Journal of Wind Engineering and Industrial Aerodynamics, 38 (1991) 235-247 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

235

Pressure measurements on the Texas Tech Building: Wind tunnel measurements and comparisons with full scale

D. Surry

Boundary Layer Wind Tunnel Laboratory, University of Western Ontario, London, Ontario N6A 5B9, Canada

Summary

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 experi- ments. In advance of full-scale data from the test building, a model scale experiment was under- taken 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 com- parisons are included with the first reliable data available from the full-scale experiment.

1. Introduction

The past decade has seen extensive research in the area of wind-induced loads on low buildings, primarily using wind tunnel testing techniques, and the results have begun to have significant impact on codification. Nevertheless, problems remain, as discussed by Surry [ 11. Notable among these are the con- tinuing need to fully validate model-scale results for low buildings. While sev- eral full-scale studies have been made and many model-scale experiments have followed, of which perhaps the most publicized is the Aylesbury experiment [2,3], and while there is a reasonable degree of agreement between many of these results, there are also facets of the comparisons that are disturbing. In some instances, particularly for very local high suctions [ 41, agreement is not very good. This is also true of wind tunnel comparisons with results from Mar- shall’s mobile home experiment [5]. Even for more ordinary pressures; the model experiments are often a matching process, where wind tunnel simula- tions are varied until reasonable agreement is obtained, rather than being truly independent simulations. For example, in the Aylesbury experiments, the mo-

0167-6105/91/$03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

236

delling of the upstream hedges was a crucial component of the wind tunnel simulation.

There are many difficulties in such comparisons. There are potential prob- lems in experimental technique, both in the model case and, perhaps to a greater degree, in the full-scale case - as again evidenced by the considerable difficul- ties with data quality in the Aylesbury experiments [4]. The full-scale exper- iment faces a much less controlled upstream environment, including effects on the approaching boundary layer due to stability and far-field influences [ 61, and the difficulty of repeating experiments when suspect data are revealed.

In this context it is notable that at least two significant efforts are underway to improve the situation. These are the experiments undertaken at AFRC in the U.K. and the experiments underway at Texas Tech University in the U.S.A. Both are described in separate papers. This paper concentrates on describing wind tunnel experiments in support of the latter, made prior to the availability of any full-scale pressure data.

The most important characteristic of the Texas Tech Experiment (TTE ) is that it is ongoing and hence provides the possibility of defining full-scale ex- periments to verify model findings; i.e. the reverse of most historical compar- isons. The TTE has also been designed to avoid mistakes made in earlier com- parative studies. The wind environment is being monitored particularly well, including its stability, while the upstream environment, consisting of the Texas plains, is probably as straightforward an upstream exposure as is possible. Thus, it presents an almost ideal case for investigating the nuances of the model/ full-scale comparison problem, and hence the verification or evolution of ap- propriate modelling techniques.

It is the author’s opinion that it is the demonstration of this link between model and full scale that is paramount, since another of the fundamental prob- lems in defining appropriate wind loads for low buildings is dealing with the inordinate amount of variability in their wind loads, due to the enormous va- riety of geometries and surroundings characterizing low buildings. This vari- ability can best be attacked by wind tunnel techniques like those currently underway by Ho et al. [ 71. It would be a mistake to use specific worst case data from isolated idealized experiments - even if they represent full-scale truth in that specific instance - as the sole basis for the general specification of design loads.

In a companion paper by Levitan, Holmes, Mehta and Vann, the TTE is described in detail. In what follows the model experiments undertaken at the University of Western Ontario will be described, some representative data will be shown, and some early full-scale results will be compared with the experiments.

2. The model-scale experiments

A replica of the full-scale building was built from plexiglass at a scale of 1: 100. An exploded view of the model and its pressure tap locations are shown

237

in Fig. 1. One corner of the model, indicated by the dashed line, was designed as a replaceable section to facilitate alterations in the detailed instrumentation for future experiments.

The model has been tested, using two different upstream exposures, in the original Boundary Layer Wind Tunnel (BLWT I) [8]. Both included large spires near the upstream end of the fetch to increase the boundary layer thick- ness and to increase the turbulence scales. One utilized about 16 m of bare wind tunnel floor in front of the model; the other used 5 mm thick, tight-weave carpet to within 1.5 m of the model and then 7 mm high machine nuts spread randomly over the remaining distance. Previous experiments [ 31 indicate that the resulting simulation is reasonably representative for 1: 100 to 1: 200 scaled models.

