ARTICLE IN PRESS Journal of Wind Engineering and Industrial Aerodynamics 91 (2003) 1401–1417 Field measurement data of wind loads on rainscreen walls K. Suresh Kumara, T. Stathopoulosb,*, J.A. Wissec a b RWDI Inc., Guelph, Ont., Canada Centre for Building Studies, Concordia University, 1455, de Maisonneuve Blvd. West, Montreal, Que., Canada H3G 1M8 c Faculteit Bouwkunde, FAGO, Technical University of Eindhoven, The Netherlands Received 30 November 2001; received in revised form 18 September 2002; accepted 31 July 2003 Abstract Rainscreen wall design is still at its infancy stage even after its introduction about four decades ago. Research continues in an effort to set out appropriate design guidelines for rainscreen walls. This paper presents the key results of yearlong full-scale measurements of wind loading on rainscreen walls. The objective of this study is to estimate the impact of various design parameters on the wind loading on rainscreen. This paper also presents the current status of the available codes and standards regarding the wind loads on rainscreen walls and compares the full-scale results with some available provisions. r 2003 Elsevier Ltd. All rights reserved. Keywords: Pressure equalization; Rainscreen; Walls; Wind loads 1. Introduction A major source of concern in the performance of building envelopes is their susceptibility to rainwater penetration. Screened wall systems use an additional exterior layer, the screen (outer wall layer or rainscreen), to keep the rainwater out of contacting the structural leaf (inner wall layer or air barrier). Further, a cavity is often placed in screened wall systems to provide gravity drainage and capillary break in case of any rainwater penetration through the screen. Pressure equalized rainscreen (PER) approach to wall design has been suggested to minimize the *Corresponding author. Fax: +1-514-848-7965. E-mail address: statho@cbs-engr.concordia.ca (T. Stathopoulos). 0167-6105/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jweia.2003.07.001 ARTICLE IN PRESS 1402 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 wind-induced pressure difference across the outer wall layer and thereby reduce the rainwater penetration through the screen. This wall system is a special case of the modern screened wall systems. The state-of-the-art information concerning PER approach to wall design has been documented elsewhere [1–3]. PER wall design has mainly been based on water penetration resistance. Increased concern regards the higher construction costs of PER wall due to its two wall layers; however, through better pressure equalization, the design loads for the rainscreen and subsequently the construction costs can be reduced. Very few attempts have been made in the past to establish design wind loads for PER walls [4–7]; however, it should be mentioned that Ganguli and Dalgliesh [5] presented some of the first field measurement data on PER walls. Most recently, Lawton [8] and Inculet et al. [9] reported design schemes for venting and compartmentalization of rainscreen walls based on analytical formulation and wind tunnel measurements. Current design guidelines in codes and standards provide only sketchy and unsubstantiated stipulations for the design wind loads of PER walls. Clearly, further research on the effect of various parameters is necessary to establish objective standards or code provisions for the structural design of PER walls. Experimental and theoretical research can yield very useful information through systematic investigations. Therefore, an extensive investigation, consisting of full-scale monitoring and computer simulations, has been carried out at the Technical University of Eindhoven (TUE) in the Netherlands [10–12]; parameters such as cavity volume, venting area and leakage area can be varied in this unique field facility at TUE. The analysis of the full-scale data to provide actual wind loads acting on PER walls is the main subject of this paper; the results are provided in the form of pressure coefficients. This paper also includes current status of the wind load provisions for rainscreen walls and their comparison with full-scale results. Detailed investigation of the field data in the frequency domain was recently reported in Ref. [12]. 