REPORT ON BUILDING DAMAGE FROM DOWNBURSTS, WESTERN AUSTRALIA, 25 FEBRUARY 2018

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1 REPORT ON BUILDING DAMAGE FROM DOWNBURSTS, WESTERN AUSTRALIA, 25 FEBRUARY INTRODUCTION At around 10:30 am Western Standard Time (WST) on Sunday 25 th February 2018 a thunderstorm that had generated over the Indian Ocean passed over Rockingham. People reported hearing the thunder, seeing very heavy rain, hail and very strong winds that damaged trees, vehicles and some buildings. The strong winds were Westerlies and debris in the Rockingham area moved generally towards the east. Figure 1 shows Bureau of Meteorology radar images at the time of the wind event in Rockingham and immediately afterwards. The black ellipse indicates the area with most noticeable wind damage Radar image 25/2/ :24 WST Radar image 25/2/ :36 WST Figure 1 Radar images of storm near Rockingham WA

2 Building damage Rockingham February The storm then travelled in a generally ESE direction close to the town of Pingelly at around 12:42 pm as shown in Figure 2. Little damage was reported in Pingelly, but later in the day, the same event appeared to have regenerated to cause significant damage to a number of properties in the Kulin area (around 250 km east of Rockingham). Radar image 25/2/ :42 WST Map showing likely passage of storm from Rockingham to Kulin Figure 2 Passage of storm from Rockingham to Kulin A wind speed of 70 knots (around 130 km/h and 80% of V R for the area) was reported at the BoM Automatic Weather Station on Garden Island, which is five kilometres to the north of the damaged buildings shown in this report. In the area of Rockingham that had most damage (indicated by the ellipse in Figure 1), some branches had broken, as shown in Figure 3, but few trees had lost many leaves. A few real estate signs had blown over and some tiles were lifted from roofs. This damage is typical of winds at around 100 to 110 km/h (70% of V R for the area) so it is likely that the damage to buildings occurred at wind speeds lower than the maximum gust recorded and reported in the media. Even so, both the estimated wind speed from the damage and the measured wind speed at Garden Island were both lower than the design wind speed (V R ) for wind region A.

3 Building damage Rockingham February Figure 3 Damage to small limbs on Norfolk Is Pines 2. DAMAGE TO BUILDINGS IN THE ROCKINGHAM AREA Rockingham State Emergency Service had around 20 calls for assistance on 25/2/2018 ranging from roof loss to tree damage. The calls were mainly from beach-side areas. The CTS investigated damage to some buildings in this area on the day the damage occurred Roof loss from flats Figure 4 shows roof loss on a block of flats that was widely reported in the media. The roofing appeared quite new. A previous fibre-cement roof had been replaced 3 to 5 years ago with a corrugated profile metal sheet roofing system. The roof debris was found behind the building. Figure 4 Roof loss from units with a relatively new sheet roof At the time the roof was replaced, extra tie-downs were installed into the brickwork to secure the lighter weight roofing against wind loads. The rafters remained well anchored by the improved tie-downs throughout the building. Around half of the building lost the roof with some of the jarrah purlins staying attached to the roofing and others remaining connected to the rafters as show in Figure 5.

4 Building damage Rockingham February (a) Purlins attached to sheeting (b) Purlins attached to rafters Figure 5 Some purlins remained attached to sheeting, others to the rafters The purlins were connected to the rafters with skew nails, and with Type 17 screws near butt joints in the purlins as shown in Figure 6. Figure 5(b) shows a framing anchor that was used to tie the purlin to a rafter at a butt joint in the purlin, but few of these were seen, and this one had roofing screws to connect the framing anchor to both members. (a) Skew nails through purlin (b) Type 17 screw Figure 6 Connection between purlins and rafters Figure 7 Screws that had split the purlin and did not load the sheeting at all