For each terrain exposure the mean speed and longitudinal turbulence in- tensity profiles were measured, and are presented in Fig. 2. Furthermore, wind spectra were measured at 3 heights, equivalent in full scale to 3.96 m, 10 m, and 48.8 m, which correspond to heights at which full-scale data will be avail- able. The lowest height corresponds to the roof height, at which the reference

+

I : 1

+ +;

4 45.25’ * M-12.83’-

++

0 f

’ +

’ I

I f

: 1

3 +

+

Fig. 1. Exploded view of the building model showing the initial pressure tap locations and the major full scale dimensions ( 1 m = 3.28’ ) .

Exposure 1 ;

0:

x 0.0 0.1 1.0

NORMALIZED VELOCtTY

LOCAL INTENSITY 0 5.0 10.0 15.0 20.0 21.0

x :

\ Exposure 2 /

x :

<’ 0 Mean vetocity

X Turbulence Intensity “j, ,L?

.-- ESDU.z. = 0.018 ,,,

-arr ,“,e

‘%I

0 0.5 1.0 NORMALIZED VELOCITY

Fig. 2. Vertical profiles of mean velocity and longitudinal turbulence intensity at the model loca- tion for the two surface roughness cases.

mean speed for normalizing the pressure coefficients is measured, both in model and full scale.

During all experimental runs, a reference pitot-static tube was used to mea- sure the speed above the model near the top of the boundary layer, and to provide reference static and dynamic pressures in the initial definition of the pressure coefficients. The mean speed profiles of Fig. 2 were used to infer the speed at roof height for use in re-referencing all the pressure coefficients after the experiments. In addition, another pitot-static tube was mounted at roof height to provide a check on this speed at roughly the same distance away from the side of the model as the full-scale anemometer tower.

Local pressure data were measured at all locations on the model using the standard laboratory data acquisition system comprising Statham 2.5 psi trans- ducers embedded in Scanivalves. Despite using only about 1% of their range, at a reference speed at the top of the boundary layer of about 14 m s-l, these transducers provide pressure coefficients accurate to within 0.01-0.02 of roof height dynamic pressure.

Each tap was sampled at about 500 samples s-l for 60 s in the scale model, from which mean, rms and extreme values were determined. The experimental

239

record was divided into segments and the segmental peaks were fitted using extreme value theory to extrapolate the expected value of the 60 s peak. Hence, the variability of the peaks is reduced over the actual peak measured during the 60 s sample. The large number of closely spaced pressure taps for which peak results are available also helps to provide a smooth picture of the spatial distribution of the peaks. The signals were filtered through a notch filter at about 60 Hz corresponding to the fundamental fan blade crossing frequency, and then were low-pass filtered at 100 Hz to remove higher harmonics of the blade crossing acoustic output. The fan drive has a fixed rotational speed with a variable pitch fan. Each tap was connected to the transducers using a 61 cm long, 1.6 mm I.D. tube containing a restrictor. The resulting pneumatic system has a slight resonance at about 90 Hz, after which the response falls off, but is essentially flat to beyond where the low-pass filters dominate. At a scale of 1: 100, and for typical roof height full-scale mean speeds of about 10 m s-l, full-scale and model eave height speeds are comparable. Hence, the time scal- ing must be similar to the length scale of about 1: 100. Thus, the model should reproduce full-scale frequencies accurately up to about 1 Hz, but will omit fre- quency components above that. Extending the frequency response of the model measurements is planned for a future testing phase. The model sample lengths correspond to about 1 f h of record in full scale.

In addition, pressure spectra were measured at 6 taps for wind angles of 0”, 30’ and 45 ‘. No spectral data will be presented in this paper.

Finally, static pressures were measured at three locations on the surface of the wind tunnel floor on a line between the building and the anemometer tower location at distances equivalent to about 75,105 and 135 ft from the building’s centre. The first of these corresponded approximately with the location of the reference static pressure pick-up for the full-scale experiment. These mea- surements were aimed to determine whether the building would have any sig- nificant effect on the reference static pressure measurement. In the wind tun- nel, “penny” taps were used; these are so called because they are flat disks with chamfered edges and a central pressure tap. This unit can be stuck on the surface at any desired location and measures the local static pressure with only a negative bias of about 2% of free stream dynamic pressure.