2. Wind load provisions for rainscreen walls Few codes and standards address the design of PER walls. The British Standards 8200 [13] is the only code with specific provisions for the design of non-load bearing vertical walls. As far as compartmentation of the cavity is concerned, this code recommends the following compartment dimensions in order to minimize air movement: The largest lateral dimension of air spaces within 25% of the corner or top of the enclosure should be about 1.5 m, and elsewhere, about 5 m. The Dutch standard for loadings and deformations [14] included a pressure equalization factor (Ceq ) for the estimation of wind pressure on building envelope consisting of two layers with an air column in between. However, this standard did not provide any specific values for Ceq ; instead, it suggested to use a value of one until pertinent information becomes available. On the other hand, the dutch code for fixing of roof coverings [15] proposed a set of values for Ceq based on roof angles and roof zones for the estimation of wind load on roofs of two layers with an air space in between; however, there are concerns regarding the scientific or technical ARTICLE IN PRESS K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 1403 background of these provisions. The Australian Standard for wind loads [16] recommended reduction factors for the estimation of design wind loads on porous cladding. These factors depend on the cladding porosity and the horizontal distance away from windward building edge. In 1987, the ECCS [17] proposed internal pressure coefficients in the inside air layer of a wall or a roof (i.e., cavity pressure) with respect to porosity and stiffness of outer and inner wall layers, as well as thickness of the air layer and any other entrances of air. Later, the Eurocode ENV 1991-2-4 [18] recommended almost the same values. Recent revisions of Eurocode, currently under consideration, reflect the uncertainty associated with the previously recommended provisions. Finally, the German Wind Code [19] also provides design pressure coefficients for building envelopes with permeable facades. These values are based on the study by Gerhardt and Janser [20]. Note that other major international codes and standards for wind loads in Canada and USA [21,22] do not give any design provisions for rainscreen walls. According to ASCE 7-98 [22]: if the designer desires to determine the pressure differential across the air-permeable cladding element, appropriate full-scale pressure measurements should be made on the applicable cladding element, or reference be made to recognized literature [y] for documentation pertaining to wind loads. The lack of consistent and ready to use wind design guidelines for PER walls in codes and standards is clear. More research has to be carried out to generate and formulate the basis for codification of design guidelines appropriate for PER walls. 3. Experimental procedure The experiments have been carried out on the main building of TUE, Eindhoven. Prevailing strong wind directions are west (270
) and south-west (225
). Upstream terrain conditions for prevailing wind directions can be characterized as suburban. A SOLENT ultrasonic anemometer mounted at the top of a 30 m high mast placed on the top of a 14 m high building, 127 m westward of the main building of the university is used for three component wind velocity measurements. Fig. 1 shows the sketch of the field facility; few photographs of the field facility are shown in Figs. 2 and 3. For pressure measurements, a wooden panel 1 m  1.3 m (panel area, Aw ¼ 1:3 m2) was mounted approximately on the middle of the west facade at a height of about 39 m above the ground. The test panel consists of three components: (1) rainscreen, (2) air barrier and (3) air space (cavity) between them; the cavity depth can be varied. Four pressure taps are installed on the rainscreen and another four on the air barrier for pressure measurements. Fig. 4 shows the details of the used test panel. Many venting and air barrier leakage configurations have been used for the measurement. Pressure and velocity data have been collected for six panel configurations, the details of which are provided in Table 1. The flow characteristics of venting and air barrier leakage have been determined using simple static pressurization tests; details can be found in Ref. [10]. For all these configurations, the cavity depth was kept constant at 0.15 m. Note that the venting area of configuration 1 is about 4.9 times the venting area of configuration 2. Comparing configurations 2 ARTICLE IN PRESS 