Full scale measurements of pressure equalization on air permeable façade elements

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1 Full sale measurements of pressure equalization on air permeable façade elements Carine van Bentum, Chris Geurts Department of Strutural Dynamis, TNO, Delft, The Netherlands ABSTRACT: Wind-indued pressure differenes over rain sreens are determined by the external pressures and the pressures inside the avity. Minimizing this pressure differene dereases the risk of water leakage and also helps to minimize the loal loads on the façade elements. Current rules to determine the wind loads on suh strutures are based on very little experimental evidene. Some guidane using very rude rules is given in EN []. A full sale measurement has been set up on a 58 m high residential building situated in Rotterdam, The Netherlands. The façade is lad with slabs of natural stone, whih typially have a size of m eah. The openings between the elements are about mm wide and the air avity has a depth of (nominally) mm. At a height of approximately m above the ground, the façade elements have been equipped with 4 pressure transduers: for the external pressure and for the avity pressure. From this experiment pressure equalization oeffiients per pressure tap have been derived, whih are defined as the maximum peak differential pressure oeffiients divided by the maximum peak external pressure oeffiients. Also wind diretion dependent values have been derived. For overpressure, a pressure equalization oeffiient of.6 has been found, whih is omparable to the ratio EN gives for the net pressure oeffiient of an permeable façade ( p,net =/3 pe ). For underpressure, values in the same order of magnitude have been found, with a maximum value of.8 in the middle zone. One value above has been found, whih lies in a orner zone ( eq =.). The measured pressure equalization oeffiients for underpressure are higher than the net pressure oeffiients in EN for permeable faades ( p,net =/3 pe ). KEY WORDS: ICWE4; PRESSURE EQUALIZATION; FULL SCALE MEASUREMENTS; FACADE. INTRODUCTION Façades often onsist of multiple layers. The inner layer is generally airtight and stiff, thus onstituting a barrier for wind and water. Thermal insulation is usually present in front of this layer, whih is then overed by an air-permeable layer that serves as a rain sreen. Between the inner layer and outer layer, there usually is a ventilated avity. The wind-indued pressure differenes over the rain sreen are determined by the external pressures and the pressures inside the avity. Minimizing this pressure differene dereases the risk of water leakage and also helps to minimize the loal loads on the façade elements. Current rules to determine the wind loads on suh strutures are based on very little experimental evidene. Some guidane using very rude rules is given in EN []. Gerhardt and Kramer (983) [] arried out wind tunnel tests on pressure equalization in different sale models of buildings with a relatively small height to width ratio. From the ratio of measured peak pressures it was onluded that pressure equalization takes plae on muh smaller timesales than the typial gust duration, relatively independently of façade permeability. When avities on different sides of a building are in diret ontat with eah other, large differential pressures over the outer layers an be found in orner areas, more or less independent of façade permeability. Gerhardt and Janser (994) [3] performed wind tunnel tests on buildings with ompartmentalized and non-ompartmentalized avities. They ompared their results with full sale experiments. The maximum time-averaged pressure oeffiient redution fator was between 5% and 5% for ompartmentalized avities. For non-ompartmentalized avities, the peak pressures in orner areas were found to inrease by % to 5%. Inulet and Davenport (994) [4] dedued from wind tunnel tests that pressure equalization is diffiult to ahieve in orner areas beause equalization is inomplete for fast flutuations (> Hz). Although ompartmentalization has a benefiial effet on the differential pressures over the façade panels they stated that residual values of Pa or more are pratially unavoidable. One of the first full-sale measurements of the flow veloity in a avity was performed by Popp et al.[5] in 98. From this experiment, it was onluded that the flow veloity mainly depends on the geometry and size of the avity openings, the veloity and diretion of the wind and any temperature differenes inside the avity. Van Shijndel and Shols (998) [6] studied pressure equalization at normal inflow over a façade panel loated in the middle of the faade of the main building of Eindhoven University of Tehnology. For the investigated geometries, the redution in peak load due to pressure equalization lies between 5% and 95% for façade panels at overpressure. These values oinide reasonably well with full sale experiments 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

