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Wind Engineering for Better Buildings

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The importance of “Wind Engineering” is emerging in India with the growing need for taller and slender buildings. In metros like Mumbai, due to scarcity of land, horizontal expansion is no more a viable solution. Buildings have to grow upwards. According to. K. SureshKumar, Ph.D., P.Eng., M.ASCE, Principal / Managing Director,RWDI Consulting Engineers (India) Pvt. Ltd., though there is enough technology to build super-tall buildings in India today, it is yet to catch up with the globally established technology practices.

Wind engineering is unique as it examines the impact of wind on structures and its environment. Wind loads on facades are required for the selection of the cladding systems and wind loads on the structural frames are required for the design of beams, columns, lateral bracing and foundations.Wind loads is generally combined with other types of typical loads i.e. dead load and live load using load combination procedures. Earthquake loads are another type of lateral load which is considered for design as well. However, considering the rare chance of simultaneous occurrence of both earthquake and high wind, both earthquake and wind loads won’t be combined together in any structural design.

Wind Tunnel: In wind tunnel studies, scaled models of structures are subjected to scaled atmospheric wind in a controlled laboratory set-up. The sensors installed on the model measure the physical quantities of interest such as shear, moment, pressure etc. Later in the analysis, these model scale quantities are converted to prototype using model scale laws. Most of the complex architectural and structural innovations are being confirmed through wind tunnel tests. Globally, wind tunnel tests are done for almost all buildings above approximately 100m. Even low buildings are being tested for places like Miami where severe wind conditions are expected.

Full-Scale: With full-scale studies, actual buildings already built are instrumented and tested in natural wind flow for several months to get decent measurements. The time consumption in full-scale study is one of the issues; many times one has to wait for the wind to blow from the directions that you need since you don’t have an opportunity to rotate the building like in a wind tunnel. Full-scale studies are good to improve our understanding of the science as well as the simulation in wind tunnel but is not practical in day-to-day life.

Analytical: With analytical studies,the structures are modelled instructural dynamic sense and the wind flow is modelled as stochastic time series. There after,the response of the structure is obtained by random vibration techniques. This is a very useful tool in research since parametric studies can be easily carried out. For any analytical study, a few parameters have to be determined through wind tunnel tests.

CFD: In Computational Fluid Dynamics (CFD) studies, like in analytical studies, the structures are modelled in structural dynamic sense. But the wind flow is modelled using basic fluid dynamic equations such as continuity, energy and momentum equations. There after a specific turbulence model is used and the equations are solved using some numerical techniques and the responses are obtained. This technique is becoming popular and being used in studies of pedestrian level wind flow, internal flows for air quality, pollution studies, topographical studies etc.

Damage Survey: This is a rather new science that has emerged in the last two decades. Once the damage has occurred, pool of experts visit the place and try to investigate how it happened. Through this we try to relate the extent of damage with the wind speed. They may carry out some simple laboratory tests as apart of the survey.

Codes / Standards : Code s /standards are widely used in India for determining wind loads for design. They are sufficient for preliminary design however, considering the numerous non-typical geometries of the structures, complex surroundings and complex structural design which are not covered by the codes/standards,for final design, other established means such as wind tunnel tests are essential.

Wind Tunnel Testing

Wind tunnel tests’ capability to replicate building geometry, local climate and surrounding details leads to cost-effective and accurate wind loading on cladding and structural frames of tall buildings. Code analytical methods are helpful for preliminary design, but provide conservative wind loads. Presently, wind tunnel model studies offer the best estimate of the wind loading acting on a building for cladding as well as structural frame design. In addition to the potential cost savings and accurate results,wind tunnel studies confirm that the architect’s vision can be safely built and elevate litigation protection. Most of the codes themselves recommend wind tunnel testing for complex and tall buildings.

Wind Climate Studies: In the chain of wind loading studies, wind climate is the first item to be dealt with. It helps identify the wind speed for a specific location as well as the wind directionality. Depending on theorientation of the structure, winddirectionality has a huge influenceon the local as well as overall loadson the structure.

Cladding Wind Load Study: In this study, pressure taps are installed at various locations of interest on the building models. Later on, during the wind tunnel experiments, the statistics of pressures are measured at these locations and subsequently,combined with the met climate to determine the design external pressure. There after, these external pressures are combined with the internal pressures to come up with the recommended cladding loads.Wind tunnel testing is beneficial in specifying the required strength of the membrane or curtain wall material. This provides a potential saving to the owner by avoiding the over specification of material.

