WIND ENGINEERING:
A Modern Tool Aiding Architecture and Structural Engineering

- Amit Mishra  /  amishra@cppwind.com

Introduction

Modern day construction is scaling new heights and so is the need for improving efficiency and reliability of these structures. Wind engineering is a well-established tool that helps increase the safety, reliability and usability of any structure before it is actually constructed. A wind engineer investigates how wind and moving air affect - and are affected by - buildings, structures, and terrain. This investigation provides solutions to wind related problems that could arise after the building construction.

Although many people are associated with the construction of a building, the architect, structural engineer and the owner of the building play the most important roles, from the design to the completion phase. Thus it is important that they understand the tools available to make their design more efficient.

The architect designs the building with a creative and aesthetic point of view. His main focus is on the overall look of the project, the type of cladding material, location of plazas and fountains, usability of the spaces and so forth. The wind engineer provides the architect with a snapshot of how the wind would affect his design. For example, he might find out that the spot chosen by the architect for a water fountain or a pool might be extremely windy and unacceptable to the clients, or the cladding chosen is not strong enough for the local wind conditions etc. The wind engineer can then help improve the building and the surroundings with his suggestions.

The wind engineer provides the structural engineer with the structural load on the building due to the local wind conditions. The serviceability of tall buildings can be impeded by motion-induced occupant discomfort. This motion is caused by wind turbulence and high velocity winds approaching the building site. Tenants living in penthouses on higher floors of tall buildings may complain about the extreme swaying motion of the building. The wind engineer can estimate the acceleration on the top floors for different sets of building strength properties and advise the structural engineer about structural properties that would better suit the project.

The owner tries to concentrate more on customer satisfaction like location of a plaza, balconies or an entry to the shopping complex and would like these spots to be pleasant for the clients, wind engineering can provide a detailed analysis of these locations and advise the owner about the usability of these locations during the design phase.

Wind Tunnel Study

The first step in wind engineering is to study and reproduce the local wind conditions around the building project. The most widely used method is the physical simulation of the atmospheric wind using a wind tunnel. The atmospheric wind flow follows a particular pattern called the boundary layer flow in which the wind velocity near the ground is low and gradually increases with height to a certain point beyond which it is almost constant. Techniques have been developed which permit boundary-layer wind tunnel testing of proposed buildings to determine wind pressure on cladding and windows, overall structural loading, and wind velocities in pedestrian areas. As examples, the wind tunnels used at CPP Inc., are shown in (Figure 1).

To perform a wind tunnel test, a model of the building under study is constructed of epoxy resin material at a scale of around 1:300 (Figure 2).

Because nearby buildings or terrain can have an important influence on wind loads, the surrounding area is modeled in detail to a radius of about 500 m using styrofoam and wood (Figure 3). A prudent wind engineer also attempts to model new construction in the vicinity of the structure so the wind study is as physically accurate as possible in the near future. The wind moving close to the earth is influenced by the earth’s roughness, which varies according to the location of the building. The surface roughness is simulated in the wind tunnel using generic roughness cubes (Figure 3).

Pedestrain Wind Conditions

An important issue tackled by the wind engineer is the pedestrian wind environment. The architect designs the building by keeping the surroundings in mind, like plazas, porte cochere, swimming pools, water fountains and running tracks. A windy environment around the base of a building, particularly near a main entrance or plaza area, will detract from the appeal of the site and perhaps discourage clients and shoppers from visiting the area. A number of high-rise condominium projects have had pool decks that were so frequently windy that residents complained the areas were unusable. Even if the conditions surrounding the building are not very windy, the design of the structure itself can cause regions of high wind speeds, decreasing pedestrian comfort and, in extreme cases, safety issues.

In order to understand the wind conditions for the pedestrian, the wind engineer reviews the architectural drawings and after discussion with the architect decides the locations where people are likely to congregate. He then tests the building model with its surroundings and provides a detailed analysis of the wind speeds in different pedestrian locations. Priority is often given to points near the pool, on balconies, plazas and walkways. An initial visual assessment is made by using smoke to observe the wind pattern in regions of utmost importance, (Figure 4), where the pool deck area of a condominium is being visually observed to understand the windiness of the region. Instruments like the hot-wire anemometer are then used to measure quantitatively the velocity of wind. Results are compared with peer-reviewed guidelines to help determine the acceptability of a given location. The wind engineer then suggests potential mitigation methods. These include using porous fences or foliage to block or direct the wind flow away from the area of concern. Sometimes measures like using dense foliage is also recommended. For some flow types a canopy or porous trellis is a better solution than a porous barrier. These methods may be quite helpful in improving pedestrian conditions for many condominiums and hotels.

Cladding Pressures

It is common for wind pressures to drive the design of the cladding. The wind loading on a structure may be estimated by using the local wind code. However, simply applying the codes does not account for any unique building geometry or unusual wind conditions and so limits its use. The code may indicate a particular cladding pressure for a whole portion of a building, even if these regions of the building do not all need the more expensive cladding materials. On the other hand, some regions might need a stronger cladding than the code indicates due to extremely high local wind pressures. Either scenario leads to inefficient cladding design. When a building is wind tunnel tested, the cladding is divided into separate pressure zones according to the specifics of the design. Where higher wind pressures occur, a stronger cladding can be used. In regions of lower wind pressure, less expensive cladding material may save the owner unnecessary expense.