1404 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 Fig. 1. Sketch of the field facility. Fig. 2. Test building, view from southwest. and 3, venting areas are the same; however, their leakage characteristics are quite different. Configurations 4 and 5 have no leakage, but their venting geometries are different; the venting area of configuration 5 is about 2.3 times the venting area of configuration 4. Configuration 6 is the same as configuration 5 but with leakage. Differential pressure transducers have been used to measure the differential pressures across the panel and across the air barrier. For data acquisition, a Physics ARTICLE IN PRESS 1405 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 Fig. 3. Westward fetch of the test building. Orientaion of Panel S (180o) E (90o) W (270o) Panel N (360o) Fig. 4. The test panel. Data Acquisition System (PhyDAS) developed at the Faculty of Physics of the TUE was used. A PARSAM 25 (Parallel sampling A/D conversion board) was used to capture the incoming signal. In each run, the exterior and cavity pressure data were simultaneously measured at four taps each at a sampling rate of 20 Hz for 10 min. The velocity data were also acquired by PhyDAS at a rate of 20.83 Hz. The measurements were carried out between May 1998 and July 1999. During this period, each of the six configurations was set for at least 2 months for measurements. About 1500 full-scale runs have been registered. The records were analyzed in time, frequency and amplitude domains. Representative mean, rms and peak pressure values have been chosen for the demonstration of the results reported mainly in the form of non-dimensionalized ARTICLE IN PRESS 1406 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 Table 1 Panel configurations used for full-scale measurements Configuration 1 2 3 4 5 6 Venting Circular holes Ars ¼ 0:009613 Cd ¼ 0:61 n1 ¼ 0:5 Circular holes Ars ¼ 0:001979 Cd ¼ 0:61 n1 ¼ 0:5 Circular holes Ars ¼ 0:001979 Cd ¼ 0:61 n1 ¼ 0:5 Circular holes Ars ¼ 0:001979 Cd ¼ 0:61 n1 ¼ 0:5 Rectangular slits Ars ¼ 0:004577 Cd ¼ 0:61 n1 ¼ 0:5 Rectangular slits Ars ¼ 0:004577 Cd ¼ 0:61 n1 ¼ 0:5 Air barrier leakage Straw Straw Filter No leakage No leakage Filter Cab ¼ 0:000314 n2 ¼ 0:71 Cab ¼ 0:000314 n2 ¼ 0:71 Cab ¼ 0:000171 n2 ¼ 1:0 Cab ¼ 0:000171 n2 ¼ 1:0 Note: Ars =venting area (m2), Cd =discharge coefficient, n1=flow exponent of air barrier, Cab =flow coefficient of air barrier (mPan2/s), n2=flow exponent of air barrier. pressure coefficients using the dynamic pressure at panel height (q). Mean, rms and # * and P=q; % peak pressure coefficients are defined as P=q; P=q respectively; where -, B and ^ represent mean, rms and peak, respectively, and P represents differential pressure acting on panel or air barrier or rainscreen. In this paper, the differential pressures acting on the panel, air barrier and rainscreen are denoted by Pe 2Pi ; Pc 2Pi and Pe 2Pc ; respectively; where, Pe is the external pressure, Pc the cavity pressure and Pi the internal pressure. Details of the experimental procedure and data analysis can be found in Refs. [10,12]. When the cavity pressure equals external pressure, full pressure equalization (i.e., no pressure acting on the rainscreen or zero rainscreen pressure coefficient) occurs. Pressure equalization performance is also indicated by the difference between the pressure acting on the panel and the rainscreen. The value of rainscreen pressure closer to the value of panel pressure, which happens when the cavity pressure is away from the external pressure, indicates poor pressure equalization performance. On the other hand, lower value of rainscreen pressure compared to the panel pressure, occurring when the cavity pressure is closer to the external pressure, indicates good pressure equalization performance. 