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Journal ArticleDOI

Wind pressure coefficients for roof ventilation purposes

01 Apr 2018-Journal of Wind Engineering and Industrial Aerodynamics (Elsevier)-Vol. 175, pp 144-152

Abstract: Wind pressure coefficients (cp) are important inputs for analytical calculations of wind load. The aim of this research is to investigate wind pressure coefficients on a test house located in Norway in order to pave the way for improved analysis of wind-driven roofing ventilation. The large-scale test measurements show that the wind pressure coefficient along the eaves of the house varies with different wind approach angles. Assuming wind-driven air flow through the air cavity beneath the roofing, an average Δ c p ¯ value of 0.7 is derived for practical engineering purposes. The results from the study are applicable for single or two-storey houses with pitched roofs at different roof angles.
Topics: Wind engineering (63%), Roof (54%), Ventilation (architecture) (51%), Airflow (51%)

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1
Windpr essur ecoefficientsforr oof
ventilationpurposes
LarsGullbrekken
1
,SivertUvsløkk²,ToreKvande
1
,KajPettersson³,BeritTime²
1
DepartmentofCivilandEnvironmentalEngineering,NorwegianUniversityofScienceandTechnology(NTNU),Trondheim,
Norway
2
DepartmentofMaterialsandStructures,SINTEFBuildingandInfrastructure,SINTEF,Trondheim,Norway.
³DepartmentofArchitectureandCivilEngineering,ChalmersUniversityofTechnology,Gothenburg,Sweden
Abstract
Windpressurecoefficients(c
p
)areimportantinputsforanalyticalcalculationsofwind load.The
aimofthisresearchistoinvestigatewindpressurecoefficientsonatesthouselocatedinNorwayin
ordertopav e the way forimprovedanalysisofwinddrivenroofing ventilation. The largescaletest
measurements show that the wind
pressure coefficient along the eaves of the house varies with
different wind approach angles. Assuming winddriven air flow through the air cavity beneath the
roofing,an average
p
c
valueof0.7 is derivedforpractical engineeringpur poses. The results from
thestudyareapplicableforsingleortwostoreyhouses withpitchedroofsatdifferentroofangles.
1. Introduction
1.1Backgroundandscope
A loadbearing wooden roof is a proven and widely used type of construction in the Nordic
countries.ThevariousdesignprinciplesforwoodenroofsarethoroughlydiscussedbyEdvardsenand
Ramstad(2014). Thebasicprinciple is that the air cavitybeneath the roofing must be ventilatedto
transport:
1. Moisture
fromtheroofandthuspreventthegrowthofmouldandmoisturedamage
2. Heatandthuspreventunwantedmeltingofsnowandicingattheeavesandg utters
Roofventilation guidelinesforNorwayaregivenby Bøhlerengen (2007,2012) andarevalid for
roofs with a span less than
15 m and roof angle greater than 10° to 15°. A strong focus on CO
2
emissions from buildings most often favours woodbased materials and timber structures. Use of
woodfortheloadbearingsysteminroofswithincreasinglylongerspansandmorecomplicatedroof
geometryis becomingmorepopular. In order to improveair cavity design guidelines for ventilate d
roofsitisnecessaryto
increasetheknowledgebaseforwinddrivenventilationofpitchedroofs.The
air change rate of the cavity is given by driving forces from wind and temperature differences
(natural convection) together with pressure losses in the system. To calculate the winddriven
ventilationofsucharoofitisnecessaryto
calculatethedifferenceinwindpressurecoefficient,c
p
,at
theinletandtheexit oftheaircavity.Thisdifferenceishereafterdefinedas
p
c
.
ThisworkisbasedonmeasurementsperformedbyUvsløkkin19 85andtheworkwaspreviously
partlypublishedinUvsløkk(1996).However,thescopeofUvsløkk(1996)waslimitedtoexaminethe
windpressuregradientsintheaircavitybehindaventilatedcladding.
Theaimofthisresearchhasbeen
toinvestigatec
p
atthewallsurfaceofatesthouse locatedin
Norway. In section 1.21.6 the theoretical framework for the wind pressure coeffici ent (c
p
) and
relevantpastresearchonitarepresented.Insection2theexperimentaldesignandimplementation

