Cyclones, Typhoons, Hurricanes or Thunderstorms. It doesn’t matter what you call them, but they all have one thing in common - the ability to destroy structures and can be fatal to 100’s if not 1000’s of people around the world. Every year about 80 Cyclones occur each year, with a variable of (+5) or (- 5) cyclones per year. As they can destroy buildings as if they were cards, we must find methods of construction that can eliminate the destruction that they cause.
The biggest problem about cyclones is the wind pressure that they exert on buildings. If the building can be designed to reduce these pressures then the chances of the building failing are minimised.
Secondly the building must be tied down as a whole structure, from the footings to the roof tiles. This principle applies to most buildings which occur in either cyclonic or seismic areas, but is a logical one as then mass will now become greater thus eliminating the damage which could of happened.
Finally all of the materials and individual components should be secured and fixed with the required fasteners to achieve this building as a whole structure. If all of these guidelines are observed, then the building should be strong and withstand many of the winds pressures.
The following pages will go into more detail regarding the above topics and other cyclone related topics.
The Effects of Cyclones in Terms of Dollars and on Humans
As stated before cyclones are the single biggest destroyers of communities, and killers of humans out of all the natural disasters that occur annually. Although towns and cities can be rebuilt, it is in our interests to design and build structures which will have minimal effect on its occupants and on the building itself.
Besides cyclone Tracy of Darwin in 1976, Hurricane Andrew which occurred in Florida in August 1992 caused $25b damage and also caused 62 deaths. Many businesses were closed and people lost jobs which effected the economy in Florida. Although it may seem as the only side effects are damage, other consequences such as the economy have to be taken into account.
Wind Velocity and the Related Wind Pressure
When a gust of wind hits a building it does not only have a pushing effect but one of suction also. The suction is caused by the wind pressure being less than the current atmospheric pressure while the positive wind pressures occur when the atmospheric pressure is less than the wind pressure.
The reason for the different pressures (positive and negative) is when a wind is travelling on open ground it either picks up or decreases in speed. When the wind encounters an object in its path it has to go around it. When the surface the wind is travelling on changes the suction or positive wind pressures occur.
Negative pressures usually occur on buildings which have a low-pitched roof (less than 25 deg), the wind pressures that will be encountered will usually be negative. Being negative pressures, the roof structure will be subject to an uplift and cause tension and torsion stresses, which will in very strong winds (Cyclones) will cause the roof to be ripped off the rest of the building.
Positive pressure usually occur on the windward side of the building, or the side that the wind first hits. This immense pressure pushes on the building and tries to move it back. If the building is on wooden or steel struts the pushing effect causes stress on the struts and the footings and therefore can sometimes cause the struts to fail and collapse the building.
Another thing about positive pressure is that if there is an opening in a windward wall, the positive pressure will force air into the building thus exerting pressure outwards and pushing the building from the inside to go out. This pushing effect plus the negative pressure of suction on the roof will cause the chances that the roof will dislodge double.
An opening on the leeward wall or the wall that is not receiving the wind will decrease the uplift effect on the roof. The reason for this is because the wind pressure is trying to act like suction on the leeward side the inside of the building will be sucked from inside to out and this effect will contain the effect of the suction acting on the roof.
The dynamic effect of gusty winds on roofing and claddings
The majority of the effects
that strong winds have on roofing and claddings has been explained in the
previous section. An overview is that the roof will try to be sucked off
from the rest of the house, the side which absorbs the force of the wind
will be pushed in with positive pressures and the sides and the leeward
side will have negative pressures acting upon it thus pulling cladding
of its support. Those effects applied to buildings with a relatively low
pitched roof (under 300).
If the building has a pitch higher than this, the front side will be pushed forward, but its pressure will be less as the wind will lose all its velocity when it hits the higher pitched roof and therefore will not have the suction effect on the other side.
The shape of buildings and special areas effected by high winds
The most effective shape for a building in cyclonic areas would be to have a building with very little or no eaves and a relatively high pitch roof (30deg to 40deg).
Hip roofs also act better than gable design roofs in storms as a hip roof makes the wind not fluctuate its velocity as it is more flat. A gable designed roof would increase the negative pressure around the vertical edge of the gable thus increasing velocity and causing more damage.
The major areas effected by
strong winds are in the north-west of Australia from just south of Carnarvon
to half way between Port Hedland and Broome (20th parallel). The wind velocity
in this region ‘D’ category can go from 50m/s to 85m/s and when tropical
cyclones hit the winds can reach in excess of 200km/h. In this area roofs
must be made out of trusses as if it does fail the roofing timbers would
not become flying debris. Buildings must also be anchored down from the
roof structure right down to the footings.