4. Experimental results Fig. 5 shows typical simultaneous measurements of the time variation of the pressure coefficients across the panel, air barrier and rainscreen for configuration 2. Because of the low venting area of this configuration, the low-frequency pressure fluctuations are not transferred into the cavity completely. As a result, the mean as well as the low-frequency pressure coefficient variations corresponding to the air barrier are lower than those corresponding to the panel. It is also noted that irrespective of the amount of venting, the higher-frequency fluctuations are not transferred into the cavity and as a result, the pressure coefficient variation across ARTICLE IN PRESS K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 1407 Configuration 2, Wind velocity at panel height = 12.5 m/s, Wind direction = 252.1o 2.5 panel air barrier rainscreen Pressure coefficient 2 1.5 1 0.5 0 -0.5 0 20 40 60 80 100 Time (sec.) Fig. 5. Time variation of pressure coefficients. the air barrier is smooth compared to that across the panel. Consequently, the higher-frequency pressure fluctuations have been transferred to the rainscreen. Extensive analysis of the field data in the frequency domain has been recently presented in Ref. [12]. For simplicity, only the pressure coefficients applicable to the panel and rainscreen are discussed; one of the intentions of pressure equalization to reduce pressure load on the rainscreen is another good reason for presenting results in this format. 4.1. Mean pressure coefficients The distribution of mean pressure coefficients applicable to the panel over the wind direction shown in Fig. 6 is nearly symmetrical about y ¼ 270
(y =wind direction; y =270
represents normal to the west facade); this is due to the exact north–south (0
–180
) orientation of the building. As expected, the largest mean pressure coefficients of the panel occur when wind blows nearly normal to the facade, i.e., y around 270
. The mean pressure coefficients of the panel consistently decrease as the wind direction increases or decreases from 270
. Fig. 6 also shows that the mean rainscreen pressure coefficients are independent of wind direction for configurations 4 and 5 with airtight air barrier and for configuration 1 with high venting to leakage area ratio. In other cases, mean rainscreen pressure coefficients depend on wind direction. This is clear from configurations 2, 3 and 6. For instance, the mean rainscreen pressure coefficients are higher when the wind blows normal to the facade and lower when wind blows parallel to the facade. For all configurations, in case of mean pressure coefficients, it appears that the difference between the mean rainscreen pressure coefficient and the ARTICLE IN PRESS K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 1.2 Pe-Pi (Config. 2) Pe-Pc (Config. 2) 0.8 0.4 0 -0.4 -0.8 180 Mean pressure coefficients Pe-Pi (Config. 1) Pe-Pc (Config. 1) Mean pressure coefficients 1.6 210 1.6 240 270 300 Wind direction Pe-Pi (Config. 4) Pe-Pc (Config. 4) 1.2 330 Pe-Pi (Config. 5) Pe-Pc (Config. 5) 0.8 0.4 0 -0.4 -0.8 180 210 240 270 300 Wind direction 330 360 1.6 Pe-Pi (Config. 2) Pe-Pc (Config. 2) 1.2 Pe-Pi (Config. 3) Pe-Pc (Config. 3) 0.8 0.4 0 -0.4 -0.8 180 360 Mean pressure coefficients Mean pressure coefficients 1408 210 1.6 240 270 300 Wind direction Pe-Pi (Config. 5) Pe-Pc (Config. 5) 1.2 330 360 Pe-Pi (Config. 6) Pe-Pc (Config. 6) 0.8 0.4 0 -0.4 -0.8 180 210 240 270 300 Wind direction 330 360 Fig. 6. Mean pressure coefficients acting on panel and rainscreen as a function of wind direction. corresponding mean pressure coefficient for the panel decreases as the wind direction changes from 270
. This may signify poor pressure equalization performance of the panel in terms of mean pressure loads for wind directions away from normal to the facade; however, the associated pressure coefficients are low and therefore, this poor performance does not have any consequences as far as design is concerned. On the other hand, the high mean rainscreen pressure coefficients occurring close to y ¼ 270
should be carefully considered for design purposes. Note that the amount of mean pressure load shared by the rainscreen varies depending on the venting area and the air barrier leakage configurations. The corresponding mean rainscreen pressure coefficients of configurations 1 and 5 are almost the same; these values are low compared to all other configurations which reveals that the rainscreen takes only a minor part of the total mean pressure load acting on the panel because of good pressure equalization. In case of configuration 4, when the venting area is reduced compared to configuration 5, the mean pressure load is increased. Note that even if the air barrier is airtight (e.g., configurations 4 and 5), sufficient venting area is required for better pressure equalization and consequent reduction of mean rainscreen load. The highest mean rainscreen loads are obtained in case of configuration 3; this is expected because of its leaky air barrier and smaller venting area. Configuration 2 experiences low mean rainscreen pressure coefficients compared to configuration 3 because of its better air barrier ARTICLE IN PRESS 1409 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 configuration. Configuration 6 experiences low mean rainscreen pressure coefficients compared to configuration 3 because of its larger venting area. 4.2. Rms pressure coefficients 0.6 0.4 0.2 210 240 270 300 Wind direction Pe-Pi (Config. 