Structural Load & Floor Accelerations

Design of the structural frame for a building requires that the frame wind loads be accurately determined. The critical condition often involves simultaneous loading along both perpendicular horizontal axes in the presence of a significant torsional loading. These conditions may occur even on buildings that are symmetrical in shape and structural properties. Current codes do not adequately address simultaneous loading in perpendicular directions or torsional loading. Wind tunnel testing techniques addresses these concerns and provides realistic frame wind loads for the structural designer. The maximum moments and torsions, as well as floor-by-floor loads are provided to the structural engineer based on the information obtained from the wind tunnel.

There is no satisfactory analytical method available to predict the vibration of tall buildings due to wind. However, a wind tunnel test of tall buildings before construction helps determine the acceleration response. This acceleration can be computed for a range of recurrence intervals from, say, 1 to 20 years. The result is then compared with internationally recognized guidelines. If the building is found to have excessive acceleration response at the top floor, remedial measures can be undertaken like increasing the stiffness or providing shear walls in required spots, thus improving the serviceability of the building.

Myths & Facts

A general misconception about wind engineering is that it may interfere with the architect’s vision for the building design. The fact is that wind engineers try to find realistic solutions to problems so the architect can maintain his design with fewer alterations while having improved efficiency and reliability at the same time.
Wind tunnel testing in the 1970’s and 1980’s was seen as an uncommon and expensive methodology only meant for elite and expensive landmarks, like the world’s tallest buildings. But with the advancement in technology and a better understanding behind the physics of the wind-structure interaction, more and more architects and owners are testing their building designs to increase safety and efficiency. The testing also creates better curtainwall designs and helps to avoid expensive repairs and replacements.

Increased Scope of Work

The last few decades have seen rapidly increasing use of wind tunnel testing for tall buildings, but recently, the trend has included the testing of much shorter structures as well. A very good example is the Brooklyn Museum of Art (Figure 5).. The museum planned a new glazed entry to the building which was wind tunnel tested for wind loads on the glass elements. Stadiums have also entered the wind tunnel in recent years. Any wind-induced damage to the roof and stands of a large stadium can lead to losses of millions of dollars for repairs and liability. To avoid those costs, the stadium can be tested for wind-induced dynamic loads before the structural engineer finalizes the design. Wind loads on a complicated structure like the new Denver Art Museum wing (Figure 6). also need special attention since the conventional building codes cannot be applied to such an unusual geometry.

Computational Wind Engineering

Computational wind engineering (CWE), the latest development in the field, is the application of computational fluid dynamics, or CFD, to wind engineering topics. High-speed computers solve the non-linear equations of turbulent fluid motion and simulate moving air. The field is in its early stages and lot of research needs to be done before CWE can be used extensively for wind engineering purposes. Currently this method has been proven to be adequate for indoor airflow conditions and flow over complex, mountainous terrain is in research stage. In the former case, a 3D computer aided design of the model is generated using software applications like Solidworks and AutoCAD. The model is then used by the CWE software to predict the internal airflow conditions driven by cooling and heating, the dispersion of hazardous materials, or the spread of smoke in case of fire. This helps improve the safety of the building even before it is built.

Hurricanes & Wind Engineering

Hurricanes are the most powerful large-scale storms on earth. Global insurance statistics have shown that windstorms account for more losses than earthquakes, floods, and human damage combined. Severe tropical cyclones are responsible for a large proportion of wind-induced losses. In hurricane-prone regions, the sustained wind speeds during a storm can reach 145 mph (65m/s) or more. In such high-wind events, building damage often occurs when a small section of the building envelope fails. This often leads to failure of much larger areas of the envelope because of increased internal pressures and additional damage to the building’s contents from encroaching water. This leads to heavy insurance losses and can hurt the reputation of the architects and the owners.

A clear example can be seen in the recent hurricane Katrina in the United States. In 1996, the design of the New Orleans Arena, which is adjacent to the New Orleans Superdome, was tested for wind loads on the building’s cladding and structural frame from 36 directions. Design recommendations to enhance the arena’s resistance to wind damage and to increase the efficient use of materials were made. The arena withstood the hurricane winds unlike the adjacent Superdome, which was not wind tunnel tested and did not fare as well in the high winds (Figure 7).. During Hurricane Katrina, the roof weather covering failed while thousands were sheltered within the building. The repairs and damage to such structures can be avoided by comprehensive wind tunnel testing. This is especially true for the US hurricane coast, which is hit by many hurricanes during the life of a building.

Overview

Wind engineers use their knowledge to improve the safety, efficiency, and comfort of the built environment. Pedestrian wind conditions, cladding designs, structural loads and motion due to wind can be estimated using this tool during the design stage. Early detection of high wind pressure on claddings and unacceptably windy conditions for pedestrian can give the architects the necessary information to improve their designs and avoid sub-optimal designs, which would be much more costly to correct after the project is completed.
The structural engineer is provided with the wind loading that they would then incorporate in their analysis, which helps in improving the safety and stability of the structure. A wind tunnel test also assures the owner of the building about the overall feel and usability, thus creating a win-win situation for everyone involved in the process of the property development.