Buildings are obstructing the motion of air called wind, and as a result the wind is exerting force on buildings. Wind force can be classified into two: the first one represents the overall wind action on the building inducing overall integrated loads called structural loads on the structural systems. The second one refers to the action of wind on the building façade known as cladding loads. As we all know, the cladding and the various building components are relatively small elements and their size is typically very small in relation to the entire structure. So the localized wind pressure variations are very crucial for their design which is the subject matter of discussion in this paper.
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In general, in India, less attention goes into the wind loads acting on façades in contrary to the overall structural loads. This translated into several cladding failures in India recently. Though the high wind speeds are being blamed for the failures, the reality lies with inadequate design and poor workmanship.
pressures acting on building façade
Figures 1 and 2 pictorially show the localized wind phenomenon on claddings and building components. Depending on the location of the cladding elements and wind direction, the subjected wind pressure can be different. The maximum positive pressures generally occur on the windward wall and all the façades will become windward for certain wind directions. As far as positive pressures are concerned (Figure 1), the wind pressure increases as the height increases. This means the lower portion of the building will be subjected to lower pressure and the higher portion of the building will be subjected to higher pressure except the top edges where the wind tries to negotiate with the edges which results into lower positive pressures. Further, the plan also shows the positive wind pressures are lower at the edges and higher at the middle of the façade. On the other hand, the building façades on the leeward side as well as the side façades are subjected to negative pressures (Figure 2). The negative pressures (suction) can be very high at the corners due to the flow separation/vortex shedding. Suction pressures can also be very high at the top and bottom corner regions due to intense flow separations. So generally the high suctions need not be associated with the higher heights/higher speed, instead associated more with the aerodynamic reasons.
How to Find Wind Loads on façades?
The most common way to find wind loads on façades is to estimate the loads based on local codes/standards. In India, we can use IS:875 (Part3) code to estimate wind loads on façades. Like any other Standard/Code, the Indian Standard also states its limitations as follows:
As per IS:875 Part 3 1987 (Page 5), “Note 1 – This standard does not apply to buildings and structures with unconventional shapes, unusual locations, and abnormal environmental conditions that have not been covered in this code. Special investigations are necessary in such cases to establish wind loads and their effects. Wind tunnel studies may also be required in such situations.”
In summary, IS:875 (Part3) can be utilized for preliminary estimation of loads. Later, wind tunnel tests shall be carried out to finalize the loads acting on the façade.
Wind Tunnel Test
In wind tunnel tests, scaled models of structures are subjected to scaled atmospheric wind in a controlled laboratory set-up. Then sensors installed on the model can measure the physical quantities of interest which is wind pressure acting on the façade. 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 constructed only after being confirmed through wind tunnel tests. As a general practice, wind tunnel tests are being done for almost all buildings above approximately 100 m.
Typical model scales are in the range of 1:300 to 1:500. Since the response of the structure is significantly influenced by its geometry, utmost care has to be taken in modelling the exact shape of the structure including all the external architectural ornaments such as fins, balconies etc. Typically, all elements more than 1ft can be modelled with the typical scale range noted above. However, certain simplification of the external architectural features is allowed/suggested at the modelling stage by wind tunnel experts.
Selected wind tunnel test cases in India are shown in Figure 3. Note that the fan behind the spires is used to blow the wind through the test section. The upwind spires and appropriate floor roughness elements are used to generate the full-scale wind characteristics (i.e., mean and turbulence profiles). Surrounding buildings for half a kilometre in radius around the study building are also modelled and placed in the disk during the test in order to get the influence of immediate surroundings. Further, the disk is rotated at every 10 degree in order to subject the building to various angles of attack similar to full-scale condition.
When to Carryout Wind Tunnel Test
Interestingly enough, nowadays developers and architects are coming up with unconventional building shapes with offsets, setback, various corner shapes, balconies, fins etc. Further, the buildings are mostly located in complex surroundings along with other structures. These conditions were not addressed in any of the international codes and standards including Indian Standards IS: 875 (Part3). In addition to this, the effect of building response due to its orientation with respect to the wind directionality of the site is not covered in detail in any of the International codes. All the codes and standards are based on box shape buildings in isolated condition. In general, Code analytical methods are helpful for preliminary design and for simple situations, but provide conservative wind loads in most cases; underestimating in others. Presently, wind tunnel 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.
The results from wind tunnel tests can be lower than code predictions due to unconventional geometry, complex surroundings as well as wind directionality. However, some allowance for possible future changes in surroundings has to be provided. Therefore, finally a minimum load need to be derived and recommended for design based on wind tunnel results and code predictions. This recommended load supersedes code predictions and should be used for design. Note that wind tunnel testing is a proven methodology for the prediction of building response and this technology has been under use for the last few decades and most high-rise buildings are constructed and safely serving occupants for many decades around the world.
As far as wind loads on façades are concerned, it is recommended to carry out wind tunnel tests when buildings are above 120 m above grade, their geometry is complex and they are in the midst of complex surroundings.
Construction industry in India slowly understands the need for wind engineered buildings. Time and money are only secondary to safety and accuracy while designing building’s façade system for wind. On the other hand, optimization of the building façade system could also result into substantial savings which can be a by-product of the wind tunnel test. Wind tunnel tests are the most advanced and the only reliable way of finding out realistic wind-induced loads acting on building façades. I encourage everyone to pay due attention to the subject matter.
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