REPORTONBUILDINGDAMAGEFROMDOWNBURSTS,WESTERNAUSTRALIA,25FEBRUARY2018

1. INTRODUCTIONAt around 10:30 am Western Standard Time (WST) on Sunday 25th February 2018 athunderstorm thathadgeneratedover the IndianOceanpassedoverRockingham.Peoplereported hearing the thunder, seeing very heavy rain, hail and very strong winds thatdamagedtrees,vehiclesandsomebuildings.ThestrongwindswereWesterliesanddebrisinthe Rockingham area moved generally towards the east. Figure 1 shows Bureau ofMeteorology radar images at the timeof thewind event in Rockinghamand immediatelyafterwards.Theblackellipseindicatestheareawithmostnoticeablewinddamage

Radarimage25/2/201810:24WST

Radarimage25/2/201810:36WSTFigure1–RadarimagesofstormnearRockinghamWA

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ThestormthentravelledinagenerallyESEdirectionclosetothetownofPingellyataround12:42pmasshowninFigure2.LittledamagewasreportedinPingelly,butlaterintheday,thesameeventappearedtohaveregeneratedtocausesignificantdamagetoanumberofpropertiesintheKulinarea(around250kmeastofRockingham).

Radarimage25/2/201812:42WST

MapshowinglikelypassageofstormfromRockinghamtoKulinFigure2–PassageofstormfromRockinghamtoKulinAwindspeedof70knots(around130km/hand80%ofVRforthearea)wasreportedattheBoMAutomaticWeatherStationonGardenIsland,whichisfivekilometrestothenorthofthe damaged buildings shown in this report. In the area of Rockingham that had mostdamage (indicated by the ellipse in Figure 1), some branches had broken, as shown inFigure3, but few trees had lostmany leaves.A few real estate signs hadblownover andsometileswereliftedfromroofs.Thisdamageistypicalofwindsataround100to110km/h(70%ofVRforthearea)soitislikelythatthedamagetobuildingsoccurredatwindspeedslower than the maximum gust recorded and reported in the media. Even so, both theestimated wind speed from the damage and themeasured wind speed at Garden Islandwerebothlowerthanthedesignwindspeed(VR)forwindregionA.

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Figure3–DamagetosmalllimbsonNorfolkIsPines

2. DAMAGETOBUILDINGSINTHEROCKINGHAMAREARockingham State Emergency Service had around 20 calls for assistance on 25/2/2018rangingfromrooflosstotreedamage.Thecallsweremainlyfrombeach-sideareas.TheCTSinvestigateddamagetosomebuildingsinthisareaonthedaythedamageoccurred.

2.1. RooflossfromflatsFigure 4 shows roof loss on a block of flats that was widely reported in the media. Theroofingappearedquitenew.Aprevious fibre-cement roofhadbeen replaced3 to5yearsagowithacorrugatedprofilemetalsheetroofingsystem.Theroofdebriswasfoundbehindthebuilding.

Figure4–RooflossfromunitswitharelativelynewsheetroofAt the time the roof was replaced, extra tie-downs were installed into the brickwork tosecurethelighterweightroofingagainstwindloads.Theraftersremainedwellanchoredbythe improved tie-downs throughout thebuilding.Aroundhalfof thebuilding lost the roofwith some of the jarrah purlins staying attached to the roofing and others remainingconnectedtotheraftersasshowinFigure5.

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(a)Purlinsattachedtosheeting (b)PurlinsattachedtoraftersFigure5–Somepurlinsremainedattachedtosheeting,otherstotheraftersThe purlinswere connected to the rafterswith skewnails, andwith Type 17 screwsnearbuttjointsinthepurlinsasshowninFigure6.Figure5(b)showsaframinganchorthatwasusedtotiethepurlintoarafteratabuttjointinthepurlin,butfewofthesewereseen,andthisonehadroofingscrewstoconnecttheframinganchortobothmembers.

(a)Skewnailsthroughpurlin (b)Type17screwFigure6–Connectionbetweenpurlinsandrafters

Figure7Screwsthathadsplitthepurlinanddidnotloadthesheetingatall

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Whiletheseconnectionswereinadequatefortheloadsonthepurlins,thelikelycauseofthefailurewasby splittingof thepurlinsat rowsof roofing fasteners.A significantnumberofroofingfastenersremainedintheroofing,butdidnotshowanysignsofdamagingthecreststhroughwhichtheyhadbeenfixedasshowninFigure7.This indicatesthattheywerenotloadedat the timeof failure.As the jarrahpurlinswouldhavebeenwell seasonedat thetimetheroofingwasreplaced,itislikelythatsomepurlinsweresplitalongthelinesofthefasteners; the investigatorsawevidenceofpurlins thatweresplit longitudinallybyroofingfasteners (an example is shown in Figure 8. If the edge purlin had split, then subsequentoverloadingofotherpurlinsduringthewindeventmayhavepulledouttheskewnailsandtype17screwsthathadbeenusedtoconnectthepurlinstotherafters.

