D. Surry

The distribution of pressure near roof corners of flat roof low buildings

An experimental study on low building roof pressures was carried out with a series of models of varying height and plan size in two different flows in the Boundary Layer Wind Tunnel Laboratory (BLWTL) at the University of Western Ontario (UWO). Some of the results are presented in this paper. Comparison with the Texas Tech University (TTU) full-scale results for the critical corner region shows that better agreement has been achieved than those seen in previous publications. This is attributed to the use of a larger model and improved flow simulation in this study; however, certain discrepancies still remain for the peak and rms pressure coefficient (Cp) in this corner region. The possible reasons are discussed. Arguing that the characteristic length for normalizing the roof coordinates, x and y, should be the height, H, the pressure distribution, Cp(xH, yH), over the corner area is studied in detail. Under similar approaching flow conditions, Cp(xH, yH) collapses for models with different dimensions. An effort has been made to obtain unified descriptions of the experimental data that provide a suitable form for codes. Area-averaging effects are also examined.

An experimental approach to the economical measurement of spatially-averaged wind loads

Abstract A pneumatic manifolding technique is suggested as an economical means for evaluating time-varying spatially-averaged pressure loads on tributary areas of design interest on building models. Results of some exploratory experiments are presented which show that mean and r.m.s. pressures can be determined accurately by such manifolds. Furthermore, the frequency response of the devices tested is virtually flat over the range for which the frequency response of the component tubes is flat, and shows little non-linearity. A pseudo-amplification of high frequency information takes place due to the finite grid of pressure taps; however, in practice this is shown to occur above much of the response range of interest, and to be correctable analytically.

Full/model-scale comparison of surface pressures on the Texas Tech experimental building

Abstract Wind characteristics for the Texas Tech University field site and four different flow simulations are discussed in order to evaluate the full/model-scale comparison of the surface pressures on the roof of the experimental building at this site. A total of 700 data records, each of 15 min duration, are used to describe the wind characteristics at the field site. These results are compared with those from the wind tunnel flow simulations at a geometric scale ratio of 1/50. The four simulations were achieved with the standard spire-roughness method and with three modifications of this technique. The final evaluation of each simulation is based on full/modelscale comparison of the extreme suction pressures observed on the roof near the leading edge for normal and oblique angles of attack.

Pressure measurements on the Texas tech building: Wind tunnel measurements and comparisons with full scale

Abstract The full-scale test building at Texas Tech University represents one of the best instrumented full-scale installations for determining wind loads on low buildings. In particular, it is intended to provide data of the highest quality for comparison with and verification of model scale experiments. In advance of full-scale data from the test building, a model scale experiment was undertaken to define the characteristics of the local pressures, partly to aid in the definition of the full-scale experiment and partly to provide an “unbiased” set of pressures for comparison. This paper describes the model and documents the test procedures in which results were determined for two simulated terrain roughnesses. Representative wind tunnel results are presented, and some comparisons are included with the first reliable data available from the full-scale experiment.

Simple approximations for wind speed-up over hills

There is a need to properly assess design wind speeds and correctly evaluate wind loads for structures on hills. In this paper, a simple method, called the LSD approach, is proposed, based on computational and wind tunnel data obtained for various shapes of hills. Comparisons of the LSD approach with other models are also given.

The use of aerodynamic databases for the effective estimation of wind effects in main wind-force resisting systems:: application to low buildings

Abstract As a part of the ongoing National Institute of Standards and Technology research on the development of a new generation of standard provisions for wind loads, we present results of a pilot project on the estimation of wind effects in low-rise building frames. We use records of wind pressure time histories measured at a large number of taps on the building surface in the boundary layer wind tunnel of the University of Western Ontario. Time histories of bending moments in a frame are obtained by adding pressures at all taps tributary to that frame multiplied by the respective tributary areas and influence coefficients. The latter were obtained from the frame designs provided by CECO Building Systems. We compare results obtained by using the pressure time history records with results based on ASCE 7 standard provisions. The comparison suggests that provisions which use aerodynamic databases containing the type of data described in this work can result in designs that are significantly more risk-consistent as well as both safer and more economical than designs based on conventional standard provisions. We outline future research on improved design methodologies made possible by the proposed approach to the estimation of wind effects. We note that the use of the proposed methodologies is fully consistent with the ASCE 7 Standards insofar as these allow the use of wind-tunnel data for estimating wind load effects. Finally, We note that the proposed methodologies may be used for damage assessment for insurance purposes.

The magnitude and distribution of wind-induced pressures on hip and gable roofs

Abstract Some recent post-disaster studies have suggested that in severe wind storms hip roofs survive better than gable roofs. This has stimulated the present study, which has derived aerodynamic data for otherwise similar 4:12 hip and gable roofs. These show that for this common roof geometry there is little difference in overall lift and overturning loads; however, there are dramatic differences in local pressures and in the loads applied to the primary structural elements. These differences appear to explain the improved survival of hip roofs.

Scale effects in wind tunnel testing of low buildings

Abstract Further analysis of wind loads derived from boundary layer wind tunnel experiments on low buildings sheds light on the question of scaling. Results are available for building models tested at three geometric scales under two different terrain exposures representing open country and suburban areas. Both mean and fluctuating pressure coefficients appropriate for both local and area loads have been examined. The errors introduced by using larger models are discussed and evidence is presented that a small relaxation of scale (up to a factor of 2) leads to errors of the order of 10% or less for all cases examined, except for the particular case of local pressure coefficients acting on building walls, which may be subject to non-conservative errors of the order of 25%.

Characteristics of mean and fluctuating pressure coefficients under corner (delta wing) vortices

This paper deals with the magnitude and the distribution of mean and fluctuating pressure coefficients associated with corners and edges on the top surface of surface-mounted rectangular prisms immersed in a variety of turbulent shear layers. The discussion of the observed pressure data includes its variation with the turbulence intensity, Su/U, and the azimuth angle, a, of the incident flow. In addition, roof pressure coefficients from 150 and 1100 scale models are compared with field data from the experimental building at Texas Tech University (TTU). Finally, some recommendations are presented for the reduction of the extreme suction pressures observed under the corner vortices. A companion paper by the same authors is in preparation which will emphasize in detail the variation of the roof-corner pressure distribution with prism geometry (primarily the prism height, H).

Variability of low building wind loads due to surroundings

Abstract A myriad of surface obstructions form the basic roughness that generates the Earths planetary boundary layer. Among these are the class of structures termed low buildings. Ironically, attempts to determine wind loads on low buildings have been hampered by the random nature of these surface elements and researchers have been forced to simplify to deal with the complex problem. As a result, almost all low building research to date has been carried out on simple rectangular block-like buildings in homogeneous surroundings. While the aerodynamic behaviour of wind on isolated low buildings has been well researched, the effect of realistic surroundings has not been examined in detail. Whether the results from these simplified tests lead to the most appropriate design values remains an open question. With the establishment of a set of rational design values for low buildings in mind, it is proposed that a series of experiments be carried out to determine the wind loads experienced by low buildings in realistic environments. The experiments will be designed to collectively represent the entire spectrum of full-scale situations, i.e. the statistical distribution of parameters associated with geometry and surroundings will be matched between model and full scale. The wind loads measured in the wind tunnel will then be statistically representative of the wind loads expected on all low buildings. These expected wind loads and their variability will form a good basis to further examine the determination of appropriate wind loads for design. The first phase of this proposed study has been to examine the effect of the surroundings on the wind loads on flat-roofed low buildings, as well as to determine the relative significance of some of the more basic wind load parameters, such as effects of simulated upstream terrains, in relation to the variation introduced by the local effects of different realistic sites.