5 Building damage Rockingham February While these connections were inadequate for the loads on the purlins, the likely cause of the failure was by splitting of the purlins at rows of roofing fasteners. A significant number of roofing fasteners remained in the roofing, but did not show any signs of damaging the crests through which they had been fixed as shown in Figure 7. This indicates that they were not loaded at the time of failure. As the jarrah purlins would have been well seasoned at the time the roofing was replaced, it is likely that some purlins were split along the lines of the fasteners; the investigator saw evidence of purlins that were split longitudinally by roofing fasteners (an example is shown in Figure 8. If the edge purlin had split, then subsequent overloading of other purlins during the wind event may have pulled out the skew nails and type 17 screws that had been used to connect the purlins to the rafters. Figure 8 Example of split purlin still attached to rafters 2.2. Verandah loss in Rockingham One of the inspected houses lost a verandah on its western side. The verandah had been secured to jarrah verandah beams that were bolted to jarrah posts. The base of the posts had been bolted to steel stirrups. However, the base of the posts had been buried into some concrete and the lawn, so that the connection with the stirrups was not free draining. Figure 9 shows the condition of the stirrups, bolts and the base of the posts. (a) Deterioration of stirrups and bolts (b) Deterioration of posts Figure 9 Failure of verandah posts due to corrosion and rot

6 Building damage Rockingham February Another house lost a roof from a verandah that was enclosed on 3 sides, but open on the windward side. In this case, the roof battens were fastened to the rafters with single nails as shown in Figure 10. Parts of this roof were embedded into a lawn while other parts bounced over roofs damaging PV solar panels, and were found on a road reserve around 100 m to the east. (a) Largest part of verandah roof (b) Roof embedded in lawn (c) Nailed batten to rafter connection Figure 10 Verandah loss with opening on windward side 2.3. Internal pressures A few garage doors failed in the event as shown in Figure 11. In this case, the internal pressure was relieved by failure of the garage door on the opposite end of the garage. Figure 11 Failure of garage door on the windward side of the building

7 Building damage Rockingham February Windows had been left open in a number of houses. In one house, internal pressure was relieved by two sets of French doors being blown open as shown in Figure 12. The top of the bolt tore through the door frame as highlighted by the red ellipse in Figure 12. Figure 12 Failure of a door frame at a bolt The investigation focused on houses where damage had been reported to the SES, rather than a systematic street survey. It is therefore not possible to quantify the role of internal pressure in damaged and undamaged houses in this event Tile anchorage Figure 13 shows a carport roof viewed from the windward side. The roof was unlined and blocked under by the house on the leeward side. Only alternate tiles were nailed and a number of other tiles were lost. Figure 13 Tile loss from carport roof photo Janice Dobra

8 Building damage Rockingham February Figure 14 shows a two-storey block of flats that had tile damage near an edge where the eaves were exposed. Ridge capping had lifted on the same building. Figure 14 Tile damage at exposed eaves 2.5. Water ingress During interviews with the media, a number of people in Rockingham reported water coming through closed windows. As the investigation did not systematically interview occupants it is therefore not possible to quantify the incidence of wind-driven water ingress in damaged and undamaged houses in this event Performance of new houses There were several recently constructed houses in the area investigated. Figure 15 shows new houses without signs of damage that were close to older houses that had been damaged. AS 4055 indicates that houses in the area should have been classified as N2 or N3. Figure 15 Undamaged new houses

9 Building damage Rockingham February Figure 16 shows some minor damage to roof sheeting on a new house. Figure 16 Damaged roof sheeting on a new house 3. DAMAGE TO BUILDINGS IN THE KULIN AREA As shown in Figure 2, the same cell travelled eastwards to the town of Kulin. While the CTS did not visit this area, media reported widespread damage to a farm in Kulin North. Figure 16 shows media photos of damage to a relatively new shed and a verandah in Kulin North. The media also reported loss of roofs on houses and a school in the town Kondinin (around 20 km NNE of Kulin) which may have been caused by winds from a separate storm cell visible to the NE of Brookton in Figure 2. Figure 17 Damage to a relatively new shed Kulin (ABC news photo)

10 Building damage Rockingham February CONCLUSIONS The wind that caused the damage to buildings in both Rockingham and the Kondinin / Kulin area appears to have been a number of downbursts associated with a thunderstorm. Wind speeds in the Rockingham area were estimated to be around 70% to 80% of the design wind speed. There was little or no obvious damage to new houses. The inspected buildings that sustained damage in Rockingham had the following features: Re-roofing that had split seasoned hardwood purlins; Posts that had deteriorated due to corrosion of bolts and rotting of timber. Fill around the metal stirrup had prevented drainage that led to the deterioration over a number of years; Inadequate batten to rafter connections; or Doors that did not have wind-rated latches or tracks. Geoff Boughton 28 February 2018