BAMOS
Jun 2020
Figure 3: Google Earth aerial
view of the location of damage
indicator 1 (blue marker), along
24
Article
with photographic evidence
of the damage inflicted
(insets) including drone
aerial photography (top right)
provided by the Metropolitan
Fire Brigade.
Damage Indicators
Of the 21 DIs identified, 10 DIs were selected that represented
the highest wind speeds within the tornado damage path and
had sufficient evidence obtained during the ground survey to
confidently assign an estimated wind speed. For brevity and
to illustrate the assessment process, examples of each damage
indicator type (one‐ and two‐family residences and hardwood
trees) are summarised in this section. Refer to Table 4 for an
overview of the tornado intensity ratings for each of the 10 DIs.
Damage Indicator 1 (DI1): One‐ and two‐family
residences (FR12)
Residential building stock within the affected area was modern
and resided within region A5 of the Australian wind loading
standard AS/NZS 1170.2 (AS/NZS 1170.2, 2011) which is
referenced by the Building Codes of Australia and New Zealand.
Regional basic design wind speed for this region is defined as
a 500 year average return period corresponding to a 3‐second
wind speed of 162 km/h (Yang et al., 2014) which resides within
the upper‐bounds of the EF‐scale EF1 tornado intensity rating
(Table 1).
DI1 was a brick constructed building with metal roofing, steel
joist and rafter construction, and an attached double garage
facing NNE. The front roller garage door blown inwards, and
rear garage roller door pushed outwards. The front patio was
exposed to strongest rotational winds on the northern flank
of the tornadic circulation was uplifted and appeared to have
allowed wind into the roof cavity, promoting metal roof sheets
to be ripped from the steel framing. The attached pergola on
western side was destroyed including displacing of roof framing
and material.
Based on the DOD descriptors in Table 2, DOD 4 “uplift of roof
deck”, "garage doors collapse inward" and "failure of porch" are
satisfied suggesting a 3‐second wind gust speed range of 130–
187 km/h with an ‘expected’ wind gust speed of 156 km/h. The
‘expected’ wind gust speed therefore resides within, and thus
the tornado intensity rating based upon the observed damage
being, an EF1 rating from Table 1 with estimated 3‐second wind
gust speeds of 138–178 km/h.
Damage Indicators 9 and 10 (DI9, DI10): Hardwood
Trees (TH)
Several hardwood gum trees in Mount Duneed had their
branches snapped aloft, some of reasonable size >10 cm in
diameter, defoliated and in a handful of cases, trunks were
snapped 1–3 metres above the ground.
Additional photographic and first‐hand damage information
was provided by storm researcher and amateur meteorologist
DOD Damage Description Expected 3‐second
Wind Gust Speed
(km/h)
Lower Bound
3‐second Wind
Gust Speed (km/h)
Upper Bound
3‐second Wind Gust
Speed (km/h)
1 Threshold of visible damage 105 85 129
2 Loss of roof covering material (<20%), gutters and/or
awning; loss of vinyl or metal siding
127 101 156
3 Broken glass in doors and windows 155 127 183
4 Uplift of roof deck and loss of significant roof covering
material (>20%); collapse of chimney; garage doors collapse
inward; failure of porch or carport
156 130 187
5 Entire house shifts off foundation 195 166 227
6 Large sections of roof structure removed; most walls remain
standing
196 167 229
7 Exterior walls collapsed 212 182 246
8 Most walls collapsed, except small interior rooms 245 204 286
9 All walls 274 229 319
10 Destruction of engineered and/or well‐constructed
residence; slab swept clean
322 266 354
Table 2: Degree of Damage (DOD) categories and descriptors for the 'One‐ and Two‐Family Residences
(FR12)' Damage Indicator and associated 3‐second estimated wind gust speeds adapted from McDonald
and Mehta (2004) and converted to metric units.