Journal of Wind Engmeenng ~ ~ ELSEVIER and Industrial Aerodynamics 53 (1994) 37-48

Wind loads on wind permeable facades

H.J. Gerhardt*, F. Janser

I F I Instttutfur Industr, eaerodynamtk GmbH, Welkenrather Strafle 120, D-52074 Aachen, Germany

Abstract

Th~s paper presents results of a fundamental, experimental study on wind loading of porous facade systems The important parameters - relatwe building dimensions, porosity, and gap width - have been vaned systematically m a model scale study Both time averaged and fluctuatmg pressures have been measured Therefore, wind loads using the quasi-static ap- proach and the peak pressure approach may be derived The validity of the model scale investigation has been checked by full scale experiments

1. Introduction

Wind load data for the external surfaces of buildings (walls, roofs) as presented In building codes and standards are based on wind tunnel measurements on models with smooth, impermeable surfaces Those data have been traditionally used to calculate design wind loads for porous cladding systems like loose-laid pavers on roofs and shingles on facades. The use of porous surfaces has greatly increased over the past years in particular for the rehabilitation of roofs and facades. To provide additional thermal insulation, the insulation is usually fixed to the outside of the original wall The Insulation layer has to be protected against adverse weather conditions, in particular against rain The rain screen itself consists usually of a porous outer sheeting allowing for extraction of condensed moisture Typical systems are the pressure equilibration rain screen (PER) commonly used in Nor th America and the back-vented ram screens (BVR) commonly used in Western Europe The wind load on the protection layer is due to the difference between external pressure and Internal pressure in the gap between the porous sheeting and Impermeable building wall Another recent application with increasing importance is the so-called convective facade Convective facades are used on medium and high-rise office buildings to avoid air conditioning. A second glass envelope is added resulting in some air space outside

* Corresponding author

0167-6105/94/$07 00 © 1994 Elsevier Science B V All nghts reserved SSDI 0167-6105(94)00083-P

38 H I Gerhardt F lanset J Wmd lzng lnd 4etod, n 53 (1994) 3" 47¢

the office rooms During winter time this a~r space will be heated by solar radmt~on hke a greenhouse During summer time excessive heating of the rooms may be avoided by adding sunshades m the air space. 1 e outside the office windows Sufficient ventalataon of fresh air and m case of fire of smoke has to be ensured by the flow in the a~r space

Whereas sufficient anformatlon concerning the external pressure distribution is available, the internal pressure has been only very httle investigated Results of experimental research are presented in Refs [1-3] and from theoretical consider- ations in Refs [4,5] This paper will present the major results of a fundamental, experimental study on wind loading of porous facade systems The important para- meters have been varied systematically m a model scale study The validity of the model scale mvestagataon has been checked by full scale experiments

2. Wind load mechanism

The wind load actang on a claddang element is due to the difference of the external and the internal pressures, with the pressure in the gap between the permeable surface covering and the wand impermeable bulldang surface denoted as internal pressure The external pressure is mainly influenced by the building geometry and In particular by the relative building height h/a ( = heaght/wldth) and the aspect ratio b/a ( = length/width), the boundary layer properties described by the roughness length Zo or profile exponent ~v and the turbulence length Lux and possably by the Jensen number Je = h/zo and the wind direction

The internal pressure depends on the external pressure &strlbutlon and the flow equilabration process between the external bualdlng wall and the gap The flow situation as sketched in Fig 1 Small differences between external and internal pressures, 1 e small net wand loads, will occur if the pressure equllabrates mainly across the permeable sheeting and not in the gap This is the underlyang pranclple of PER systems A good pressure equilibration across the permeable sheeting as ensured if the pressure losses due to the flow through this layer are small. On the other hand, the pressure equahbrataon process m the gap will be small for large gap flow resastance Thus, the optamal satuation as given by small through-flow resistance and at the same time large gap flow resistance Here, the wind load mechanism IS similar to the situation for pavers or thermal ansulation boards loosely laid on flat roofs [4,6]

