Peter A. Irwin

A comprehensive assessment of pedestrian comfort including thermal effects

Abstract The effects of wind force on pedestrians have been a concern since it was realized that tall buildings could greatly accelerate the wind at grade. The use of wind tunnels to assess pedestrian level winds has lead to the development of criteria not only for safety but also for comfort. More recently there have been increasing attempts to develop more comprehensive criteria that include more of the overall microclimate rather than wind in isolation. This paper describes new methodology developed by the authors. It includes the effects of wind speed, temperature, relative humidity, clothing, activity, solar radiation, and exposure time. Thus not only is wind force considered but also the impact of wind chill on exposed skin and a persons thermal comfort. The assessment model has been designed to allow for customization by the various end-users and is capable of being upgraded to accommodate other input parameters of importance to pedestrain comfort, such as noise and air quality.

Application of the force balance technique to a building complex

Abstract A general approach using a multi-high-frequency-force-balance system (MHFFB) is presented to determine the wind-induced response of a complete building complex from wind tunnel tests. A building complex consists of several building towers being structurally connected either through skybridges or by a common podium. By dividing the whole structure into several substructures, the overall generalized wind loads can be determined from simultaneous measurements on each individual substructure. A treatment for nonlinear mode shapes is also presented. A qualitative analysis for a twin tower structure reveals a general phenomenon that the structural connections between towers tend to increase the wind-induced response for the tower which experiences relatively lower wind loads but tend to decrease the wind-induced response for the tower which experiences relatively higher wind loads. The presented method provides a tool to quantitatively estimate this effect. The application of the presented method can also be extended to some torsionally sensitive structures, for which the traditional force balance technique can hardly provide satisfactory estimates.

Ultimate Wind Load Design Gust Wind Speeds in the United States for Use in ASCE-7

This paper presents an overview of the approach employed in the development of the wind speed maps for use in ASCE 7-10. The reason for a reduction in the wind speeds in the new standard as compared to those given in the ASCE 7-98 through 7-05 standards is presented, as well as the reason for the reintroduction of Exposure D along the hurricane coastline. The most significant change in the wind speed maps from the previous version is the shift from a single map for an importance factor for buildings and other structures of 1.0 to three separate maps, one for each category of occupancy, thus eliminating the need for importance factors that vary between hurricane and nonhurricane regions.

Pressure model techniques for cladding loads

Abstract The pressure model test is now used as a well accepted procedure for determining wind pressures on the exterior cladding of tall buildings. The methods used in a pressure model study are reviewed including measurement system frequency response, the determination of peak pressure coefficients, combining wind tunnel and meteorological data and evaluating internal pressures. In addition, an assessment is made of the uncertainties involved in wind tunnel testing as compared with using building code methods.

The use of integrated pressures to determine overall wind- induced response

Abstract A new wind tunnel testing technique has been developed which makes use of integrated local pressures, measured by a Synchronous Pressure Acquisition Network (SPAN), to determine overall wind-induced response. The integrated pressure modal load or IPML technique has the potential of addressing all of the limitations of the conventional high-frequency force-balance technique while still maintaining the same advantages that that technique has over the aeroelastic modelling. This paper outlines the approach and presents several experimental results including comparisons with data from matched high-frequency force-balance tests.

Risk considerations for internal pressures

Abstract An approach, in which the probability of there being an opening is treated as one other factor in the risk calculation, rather than as an “all or nothing” choice between one possibility and another, is described. The approach is well suited to wind tunnel studies and uses a modification of the upcrossing method to incorporate the risk associated with there being an opening.

Wind induced Building accelerations

Abstract This paper examines the ability of two semi-empirical approaches to estimate wind induced accelerations in tall buildings as compared to results obtained from wind tunnel measurements on scale models. The estimating methods include a code based approach found in The National Building Code of Canada (NBCC) and an alternate semi-empirical approach which incorporates more detailed building and wind properties in the analysis steps. These techniques are applied to 48 buildings tested to date.