Structural Wind Load Study: Similar to cladding studies, series of pressure taps are evenly distributed on a building to measure simultaneous loads on different parts of the building. These pressure variations can be combined to come up with over all loads acting on buildings. Another method would be to use very light weight and stiff model known as High frequency Force Balance model and put it on a stiff balance to measure the moments and shears at the base using gauges. Wind tunnel testing is almost universally used on tall structures in order to optimize the structural design. This has the dual benefit of potentially providing cost savings to the owner or identifying any unusually high wind loads.

Pedestrian Comfort Wind Study: Pedestrian comfort, in terms of wind speed and direction at the base of the structure, entrances and roof are simulated and the comfort level of the wind environment in these areas are assessed. Complaints due to excessive winds at key pedestrian areas can compromise the image of the development in the eyes of the public. A Pedestrian Wind Study will assess the comfort level around the proposed development and provide recommendations for improving it.

Effects of geometr, surroundings and wind directionality on the wind loading on facades can only be accurately determined through wind tunnel study. For example, traditional 1:400 scale wind tunnel testing was adopted for the over all cladding pressure study of a 140m high tall building development consisting of a two layer facade system with a gap of about 2.0m between the layers. The external facade was quite complex made of flat panels placed with a clear gap of 0.25m in-between and glass fins placed with a clear gap of 1.2m between them.

In this case, wind will flow through the external facade gaps and pressurize the gaps between the layers. This will result in a change in pressure on the internal facade as well as net pressure across the outer facade. At a reduced scale of 1:400, the through-flow gaps on the exterior facade as well as gap between the two facades become quite small, as a result, the flow through these gaps can be laminarat model scale rather than turbulent as expected at full scale. To address this concern, the external pressure values on the interior facade behind the flat panels as well as the net pressure across the exterior facade measured were confirmed and/or adjusted through wind tunnel tests of selected sections of the two layer facade system at a larger scale (1:100). These larger scale tests appropriately simulated the flow through the gaps in exterior facade.

Based on both of these scaled model wind tunnel tests and further detailed analysis, wind pressures on facades were recommended for this project. During the final stages of the analysis, the derived peak negative pressures and peak positive pressures were plotted at corresponding tap locations.Thereafter, taps with similar pressure values have been categorized into a particular pressure zone. The pressure zones were created with 0.25kPa increments so that the pressure indicated the maximum pressure in that particular zone. A sample recommended peak negative pressure corresponding to the top portion of the east elevation facade, where the recommended cladding for design was provided for both exterior and interior facades. Similar pressure diagrams corresponding to all the elevations and roof plan were provided tocurtain wall consultant for the appropriate selection of glass.

Scenario in India

Currently, the general trend is not to carry out any special wind tunnel studies and the building design is simply based on the wind load provisions in the IS: 875 Standard.Even with the Standard, there are cases where the wind load code provisions are often misunderstood.This lack of understanding could be resolved easily by including a course in the undergraduate curriculum addressing dynamic lateral load sand their calculation using IS codes.I think this is essential considering the increasing number of high-rise buildings currently being proposed in India.

Most users are unaware that the Standard IS: 875 is derived from a set of wind tunnel experiments of buildings with simple geometries.One of its limitation notes is as follows: IS:875 Part 3 1987– This standard does not apply to building sand structures with unconventional shapes, unusual locations, and abnormal environmental conditions that have not been covered in this code.” Regardless of this warning,the practitioners are using this code over looking this crucial aspect.

In my recent papers I have addressed the major pitfalls of the existing code. In addition to its stated limitations, the current code is seriously underestimates the local façade loads on edges. It is recommended that peak pressure coefficient of -1.8 is used in contrary to the -1.2 in the current code for the design of façade at the edge zone of 20% building width. The existingvalue of -1.2 seems good for thecentral portions, probably slightly onthe conservative side. Code needs immediate revision before it gets too late since IS875: Part 3 code has been widely used in façade design.

Another issue is the cost. Frequently when clients request a wind tunnel test, the first item for discussion is the cost related to the tests. In many parts of the world, the cost of the wind engineering studies is mere 10% of the savings that can be realized in just the structural system alone. Also, in general, the code derived loads are conservative but by doing wind tunnel studies one can further reduce loads and save in construction costs.

As we are now designing tallerbuildings with more challenging designs, it is imperative India move towards meeting international design standards. We require stringent guidelines and enforcement policies to promote this science effectively for the benefit of the public. The immediate requirement is to update the existing 27-year old wind code, since many of the on going designsare based on this leading to cladding failure instances.

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