The main influence parameters governing the wind load on permeable sheeting systems have been investigated by the authors in a fundamental study supported by the Deutsche ForschungsgemeInschaft (DFG) The main results of this still ongoing investigation will be presented

3. Experimental methodology and similarity considerations

The measurements were conducted In the boundary layer wind tunnel of the Fachhochschule Aachen The test section has a width of 1 75 m, a height of 0 9 m and

HJ Gerhardt, F Janser/J Wind Eng lnd Aerodyn 53 (1994) 37-48 39

-I -Cp

Fig 1 Flow situation

a length of 2 m, the fetch length is approximately 6 m and the maximum velocity Umax = 23 m/s. The boundary layer is produced according to the method of Cook [7]. Three boundary layers have been used in the present tests, having profile exponents o% = 0 1, 0.2 and 0.3 The boundary layer scale is approximately 1"350 Models with relative dimensions h/a = 0.5, l, 1.5, 2 and 4 and b/a = 1, 2 and 4 with a constant width a = 100 m m have been investigated All models were sharp edged The pressure distribution was measured for various flow directions with flow direction perpendicu- lar to the small side of the building defined as 0t = 0. The flow direction was varied in steps of A0t = 10 ° The same models were used to determine the external and internal pressures To measure the internal pressures a porous wall was added where the porosity was obtained by regularly spaced holes. Relative permeabihtles e -- 0.5%, 0.75% and 1% (based on the area of the building side under consideration) have been Investigated The gap flow resistance was varied by varying the gap width between the impermeable building wall and the porous facade (s/a = 0 0025, 0005 and 0.01) Typical full scale gap widths for BVR systems and convective facade systems are In the range s/a = 0.001 to 0 01

Each model was equipped with 180 pressure taps The pressure tap locations were determined from preliminary experiments and chosen in such a way that the expected locations of the maximum and minimum pressures were Included The data were sampled with a data acquisition system consisting of three scanivalves with internal

40 H I Gerhardt, f Janser/J WmdEng Ind 4erod~n 53 (1994)37 48

pressure transducers and a PC A 100 Hz low pass filter was used Each of the scanivalves was connected to the wind tunnel Prandtl-tube to measure the dynamic pressure The reference pressure for the transducer was the static pressure port of the Prandtl-tube at the roof hezght of the investigated building Each pressure tap was scanned w~th a frequency of 500 Hz From the accumulated data the time averaged pressures and the rms values of the pressure fluctuations were calculated and non- dlmensIonahsed with the wind stagnaUon pressure at the bmldmg roof height To ensure a good frequency response the tube lengths between the scanner and the pressure taps had been opUmlsed following the recommendations by Cook [8] and Knoch [9] The maximum gain for frequencies up to 100 Hz Is below 1 18

To ensure the similarity of the net pressures m the full scale and the model scale expertments the raUo of through-flow resistance and gap flow resistance have to be the same m both cases For the through-flow resistance the s~mdarlty ~s saUsfied tf the pressure loss (Ap)-volume flow (I?) relauon,

A p = CV" , (1)

for model scale and full scale facades is equivalent, I e exhibits the same values C and exponent n The range of those characteristic values has been checked by Gerhardt and Kramer [2], the model facades have been manufactured accordingly

The similarity condmon for the gap flow resistance IS more complex The flow in the air space is governed both by the gap flow and the outflow (in the critical areas of large external suction) through the porous cladding The situation IS sketched m Fig 2 (left facing flow) The gap flow may be treated as the flow in the inlet section of a two- dimensional channel Here, the pressure losses are mainly due to the establishment of the boundary layer The pressure loss at the channel entrance is approximately 2 16 times the stagnation pressure of the gap flow for the laminar model scale flow [10] For the full scale situation with turbulent flow the pressure loss is only approximately 1 4 times the stagnation pressure Thus, equivalent flow s~tuat~ons m model scale and full scale, t e the same gap flow resistance, may be achieved by adjusting the gap

