EHJ July 2002, pages 196-199 The use of toughened glass in building structures can lead to serious health and safety issues should it fail. Ian Wheeler explains

Glass is increasingly being used as an architectural element in building structures and toughened glass in particular continues to be used in overhead situations. However, should it fail, there is a risk of serious personal injury which can result in protracted legal proceedings and, potentially, expensive remedial measures.

Toughened glass used in overhead situations may fail for a number of reasons, but one cause in particular, due to nickel sulphide inclusions, is of note. Until recently, this phenomenon was not well known outside of the architectural or glazing industries and engineering circles; with little literature or guidance available other than through specialist sources.

An additional problem with broken toughened glass in overhead situations is that when it breaks, whether for example due to a nickel sulphide inclusion or a hard body impact such as a stone, it tends not to break into its constituent diced parts. It may fall in large clumps of diced glass, which hold together until its progress is obstructed. Where clumps of such glass strike a person below, serious injury can result.

THE ACCIDENT
In August 1999, a panel of toughened glass fell from the roof of a busy two-storey shopping centre in Cirencester, Gloucestershire. The glass struck three young brothers, who were taken from the site of the accident to hospital. Two of the boys sprayed with glass were released on the same day after treatment for lacerations, bruising and removal of subcutaneous glass. The older boy, who shielded the other two boys and who took the majority of the glass impact, was detained in hospital. He received treatment for extensive lacerations, subcutaneous glass, a fractured clavicle and shock.

INITIAL INVESTIGATION
The police informed the local authority of the accident, enabling an immediate site visit to be made. A visual examination of the shopping centre roof revealed that a panel of glass (0.6m by 0.9m) was missing from the upper section of the atrium roof.

A detailed examination revealed two areas of roof glazing - a main atrium and a passageway some 40m long - with total glass coverage of 370m². The glazed roof was fabricated from a patent glazing system consisting of extruded aluminium alloy glazing bars, laid to falls, with single panes of reflective thermally toughened glass. At its highest point the atrium measured 11m and contained predominantly shallow sloping panes, with some small areas of vertical glazing panes forming an architectural feature.

Building control officers were requested to attend the scene in relation to enforcement responsibilities for dealing with dangerous structures. Following advice given to the centre manager concerning the potential risk of injury due to further glass failure, the centre was voluntarily closed.

Evidence obtained at this time included witness details and the CCTV security surveillance video tape, secured under section 20 powers,¹ which graphically captured the accident in real time. Subsequent witness statement accounts and the CCTV video tape both identified that a large clump of glass had fallen and struck the older boy. Further evidence obtained (also under section 20 powers) included glass panel shattered fragments and full site file documentation. Analysis of the site file documentation revealed that two further previous glazing failures had occurred in September 1998. Both of these failures, however, had been attributed to vandalism.

Specialist technical advice was sought from John Willbourne, specialist HSE inspector for construction engineering. In particular, advice was obtained on suitable measures to form a temporary catchment system in case of further glass failures. Positioned directly underneath all centre glazing, the system consisted of safety netting and "monarflex" reinforced polyethylene sheeting, which in the event of further glass failures would break up any clumps of glass and prevent the resulting glass fragments from falling. As the catchment system adequately controlled the risk of injury from further glass failure the shopping centre could be reopened, which was important as businesses were losing income. A leading expert, Barry Josey, was instructed to undertake an inspection of the shopping centre roof. Although this initial examination could not identify the exact cause of the glass failure, his report did identify several reasons why thermally toughened glass may fail, as well as reasonably practicable permanent remedial measures which would reduce the risk of injury due to any further glazing failures.

MAIN INVESTIGATION
Once the temporary catchment system was in place and the centre had been reopened, a review of the initial investigation determined an investigation plan which fell into three distinct phases:

• to ascertain the cause of the glass failure;
• to determine what reasonably practicable permanent remedial measures were available; and
• to establish what permanent remedial measures were proposed by the centre owners.

CAUSE OF THE GLASS FAILURE
As glazing technology is a highly complex discipline, it was necessary to undertake a literature review. This enabled a clearer understanding and analysis of all evidential material gained, facilitated liaison with councils and industry experts, and helped inform decisions on the cause of the glass failure to be made.

Toughened glass, sometimes referred to as tempered glass, is produced by heating ordinary annealed glass to just below its softening point, about 650^oC, and then cooling it rapidly with blasts of air. This causes the surface of the glass to cool more rapidly than the inner core, which in turn causes the outer zones of the glass to be in compression while the inner zone is in tension. The compressive and tensile stresses balance one another and in doing so impart to the glass a bend strength some four to five times greater than ordinary glass. The increased bend strength provides resistance to breakage. In addition, when glass toughened in accordance with the provisions of BS6206 breaks, it does so "safely".² In effect, the glass dices into small pieces with a diameter no larger than 10mm and without sharp edges. The standards and codes of practice however, deal with safety in relation to human impact; they do not deal with the effect that broken safety glass may have when it falls from height.

Two relevant principle causes for glass failure were identified in the report prepared by Mr Josey. First, "hard point glass to metal contact" was identified, where glass was directly bearing onto the glazing bars. This was found to be occurring in several places. Second, "inclusion expansions" were noted. From the literature review, it was identified that in the manufacture of soda-lime glass minute inclusions may be formed, with at least 50 identified chemical types. These are generally simple appearance imperfections that are of little consequence. However, in cases where contamination of raw materials (or melt) by nickel containing metallic alloys, eg nichrome wire or thermocouples, occurs, the nickel and sulphur can react under the high furnace temperatures and produce nickel sulphide inclusions.³

Inclusions can become a serious problem when the glass is thermally toughened. Some of the inclusions are stable after crystallisation, but others are unstable and change under the influence of heat and time to another and larger configuration type. The change in volume from one configuration to another is considered to be in the order of 2.4 per cent to 4 per cent.