2
usedinthispaperispresented.Insections3and4theresultsandadiscussionoftheimplicationsare
given, respectively. Section 5 consists of final thoughts and conclusions. The c
p
at the air cavity
openings of theroof,together with thewindvelocityand wind approachangle, defines the driving
forces of winddriven air cavity ventilation. Therefore, knowledge about such parameters is
necessarywhencalculatingwinddrivenaircavityventilationofpitchedroofs.
1.2Windpressurecoefficient
Thewindpressurecoefficientatapointisdefinedaswindpressureatthepointdivided bythe
dynamicpressureinfreewindatareferenceheightaboveground,normally10m(NSEN19911
4:2005):
0x
px
d
P
P
c
P
(1)
2
2
d
U
P
(2)
p
x
c
isthewindpressurecoefficientofapoint(),
x
P
isthestaticpressureatpointxonthebuilding
facade(Pa),
0
P
isthestaticreferencepressure(Pa)(at10mheight),
d
isthedynamicpressure(Pa),
istheairdensity(kg/m³)and
U
isthewindspeedat10m(m/s).
p
c
isdefinedbyequation(3)and(4),wherec
px1
andc
px2
arewindpressurecoefficientsatthe
positionsshowninFigure1.
12
p
xpxpx
ccc
(3)
() () (180 )
ppp
ccc


(4)
Whereθisthewindapproachangle(horizontal)
Fig.1Crosssectionofahouseshowingthelocationofthetwopressurepointsforcalculationof
p
x
c
.
Therearethreemethodstoestimatec
p
:fullscaletest,amodeltestinalaboratorywind
tunnelandbyparametricequationsderivedfromexperiments.Foraspecificbuilding,afully
accuratedeterminationofc
p
canonlybedoneusingafullscaletest(Bartkoetal.2016,Uvsløkk
1996)oramodeltest(Tominagaetal.2015,Quanetal.2011)ofthespecificbuilding.Fullscale

3
measurementsarecostly,difficultandrequireexpertise,andconsequentlyareonlyperformedon
complexandhighrisebuildingsinordertodevelopparametricequations.
1.3Fullscaletests
Wellsand Hoxey (1980)performedfullscale wind coefficient measurementsmainlyon roofsof
fivedifferentglasshousessituatedintheUK.Themeasurementsweredonewiththeaimtoincrease
knowledge about design values of wind coefficients of such buildings. Richardson and Surry (1991)
performed comparisons of fullscale and model
scale measurements, focusing on mean wind
pressure coefficients of four lowrise buildings. The mean pressure coefficients presented for four
buildingssuggestedthatawindtunneldoesnotaccuratelymodeltheseparationoftheflowonthe
windward roof. Fullscale measureme nts of side wall pressure coefficients reported for three
buildingsindicateda
p
c
acrosstheaircavityoftheroofof0.61.0.
Wind pressure coefficients on a specific part of a building were calculated by Caracoglia and
Jones(2009).Thefacademeasurementswereperformedclosetoacornerofthebuildingwhichhad
a complex geometry. Windinduced response on lowrise buildings
by use of load cells in the
foundation system of the building and in the wall/roof joint was investigated by Zisis and
Stathopoulos (2012). For the mean wind pressure coefficient, the fullscale measurements showed
excellent agreement with the model (wind tunnel) results given a suburban terrain. Given an open
andlightsuburbanterrainandawindangleof60degreestothelongwallof thetestmodel,Figure6
in Zisis and Stathopoulus (2012) indicates a
p
c
across the air cavity of 0.5. Further, the results
interpret a larger
p
c
in a more suburban terrain however it should be noted that the
measurementsonlyincludedonewinddirection.
1.4Modeltest
Kanda and Maruta (1993) performed model measurements of wind coefficients on a long low
rise building with a gable roof. On the windward wall they measured a wind pressure coefficient
between 0.5 and 0.8 depending on the roof pitch. A thorough literature review of wind pressure
measurements,usingbothfieldstudies
andmodelstudies,wasconductedbyUematsuandIsyumov
(1999).Theyfoundanumberofresearcheffortstryingtodeterminewindloadsonlowrisebuildings.
However, the authors still pinpointed the need for more measurement data to cover different
variables.
Blom (1990) also performed model measurements using an identical
downscaled model of the
test house used in this study. However, the measurements were simplified and included, fo r
example,onlyonewindapproachangle.
Yang et al. (2008) performed model testing in a wind tunnel and compared the results to
calculations,however,themeasurementswereonlyperformedona limitedrange
ofwindapproach
angles.
Tominagaet al.(2015) conductedwindtunnelexperimentstoe xamineairflowaroundbuilding
models and the results were used to validate a Computational Fluid Dynamics (CFD)model. The
modeldimension wereLxW=6.6 X 6.6mandheight fromgroundtoeavesof
6m.Three different
roof angles of 16.7, 26.6 and 36.9° with no roof overhang were tested. The model was oriented
perpendiculartotheflow.Althoughtakingmeasurementsofthedrivingforcesforventilationtheair
cavitybeneaththeroofingwasnotinthescopeofthework,thestudyindicatesa
p
c
acrosstheair
cavityof1.11.4foralltheinvestigatedroofangles(Figure2).

