University of Queensland, as well as professional groups of architects, engineers and developers via CTBUH/ASCE. Professional volunteer roles include: past secretary of the American Society of Civil Engineers (ASCE) Environmental Wind Engineering Committee, 2000-2006; past chair of the ASCE Structural Wind Engineering Committee, 2005-2011; past president of the American Association for Wind Engineering (AAWE), 2007-2008; past board member for the Applied Technology Council, 2011-2017; past Americas Regional Coordinator for the International Association for Wind Engineering, 2009-2017; and current board member of the Australasian Wind Engineering Society. He is the recipient of the AAWE Michael Gaus Award (2013), and the coauthored the AAWE Best Journal Paper in 2013. Number of Words = 6,622 (<8,000)
HIGHLIGHTS
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Wind engineering considerations for tall and supertall residential buildings Upper floor motion perception and wave action in rooftop pools Open windows, increased internal pressures and partition failures Sunshade and photovoltaic panel fatigue wind failure Pedestrian wind conditions at base of tall buildings
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Dr Leighton Cochran 07 October 2019