2 performed by Ganguli and Dalgliesh (988) [7], who found redutions in peak loads varying between 5% and 6%, depending on the position on the panel. Geurts et al. (5) [8] found a pressure equalization fator between.3 and.5 for a typial brik wall with a ventilated avity. Aording to a review artile by Suresh Kumar () [9] it an be onluded from measurements performed by Straube and Burnett that pressure equalization is feasible on timesales larger than 5 minutes. The equalization of gusts, however, is limited. The authors attributed this to the spatial flutuations in external pressure rather than temporal flutuations. In ontrast with these findings, Brown et al. (99) [] onluded from measurements on a high-rise building that equalization performane is good for both stati and dynami pressures as long as the ventilation openings are large and the avity volume is small. A researh projet is urrently undertaken in the Netherlands to better understand the basi mehanisms of pressure equalization over air permeable façade elements. As part of this projet, a field test has been set up on a 58 m high building in Rotterdam and both CFD alulations and wind tunnel tests have been performed. This paper presents data from the full sale experiment. To get a better understanding of the results, the external pressures of the full sale experiment are ompared with wind tunnel data. EXPERIMENTAL SET-UP A full sale experiment has been set up on a high-rise residential building situated in Rotterdam, The Netherlands. This building, alled the New Orleans, has a height of 58 m, a width of 9 m and is situated on a small peninsula in a former harbour area (see Figure and ). Prevailing winds are from the south-west over a relatively open feth. The façade is lad with slabs of natural stone, whih typially have a size of m eah. The openings between the elements are about mm wide. The air avity has a depth of (nominally) mm and the thermal insulation onsists of stone wool. At a height of approximately m above the ground, the façade elements have been equipped with 4 pressure transduers: for the external pressure and for the avity pressure. The external pressures are measured near the openings between the elements and the avity pressures are measured behind the façade panels in the avity. The position of the pressure taps in the avity is in the enter of the slab at the bakside of the façade element. The pressure taps are onneted to the pressure transduers by flexible tubes, whih are loated at the balonies due to pratial onsiderations. The tubes have an internal diameter of 6 mm. The tube length has been restrited to 4 m to limit resonane effets of the tube. A laboratory test has been arried out for different tube lengths to determine the amplifiation fator of the pressure signal per frequeny. Using this data, a tube length orretion has been applied. The sampling rate of the pressure measurements is Hz. A referene pressure vessel is situated inside the building, to whih all differential pressure transduers are onneted. An anemometer is installed on top of the building at a height of.5 m above roof level, whih is the maximum allowable height due to arhitetural restritions. Sine this position is in the lee of the building for most wind diretions, data from a nearby meteorologial station (Rotterdam airport, approximately 5 km away) are used in the data analysis. The sampling period of the measured data is minutes. Details of the full sale experiment an also be found in [4]. The external pressures are ompared with wind tunnel results. The wind tunnel study was performed in the atmospheri boundary layer wind tunnel of TNO in Apeldoorn. The tunnel has a ross setion of m x 3 m with a the test setion of.5 m. For this researh the roughness of the surrounding area is simulated with legoboard, giving a roughness length z =.3m at full sale. The geometrial sale of the model is :5. The pressures are simultaneously measured for 4 angles of inidene in steps of 5 degrees between (North) and 345 degrees. The sampling rate and period were 4 Hz and.4 seonds. The undisturbed veloity at building height was determined with a pitot-stati tube, approximately meters in front of the model. This veloity was 3.8 m/s. Figure. Left: View of the full sale building (photo by Tom Kroeze). Middle: Detail of a orner of the building. Right: Detail of the building façade, showing the thermal insulation, the air avity and the natural stone façade ladding. 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

