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CROSS Safety Report

Use of water filled containers to anchor temporary structures

Report ID: 255 Published: 1 July 2011 Region: CROSS-UK

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Overview

The reporter who is a local authority Environmental Health Officer raises concerns over the use of water filled plastic containers being used as anchors/ballast at festivals.

Key Learning Outcomes

For event organisers and design engineers:

  • If water is being used as ballast for significant structures the support and rigging systems should be properly designed and constructed

  • General advice on temporary structures can be found in the Institution of Structural Engineers publication: Temporary demountable structures

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The reporter is a local authority Environmental Health Officer who is responsible for licensing the safety aspects at a local festival. As part of the structure of the main stage, water filled plastic containers are used as anchors/ballast (Figure 1). They say that they have concerns over the suitability of these as they believe they could become crushed quickly and not provide the weight that is intended.

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Figure 1: typical water filled containers used as ballast

The rigging, straps, lines and other components, which have all been tested for their suitability and strength, are secured to a riveted fixed aluminium frame around the container. Neither the frames nor the containers, so far as the reporter is aware, have been tested for their capacity as fixing points.

When the reporter has questioned the suitability of the system, they have been told it is industry practice. The reporter would if possible, like guidance for the sector with which they work closely.

Expert Panel Comments

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There are systems that use water in purpose made steel tanks. Some vehicle based fairground rides contain water (typically 12,000 to 15,000 litres) into ‘belly’ tanks in the chassis to aid stability. Also, some very large tower base systems have in built steel tanks to provide stability and in both of these cases there are proper valves and methods to make sure the water stays in place.

However, in the example given by the reporter the water ballast appears to be in plastic 1,000 litre ISO containers. These are quite expensive to buy new but are widely available on the second-hand market. A significant premium is charged to provide certification that they are not contaminated so many that are in use may not have been cleaned and this may present a number of hazards.

The valve mechanisms are prone to damage, to failure due to contamination of the seals, and to simple vandalism. The tanks, if in vulnerable positions, could be damaged by vehicle impact. Also, the walls of the tanks are very easily pierced with a screwdriver or similar tool. In one city with an annual event requiring a large number of such tanks the problem of vandalism was so severe that the plastic containers had to be encased in protective steel jackets.

A secondary problem with tanks in winter is that the water may freeze and be impossible to remove. A practical solution at one site was to add plenty of salt to the water. How to fill the tanks, and how and where to discharge water after an event, are issues that have to be considered.

It may be difficult to attach the tanks externally to a structure in an effective way. Examples have been seen where there is no proper load path from the ballast to the structure, or where the load is transferred through inadequate arrangement of ropes. The commercially available tanks often have surrounding lightweight alloy cages which are intended to stop them bulging outwards when full but are completely inadequate as fixing points. Any tensile system for transferring the ballast load it should be through a properly designed and robust structure.

An alternative way of doing this is to place the ballast within the structure, so that all the weight is properly transferred to the structure. Figure 2 is of concrete blocks used in this way. Advantages of concrete are that it is 2.2 times denser than water and hence less bulky, it cannot leak away, be vandalised, or potentially contaminate the ground.

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Figure 2: concrete ballast blocks within a structural frame

In short if water is being used as ballast for significant structures the support and rigging systems must be properly designed and constructed. General advice on temporary structures is to be found in the Institution of Structural Engineers publication: Temporary demountable structures.

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