panel /

~surface fnctJon + pressure loss = gap flow resistance

Fig 2 Flow between building wall and facade element

HJ Gerhardt, F Janser/J Wind Eng Ind Aerodyn 53 (1994) 37-48 41

width The model scale gap width should be larger than the full scale gap width by

a factor of x/2 16/1 4 = 1.24 If addmonal pressure losses occur at full scale due to the support structure (e g battens, see right facing flow in Fig 2) of the porous cladding system, the correction factor for the gap width will decrease. As a first estimate the gap width in model scale tests should be about 10% larger than in the full scale situation

4. Results

The influence of the wall permeabihty, the gap width and the relative building dimensions will be discussed for open country exposure flow The following results are for the lowest locally measured, time averaged pressure coefficients (maximum suc- tion) for the flow angle ~ = 10 ° The angle ~ -- 0 ° is defined for the flow parallel to the long sides of the building. For most building dimensions investigated the flow angle

= 10 ° led to the largest time averaged suctions occurring on the short side at about 15% to 20% of the building width downstream of the windward edge and at about 90% of the bmlding height. For the discussion of the various influence parameters the constant flow angle ~ = 10 ° has been chosen

4 1 Influence of the wall permeabthty (through-flow reszstance)

Fig. 3 shows the pressure coefficients (maximum suction) for a building with rectangular plan form (b/a = 2) and constant gap width s/a = 0.01 versus the relative building height h/a The external suction increases with relative building height The internal pressure 6p,,t is nearly independent of the relative building height and

- 1 4 I , i i

,Ib/a =2, a.p= 0 1, s/a : 0.011 12. I

~ ~p,ex /

- 1 . 0

- 0 8

p,net I0°" - 0 6

~p,mt . -OA.

......~ 8 = 1%

¢ = 075% I 0 ~

! . - - E = 0 5 %

0 . 0 I I ! I 0 1 2 3 4 5 ~a

Fig 3 Influence of the through-flow reststance on the net pressure cocfliclent

42 H J Gerhardt, F Jan~e¢/J ~m dE ng lnd Aerodvn 53 (1994) 37 4~

Increases slightly w~th decreasing wall permeability For the relatively large gap w~dth s/a = 001 the gap flow resistance is obviously small Therefore, the through-flow resistance does not alter the gap pressure appreciably

4 2 Influence of the gap wtdth (gap [low resistance)

The importance of the gap flow resistance is clearly seen in Fig 4 The external and internal pressure coefficients 5pex and 6p..t for b/a = 2 and s = 1% are plotted versus the relative building height h/a The smaller the gap width, i e the larger the gap flow resistance, the better the gap pressure follows the external pressure. For s/a = 0 0025 the net pressure coefficient is less than half the external pressure coefficient Even for the relatively large gap width s/a = 0 01 the net pressure coefficient is about 20% smaller than the external pressure coefficient

4 3 Influence of the butldmg dtmenstons

Fig 5 shows the maximum net pressure coefficient for relatwely large porosity (5 = 1%) and relatively small gap width (s/a = 0 0025) versus the relative building height h/a with the relative budding width b/a as parameter Obviously, the smallest net pressures have to be expected if the building walls under consideration are in completely separated flow regimes This is the case for relatively short buildings (b/a < 2) If the building wall ~s sufficiently long to allow for flow reattachment, the external pressure distribution will exhibit a distinct pressure minimum near the windward wall, see Fig 1

- 1 . 4 = = i I

_Lz [b/a=2, %=01, 8=1% l ~ ~'p,ex

- 1 . 0 Cpdnt

--0.8 s/a=O 0025

-0.6 s/a=O 005

~Og~ s/a=O 01 --0.2.