The capability of rotating the full-scale building relative to the wind, and hence also with respect to the anemometer tower and static pressure port, pro- vides an extra degree of freedom over that required for normal wind tunnel tests of fixed buildings. This was not explicitly modelled. For the pressure mea- surements on the model, only the two homogeneous exposures were examined, representing full-scale winds approaching over the open terrain to the north to northwest. Rotation of the model then simulates the building being rotated to gather data from different angles of attack. In full scale, other wind direc- tions can introduce more complex upstream exposures involving subdivisions and the occasional institutional building.

240

In the model case, the ground surface static pressure ports rotated with the building, so these correctly simulate only a single full-scale case which was chosen to be the predominant wind direction with the building axis oriented to the north, at about 90” to the line joining the building centre and the ane- mometer tower. The data taken for other model wind directions approximately simulate other full-scale wind directions for this fixed building/instrumenta- tion orientation. The approximation arises because the upstream terrain model no longer represents reality. The effect on the reference static pressure port of rotating the building for the predominant wind directions was not examined during these initial experiments.

3. Results and discussion

The speed profiles presented in Fig. 2 are best fitted by power law exponents of 0.13 and 0.17, and have equivalent full-scale z. values of about 3-4 mm and 14-18 mm, depending on the details of the fit. The first case has a much smaller z. resulting from the smooth floor; the mean flow exponent is maintained by the upstream spires.

Early full-scale data by Mehta and Levitan [9] for winds from the open north to northwesterly fetch indicated an average power law exponent of about 0.15 and a roughness length of about 14 mm, suggesting that the rougher ex- posure modelled here would be the most appropriate.

The results of the ground surface static pressure investigation are shown in Fig. 3, where the data from the smoother simulation are shown. The rougher exposure has identical trends but has much more scatter due to the machine nuts spread over the nearby surface to maintain the local surface drag for this simulation. Also shown is the static pressure measured at eave height from a pitot-static tube which did not rotate with the model. All pressures are refer- enced to the free stream static pressure above the boundary layer, but are nor- malized by the roof height dynamic pressure.

At 0’) the negative bias of the penny taps is evident; however, within the accuracy of the measurements there is no difference between the three loca- tions, indicating that the building has no significant effect on the full-scale reference port for this orientation. Although the case examined may not have been quite as severe as if the building were turned 90”) it is considered unlikely that the full-scale static port will be appreciably affected for winds from the important north through northwesterly directions. As the model was rotated, the surface static pressures exhibit a surprising response, particularly at the full-scale static port position when the wind is blowing over the building at 90’) where this pressure goes positive, presumably since it is close to the reat- tachment of the flow going over the roof and back down onto the ground. The other two locations show little effect for all angles examined; however, the full-

241

75’

105’

135

150’ 3

STATIC

7,. .“. 8. .‘.‘,‘.“‘I ““‘U’.“‘I. “‘C 0.0 60.0 120.0 160.0 210.0 300.0

Azimuthal Angle 0.0

Fig. 3. Comparison of the roof height and ground surface static pressure variation with wind angle (uncorrected for the “penny” tap negative bias).

scale static port position shows an expected small positive increase at 270’ due to the upstream effect of the building.

The effects of the two terrain models on the pressure coefficient distribution

242

over the central cross-section of the building (see Fig. 1) for winds normal to the ridge line can be seen in Fig. 4. Both sets of results are referenced to the mean dynamic pressure at 3.96 m. Only the peaks having the same sign as the means are plotted. The rms values are plotted within the building outline. The results reflect the fact that the two terrain models are not all that different, although the slightly rougher exposure does lead to slightly higher peaks and a slightly more rapid reattachment onto the roof. In light of the full-scale wind speed results mentioned above, only the rougher exposure data will be pre- sented from here on.

Figure 5 compares the 90” wind tunnel test results on the same central cross- section with some early full-scale data, for winds within 2 10” of being normal

n

6

1 0 MEAN - EXPOSURE 2

0 vi Cl PEAK - EXPOSURE 2

0 RMS - EXPOSURE 2

* MEAN - EXPOSURE 1

X PEAK - EXPOSURE 1

0 i + RMS - EXPOSURE 1

Fig. 4. A comparison of mean, peak and rms pressure coefficients measured in the two different terrain roughneeees (90 ’ ) .