4) Pe-Pc (Config. 4) 1 330 Pe-Pi (Config. 5) Pe-Pc (Config. 5) 0.6 0.4 0.2 210 240 270 300 Wind direction 330 360 Pe-Pi (Config. 2) Pe-Pc (Config. 2) 1 Pe-Pi (Config. 3) Pe-Pc (Config. 3) 0.8 0.6 0.4 0.2 0 180 360 0.8 0 180 Rms pressure coefficients Pe-Pi (Config. 2) Pe-Pc (Config. 2) 0.8 0 180 Rms pressure coefficients Pe-Pi (Config. 1) Pe-Pc (Config. 1) 1 Rms pressure coefficients Rms pressure coefficients The distribution of rms pressure coefficients of the panel over the wind direction shown in Fig. 7 is nearly symmetrical about y ¼ 270
as previously noted in case of mean pressure distributions. Similar to the mean pressure distributions, the largest rms pressure coefficients of the panel occur for wind blowing normal to the facade; and the rms pressure coefficients decrease as the wind direction increases or decreases from 270
. As noted in case of mean rainscreen pressure coefficients, the rms rainscreen pressure coefficients do not depend on wind direction for configurations 1, 4 and 5. In case of configurations 2, 3 and 6, rainscreen pressure coefficients depend on wind direction; the rainscreen pressure coefficients shoot up when wind blows normal to the facade and lower when wind blows parallel to the facade. Similar to the mean pressure coefficient distributions, pressure equalization performance of the panel is poor for wind directions away from normal to the facade since the difference between the rms rainscreen pressure coefficient and the corresponding rms pressure coefficient for the panel decreases as the wind direction changes from 270
. Note that since the associated rms pressure coefficients are low for wind directions away from 210 240 270 300 Wind direction Pe-Pi (Config. 5) Pe-Pc (Config. 5) 1 330 360 Pe-Pi (Config. 6) Pe-Pc (Config. 6) 0.8 0.6 0.4 0.2 0 180 210 240 270 300 Wind direction 330 360 Fig. 7. Rms pressure coefficients acting on panel and rainscreen as a function of wind direction. ARTICLE IN PRESS 1410 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 normal to the facade, high rms rainscreen pressure coefficients occurring close to y ¼ 270
may be important for design purposes. The amount of pressure load shared by the rainscreen varies depending on the venting area and the air barrier leakage configurations. The corresponding rms rainscreen pressure coefficients of configurations 1 and 5 are almost the same; these values are low compared to all other configurations which reveals that the rainscreen takes only a minor part of the total rms load acting on the panel because of good pressure equalization. The highest rms rainscreen loads are obtained in case of configuration 3; this is expected because of its leaky air barrier and smaller venting area. Configuration 2 experiences low rms rainscreen pressure coefficients compared to configuration 3 because of its better air barrier configuration. Configuration 6 experiences low rms rainscreen pressure coefficients compared to configuration 3 because of its larger venting area. 4.3. Peak pressure coefficients Peak pressure coefficients as well as peak factors ((peak pressure coefficientmean pressure coefficient)/rms pressure coefficient) acting on the panel and rainscreen have been estimated for all configurations. In this analysis, the peak pressure coefficients were selected based on 1 s averaging time. The dependence of the peak panel and rainscreen pressure coefficients on wind direction is similar to that of mean and rms rainscreen pressure coefficients on wind direction already described. In general, as the wind deviates away from the normal, the peak panel and rainscreen pressure coefficients decrease. The magnitudes of peak panel and rainscreen pressure coefficients depend on the venting and leakage characteristics of the configuration. For instance, peak panel and rainscreen pressure coefficients of configurations 2 and 3 are higher than those of configuration 6. It is conjectured from the values of mean, rms and maximum pressure coefficients that the pressure fluctuations measured for wind directions close to 270
