Nicholas Isyumov

Wind pressures acting on low-rise buildings

Abstract Data gathered from a number of field and laboratory experiments concerned with wind pressures acting on low-rise buildings are reviewed, and selected experimental results are presented in this paper. Particular attention is paid to works related to cladding design. Only either full-scale studies or those done under conditions simulating the atmospheric boundary layer have been considered. Comparisons of the data from various sources are made for the characteristics of the mean and fluctuating wind pressures. The results indicate that the statistical properties of fluctuating pressures on the roof edges and corners can be predicted by a quasi-steady approach. Furthermore, the peak-factor approach is found to perform adequately in evaluating the design wind loads. The relation between the spatial and time averages is also discussed.

Wind induced torque on square and rectangular building shapes

Abstract This paper examines the anatomy of the wind induced torsional moments on buildings of square and rectangular cross-section. This is achieved by using weighted pneumatic averaging to measure the mean and dynamic torque components contributed by particular portions of the building perimeter. Changes in the lift force and its centre of action are major sources of torsional moments and quasi-static assumptions are found to provide good estimates of the dynamic torque in some situations. An important finding is that vortex shedding induced pressure fluctuations on the back face can provide an important contribution to the dynamic torque.

Reduction of tall building motion by aerodynamic treatments

Abstract In recent years the subject of tall building motion and its reduction has received considerable attention. With present trends towards taller, lighter, and more flexible structures, the importance of this topic is ever-increasing. Of particular interest are methods to reduce the potentially severe across-wind motion due to vortex shedding excitation. One such method presently under investigation at the Boundary Layer Wind Tunnel Laboratory (BLWTL) is the aerodynamic modification of the building shape to disrupt the vortex shedding process. This paper reports on a wind tunnel model study of a slender tall building of square cross-section. The aerodynamics are modified by introducing openings or “gaps” in the upper half of the building. These gaps extend through the entire building. Different gap widths are examined and two types of data are presented. First, information is presented on the effects of these gaps on overall forces and responses of a 9:1 aspect ratio building, tested on a high frequency force balance. Second, the effects on the time-varying pressures are examined with a 2-dimensional model tested in uniform flow. This study is ongoing and further findings will be reported elsewhere.

Experimental study of the wind forces on rectangular latticed communication towers with antennas

Abstract With todays expanding communication systems, a large number of lattice towers to support cellular antennas are being constructed in Brazil. Due to the lightweight of these structures, wind forces are the primary concern in the design. An experimental investigation on the subject was carried out at the Boundary Layer Wind Tunnel Laboratory, University of Western Ontario (UWO), Canada. Three section models were designed and constructed based on existing latticed towers built in Brazil. The wind incidence angle; the tower solidity; the shielding effect; the influence of the wind turbulence on the drag coefficient were analyzed. Measurements were made of the mean and RMS drag and crosswind forces. The results were compared with some existing codes and standards including the Canadian (NBCC, 1995), American (ASCE 7-95, 1995), Australian/New Zealand (AS/NZS 1170.2-2002), Australian (AS 3995-1994), British (BS8100, 1986), Eurocode 1 (European Committee for Standardization, 1995) and Brazilian (NBR 6123, 1988). It is a common approach to consider the wind forces on antennas independent of the lattice tower, without considering the effects of their presence on the computation of the wind forces. The question arises whether this is a good approach or not. These effects can be described by introducing an interference factor. This factor depends, among other things, on the tower solidity. Two models with different solidity were tested for wind incidence angle of 0 degrees and antenna dishes simulated with disks made of Styrofoam attached to the windward face. The results were compared with ESDU.

Performance of a tall building under wind action

Abstract Predictions of the wind-induced response of a projected 390 m high office tower have been made using high frequency base balance model and aeroelastic model techniques. Comparisons of the results from these two types of tests show that additional motion-induced forces in this case are small and that satisfactory estimates are obtained with the much simpler high frequency base balance model. Various aspects of the wind-induced response, including peak and joint action factors, the aerodynamic damping, the influence of building top geometry, and the effect of other buildings in the near field, are discussed.

Performance of a tuned sloshing water damper

Abstract A study was carried out to investigate and evaluate the performance of a one-dimensional tuned sloshing water damper (TSWD). The study examined the performance of a 1:13 scale model of a full scale TSWD. During the course of the study, the frequency and amplitude dependent response characteristics of the TSWD were determined experimentally, an effective method of increasing the inherent damping, and therefore the performance of the TSWD, was developed and verified, a theoretical model based on the linear theory was formulated, and the effect of the TSWD on a model structure was quantified.

Empirical aerodynamic damping function for tall buildings

This paper describes an empirical aerodynamic damping function for tall buildings and prisms. Empirical aerodynamic impedance functions (damping and stiffness) have been proposed. Their applicability has been examined using experimentally obtained aerodynamic damping values for prisms. It has been demonstrated that the proposed aerodynamic damping function can simulate aerodynamic damping of tall buildings quite well. Some physical conditions such as tip-amplitude, aspect ratio, shapes of cross section of the building and intensity of turbulence of the wind have been related to the parameters of the aerodynamic damping function.

Comparison of full-scale and wind tunnel wind speed measurements in the commerce court plaza

Abstract This paper presents comparisons of full-scale measurements of the mean wind speed and mean wind direction at several points in a plaza located in a built-up urban environment with similar measurements obtained in a scaled boundary layer wind tunnel model study. Full-scale measurements were carried out with a portable propeller vane anemometer positioned at a height of 9 ft. above local ground. Full-scale measurements were carried out over approximately a two-week period. Wind tunnel measurements of wind speed were carried out with special single-ended hot film anemometer probes. Measurements of wind direction in the wind tunnel were made with a flow indicator consisting of a light sphere mounted on a flexible support. Although the full-scale data base was not adequate to allow a comprehensive comparison, the agreement between the full-scale and model observations was encouranging, particularly in windy areas of the plaza. The agreement between wind tunnel and full-scale mean wind speeds was found to be within about 10% for relatively windy areas of the plaza.

A scour technique for the evaluation of pedestrian winds

Abstract A “scour” technique can be used as a convenient method to provide an overall indication of wind conditions at pedestrian level. This visual technique is particularly useful in situations where a large area is to be examined. In experiments using the scour technique, a model of the site of interest is covered with a uniform thin layer of some particulate material. The wind tunnel speed is increased in several stages; at each stage, scouring is allowed to reach a steady state. The resulting scour indicate areas where the wind speed is greater than or equal to the predetermined threshold scour speed of the material. This paper describes comparisons of results of this technique with quantitative data obtained from hot-wire anemometer tests. These comparisons indicate that this technique yields numerically reliable results, as well as an overall visual representation of the pedestrian-level environment.

Wind tunnel model tests of snow drifting on a two-level flat roof

Abstract Consistent similarity of the flow regime and the behaviour of individual snow particles is difficult to achieve at a reduced geometric scale. As a result, model studies of snow drifting and accumulation invariably require the relaxation of some of the similarity parameters. This paper examines drift formation on the lower level of a large-area two-level roof at different wind speeds and in two different approach terrains. In relatively rough terrains, the effective surface shear stress is determined primarily by the collective drag of various buildings and the loss of horizontal momentum during saltation is less significant. In such situations both the deposition and drifting of snow at roof level is strongly influenced by the overall aerodynamics of the building shape and its surroundings.