Hazard to Houses through wind borne debris
Although there should be concern
when a piece of debris is flying around, more attention should be given
to the impact it will create and the amount of wind pressure that will
therefore be exerted in the house (negative or positive). Also the external
claddings should be strong enough to withstand a reasonable force and the
fixing of them to the backing should be according to the various Australian
(AS 1684 National Timber framing Code).
If debris is expected (Category D) during storms the windows and other easily broken openings should have some sort of shutter over them to decrease the chances of it breaking during a storm and therefore subject the interior of the building to either negative or positive wind loads.
Choice of structural materials and quality control of materials
As the area discussed is one with changing weather conditions the Australian Standards should be referred to when making decisions about materials.
Manufacturers instructions should also be followed as they have usually subjected their products through rigorous testing before they are released to the public. The manufacturer can supply all sorts of technical information about when the item will fail and under what load etc. This is important when choosing the fasteners for roof and wall frames as if these items fail the material used for it is also useless and could pose a problem.
In building in cyclonic areas an M12 Bolts and 0.8mm thick galvanised steel straps that are 30mm wide should be used where necessary. Also before applying the material the builder or supervisor should check the material(s) for any visible defects. If there are defects the material must not be used.
Usage of materials in general
The major material that will differ from normal construction are the fasteners and fastening procedure. The common nail gun is usually inadequate on its own in high wind areas. The commonly accepted fasteners in industry is the Galvanised steel strap and the framing anchor (Trip ‘L’ Grip). These two items will be explained later in Bracing requirements.
Other materials that are used are the Cyclonic rod which is fixed and set from down in the footing runs up the struts through the floor plate up the wall and tensioned and bolted to the top plate.
Stability Þ bracing requirements (walls and roofs)
The bracing requirements for timber framed walls is explained in [AS1684 - Section 4: Timber wall framing: 4.9 Permanent bracing of walls.]
Once the cross pieces have gone in (either timber or metal) the studs nearest the intersection of the brace must be also tied to the wall plate to strengthen and square up the structure. The minimum distance for bracing like this is 1.8 metres and the maximum is 2.7 metres. If we have a span under the 1.8m plywood sheeting is used being fixed at the corners by M10 coach screws and then fixed with 2.8mm diameter, 30mm long at 50mm centres.
If the bracing is to withstand raking pressures of 4kN then only metal can be used as a brace and the diameter of the nails will be 3.15mm diameter.
Windows and shutters
As mentioned earlier the windows are an integral part of the building because if they get smashed by debris or are broken by the wind pressure acting on the glass an opening in the building will allow positive or negative pressures to take part. There are tables that can be referred to when ordering windows. The surface area of the window will be asked, and then the wind in metres per second. Once these two variables are found the thickness can also be found. In a cyclonic area a large area window could be better off decreasing the section size and have a special lamination put on the glass to toughen it. Nowadays toughened safety glass can be obtained which breaks but due to the lamination remains intact in one piece and would not allow the wind to enter the building.
Alternatively shutters can be used as a physical barrier to stop the debris from reaching the window and most probably save it from breaking. These used on their own are effective to an extent but small objects such as Tennis ball sized rocks etc. can penetrate through this and still cause sizeable damage. These shutters would best be used in combination with the laminated glass.
The fixing requirements which were explained earlier are either galvanized steel straps, fixing anchors bolts and screws. All of these materials are more time consuming for the tradespeople and in some cases they are not fixed correctly.
If the framing anchor is used at least 4 nails should be used to fasten it, while if a strap is used at least 10 nails should be fixed on it depending on the situation.
When using cyclonic rods they must be set in the footing (if concrete slab) it must be bolted to the floor plate run up the wall then fixed to the top plate with nuts and washers.
Other Critical construction details
Other items which come into construction when building in cyclonic areas are the external fixtures such as flue pipes, ventilators, flashing, gutters and television aerials. These should all be fixed to the underside of the cladding to avoid extra debris during a storm.
Outer backyard sheds which are usually sheet metal should be adequately fixed as they can be easily picked up by the wind and thrown towards the house if the wind is in the right direction.
Regular maintenance should be carried out on the structure to check for any deterioration in the fixing materials or the timber used for framing. If any defects are found they should be repaired immediately.
Finally when building in cyclonic and high wind zones the general requirements explained throughout this report should be followed and the various Australian Standards referred to for any extra information. In the end building in a cyclonic zone comes down to the economics of the job. Because the majority of this type of construction occurs in remote areas prices for transporting these items increases the cost and not all the requirements are provided for. Whether or not the requirements are used is a choice for the builder and owner, but if they are taken into account the chance of failure in a house during a storm will be dramatically decreased.
Þ AS 1684 National Timber Framing Code
Þ Notes on the Science of Building (CSIRO)
Þ Building Construction Vol. 1.
Þ Lessons from Hurricane Andrew Leighton Cochran & Marc Levitan.