4
Fig.2Blackdotsrepresentmeasurementsofc
p
foramodelhouse.Threedifferentroofangleswere
studied.FigurefromTominagaetal.(2015).
The c
p
of the pitched roofis significantly influenced by the roof geometry of lowrise buildings
(Xu and Reardon 1998, Blom 1990). Xu and Reardon (1998) performed wind tunnel measurements
onabuildingmodel with15°,20°and30°roofpitchesandlargeoverhangs.Theyfound thataroof
angle of
30° experienced the highest negative c
p
at the roof corner compared to the 15°‐ and 20°‐
roofangle.Blom(1990)performedmodelmeasurementsusinganidenticalmodelofthetesthouse
usedinthecurrentstudy.Healsoconcludedthattheroofangleinfluencesthedistributionofc
p
ofa
pitchedroof.
Furthermore,AhmadandKumar(2002)studiedthemeanpressurecoefficientsonelevatedand
singlestoreyhouses.ByassumingthesituationgiveninFigure1,theresultsfromtheirstudyindicate
a
p
c
acrosstheaircavityof0.61.3dependingontheheightofthebuilding.
1.5Parametricequations
Muehleisen and Patrizi (2013) developed simplified parametric equations that more accurately
describetheperformanceofisolatedbu ildings.Thestudyonlyincludedaflatroofconfiguration.
A thorough overview of pressure coefficient data and to what extent the data is currently
implementedinbuildingenergysimulationandairflownetworkprogramswasperformed
byCóstola
etal.,(2009).Thefollowingprimarysourcesofdataweremapped:fullscalemeasurements,reduced
scale measurements in wind tunnels and CFD (computational fluid dynamics) simulations. In
addition, secondary sources such as databases and analytical models were studied. Cóstola et al.,
(2009)foundthatawiderangeof
parametersinfluencethepressurecoefficientsonbuildingfacades.
A high uncertainty was also associated with pressure coefficients of buildings sheltered by
neighbouringbuildings.
TheEurocode1standard(NSEN199114:2005)givesinstructionsforcalculationofwindstrains
onbuildingfacades.
1.6Knowledgegap
Several fullscale and model scale studies of c
p
on facades and roofs have been conducted.
However,totheauthors'knowledge,therearefewstudiesofspecificmeasurementsofthec
p
atthe
inletandtheoutletoftheaircavitybeneaththeroofingofapitchedroof,definedbytheauthorsas
p
c
. Analysis of the measurements by Tominaga et al. (2015), Zisis and Stathopoulos (2012) and
RichardsonandSurry(1991)indicatesa
p
c
of0.5to1.4,whichrepresentsaratherlargespan.This
studyisundertakeninordertomoreprecisely derivea
p
c
applicableforan engineeringevaluation
ofaircavitydesign.