0 . 0 I I I I 0 1 2 3 4. 5 h/a

Fig 4 Influence of the gap flow resistance on the net pressure coefficient

HJ Gerhardt, F Janser/J Wind Eng lnd Aerodyn 53 (1994) 37-48 43

- L 4 i i i i

] ap= 0.1 s/a = 0.0025 e= 1% i

p,net -LO

~ b/a=4

Q.-O 8 ~ + ~ b/a=2

to b/a=1 _ o ,

-O4

-.41.,7.

0.0 I I I I 0 1 2 3 4 5

h/a

Fig 5 Influence of the building aspect-ratio on the net pressure coefficient

4 4 Influence of the oncommgflow

For typical full scale values of relative gap widths (s/a <_ 0.003) the gap flow contributes little to the pressure equilibration which is dominated by the through flow However, the pressure difference IS small compared to the ambient pressure Thus the volume flow across the porous cladding is small, too The pressure equlhbra- tlon process across the porous cladding is then mainly due to compressibility effects and occurs at the speed of sound The equilibration time is therefore much shorter than the typical gust duration tame Hence, the design wind load for porous facade systems should be determined using the peak factor approach [11]

The influence of the oncoming flow on the wind loads for permeable facade systems should be judged according to the peak pressures. Fig 6 shows the maximum peak suction coefficients for the external pressure for three different approach flows versus the relative building height h/a The pressure in the air space varies little with varyang approach flow conditions. Thus, only the average internal peak pressure coefficient for all exposures Investigated IS given In Fig 6 The external and internal peak pressures shown were not measured simultaneously The net peak pressures deter- mined from those data are "worst case" values The smallest net peak pressure coefficient occurs for relatively smooth exposure with ~p = 0 1 Obviously, the pres- sure fluctuations of the external pressures are dominant. The pressure fluctuations in the gap are attenuated by the through-flow resistance and the gap-flow-resistance. The information of Fig 6 indicates that the net pressure coefficients are almost inde- pendent of the oncoming flow and that the most critical condition for porous cladding systems will therefore occur in open country exposure flow, where the stagnation pressure at a given height is larger than for suburban or urban flow conditions

44 H J Gerhardt F Janaer/J Wmd Eng lnd Aerodvn 53 (1994) 37 4~

- 60 i i i J - -

"50 -I b / a = 2 ~ : : 0 0 1 s / a = O 0 0 2 5 I Cp,ex

~ C~p=03 -40 + * ~ p = 0 2

k O~p= 0 1 -30

-2.0 O.

-1.0 CP, net

0.0 . . . . . . . . . . . . . . . . . . . . ^

LO × v ~ Cp,lnt

20

3 . 0 I I 1 I 0 1 2 3 4 5

h / a

F~g 6 Net pressure coefliclent derived from the peak factor approach

4 5 Correlatton between external and mternal pressures

The evaluation of the correlation ume histories of external and mternal pressures measured simultaneously at the same budding locauon for the critical flow condmon (~t = 10 °) are being analysed at present The first results show that relatwely high correlauon coefficients (p ~ 0.9) are observed when the gap flow resistance is relatwe- ly high (s/a < 0.0025). For small gap flow resistance, i e. large gap width (s/a = 0 01), the correlation coefficients are smaller (p ~ 0.5) when the gap between the windward face and wmd parallel side is sealed Negative correlataon coefficients may occur lmmedmtely downstream of the leading edge when the vertical gap is not sealed and the gap flow resistance Is relatively small (s/a = 0 01) Detailed reformation of the correlation wdl be presented shortly in the Ph D thesis by Janser [12]

5. Comparison between full scale and model scale results

5 1 Test setup

To compare the wmd loads acting on porous cladding systems measured m full scale and model scale the ETERNIT AG, Berhn, offered to set up a special facade on an existing office budding at the ETERNIT facdmes in Berhn The facade system added was a typical back vented ram screen system w~th vertical battens and facade panels of size 1 5 m × 1 m The gap width between facade elements and the braiding wall was 75 mm, between battens and the budding wall 20 mm Fig 7 shows a schematic of the building and a horizontal section through the facade The air space

