The Recent Adelaide Wind Storms Related to Australian Wind Loading Code

Relating the recent Adelaide storms to the Adelaide Metro Wind Speed Map, the simplified wind classification system (AS4055), and our wind loading code (AS1170.2)

The map typically classifies sites as either:

N1 Vzp = 28 m/s [100.8 km/h]
N2 Vzp = 33 m/s [118.8 km/h]
N3 Vzp = 41 m/s [147.6 km/h]

These commonly used speeds are not real design wind speeds, these are the wind speeds for a now obsolete method of structural design known as permissible stress design. When using permissible stress design the actual force produced by the wind speed is magnified, so the structure is designed for a different load than implied by the wind speed. Today [well since 1989] we use a design method known as limit states design. With limit state design we define a state-of-nature for the building or structure. The most commonly used state-of-nature is the ultimate strength of the structure. Ultimate strength being the load at fracture for most materials, or the load at collapse for a member prone to compressive buckling. At the ultimate strength loads expect the materials to deform permanently, with the result that the structure will require significant repair or replacing. The equivalent wind speeds for ultimate strength design are:

N1 Vzu = 34 m/s [122.4 km/h]
N2 Vzu = 40 m/s [144.0 km/h]
N3 Vzu = 50 m/s [180.0 km/h]

However these are the speeds adjusted to the local site, taking into consideration such factors as wind direction, terrain and building height, topography, and shielding by other buildings. These speeds should only be experienced at the sites when the regional wind speed, typically measured at an airport, reaches 45 m/s [162km/h]. The terrain at an airport is typically considered to be terrain category 2, and the speed is measured at 10m above the ground, for which M[z,cat]=1 by definition. {NB: It should be noted that when using the simplified symbolic approach of AS4055, that TC2 does not entirely mean terrain category 2, rather it is a symbolic code for M[z,cat]=0.96 where z=8.5m. Thus M[z,cat] has been reduced because it is taken at a height less than 10m above the ground.}

So until the news reports wind speeds gusting equal to or greater than 162km/h there should be no structural damage to buildings designed to the wind loading code (AS1170.2/AS4055). Whilst the Beaufort wind scale indicates loss of tiles at speeds above 75km/h, and structural damage above 88km/h, such is only for old construction, as all modern construction should be designed for a minimum wind speed of 30m/s [108km/h]. If experiencing damage to roof during the annual storms at wind speeds ranging from 90km/h to 110km/h, then should consider having the roof cladding fastenings checked and upgraded. However, if improving the cladding connections should also consider improving all other tie down connections in the roof and wall construction.

If have an attached carport or verandah, which is attached to the house roof, then should also check that the house rafters were strengthened and additional tie-down system installed. The additional tie down system is to resist the increased wind load added to the house roof by the attached canopy. Roughly half the width of the canopy roof is added to the house roof and supporting wall. The uplift on the wall adjacent to the canopy depends on the type of construction of the house roof and the canopy roof. Typically half the width of the house roof and half the width of the canopy roof contributes to the load on the house wall. If a house is located in wind class N1 area, then it most likely will require additional tie-downs installed, if located in wind class N2 or N3 then the existing tie-downs may have reserve capacity for an attached canopy.

Whilst the regional design wind speed is 162km/h, the Bureau of Meteorology (BoM) typically gives severe weather warnings when average speed exceeds 63km/h or gusts exceed 90km/h. When these warnings are issued it is typically hazardous to be outside, and expect to take shelter inside: therefore expect buildings to be serviceable at these speeds. From the Beaufort windscale, at 63km/h expect twigs to break off trees and progress generally impeded when walking. Whilst at 90km/h expect trees to be uprooted and structural damage to older construction.

Since annual storm damage occurs at speeds of 90km/h to 110km/h each and every year, it would be advisable to determine the maximum growth height of local trees and ensure the trees are at least such height from buildings. If the trees are considered significant by the local council and are not allowed to be removed, then suggest get such restriction placed in writing. Then get an arborist to assess the wind speed at which the tree is likely to be uprooted, if it is less than the site reference wind speed, then either have the tree removed or provide protective structure. Protective structure can be a cage around the tree, or a horizontal crash bar located either at the building to be protected or some distance between the tree and the building. In the main however keep trees and vegetation away from technology, by a distance equal to the maximum height of the tree. For power lines, either keep trees away from the power lines or suggest make sure the maximum tree height is 75% of the height of the cables, or if stuck with the trees ensure the cables are supported at a height of 1.5 times the height of the tree. Similarly no trees should over hang a road, a footpath or a parking area. Besides the hazards caused by the trees breaking or being uprooted in strong winds, there is the general damage done to infrastructure by roots and leaf litter. We can have trees everywhere, but let’s make sure that within 50km of a town centre that we have a built environment which is free from avoidable hazards.

For detailed weather information check out Australian weather news, Using a flat blade anemometer from Australian Geographic, mounted at the end of pergola, I gained a general awareness that the wind speed in the main is less than 5km/h, that it can occur in bursts where it sustains speeds of 30km/h for 10 minutes or more, with occasional spikes of more than 80km/h for a few seconds. The limit of the device was 80km/h, and when the wind peaked, the house creaked. Without an anemometer can estimate the wind speed using the Beaufort wind force chart.