Figure8Exampleofsplitpurlinstillattachedtorafters

2.2. VerandahlossinRockinghamOneof the inspectedhouses lost a verandahon itswestern side. The verandahhadbeensecured to jarrahverandahbeamsthatwerebolted to jarrahposts.Thebaseof thepostshadbeenboltedtosteelstirrups.However,thebaseofthepostshadbeenburiedintosomeconcrete and the lawn, so that the connection with the stirrups was not free draining.Figure9showstheconditionofthestirrups,boltsandthebaseoftheposts.

(a)Deteriorationofstirrupsandbolts

(b)DeteriorationofpostsFigure9–Failureofverandahpostsduetocorrosionandrot

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Anotherhouse lostaroof fromaverandahthatwasenclosedon3sides,butopenonthewindwardside.Inthiscase,theroofbattenswerefastenedtotherafterswithsinglenailsasshowninFigure10.PartsofthisroofwereembeddedintoalawnwhileotherpartsbouncedoverroofsdamagingPVsolarpanels,andwerefoundonaroadreservearound100mtotheeast.

(a)Largestpartofverandahroof

(b)Roofembeddedinlawn (c)NailedbattentorafterconnectionFigure10–Verandahlosswithopeningonwindwardside

2.3. InternalpressuresA few garage doors failed in the event as shown in Figure 11. In this case, the internalpressurewasrelievedbyfailureofthegaragedoorontheoppositeendofthegarage.

Figure11Failureofgaragedooronthewindwardsideofthebuilding

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Windowshadbeen left open in a numberof houses. In onehouse, internal pressurewasrelievedbytwosetsofFrenchdoorsbeingblownopenasshowninFigure12.ThetopofthebolttorethroughthedoorframeashighlightedbytheredellipseinFigure12.

Figure12–FailureofadoorframeataboltThe investigation focusedonhouseswheredamagehadbeen reported to the SES, ratherthanasystematicstreetsurvey. It is thereforenotpossibletoquantifytheroleof internalpressureindamagedandundamagedhousesinthisevent.

2.4. TileanchorageFigure13showsacarport roofviewed fromthewindwardside.Theroofwasunlinedandblocked under by the house on the leeward side. Only alternate tiles were nailed and anumberofothertileswerelost.

Figure13–Tilelossfromcarportroof–photoJaniceDobra

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Figure14 showsa two-storeyblockof flats thathad tiledamagenear anedgewhere theeaveswereexposed.Ridgecappinghadliftedonthesamebuilding.

Figure14–Tiledamageatexposedeaves

2.5. WateringressDuring interviews with the media, a number of people in Rockingham reported watercoming through closed windows. As the investigation did not systematically interviewoccupantsitisthereforenotpossibletoquantifytheincidenceofwind-drivenwateringressindamagedandundamagedhousesinthisevent.

2.6. PerformanceofnewhousesTherewere several recently constructedhouses in the area investigated. Figure 15 showsnew houses without signs of damage that were close to older houses that had beendamaged.AS4055indicatesthathousesintheareashouldhavebeenclassifiedasN2orN3.

Figure15–Undamagednewhouses

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Figure16showssomeminordamagetoroofsheetingonanewhouse.

Figure16–Damagedroofsheetingonanewhouse

3. DAMAGETOBUILDINGSINTHEKULINAREAAsshowninFigure2,thesamecelltravelledeastwardstothetownofKulin.WhiletheCTSdidnotvisitthisarea,mediareportedwidespreaddamagetoafarminKulinNorth.Figure16showsmediaphotosofdamagetoarelativelynewshedandaverandahinKulinNorth.ThemediaalsoreportedlossofroofsonhousesandaschoolinthetownKondinin(around20 kmNNEof Kulin)whichmayhavebeen causedbywinds froma separate storm cell –visibletotheNEofBrooktoninFigure2.

Figure17–Damagetoarelativelynewshed–Kulin(ABCnewsphoto)

PHOTO: A new shed on Janine and Simon Noble's Kulin farm was badly damaged. Simon was sheltering in another nearby shed. (Supplied: Janine Noble)

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4. CONCLUSIONSThewindthatcausedthedamagetobuildingsinbothRockinghamandtheKondinin/Kulinareaappearstohavebeenanumberofdownburstsassociatedwithathunderstorm.WindspeedsintheRockinghamareawereestimatedtobearound70%to80%ofthedesignwindspeed.Therewaslittleornoobviousdamagetonewhouses.TheinspectedbuildingsthatsustaineddamageinRockinghamhadthefollowingfeatures:

• Re-roofingthathadsplitseasonedhardwoodpurlins;• Posts that had deteriorated due to corrosion of bolts and rotting of timber. Fill

aroundthemetalstirruphadpreventeddrainagethatledtothedeteriorationoveranumberofyears;

• Inadequatebattentorafterconnections;or• Doorsthatdidnothavewind-ratedlatchesortracks.

GeoffBoughton28February2018