3 3 B Northwest A 85 Wind diretion 5 Southwest 5 Northeast C Southeast Figure. Left: Wind tunnel overview of the peninsula with the New Orleans building in its surroundings. Right: Plan of the building with the pressure tap positions (blue dots: external pressure taps with funtioning pressure taps in the avity and red dots: external pressure taps with non-funtioning pressure taps in the avity) and the anemometer (red triangle). D 3 DATA The data presented in this paper were measured between April st and June 7th 4 and therefore spans more than years of ontinuous measurements. The total dataset ontains,9 reords of -minute measurements. From this dataset, the data under neutral onditions have been seleted by applying a veloity riterion of 6 m/s. Sine the veloity measurements are highly affeted by the building, veloity measurements taken at the KNMI (Royal Duth Meteorologial Institute) meteorologial station at Rotterdam airport were used both as referene veloity and as filter riterion. The data whih are used for this purpose are the -minute mean veloities at m height for the last minutes of eah hour. Sine there is only veloity information about the last minutes in an hour, the veloity of this period is also used for the 33 minutes before this time interval and 33 minutes after this time interval. The measured pressure data at the building site that lie within a time interval in whih the veloities at the meteorologial station exeed the threshold of 6 m/s are used in the data analyses, resulting in,853 samples. Figure 3 shows the number of samples per wind diretion. Only wind diretions with more than 3 samples are used in the analyses. The prevailing wind diretion in the Netherlands is the south-western wind as an be seen learly. Unfortunately, the pressure taps inside the avity at side CD, the pressure tap inside the avity next to orner C at side BC and the pressure tap inside the avity next to orner D at side DA have been mounted inorretly and annot be used in the analyses (positions are marked in Figure ). The wind diretions with the most data samples mainly affet those pressure taps. Therefore, the data analyses have to rely on the other wind diretions with less data samples. That is also the reason that the data samples outside the last minutes of an hour have been inluded in the analyses. This paper fousses on two wind diretions: one wind diretion normal to a side of the building. The other wind diretion has an angle of 3 degrees with respet to the normal wind diretion. The wind diretions that have been hosen are: 5 degrees (normal at side BC) and 85 degrees (3 degrees angle of attak at side AB), see Figure. At a wind diretion of 5 degrees, high overpressures are expeted at side BC. At 85 degrees, high underpressures are expeted near orner B at side BC. The number of samples for the wind diretions of 5 degrees and 85 degrees are 5 and 69 respetively. 8 6 Number of samples per wind diretion [-] Angle of attak ( o ) Figure 3. Number of samples per wind diretion. 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

4 4 4 DATA ANALYSIS Pressure oeffiients are derived for eah minute pressures series aording to Eq. (). The pressure oeffiients originated from the external pressures are denoted as p,e, the pressure oeffiients that stem from the avity pressures as p,. p pa = ()a e p, e ρvref p pa = ()b p, ρvref In whih p is the pressure oeffiient, as a funtion of the wind diretion θ and time t; p e is the external pressure measured at the pressure taps; p the avity pressure; p a is the referene pressure, measured inside the building; ρ is the density of air, taken as.5 kg/m 3 ; v ref is the referene veloity, measured at meteorologial station Rotterdam Airport. Both mean values and peak values are presented in the present paper. To determine the peak values, the minima and maxima of the time series of minutes eah have been obtained for every wind diretion. When a minimum of 3 samples was available, a Gumbel-distribution has been fitted using the maximum likelihood method. The parameters of the Gumbel-fits of the minima and maxima separately, are used to onstrut Cook-Mayne Coeffiients aording to Eq. (). ˆ p;max ( θ ) = umax +.4 ()a a max ˆ p;min ( θ ) = umin +.4 ()b a min In whih u is the mode and a the sale of the Gumbel distribution. Using the time signals of external pressures p e and avity pressures p time series of the differential pressures are derived. The resulting differential pressure oeffiients p,diff are given by Eq. (3), in whih negative values orrespond to a diretion pointing outwards from the faade and positive values towards the faade. p p = (3) e p, diff ρvref Finally, pressure equalization oeffiients eq are derived by dividing the peak differential pressures by the peak external pressures, Eq. (4). Pressure equalization oeffiients are determined for overpressure and underpressure separately. eq; overpressure eq; underpressure ( θ ) ( θ ) ˆ ( θ ) p, diff ; overpressure = (4)a ˆ ( θ ) p, e; overpressure ˆ ( θ ) p, diff ; underpressure = (4)b ˆ ( θ ) p, e; underpressure Also the maximum value of the pressure equalization over all wind diretions, omplying with the definition of the pressure equalization oeffiient in the Duth National Annex of EN 99--4, is determined: max( ˆ ( θ )) p, diff eq = (5) max( ˆ p, e( θ )) 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