are positively skewed. Close to wind direction 180
/360
, negative mean pressure coefficients are obtained; the corresponding maximum and absolute minimum pressure coefficients are low and high, respectively. This shows that the pressure fluctuations measured for wind directions close to 180
/360
are negatively skewed. It is also noted that the magnitudes of the highest maximum pressure coefficients occurring near 270
are much higher than the highest absolute minimum pressure coefficients occurring near 180
/360
. This result is probably due to the panel’s location on the middle of the facade; further, the results are based on measurements made only for wind directions between 180
and 360
. On the other hand, the highest absolute minimum pressure coefficients are expected to be higher than the highest maximum pressure coefficients when the panel is located close to the corner on the separation region. Typical variation of positive and negative peak factors with wind direction is shown in Fig. 8. Note that positive and absolute negative peak factors corresponding to panel increase as the wind deviates from 270
. The highest positive and negative peak factors obtained are close to 9 and 9, respectively. These high peak factors occurring close to 180
/360
may not be significant for design since ARTICLE IN PRESS 1411 Positive peak factors for panel 10 Config. 1 Config. 2 Config. 3 8 Config. 4 Config. 5 Config. 6 6 4 2 Negative peak factors for panel 0 -2 -4 -6 -8 -10 180 210 240 270 300 Wind direction 330 360 Minimum rainscreen peak factors Maximum rainscreen peak factors K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 10 Config. 2 Config. 3 8 6 4 2 0 -2 -4 -6 -8 -10 180 210 240 270 300 Wind direction 330 360 Fig. 8. Peak factors acting on panel and rainscreen as a function of wind direction. the corresponding positive and absolute negative pressure coefficients are not as high as the positive pressure coefficients observed near 270
. The positive peak factors occurring close to 270
spread between 3 and 5; the peak factor above 3 signifies that the associated pressure fluctuations are non-Gaussian and positively skewed. Note that peak factor value depends on the mean pressure as well as the rms pressure value. Therefore, low rms pressure can result in high peak factor. Typical variation of positive and negative rainscreen peak factors with wind direction is also shown in Fig. 8 for configurations 2 and 3. Note that, as with the panel, positive and absolute negative rainscreen peak factors increase as the wind deviates from 270
, resulting in lower values near 270
. Typical variation of positive and negative rainscreen peak factors with rms rainscreen pressure coefficients is shown in Fig. 9. It is clear that very high values of positive and negative rainscreen peak factors are associated with low rms rainscreen pressure coefficients; this seems to be an overestimation of the values and has no physical significance. Note also that wide scattering of the positive and negative peak factors corresponds to low rms rainscreen pressure coefficients. 4.4. Rainscreen pressure coefficients Fig. 10 shows the measured mean and peak pressure coefficients across the rainscreen for all configurations shown in Table 1. It is noted in this investigation that the pressure coefficients for wind angles between 240
and 300
are higher than those measured for 180
–240
and 300
–360
. In this wind direction range, there is ARTICLE IN PRESS 1412 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 Positive rainscreen peak factors 10 Config. 2 Config. 3 8 6 4 2 Negative rainscreen peak factors 0 -2 -4 -6 -8 -10 0 0.2 0.4 0.6 0.8 Rms rainscreen pressure coefficients 1 Fig. 9. Rainscreen peak factors as function of rms rainscreen pressure coefficients. no clear pattern of variation of pressure coefficients with wind direction. As expected, the peak pressure coefficients are more scattered compared to the mean pressure coefficients. For a particular configuration, the pressure coefficient values scatter around a certain level, which is different for each configuration. In case of configurations 1, 4 and 5, most of the wind loads are transferred to the air barrier and as a result, the mean as well as the peak rainscreen pressure coefficients are low. On the other hand, in case of configurations 2, 3 and 6, the rainscreen