ADVICE TO TECHNICAL OFFICIALS AND FACILITY OWNERS
Written by Denis Wilson & Graham Dwight
Existing Cages
Many of the older cages installed in Australia were purchased from overseas and are not designed for the netting to remain in place after the completion of a competition or a training session.
World Athletics, since requiring more information from manufacturers seeking a Product Certificate for their cage, has found that some cages in fact have quite low design wind speeds. World Athletics has now determined that cages that are to be used for major international competitions shall have a design wind speed of 15m/s or greater. World Athletics is granting product certificates for cages with a design wind speed less than 15m/s but all new cages are to have a plaque attached to it stating the design wind speed at which the cage netting must be lowered. It is also recommended that at each ground there be a plan which indicates the danger zone for each cage so that users are aware of the need to prevent other activities within the danger zone when the cage is being used.
Even a PlayHard Thor 10 or Thor 9 cage, which has a design wind speed of 100kph (28m/s) located in southern Australia let alone the cyclone prone tropics, must have the netting lowered on the cage if there is a violent storm forecast. Please note that the Thor 9 cage with a smaller footprint but wider gates has an IAAF Product certificate because its danger zone meets World Athletics requirements thus can be used for all competitions.
The minimum design gust wind speed for permanent structures designed by ultimate limit state analysis is 30m/s in southern Australia but local conditions with extreme exposure may warrant a higher design wind speed.
It is therefore suggested that there should be appropriate instructions promulgated at each venue when the cage netting must be lowered so as to protect the investment.
New Cages
Before purchasing a new cage determine how you want to operate and maintain[1] the cage.
Hammer cages with metal framed gates are not recommended as the gates and their netting are very prone to damage from hammers, the gates are often very difficult to move particularly in a strong wind and if not locked firmly in the open or closed position can swing around and demolish a cage. Further some gates are unstable in strong winds.
There are some designs with top-hung netting gates or sliding netting gates available.
Can the cage structure stay in place permanently? If not, how will the cage structure be installed or removed quickly without heavy lifting equipment?
The posts can either be installed in sockets set into the post footings2 or attached to a fixed or hinged baseplate set into the footing2. The latter may be the better solution.
If a cage structure can remain in place then a cage with posts permanently set into the footings would be acceptable. However, note that the posts are longer and heavier, and accurately placing them plus keeping them vertical during footing concreting may be difficult.
Determine the make(s) of cage which meet your operational requirements with the design wind speed which meets your requirements and select the appropriate-sized footing[2] for the soil conditions existing at your site.[3]
If possible, have the cage supplier also install the required footings.
In selecting a cage, the cheapest price for supply of the cage should not be the determinant. Have a rating system for all the desirable properties. Examine an existing cage of the type and talk to its users.
Desirable Properties
The IAAF has listed the following desirable properties of a cage as:
– The cage structure and the netting meet the dimension requirements of the IAAF Rules*.
– The netting cord is strong enough so that it does not break under the impact of the implement, abrade where it is attached or deteriorate unduly under the effects of ultra violet ray exposure*.
– The net can be quickly raised and lowered*.
– There is positive attachment of the netting at ground level which maintains the net in correct relationship to the throwing circle(s) particularly in windy conditions and there is sufficient netting at ground level so that an implement cannot penetrate under the netting*[4].
– The netting when blown by wind does not impede the athlete making a throw*.[5]
– The gate pivot posts and any gate metal frame is not be exposed so that it can be hit by an implement causing damage to both.
– The netting is hung clear of the support posts by at least 0.60m and arranged so that when the gate is closed that the posts are not struck by a thrown implement*.
– The volume of netting needs to be sufficient to minimise rebound of implements but not slack enough to allow implements to push netting against the supporting structure.*
– The gates are easy to open and close manually quickly with a positive positioning arrangement in the fully open and closed positions.
– The leading edge of the gate netting must maintain a vertical line in order to maintain the integrity of the danger zone. This means that a tensioning of the leading edge will generally be required at each anchor point i.e. open and closed position.*
– The gates maintain their integrity under long term usage.
– The supporting frame is stiff enough so that it does not deflect out of position unduly under the weight of the net and the force of wind*.
– The cage is easy to install and remove preferably without heavy lifting equipment*.
– The cage structure does not unduly block the spectator view of the track events when the netting is lowered*.
– Maintenance procedures required to maintain the safety of cage before, during and after competition must be provided. The users and maintenance staff should be regularly reminded of these procedures
* These properties also apply to a stand-alone discus cage
Denis Wilson & Graham Dwight
Updated November 2020
[1] Selection of cage type will also depend upon:
- Security of site
- Accessibility of public/unsupervised users
- How cage and netting is to be maintained
- Potential for damage e.g.; fire risk (nearness of vegetation), vandalism, misuse by users
Use of chain wire instead of cord netting could be a considered as an alternative particularly for training venues.
[2] For both poles and anchor points. A soil analysis is desirable to ensure that the footings are properly sized.
[3] See Note 2.
[4] Preferably a minimum of 30cm adequately anchored.
[5] At no time should netting be tied to the cage superstructure.