5 5 5 RESULTS Figure 4 and 5 show the external pressure oeffiients for the wind diretions of 5 degrees and 85 degrees respetively. The offset in the full sale mean pressure oeffiients ompared to the wind tunnel values is presumably aused by an internal underpressure, sine the referene pressure p a is measured inside the building and the building is in a ondition of underpressure due to its mehanial ventilation. The differene between the external mean pressure oeffiients measured at the building in full sale and the external mean pressure oeffiients measured in the wind tunnel is used to redue the offset in the full sale measurements. The average of the differenes of all external pressure taps is used as offset orretion ( ). The differene is wind diretion dependent, see Figure 6, and applied on both the mean pressure oeffiients and peak pressure oeffiients. After applying the offset orretion, the mean values of the full sale data and the wind tunnel orrespond well, nevertheless the peak values still show differenes. A ouple of explanations ould be given. Firstly, the wind tunnel peak values are based on -minute peaks that are saled up to -minute values. Seondly, the wind tunnel peaks represent.4s peaks, whereas the full sale values represent.s peaks. Finally, the full sale referene veloity is taken from a meteorologial station that only provides -minute values of the last -minutes of eah hour. For pressure data outside this time interval, the wind veloity of losest hour has been taken, resulting in a lower orrelation between pressures and veloities than in the wind tunnel. The effets ould work in both diretions, either lowering or inreasing the peaks. The mean and peak pressure oeffiients at side BC, that are used in the further analyses, orrespond well for both wind diretions..5 Wind diretion 5 o wind tunnel full sale full sale + ( -.3).5 Wind diretion 5 o wind tunnel full sale + ( -.3).5.5 pe;mean pe;peakev Figure 4. External pressure oeffiients at 5 degrees. Left: Mean values Right: Peak values.5 Wind diretion 85 o wind tunnel full sale full sale + ( -.68).5 Wind diretion 85 o wind tunnel full sale + ( -.68).5.5 pe;mean -.5 pe;peakev Figure 5. External pressure oeffiients at 85 degrees. Left: Mean values Right: Peak values 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

6 6 Cp WT;mean - Cp FS;mean.5 [-] Angle of attak [ o ] Figure 6. Differene between the wind tunnel and full sale external mean pressure oeffiients. The pressure inside the avity is largely dependent on the external pressures at the gaps. The distribution of the gaps with respet to the external pressure (gradient) determines to a large extent the pressure equalization. In partiular in orner areas where a large pressure gradient is present, the position of the gaps is very important. If there are gaps near the orner at the onsidered sides, pressure equalization an our and the resulting fore on the outer layer is probably lower than the external pressure. If the gaps are loated just around the orner, a negative situation an our. For example when an overpressure of the adjaent side enters the avity, while the side itself is subjeted to underpressure. This will lead to a high outward pointing fore as is shown in Figure 7. Figure 8 shows on whih side of the orner the gaps are loated in the New Orleans building for the partiular floor that is equipped with pressure taps. The loations of the gaps, however, alternate for eah row of panels. Where the onsidered panel row has gaps at side BC and DA, the panel rows below and above the onsidered panel row have openings at side AB and CD. Figure 8 also shows that the avity is not ompartimentalized and that the extremities are open. Therefore, both an internal avity flow around the orner and an inflow from external air is possible. Due to the tube length restrition no pressure taps are present near the orners of side AB and CD. Therefore, it annot be measured whether the negative situation, with overpressure in the avity, ours at New Orleans building. It is expeted that all pressure taps measure the positive situation where the net pressures of the outer layer are lower than the external pressures. Figure 7. Priniple of pressure equalization. Left: positive situation, low net fores. Right: negative situation, high net fores External pressure tap Cavity pressure tap B Northwest A Southwest Wind diretion 5 Northeast C Southeast D Figure 8. Left: Detail of Corner C. Right: Loations of avity openings near the orners. 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