experiences higher wind loads either due to their smaller venting area or due to their leaky air barrier and as a result, the associated pressure coefficients are high. The pressure coefficient values of configuration 3 are higher compared to configuration 2 because of the higher leakage associated with the latter. Fig. 11 shows the ratio of the load taken by the rainscreen with respect to the load acting on the panel; the load acting on the panel represents wind load on regular wall. As per this definition, low ratios indicate good pressure equalization and load reduction for rainscreen; a ratio equal to zero indicates no pressure acting on rainscreen, while a ratio equal to one indicates full pressure acting on rainscreen. In case of configurations 2 and 3 with leaky air barrier and smaller venting area, the ARTICLE IN PRESS 1413 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 Config. 1 Config. 2 Config. 4 Config. 3 Mean pressure coefficient Mean pressure coefficient 0.8 0.6 0.4 0.2 0 -0.2 0.8 0.6 0.4 0.2 0 -0.2 -0.4 3 Peak pressure coefficient -0.4 3 Peak pressure coefficient Config. 6 1 1 2.5 2 1.5 1 0.5 0 -0.5 240 Config. 5 250 260 270 280 Wind direction 290 300 2.5 2 1.5 1 0.5 0 -0.5 240 250 260 270 280 290 300 Wind direction Fig. 10. Mean and peak pressure coefficients acting on the rainscreen. pressure coefficient ratios are high compared to configuration 1. This shows that lower ratios can be achieved by providing adequate venting area for counteracting the leakage of the air barrier. 5. Design issues For each configuration, the maximum mean and peak load ratios have been selected from Fig. 11 and plotted with respect to the venting area of the configuration in Fig. 12, where the corresponding configuration numbers are indicated beside the symbols. The plot also groups the cases according to the air barrier leakage. Low values indicate good pressure equalization and load reduction for rainscreen. In general, as the venting area increases, the pressure equalization performance improves and correspondingly, the load taken by rainscreen decreases. However, note that the percentage of venting area required to produce good pressure equalization or load reduction depends on the air barrier leakage. For instance, in case of no air barrier leakage, the panel requires only small venting area to have good pressure equalization. For the same venting area with the inclusion of air barrier leakage, the pressure equalization performance worsens. This figure also ARTICLE IN PRESS 1414 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 Config. 2 Config. 4 Config. 3 (Rainscreen load/panel load) peak (Rainscreen load/panel load) mean (Rainscreen load/panel load) peak (Rainscreen load/panel load) mean Config. 1 1 0.8 0.6 0.4 0.2 0 1 0.8 0.6 0.4 0.2 0 240 250 260 270 280 290 300 Config. 5 Config. 6 1 0.8 0.6 0.4 0.2 0 1 0.8 0.6 0.4 0.2 0 240 250 260 270 280 290 300 Wind direction Wind direction Fig. 11. Ratio of the load acting on the rainscreen and the total load acting on the panel. (Rainscreen load/panel load) mean (Rainscreen load/panel load) peak Cab = 0.000314 mPa-n2/s, n2 = 0.71 No leakage 1 3 0.8 2 0.6 0.4 4 6 0.2 1 5 0 0 0.2 0.4 0.6 0.8 Venting Area (% of panel area) 1 Cab = 0.000171 mPa-n2/s, n2 = 1.00 1 0.8 3 0.6 2 0.4 6 4 0.2 5 1 0 0 0.2 0.4 0.6 0.8 1 Venting Area (% of panel area) Fig. 12. Rainscreen load reduction as a function of venting and leakage area. provides an idea about the required percentage of venting area with respect to the desired reduction in load. The results of this study can be used as a preliminary recommendation for the design of rainscreen walls. Recently, the first author used these results along with the ARTICLE IN PRESS K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 1415 Table 2 Comparison of full-scale measurements with the provisions in ENV 1991-2-4 for rainscreen walls with permeable air barrier Configuration ENV 1991-2-4 Condition Measurements % Load reduction % Load reduction 1 me ¼ 0:74%; mi ¼ 0:045% me > 3mi ; me o1% Outside overpressure 0 95 2 me ¼ 0:15%; mi ¼ 0:045% me > 3mi ; me o1% Outside overpressure 0 55 72 25 3 me ¼ 0:15%; mi ¼ 0:095% mi ome o3mi 3 4 me ¼ 0:15%; mi ¼ 0:0 me > 3mi ; me o1% Outside overpressure 0 75 5 me ¼ 0:35%; mi ¼ 0:0 me > 3mi ; me o1% Outside overpressure 0 90 6 me ¼ 0:35%; mi ¼ 0:095% me > 3mi ; me o1% Outside overpressure 0 65 Note: me =porosity of rainscreen, mi =effective porosity of air barrier