H J Gerhardt, F Janser/J Wind Eng Ind Aerodyn 53 (1994) 37-48 45

m e t . s t a t i o n - - 1 I " - " / <

~ ~ 6 c 3231 / / ca 7m

O l l ~ " 3 1 o 12¢ ~351/''~ / / - I f

/ . . IN. / / ." /

section A-A

Fig 7 Full scale test setup

between the rain screen and the bmldlng wall was open at the top and the bot tom and sealed at the vertical budding edge and around the windows.

A 1" 150 scale model of the test building and its surroundings was budt for the wind tunnel study The porous facade was simulated by a sheet metal model facade having an appropriate number of 0.8 m m diameter holes The number of holes was chosen to achieve the same effective porosity as for the full scale facade The &stance between the model scale ram screen and the building wall was calculated to achieve the same gap flow resistance as for the full scale facade taking into account the pressure losses due to the vertical battens This reqmred a gap width of approximately 1/150 of the full scale width, l e 0 5 mm

5 2 Data acquisition

The pressure differences across six facade panels were measured The locations of the pressure transducers are sketched m Fig 7. Measurements were taken without and

46 H J Gerhardt F lan~ep/J Wind Eng lnd Aerod~w 53 (1994) 37 4,~'

with alrUght sealing of the gap between the ram screen and the verucal edge at the corner of the building

The dlfferentml pressure Ap.et was measured using surface probes and a manometer with a range of 1 kPa with an accuracy of _+ 1 Pa The analogue signal was dlglused with an A/D converter Wind velocity and wind direction were measured with a cup anemometer and a vane, respectively, on a mobile meteorological station fixed to the roof, see Fig 7 The signals (pressure differential, wind speed, wind direction) were scanned at a frequency of 676 Hz for a period of 3 02 s If the pre-set conditions (threshold velocity, wind direction range, stability of wind direction) are satisfied, the ume histories are converted into wind velocity, wind direction and pressure dlfferen- tml averaged over the measuring period of 3 02 s In addition, the rms value of the pressure fluctuations is calculated Finally, the pressures are non-dimenslonallsed with the wind stagnation pressure to give pressure coefficients (averaged over 3 02 s) and rms-pressure coefficients

The pressure d~strlbutlon for the model scale study was obtained for 45 pressure tap locations In addiuon, the wind velocity and wind direction at the equivalent position of the mobile meteorological stauon were obtained for various undisturbed flow condlUons Six of the model scale pressure taps (nos 6, 11, 12. 23, 31 and 35) coincided with the six full scale pressure tap locations Standard pressure measurement tech- niques including a scanlvalve were used for the model studies

5 3 Results

A typical result of the full scale test IS shown in Fig 8 For pressure tap location 31 (distance from the vertmal edge of approximately 2 m) the coefficient of the net

02 I I J I r I

01

~wlth edge seahng

0 -- -- "~r--b.. '.-- ~__ ~ _ . . . ~ a , ~ - -

~- -01

Io - 0 2

w~thout edge sealing - 0 3

-04

-0 5 , I , I , I , I , I ~ I , I t

0 45 go 3.35 180 225 270 315 360

flow d,rection

Fig 8 Net pressure coefficient for tap 31 with and without edge seahng

H J Gerhardt, F Janser/J Wmd Eng Ind Aerodyn 53 (1994) 37-48 47

0 5 I I I I I i I

0 4

0 3

0 2 0 $ . ~ m d tunnel e x p e n m e n t

- = _ - o l o - - " "