7 7 Figure 9 shows the differential pressure oeffiients for a wind diretion of 5 degrees, a wind diretion almost normal to side BC. At side CD the avity pressure taps are not funtioning orretly, therefore the results of these pressure taps are removed in the results. This also applies for the first tap just around the orner for both orner C and D. The mean differential pressure oeffiients are lose to zero, whereas the peak differential pressure oeffiients are greater than zero, pointing towards the building at the windside of the building and smaller than zero, pointing outwards, at the other sides. The differential pressure oeffiients are not uniformly distributed over the sides, but vary between. and.6 at the windside, -.5 and -.3 at the side and -.3 and -. at the leeside. Even though EN does not reognize an edge zone for overpressures, an edge effet seems to our. Figure shows the differential pressure oeffiients for a wind diretion of 85 degrees, whih is a relative wind diretion of 3 degrees to side AB. This wind diretion will lead to high underpressures at side BC. The mean differential pressure oeffiients are again lose to zero, with a maximum differene of -. near orner B. The peak differential pressure oeffiients at side BC are high, with a maximum of -.5 near orner B. The two taps lose to orner B, with values -.5 and -., are loated in a orner zone aording to EN (zone A). The differential pressure oeffiients of the other taps at side BC, whih an be seen as middle zone taps, vary between -.4 and -.8. C pdiff;mean Wind diretion 5 o full sale C pdiff;peakev Wind diretion 5 o full sale Figure 9. Mean differential pressure oeffiients at 5 degrees. Left: Mean values Right: Peak values C pdiff;mean Wind diretion 85 o full sale C pdiff;peakev Wind diretion 85 o full sale Figure. Mean differential pressure oeffiients at 85 degrees. Left: Mean values Right: Peak values 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

8 8 Figure shows the pressure equalization oeffiients, whih are defined as the peak differential pressures divided by the peak external pressures, for those two wind diretions. The pressure equalization oeffiients at the windside, whih is in overpressure, are around.5 and at the leeward side around.3. For the other sides, higher values are found, however they are never larger than.. This is in line with the results found by other researhers [6],[7],[8]..5 Wind diretion 5 o full sale + ( -.3).5 Wind diretion 85 o full sale + ( -.68) C eq.5 C eq Figure. Pressure equalization oeffiients. Left: 5 degrees. Right: 85 degrees. In Figures and 3 the pressure equalization oeffiient are presented per wind diretion. From Figures and 3 it an be seen that the pressure equalization oeffiient is both dependent on the wind diretion and on the loation at the faade (middle zone or orner zone). in the middle zone show very similar behavior, in whih the pressure equalization oeffiient is dependent on the wind diretion only. However, for taps in the orner zone, a less onsistent behavior is notied. For overpressures the pressure equalization oeffiients vary more than for underpressures. The values for overpressure seem a bit higher than for underpressures. 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

9 9 Side BC Cpdiff;peakev Side BC Cpe;peakev Ceq;overpressure Ceq;underpressure Angle of attak ( o ) Figure. a: Peak differential pressure oeffiients of side BC per tap per wind diretion. b: Peak external oeffiients of side BC per tap per wind diretion. : Pressure equalization oeffiients (overpressure) of side BC per tap per wind diretion. d: Pressure equalization oeffiients (underpressure) of side BC per tap per wind diretion. 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

10 Side AB Cpdiff;peakev Side AB Cpe;peakev Ceq;overpressure Ceq;underpressure Side AB Side AB Angle of attak ( o ) Figure 3. a: Peak differential pressure oeffiients of side AB per tap per wind diretion. b: Peak external oeffiients of side AB per tap per wind diretion. : Pressure equalization oeffiients (overpressure) of side AB per tap per wind diretion. d: Pressure equalization oeffiients (underpressure) of side AB per tap per wind diretion. 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