estimated based on orifice platemeter equation [10]. analytical simulation results and the expert opinions of his colleagues to obtain load reduction factors for panels of a high-rise building in Chicago. In the current design practice, the wind loads acting on the rainscreen are handled by the engineering judgment of the design engineer or the panel manufacturer. As already mentioned, very few building codes and standards address or provide some design guidelines for these walls. Table 2 provides the comparison between the measurements and provisions of ENV 1991-2-4 [18] for the case of rainscreen with impermeable air barrier (configurations 4 and 5), and permeable air barrier (configurations 1, 2, 3 and 6). Effective porosity of the air barrier has been estimated in each case using traditional orifice plate-meter equation [10] for comparison purposes; note that the same difficulty occurs in real-life situations where the actual leakage area of the air barrier cannot be estimated precisely since they are in the form of punctures, cracks, etc. As per the definition of the ENV 1991-2-4 [18], configurations 1, 2, 4–6 fell in the impermeable category and code proposes no load reduction to the rainscreen though significant load reductions have been found in full-scale measurements. On the other hand, for configuration 3, code proposes 72% load reduction compared to the fullscale value of 25%. The expression of cavity pressure coefficient in this code for panels with permeable rainscreen and air barrier assumes that the load sharing between the rainscreen and air barrier is only a function of the porosity of these two layers. The data from Ganguli and Dalgliesh [5] indicate that the percentage of load ARTICLE IN PRESS 1416 K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 reduction on rainscreen walls ranges between 25% and 60% for venting area less than 1% and leakage close to zero. However, realistically, the load sharing between the layers does also depend on the total load on the panel. This is caused by the difference in flow exponents for the rainscreen and air barrier which resulted in a load sharing that depends not only on the ratio of venting to leakage characteristics but also on the absolute total pressure drop across the entire panel [2]. Therefore, quantification of the load sharing between the rainscreen and air barrier has yet to be developed for codes and standards. The current study is limited to winds favoring rain penetration (i.e., panels facing windward direction). In order to establish an appropriate wind design guideline for rainscreens, measurements of the pressure equalization performance of panels located in separation zones, corners and edges must be carried out. Further, measurements on larger size panels should also be carried out. 6. Concluding remarks Pressure equalization of mean as well as low-frequency pressures can be achieved by providing adequate venting area with respect to the panel area and air barrier leakage. Pressure equalization of the short duration pressure fluctuations seems to be difficult. Based on the extensive set of full-scale data although for a limited number of cases, some preliminary findings have been presented in the paper, regarding area of venting and design wind loads for rainscreens. The paper has also demonstrated the weakness of national and international wind codes and standards with respect to the design provisions for rainscreen walls. Acknowledgements Financial support for this project provided by the Netherlands School for Advanced Studies in Construction is gratefully acknowledged. The authors would also like to thank Mr. Eric Wijen and Mr. Wim van de Ven for their help in setting up the experimental facility. Special thanks are also due to Professor Hans Gerhardt who provided access to some of the research results obtained in Germany on the subject of rainscreen walls. References [1] J.M. Anderson, J.R. Gill, Rainscreen Cladding: A Guide to Design Principles and Practice, Butterworths, London, UK, 1988. [2] K.S. Kumar, Pressure equalization of rainscreen walls: a critical review, Build. Environ. 35 (2) (2000) 161–179. [3] A. Baskaran, Review of design guidelines for pressure equalized rainscreen walls, Internal Report No. 629, IRC, NRCC, 1992. ARTICLE IN PRESS K.S. Kumar et al. / J. Wind Eng. Ind. Aerodyn. 91 (2003) 1401–1417 1417 [4] P.A. Irwin, G.D. 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