243

0 MEAN 0 PEAK 0 RMS $ MEAN - FULL SCALE W PEAK - FULL SCALE

$ RMS - FULL SCALE

o &,

6 0040 Oc&

oat o o *loo 0 0 $0

0 QO 0

, -+0--H- ,

I=. a * Co%-+-

D 0 0

Fig. 5. A comparison of mean, peak and rms pressure coefficients between model and full scale for winds approximately normal to the ridge.

to the ridge. Only the peak data with the same sign as the mean are included. The full-scale data comprise averages and scatter bars from 11 full-scale runs. The scatter bars indicate the maximum span of the component runs. The full- scale peak data correspond to the single peak realization recorded over each 15min component run corresponding to the largest 10 sample per second da- tum as measured through a pressure system that was low-pass filtered at 10 Hz. Thus, the full-scale data have an upper frequency limit considerably higher than the equivalent 1 Hz maximum response of the wind tunnel data; however, the model experimental runs were the equivalent of about six times the length of each component full-scale run. Whereas the difference in frequency re- sponse should lead to an underestimation of the full-scale peaks by the model, the longer record length will provide a small trend in the opposite direction. These considerations are not significant for the mean and rms which are in

244

very good agreement. Even the peaks agree well, considering the above inher- ent differences.

Holmes has carried out an initial investigation of the effect of frequency response using all of the component runs for two pressure taps, one on the roof and one on the wall, using moving averages having varying time spans. These indicate an expected reduction in the peaks of about 15% on the walls to as much as 25% at the edge of the roof for the equivalent model frequency re- sponse. This would bring the average peak values into almost perfect agreement.

Not all full-scale data are in such good agreement with the model results. Data for an oblique angle are presented in Fig. 6. The wind tunnel data are for 60”) while the full-scale data represent eight component runs within 2 10’ of 60’. The wind tunnel results do not change dramatically over this angle range. While the mean and the rms data in Fig. 6 are in reasonable agreement, the peak data in full scale are very much larger than the model results. The scatter bands, indicating the complete span of the results for the eight component runs, show that the results for this wind angle are highly variable. It is most significant, however, that the differences extend to both the front walls and

0 MEAN

0 PEAK

0 RMS

Q MEAN - FULL SCALE

63 PEAK - FULL SCALE

+ $ RMS - FULL SCALE

Fig. 6. A comparison of mean, peak and rms pressure coefficients between model and full scale for oblique winds (near 60” ).

245

q MEAN - TTE MODEL, EXP 1

0 MEAN - TTE MODEL, EXP 2

0 MEAN - 13 m HIGH ROOF

x MEAN - 5.5 m HIGH ROOF

q PEAK - TTE MODEL, EXP I

d PEAK - TTE MODEL, EXP 2

PEAK - 13 m HIGH ROOF

I ” ” 3. . ” I “. ”

0 60.0 120.0 180.0 240.0 300.0 3

Wind Angle

Fig. 7. A comparison between the worst overall mean and peak suction coefficients found on the TTE model with those found on a flat roof model.

the wake. This strongly suggests that the gust structure of the approaching wind is significantly different than in the wind tunnel, since these regions should exhibit a quasi-steady type of response. This avenue of investigation is being pursued through examination of the turbulence intensity and spectra for these runs. Other factors could also contribute to these differences. For example, non-stationarity would exaggerate the peak-to-mean ratios. Half of the com- ponent runs for this wind angle failed a stationarity test applied by Holmes. Also of importance will be to carefully monitor the atmospheric stability dur- ing the experimental runs, which is a capability of the TTE facility, but is not yet fully operational. Clearly, there is a great deal of interesting work ahead.

Future comparisons will be extended to those regions that are most likely to provide the most difficulty for the model simulation; namely, regions close to edges and corners where strong vertical structures develop in the flow that are very local and hence often difficult to pick up with conventional model instru- mentation, as pointed out by Kind [lo]. The current model has the capability

246

of receiving a special corner, instrumented very much closer to the edges than those taps that were initially installed and different instrumentation can in- crease the frequency response by an order of magnitude. Nevertheless, it is interesting to plot the worst mean and peak suction coefficients found in this model vs. azimuth, as shown in Fig. 7. All of the extreme suction values were found on the roof, within 2 ft of the edge; the closest tap to an edge is at a full- scale equivalent distance of 6 in. In comparison, worst mean and peak suction coefficients as found on a 1:500 scale flat roof model of a building 12.7 m high, constructed by Stathopoulos [ 111 and tested by Surry et al. [ 121, have also been added. Results not presented suggest that the mean and peaks for a 4 m high building tested in the same conditions would be lower by about lo%-20% or so (see ref. 1). A single point is also shown for a quartering wind on a 5.5 m high roof, taken from Kind [lo]. All of these latter high peaks were also re- corded very close to edges. Both sets of model results seem very consistent. It remains to be seen whether they will agree with full-scale results for very local peaks that will strain the small scale Reynolds number simulation capability of the model and the frequency response capabilities of both model and full scale. It is, of course, a completely different question as to what the design significance is of these very intense local peaks.