WIND-INDUCED STAY CABLE VIBRATIONS - MEASUREMENT AND MITIGATION

In February 1998, stay cable oscillations were observed on the Cochrane Bridge, Mobile, Alabama in winds accompanied by rain. Rowan Williams Davies & Irwin Inc. (RWDI) was retained by A.G Lichtenstein & Associates to assist them in identifying the causes of the cable oscillations, to assess the potential of future wind-induced cable vibrations, and to design a damping system to mitigate the oscillations. RWDI performed on-site oscillation decay measurements for some of the stay cables. Inherent damping values for the cables were calculated from the decay traces. Using these damping values and a criterion based on Scruton number (mass damping parameter), the potential for rain/wind type oscillations for the stay cables was assessed. In addition, the cables’ susceptibility to other forms of wind-induced vibrations was reviewed. The program of work provided an opportunity to check the validity of criteria for wind-induced oscillations against field observations.

Variability of cladding pressures caused by adjacent buildings

Abstract A question that arises in wind tunnel testing of buildings is how to take into account the changes in wind load that might occur as a result of future buildings being built nearby or existing buildings being demolished. This paper addresses this question in so far as it affects cladding loads. The implications of applying a “lower cut off” to peak suctions determined from wind tunnel tests is explored in the context of structural reliability. It is found that high overload situations caused by future buildings are largely avoided when a “lower cut-off” is applied to wind tunnel results. Also, a technique of “correcting” wind tunnel results by an amount equal to the estimated increase in load factor due to future buildings is examined. While slightly more complicated to apply, this last approach is better in providing more uniform structural reliability.

Recent Developments Concerning Vortex-Induced Oscillations of Edge Girder Bridges

A common configuration of decks on long span cable stayed bridges is to have local stiffness provided by plate girders located near the two edges. However, these sections are susceptible to vortex-induced oscillations that can be disturbing to bridge users and may potentially lead to other problems such as fatigue damage. A recent wind-tunnel investigation was conducted using a generic edge girder section model with a variable edge girder depth. This was done to capture the response characteristics of a range of practical cross-section geometries. The paper outlines the characteristics of both vertical and torsional vortex-induced responses observed in this investigation. Incorporating these response characteristics with the results of various wind tunnel studies, the effectiveness of baffle plates as possible mitigation measures is discussed. The characteristics of the responses, specifically the differences between the vertical and torsional vortex-induced oscillations, are important when optimizing the design of mitigation measures. Approximate prediction methods using Strouhal number estimates of basic sections, which allow specific mitigation measures to be incorporated early in the design process, are discussed. Guidelines for conducting detailed wind tunnel investigations on these sections are also proposed. VORTEX-INDUCED OSCILLATIONS Vortex-induced oscillation refers to the oscillatory motions of a structure caused by the regular shedding of vortices. This shedding induces fluctuating forces around the surface of the structure. Although vortex oscillations are self-limiting in amplitude, the magnitude of response is a critical aspect. Many design codes include criteria based on perceptible motions, whereby a structure is considered unserviceable if motions exceed a given displacement or acceleration. In BD 49/01 [BD 49/01], a guideline for assessing vortex excitation effects is given in terms of the vertical acceleration. Accelerations are considered acceptable up to approximately 2.5 mm/s, while pedestrian discomfort is expected for STRUCTURES 2006 Copyright ASCE 2006 Structures 2006 those over 40 mm/s. Using pedestrian discomfort as the limit state, these values are comparable to those presented by the American Society of Civil Engineers (ASCE), which limits vibrations due to vortex shedding to 5.0% g, where g is the acceleration due to gravity [ASCE]. Wind tunnel section model testing is the primary method used to estimate the susceptibility of a bridge section to vortex-induced oscillations. Plate girder bridge sections are susceptible to excessive motions due to vortex excitation as the sharp corners of their geometries, shown in Figure 1, promote the formation of separation zones and subsequently vortices. These excitations generally occur at relatively low wind speeds, and typically excite the fundamental vertical and torsional modes of vibration. The motions have restricted amplitude that typically do not reach destructive levels, but can lead to potential fatigue problems and serviceability issues [Nakamura, 149-169]. Critical vortex induced responses of a bridge section can be examined in the wind tunnel on a section model, allowing mitigation measures to be incorporated in the section at the design stage. FIGURE 1 TYPICAL PLATE GIRDER BRIDGE CROSS-SECTION [KING] PREDICTION OF VORTEX-INDUCED OSCILLATION ISSUES The frequency of vortex shedding can be defined according to the Strouhal Number relationship. The Strouhal Number (St), specific to the cross section, relates the shedding frequency (f), the across-wind dimension of the structure (D), and the mean wind speed (U) as follows: