CROSS Safety Report
Safety concern for outdoor video screens
This report is to raise concerns about the suitability of video screen support systems being used outdoors for public displays.
Key Learning Outcomes
For owners and operators of outdoor screens:
- It is good practice to carry out independent design checks on temporary structures. A Chartered Engineer having adequate skill and experience can carry out these checks
Consider implementing a wind management plan which can assist in the safe operation of outdoor screens
Carrying out independent erection checks by a person who is competent to do so, can ensure that the temporary structure is built in accordance with the design
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This report is to raise concerns about certain video screens systems being used outdoors. In one example the screen which is approximately 3m deep and 5m long is made up of a number of modules.
It is hung from a trussed beam supported at the ends by proprietary telescopic posts. The reporter has a number of key concerns about safety as such screens are being used for public displays with large numbers of people in close proximity.
The reporter has a number of key concerns about safety as such screens are being used for public displays with large numbers of people in close proximity.
Strong winds can de-stabilise outdoor screens
The vertical supports appear to be typical for proprietary products with an axial capacity of around 200kg. Manufacturers may provide no data for use outside and do not provide any ratings for lateral loads. One manufacturer of this type of product includes the warning: Do not use the tower as a support for banners as with strong winds this can de-stabilise the tower and make it fail.
Screen systems of around 5m by 3m probably weigh about 400kg so the self-weight of the screen and truss is at the maximum capacity of the vertical supports. This is even without forces from wind actions.
From observation in one case the horizontal truss was visibly deflecting. Whilst that does not prove it is overloaded, the deflection will be inducing bending forces into the vertical supports by displacing them laterally and thus reducing the axial capacity.
Unsuitable restraint systems
Such screens are solid and not designed to allow wind ‘blow-through’. The screen may be restrained against lateral swaying with tension members to the bases of the vertical supports. As the bases are not anchored to the ground, any tension forces could cause the bases to slide, leading to immediate collapse.
The provision of ‘truck’ straps as guys from the tops of the towers at right angles to the screen (Figure 1) will result in increased axial loads in the vertical support. In the view of the reporter this may lead to overload/failure in the event of significant wind forces. There are also no apparent measures to prevent guys or other equipment being tampered with by the public.
Typical failure modes of outdoor screens
The screen system in use does not have ‘tour frames’ attached to the rear. These are designed to absorb bending forces between screen modules. With the modules restrained across the top and at the bottom corners, wind loads tend to cause the screen modules to deflect into a concave ‘saucer’ shape. This over stresses the module to module connections, an effect implicated in several screen failures to date. It is noted that this is a less likely failure mode here as the whole structure can be expected to overturn or collapse before a failure of the screen modules.
In conclusion the reporter believes such arrangements which would be worrying indoors are totally unacceptable outdoors. There clearly cannot have been any input from a structural engineer and so the fundamental concern is the failure of the regulatory system that allows this collection of equipment to be deployed regularly, and perhaps with increasing frequency due the Covid-19 restrictions on indoor gatherings.
Expert Panel Comments
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Covid-19 restrictions mean that there are many pop-up temporary structures for outdoor events such as showing films. The entertainments industry has few defined qualifications and there is a propensity to rely on experience and ‘using a product’ rather than designing a temporary structure.
Public safety risks
This is of concern as it can lead to operating outside of competence, either deliberately or unknowingly, and hence leading to public safety risks. This example looks as if a lighting rig has been used without consideration of:
The overall stability of the system
The suitability of the individual components acting as a whole
The effects of high wind
CROSS has many reports of temporary structures collapsing, often in wind. One cause is that such structures might be checked against low wind speeds however, any sudden gust above design assumptions can lead to disproportionate pressure increases.
In 2006, a large outdoor TV screen collapsed in Birmingham and this led to prosecutions. The owner/operator of a temporary facility is responsible for the safety of all concerned including staff and the public.
Measures to ensure safe operation
The Institution of Structural Engineers (IStructE) Advisory Group on Temporary Structures (AGOTS) publication Temporary demountable structures: Guidance on procurement, design and use (Fourth edition) includes guidance and recommendations on outdoor screens. It says there should be:
An independent design check carried out by a Chartered Engineer having adequate skill and experience; and
An independent erection check on the structure once it has been erected should be carried out by a competent person, who may be an employee of the supplier of the structure or a person nominated to carry out such checking by the contractor
Also, as they are wind sensitive structures, there should be a clear procedure for wind management to ensure safe operation.
In regulatory terms CDM Regulations 2015 may apply in some cases and it is likely that the Health and Safety at Work Act 1974 will apply and hence workers and the public should not be put at risk.
Whatever the regulations unqualified persons making safety-critical engineering decisions can result in catastrophic consequences. There have been collapses with multiple fatalities of temporary stages and the Standing Committee on Structural Safety (SCOSS) published an Alert in 2012 on Temporary Stage Structures.
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