5
2. Method
2.1Testsetup
A test house located at an open field test station in Tyholt, Trondheim, (63.4222N, 10.4302E)
110m above sea level was equipped with instrumentation for wind pressure measurements (see
Figure3).Thebuildingwas8mlong,5mwideandtheheighttothetopofthe roofwas6m.Theroof
angle
was 38° and the attic space was ventilated through an opening along the eaves as shown in
Figure3and4.Theroof(ceiling),wallsandfloorwereinsulated.Byuseofanelectricmotor,thetest
housecouldberotatedmakingitpossibletocarryoutwindpressuremeasurements
forallpartsof
thewallsatanywindapproachangle.Thegroundatthetestsitewasevenand openwithnotreesor
buildingswithinadistanceofabout150mina sectorfromsouthsoutheasttosouthwestwhichwas
the dominating wind direction during the measurement periods. Wind
speed was measured by a
Lambrechtanemometerpositioned6mabove groundlevelinamastpositioned40mawayinawest
northwest direction from the test house, as Figure 3 shows. Wind speed was also recorded by an
identicalanemometerlocatedinthe centreofthehouse,4mabovethe
ridgeand 10maboveground
level.Thiswastoinvestigatetheinfluenceofthetesthouseontheflowpatternofthewind.
Fig.3Positionoftesthouseandwindmastatthefieldteststation.
Four groups of pressure measurement points were located on the long wall (see
Figure 4). In
addition,twogroupsofpressurepointswerelocatedontheshortwallofthehouse. Eightpressure
points in each groupwere distributed along a vertical line: four located in the air gap between the
cladding and the wind barrier, and four on the exterior surface of the
cladding. The wind pressure
wasmeasuredusingplastictubeswhichwerecoupledtopressuretransducersinsidethetesthouse.
Allthetubeswere10mlongandhadaninteriordiameterof4mm.Ateachexteriormeasuringpoint
aplastictubewascoupledtoabrasstubewiththeopeningat
thesurfaceof thecladding.Thebrass
tubes were 50mm long and had an interior diameter of 3mm. The 20 pressure transducers were
FurnesTransducer FCO 40 (01000Pa) micro manometers.Of the 20 transducers,16 wereattached
to the different pressure tubes. The remaining four were coupled to fixed measuring
positions as
follows: dynamic wind pressure at 10m above ground level, air pressure inside the house, wind
pressureintheatticand windpressureinafixedpositiononthelongwall.Thepressuretransducers
reported the pressure difference between the wind pressure at the measuring point and the
reference
staticpressure.Referencestaticpressurewasmeasuredinamast10maboveground,4m
above the ridge of the house and in the centre of the house (see Figure 4). This was done by
attaching all the pressure transducers to a plastic tube which ended at the surface of a vertical
aluminium plate designed as a wind vane. Dynamic pressure and wind approach angle were also
registeredinthismastbasedontheprinciplepresentedinHoxeyandWells(1974,1977).

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Abstract: Wind pressure coefficients (Cp) are influenced by a wide range of parameters, including building geometry, facade detailing, position on the facade, the degree of exposure/sheltering, wind speed and wind direction. As it is practically impossible to take into account the full complexity of pressure coefficient variation, Building Energy Simulation (BES) and Air Flow Network (AFN) programs generally incorporate it in a simplified way. This paper provides an overview of pressure coefficient data and the extent to which they are currently implemented in BES-AFN programs. A distinction is made between primary sources of Cp data, such as fullscale measurements, reduced-scale measurements in wind tunnels and computational fluid dynamics (CFD) simulations, and secondary sources, such as databases and analytical models. The comparison between data from secondary sources implemented in BES-AFN programs shows that the Cp values are quite different depending on the source adopted. The two influencing parameters for which these differences are most pronounced are the position on the facade and the degree of exposure/sheltering. The comparison of Cp data from different sources for sheltered buildings shows the largest differences, and data from different sources even present different trends. The paper concludes that quantification of the uncertainty related to such data sources is required to guide future improvements in Cp implementation in BES-AFN programs.

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