11 In Figure 4 the pressure equalization oeffiient aording to Eq. 5 is presented, being the maximum differential pressure oeffiient over all wind diretions, divided by the maximum external pressure oeffiient over all wind diretions. The values are presented per tap. For overpressures a maximum value of.6 is found, whereas for underpressures a maximum value of.8 is found in orner zones and. in middle zones. However, for side BC a maximum value of.5 is found for overpressure and values of.4 and.6 are found for underpressures in the orner and middle zones respetively. EN gives rules for the net pressure oeffiient of an permeable façade: p,net =/3 pe in situations with overpressures and p,net =/3 pe in situations with underpressure. The given values for overpressure are more or less omparable with the measured pressure equalization oeffiients, however the given values for underpressure are lower than the measured pressure equalization oeffiients..5 overpressure underpressure eq Figure 4. Pressure equalization oeffiients for overpressure and underpressure aording to Eq CONCLUSIONS A full sale experiment has been set up in the Netherlands to measure differential pressures over the outer layer of a permeable façade. The external pressures have been ompared with wind tunnel experiments on the same building. It has been onluded that the external pressures of the full sale experiment show an offset, presumably aused by an internal overpressure. The wind tunnel results have been used to orret for this offset. Another adjustment has been made in the referene veloity, whih is taken from a nearby airport, sine the measured veloities on the roof appeared to be highly affeted by the building itself. Pressure equalization oeffiients per pressure tap have been derived, whih are defined as the maximum peak differential pressure oeffiients divided by the maximum peak external pressure oeffiients. Also wind diretion dependent values have been derived. For overpressure, a pressure equalization oeffiient of.6 has been found, whih is omparable to the ratio EN gives for the net pressure oeffiient of an permeable façade ( p,net =/3 pe ). For underpressure, values in the same order of magnitude have been found, with a maximum value of.8 in the middle zone. One value above has been found, whih lies in a orner zone ( eq =.). The measured pressure equalization oeffiients for underpressure are higher than the net pressure oeffiients in EN for permeable faades ( p,net =/3 pe ). More (reliable) onlusions an only be drawn when more data beomes available. Espeially, more data from northern wind diretions ould support the urrent onlusions regarding the atual values of the pressure equalization oeffiients in orner and middle zones of sides in an underpressure situtation. When more data beomes available, the measurement is still running, a striter filter riterion will be used to selet the data and the data will be reanalysed. The insights gained by this full sale experiment, the wind tunnel tests and the CFD alulations will be used to develop a predition model for avity pressures. 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5

12 ACKNOWLEDGMENTS This researh would not have been possible without the finanial support of DHV, whih also took the initiative for the full sale measurements, Blitta, Centrum Natuursteen, Kennisentrum Gevelbouw and TNO. REFERENCES [] CEN, EN (5); Euroode; Ations on strutures; wind ations. [] Gerhardt, H. J., C. Kramer. (!983) Wind loads on permeable building faades. Journal of Wind Engineering and Industrial Aerodynamis,,. [3] Gerhardt, H.J., F. Janser. (994). Wind loads on permeable faades. Journal of Wind Engineering and Industrial Aerodynamis 53, [4] Inulet, D.R., A.G. Davenport. (994). Pressure equalized rainsreen: A study in the frequeny domain. Journal of Wind Engineering and Industrial Aerodynamis 53, [5] Popp, W. E., Mayer, and H. Künzel. (98). Untersuhungen über die Belüftung des Luftraumes hinter vorgesetzten Fassadenbekleidung aus kleinformatigen Elementen. Forshungsberiht B Ho /8, Fraunhofer Institut für Bauphysik. [6] Shijndel, A.W.M. van, S.F.C Shols. (998). Modeling pressure equalization in avities. Journal of Wind Engineering and Industrial Aerodynamis 74-76, [7] Ganguli, U., W. A. Dalgliesh. (988). Wind pressures on open rain sreen walls. Plae Air Canada. J. Strut. Eng., 3(3), [8] Geurts, C.P.W., P.W. Bouma, A. Aghaei. (5). Pressure equalization of brik masonry walls, in: Proeedings of the 4th European-Afrian Conferene on Wind Engineering, published on CD rom, Prague. [9] Suresh Kumar, K.. (). Pressure equalization of rainsreen walls: a ritial review. Building Environ., 35(), 6 79, [] Brown, W.C., M.Z. Rousseau, W.A. Dalgliesh. (99). Field testing of pressure equalized rainsreen walls. Exterior Wall Symposium, Preast Conrete, Masonry and Stuo, [] CUR Aanbeveling 3 'Windbelasting op (hoge) gebouwen' (5). Stihting CUR, Gouda [] Cook, N.J., J.R. Mayne. (98). A refined working approah to the assessment of wind loads for equivalent stati design, JWEIA, 8, 5-37 [3] Staalduinen, P.C. van, A. Vrouwenvelder. (993). In situ bepaling van de vormfator van bouwwerken en onderdelen daarvan, B-9-738, TNO, the Netherlands. [4] Bentum, C.A. van, I. Kalkman and C.P.W Geurts (4). Field tests to study the pressure equalization on air permeable façade elements, in: Proeedings of ICBEST 4, Aahen, Germany. 4th International Conferene on Wind Engineering Porto Alegre, Brazil June -6, 5