(3. ~0 - 0 2 full - sca le - test

- 0 3

- 0 4

- 0 5

- 0 6

- 0 7 wi th e d g e seahng

- 0 8 I , I ~ I ~ I ~ I , I , I

0 45 90 135 180 225 270 315 360

flow direction

0 5 i i i i i i i

O4

O3

O2 full - sca le - test

~ o . . . . . . . . =. -o1

, ~ i . - 0 2

- 0 3

- 0 4 w ind tunne l e x p e n m e n t

- 0 5

- 0 6 - 0 7 w i thou t e d g e seahng

- 0 8 I , I , I , I , I , I , I , 45 90 135 180 225 270 315 360

flow direction

Flg 9 Companson between full scale test and wmd tunnel experiment (tap 31) with (upper) and without (lower) edge seahng

pressure ts plotted versus wind direction Flow &rectlon north (0t = 0 °) is defined as perpen&cular to the side including the pressure taps 23, 31 and 35. The pressure differential across the rain screen is given with and without seahng of the verUcal edge For the wind direcUon range considered, the net pressure, and thus the wind load, without edge sealing is sigmficantly larger than for the case with edge seahng. As pointed out earher [1,2], the edge seahng prevents the horizontal pressure eqmhbratlon from one budding side to the other Thus, the

48 H J Gerhardt F Jansep:] WmdEng lnd 4erodvn 53 (1994)3 7 4,~

pressure equthbrates mamly across the facade panels leadmg to very low net wind loads

Ftg 9 gtves for the same pressure tap locatton the compartson between full scale and model scale tests The upper dmgram shows the results wtth edge seahng and the lower dmgram without The agreement between full scale and model scale tests is sattsfactory It ts better for the case w~th verttcal seahng Wtthout vemcal seahng the gap flow becomes more intense and the net pressure coefficients larger One reason for the poorer agreement between full scale and model scale results could be a minor difference in gap flow resistance between those two cases For the full scale measure- ments pressure tap location 31 is close to a wmdow, see Ftg 7 The gap flow m the wcmlty of the wmdow ts not well defined and the influence of the window has not been modelled for the wmd tunnel test

References

[11 H J Gerhardt and C Kramer, Wind loads on wind permeable building facades, J Wind Eng lnd Aerodyn 11 (1983) 1-20

[2] H J Gerhardt and C Kramer. Wmdkrafte an hinterlufteten Fassaden, Betonwerk + Fertlgtell- Techmk, Heft 1/1985, pp 46-53

[3] H J Gerhardt and F Janser, Wind loads on wind permeable facade systems, in Proc 1st IAWE European and African Regional Conf on Wind Engineering, ed N J Cook (Thomas Telford, London, 1993)

[41 H J, Gerhardt, C Kramer and K K Bofah, Wind loading on loosely laid pavers and insulation boards for fiat roofs, J Wind Eng Ind Aerod~n 36 11990) 309-318

[5] N J Cook. The designer's guide to wind loading of building structures, Part 2 Static structures {Butterworths, London, 1990)

[6] H J Gerhardt, Wind loads on roofing systems roof membranes tiles and loose-laid slabs, in Proc 4th Canadian Workshop on Wind Engineering, Toronto, November 1984

[7] N J Cook, Simulation techniques for short test section wind tunnels Roughness, barrier and mixing device methods, in Wind tunnel modelhng for civil engineering applications, ed T A Rheinhold (Cambridge Uni~ Press Cambridge, 1982)

[8] N J Cook, Manufacture and calibration of restrictors and averaging manifolds for the measurement of fluctuating pressures, BRE Note No 54:80

[9] M Knoch, Entwicklung einer Methode zur amphtudenrichtigen Messung schnell schwankender Drucke mlttels pneumatlscher MeBstellenumschalter, Studienarbett RWTH Aachen, Fakultat fur Maschmenwesen (1980)

[10] BEck, Techmsche Stromungslehre (Springer, Berlin) [11] A G Davenport, The application of statlstlcal concepts to the wind loading of structures, Proc Inst

Civil Eng 19 (1961) S44%472 [12] F Janser, Windbeanspruchung belufteter Aul3enwande Ph D thesis TU Berlin, to be submitted in

July 1994