4. Concluding remarks

A set of wind tunnel model data has been developed in advance of full-scale testing of the Texas Tech Experimental Building using a model designed to allow simple reinstrumentation and retesting. The first reliable full-scale data provide interesting comparisons. For winds nearly normal to the ridge, the model and full-scale data are in very good agreement, particularly when their differing frequency responses are accounted for on the magnitudes of the peaks. For winds oblique to the ridge, the data indicate significant differences in the peak coefficients, although this may be attributable to markedly different gust- to-mean ratios in these storms or to non-stationarity.

Extensive comparisons, including wind and pressure spectra, will be made in the future, as more full-scale data become available.

Acknowledgements

The author wishes to acknowledge the entire Texas Tech team, led by Dr. Kishor Mehta, for making the full-scale data available, and for their encour- agement in preparing this paper. Particular thanks go to Dr. John Holmes, who worked on this project while a visitor to Texas Tech, and to Mark Levitan who has put so much effort into the project. The author also appreciates the efforts of Betty Stopar who designed and tested the model and carried out the initial data reduction, and to Andrew McDougall who carried out the final

241

analysis and prepared the figures. The overall encouragement of Dr. A. G. Dav- enport, Director of the Boundary Layer Wind Tunnel Laboratory, is also greatly appreciated. Financial support was provided under a Natural Sciences and En- gineering Research Council Operating Grant and from the Boundary Layer Wind Tunnel Laboratory.

References

1

2

3

4

5

6

7

8

9

10

11

12

D. Surry, Recent and current research into wind loading of low buildings at the University of Western Ontario, Proc. Sixth U.S. National Conference on Wind Engineering, 8-10 March, 1989, University of Houston, Texas. K.J. Eaton and J.R. Mayne, The measurement of wind pressures on two-storey houses at Aylesbury, Symp. on Full-Scale Measurements of Wind Effects on Tall Buildings and Other Structures, London, Canada, 1974. P.J. Vickery, Wind loads on the Aylesbury experimental house: A comparison between full scale and two different model scales, M.E.Sc. Thesis, The University of Western Ontario, London, Canada, 1984. P.J. Vickery, D. Surry and A.G. Davenport, Aylesbury and ACE: Some interesting findings, Proc. 6th Colloquium on Industrial Aerodynamics, Aachen, Germany G.L. Johnson and D. Surry, Wind tunnel tests of a mobile home and comparisons with full scale data, Proc. Fifth U.S. National Conference on Wind Engineering, 6-8 Nov. 1985, Texas Tech University, Texas. H.W. Tieleman, Wind tunnel simulation of the turbulence in the surface layer, Proc. Sixth U.S. National Conference on Wind Engineering, 8-10 March 1989, University of Houston, Texas. T.C.E. Ho, D. Surry and A.G. Davenport, The variability of low building wind loads due to surrounding obstructions, Proc. Sixth U.S. National Conference on Wind Engineering, 8-10 March 1989, University of Houston, Texas. A.G. Davenport and N. Isyumov, The application of the boundary layer wind tunnel to the prediction of wind loading, International Research Seminar on Wind Effects on Buildings and Structures, Ottawa, Canada, September, 1967, University of Toronto Press, 1968. K.C. Mehta and M.L. Levitan, Field experiments for wind pressures, Department of Civil Engineering, Texas Tech. University, Progress Report, June 1988. R.J. Kind, Worst suctions near edges of flat rooftops on low-rise buildings, J. Wind Eng. Ind. Aerodyn., 25 (1968) 31-47. T. Stathopoulos, Wind pressures on flat roof edges and comers, Proc. Seventh International Conference on Wind Engineering, Aachen, West Germany, July 1987. D. Surry, J.-X. Lin and J.R. Lankin, Pressure measurements near comer edges on a flat roof, Boundary Layer Wind Tunnel Report, 1991 (University of Western Ontario, London, Ont.) in press.