At speeds of around 20km/h expect dust to be raised and loose paper to be blown around, and due to the inconvenience of such it is generally a good time to close a house, office or workshop. At 30km/h expect small trees to sway, for flags to fly horizontal when speeds are 40km/h and for whole trees to be in motion at 50km/h. At around 20km/h various outdoors activities are likely to become inconvenient or hazardous. With appropriate wind breaks it may be possible to continue with such activity.

It should be noted that the mandatory requirement for wind load design is ultimate strength, there is no mandatory serviceability loading. After a structure has experienced its ultimate strength load it will need repair of replacing. Serviceability loads are expected to remain within the elastic limits of the materials, whilst large deflections may occur under load, when the load is removed the deflections should recover. If expected to retreat to a building when wind speeds are at 63km/h, then expect to be able to open and close a building doors at this speed. At speeds less than 20km/h may expect people to open windows and ventilate the house, when speeds increase, expect the windows to be closed. At 63km/h expect the doors to be opened and closed as quickly as possible. Traditional construction was to have a weather lock entrance. At a minimum a weather lock entrance comprises a small porch, with two doors at right angles. One door from the outside into the porch, the other from the porch into the house. The door to the outside is closed before the door to the inside is opened. It is generally not a good idea to have an entrance which opens directly into the interior of the house. If a building has appropriately protected doors and windows, then it only needs to be designed for a high internal pressure coefficient at a wind speed fo 63km/h: on the assumption that should be inside the building already and closed the building up. If expect to open the doors at speeds greater than 63km/h, then it is preferable that do have a weather lock entrance. If do not have a weather lock entrance, then the whole of the building needs to be designed for high internal pressure which could be considered wasteful.

If have indoor/outdoor living areas so that the whole house is opened up, then the house is more canopy than enclosed building. It should be noted that the wind loading code (AS1170.2) only covers buildings of relatively simple shapes, and there is no clear dividing line between a free roof and an enclosed building with dominant openings: or a free roof with walls. Amongst the hazards during a windstorm are small wind borne objects, some of which are loose items left lying around outside and others a debris. Some of the hazards can be minimised by the use of wind breaks, fabric structures, nets and mesh. It is not entirely necessary to provide a shutters on a window, protection can be provided to a window by a screen at some distance from the window. For example a 1.8m high fence will provide some protection to a window as long as it is no more than 1.8m from the window. Similarly if verandahs are provided encircling the house and the verandah is enclosed with shade cloth or heavy duty netting, then will provide some protection to the house: it would need to be appropriately designed to absorb energy from flying debris. It should be noted that a relatively large distance is required for airborne debris to reach the speed of the wind, therefore typically expect impact to be at lower speeds. In a suburban setting most houses are shielded to the sides and rear, it is their fronts that are relatively open and unshielded. If a house is on a large open block of land, then some protection can be provided by installing fencing in close proximity to the house.

It should be noted that wind class N1 requires full shielding in all directions In a typical suburban environment, with a near continuous roof scape, then have full shielding when wind blowing parallel to the road, and possibly full shielding across the back gardens between two houses, however wind blowing across the road, then most likely have no shielding to partial shielding. In which case do not have full shielding and not wind class N1. The Adelaide metro wind speed map, is for guidance only, it can only be a rough indicator, as a proper assessment needs to be made centred on the individual site: whilst such is potentially possible using a geographical information system (GIS), it is highly unlikely that such was done. When sites are assessed to AS1170.2, most of the N1 sites are at the lower end of wind class N2, and the N3 sites are at the upper end of wind class N2. Put simply most sites are wind class N2, and wind class N2 really needs splitting in two. If a house is located in the wind class N1 area on the metro map, then suggest get assessed to AS1170.2 as its design wind speed is more likely around 37m/s [133km/h] not 34m/s.

As for Adelaide not being a cyclone region, well no it isn’t its worst than a cyclone region. The difference between cyclone regions and non-cyclone regions is not the magnitude of the wind speed, the difference has to do with the nature of the wind and the forces it exerts. Tropical cyclones are seasonal phenomena, and they can be tracked upto 48 hours as they approach land. Thus each season, people can prepare for the event. The airflow is also mostly horizontal, the basis of the wind loading code. The loading of structural elements however fluctuates and as a consequence materials can fail by fatigue (eg. how many times can bend a paper clip back and forth before it breaks?). Fatigue is an accumulative phenomenon, consequently can become complacent about the resistance of a structure: as each cyclone season the structure resists the loads but accumulates fatigue, so just when think the structure is plenty able to resist the cyclone, the next cyclone tears the structure apart.

In non-cyclonic regions, the storms are not entirely seasonal, only get about 15 minutes warning if any, and the storm is more associated with violent down drafts rather than horizontal airflow, and as such is outside the scope of the wind loading code (AS1170.2). There is a lot beyond the scope of the wind loading code, pressures on solar panels for example, and a lot more research is required into pressures actually experienced on the complex shapes that real buildings have.

I would contend that if our houses do not need replacing until wind speeds are at 162km/h then I would also expect for power transmission towers to remain standing at speeds of 125km/h. More than that I would contend that power is important to post disaster recovery, and therefore the transmission towers should remain standing when our houses are destroyed: that is the towers should be designed for ultimate strength post disaster loading for 172km/h. This is not to say that power continues to be supplied, excessive deflection of the cables may make it too hazardous to supply power. However after the storm event the towers should not need replacing.


References:

  1. AS4055
  2. AS1170.2: 1983, 1989, 2002, 2011

Revisions:

  1. [12/10/2016] : Original