Rigid Model

Although the primary purpose of the rigid-model test is for obtaining cladding design pressures, the data acquired from the wind-tunnel tests may be extrapolated to get the floor-by-floor shear forces for the design of the overall main wind-force-resisting frame.

Most commonly, pressure study models are made from methyl methacrylate sheets commonly known as Plexiglas, Lucite, and Perspex. This material has several advantages over wooden or aluminum alloy models because it can be easily and accurately machined and drilled and is transparent, facilitating observation of the instrumentation inside the model. It can also be formed into curved shapes by heating the material to about 200°C. Model panels can either be cemented together or joined, using flush-mount screws.

A scale model of the prototype in a 1:300 and 1:500 range is constructed at the testing facility by using architectural drawings provided by the project architect. In a rigid model, building features that have significance in regard to the wind flow, such as building profile, protruding mullions, and overhangs are simulated to the correct length scale. Wind measurements obtained are only for the mean and fluctuating pressures acting on the building.

The model typically instrumented with a large number of pressure taps (sometimes as many as 500 to 700), is tested surrounded by a detailed modeling of nearby surroundings within a radius of 1500 ft (457 m), as shown in Fig.1.21. Flexible, transparent vinyl or polyethylene tubing of about 1/16 in (1.5 mm) internal diameter is used as pressure tappings around the exterior of the model. Pressure tap locations are generally more concentrated in regions of high pressure gradients such as around corners.

The wind-tunnel test is run for a duration of about 60 sec which corresponds to approximately 1 hr in real time. Sufficient numbers of readings are obtained from each port to obtain a stationary value such that fluctuations become independent of time. From the values thus obtained, the mean pressure and the root-mean-square value of the pressure fluctuations are evaluated.

The boundary-layer wind tunnel, by virtue of having a long working section with roughened floor and turbulence generators at the upwind end, is expected to correctly simulate the mean wind speed profile and turbulence of natural wind. The model is mounted on a turntable to allow measurement in any wind direction. Near-field characteristics around the building are duplicated, typically using polystyrene foam models.

1.5.1.1. Cladding Pressures

Measurements are taken for representative wind directions, generally spaced about 10 to 20° apart. From the data acquired, full-scale peak exterior pressures and suctions at each tap location are derived by combining the wind-tunnel data with a statistical model of windstorms expected at the building site. The results are typically given for 25-, 50-, and 100-year return periods.

In evaluating peak wind loads on the exterior of the prototype, the effects of internal pressures arising from air leakage, mechanical equipment, and stack effect should be included. The possibility of window breakage caused by roof gravel scoured from roofs of adjacent buildings and other flying debris during a windstorm should also be included. As a rough guide, the resulting internal pressure can be considered to be in the range of

Figure 1.21. (a) Rigid models of high-rise buildings in a wind tunnel; (b) close-up view of a pressure model. (Photographs courtesy of Dr. Peter Irwin, Rowan, Williams, Davis & Irwin, Inc.)

Figure 1.21. (a) Rigid models of high-rise buildings in a wind tunnel; (b) close-up view of a pressure model. (Photographs courtesy of Dr. Peter Irwin, Rowan, Williams, Davis & Irwin, Inc.)

± 5 psf (25 kg/m2) at the base, to as much as ± 20 psf (100 kg/m2) a the roof of a 50-story building.

In the design of glass, a 1-minute loading is commonly used. The duration of measured peak pressure in a wind tunnel is different from the 1-min interval; usually it corresponds to 5 to 10 seconds or less in terms of real time. Therefore, it is necessary to reduce the peak loads measured in wind-tunnel tests. Empirical reduction factors of 0.80, 0.94, and 0.97 have been given in glass manufacturers' recommendations for three different types of glass—annealed float glass, heat-strengthened glass, and tempered glass.

1.5.1.2. Overall Building Loads

The results of rigid-model tests are used to predict the design wind loads for glass and cladding. For buildings that are not dynamically sensitive to wind, the results can nevertheless be extrapolated to obtain lateral loads for the design of the main wind-force-resisting system of the building. The procedure entails introducing a gust factor for converting the mean wind load to gust loads. An appropriate gust factor estimation should take into account:

• Averaging period of the mean wind load

• Terrain roughness in relation to the building height

• Peak gust factor, which depends on the natural frequency of the building

• Effect of turbulence

• Critical damping ratio of the building.

In spite of the fact that rigid-model wind study does not take into account all of the preceding factors, it is still considered to provide adequate design data for buildings with height-to-width ratios of less than 5.

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