As toughened glass has outer surfaces that are in compression and an inner core that is in tension, the compressive and tensile forces are in equilibrium. If, however, an unstable inclusion particle is located within the tension zone of the glass the tension will be increased as the changes occur. If the size of the unstable inclusion is appropriate, the increase in tension will reach a critical level. A micro-crack will develop at the inclusion site (fracture origin) causing imbalance and followed by spontaneous failure of the glass. An unstable inclusion 80µ in diameter could cause a problem, while an inclusion of 110µ almost certainly will. Three conditions are needed for this type of failure:

• an unstable nickel sulphide inclusion;
• a particle size greater than 80µ; and
• a location within the tension zone of the glass.

The problem can be ameliorated by heat soak testing of the glass, a process that claims to cause breakage of most if not all the substandard panes of glass. Interviews with the suppliers of the centre roof glazing confirmed that no heat soak process was carried out on the original batch of glass.

Temperature data logging at a site immediately adjacent to the shopping centre confirmed that the glass failure occurred at the hottest point of the day - 30^oC - and when glass thermal expansion would be at its maximum point. This provided strong, yet inconclusive, evidence that a nickel sulphide inclusion may have been responsible for the glass failure.

The shattered glass samples were therefore submitted to an independent UKAS accredited laboratory. The analysis identified the presence of toughened glass and elemental composition but was unable to establish any presence of inclusion expansions.

To provide conclusive evidence of any nickel sulphide inclusions it was established that a piece by piece examination of all of the many thousands of shattered glass panel fragments would be required. This was carried out by the glass manufacturer Pilkington plc. The examination located both pieces of the fracture origin and further in-depth analysis using scanning electron microscopy with X-ray dispersive spectroscopy identified a nickel sulphide inclusion at the fracture origin. This was most probably the cause of the glass fracture.

PERMANENT REMEDIAL MEASURES AVAILABLE
Following an EHCnet message request concerning glazing failures, responses were received from several local authorities. Together with Mr Josey's report an appreciation and collective knowledge of investigation remedial measures previously utilised was acquired. This process also provided the necessary information with which to construct an improvement notice in anticipation of any required enforcement action. In brief, the remedial measures identified for any appropriate enforcement action to date included:

• to establish structural integrity of the glazing system by competent persons, with replacement of any panes of glass at risk of spontaneous breakage or which failed to meet safety criteria set out in British Standards: 5516, 6206, 6262 or 8000: Part 7. ^4,2,5,6;
• application of a clear blast protection film of minimum thickness 150µ to the underside of the glass, including glass located within glazing rebates;
• application of a clear blast protection film of minimum thickness 150µ to the underside of all visible glass, with the film restrained by and bonded to a secondary frame permanently attached to the glazing bars; and
• installing a mesh to the underside of glazing of sufficiently narrow gauge, so as to prevent diced glass falling through and capable of supporting the broken glass pane and any loads likely to act upon it.

MEASURES PROPOSED
The unacceptable risk to the safety of centre users from falling glass was adequately controlled by the catchment system. However, this was only a temporary measure as its presence interfered with the safe operation of the fire precaution systems. Details were therefore requested from the centre owners of all proposed permanent works. Despite a reasonable time period for receipt of this information, including an agreed time extension, no details were received. In December 1999, an improvement notice was served on the centre owners to be complied with by March 2002.

APPEAL PROCESS
In January 2000, notification of an appeal against the improvement notice was received from the employment tribunal. From the literature review and consultation with the HSE and the council's expert, the appeal grounds were critically analysed and evaluated. The result being that the council's solicitor was advised that the grounds for appeal were either technically invalid or inappropriate.

Although the appeal hearing date was subsequently rescheduled for September, during which time meetings with the centre owners were held, the appeal against the improvement notice was withdrawn by the centre owners. The council was able to facilitate this decision as the improvement notice could be extended once the appeal was withdrawn, allowing completion of all required works based upon three criteria:

• a proposed scheme of works;
• risk assessment and method statements for all works;
• and the dates for commencement, incremental progression and completion of all works.

A formally documented signed agreement satisfying these criteria was received enabling formal extension of the improvement notice.

COMPLIANCE
Although the scope of the works proposed - installation of a narrow gauge mesh sited directly underneath all centre glazing - satisfied the requirements of the notice, submitted risk assessments and method statements did not adequately or sufficiently demonstrate safe systems of work. Roof works include high risk activities requiring close attention to detail at all stages and job-specific risk assessment and method statements.^7,8 Thus, amended details were received from the contractors.

As part of the construction works was to external parts of the structure, the HSE was the enforcing authority for these elements. However, as submitted risk assessments and method statements shared commonalities in demonstration of safe systems of work, for both internal (council) and external (HSE) work phases, joint visits with the area HSE construction inspector ensured consistency for enforcement.

Regular visits to the centre during the day and at night were made to review progression of works and to inspect required safe systems of work. This was necessary as at first floor level, there is a night club and bar which required specific risk assessment and method statements from contractors concerning their night work activities. Completion of installation of a narrow gauge mesh underneath all centre glazing finally ensured compliance with the improvement notice.

LEGAL REQUIREMENTS
Under Section 3(1) of the Health and Safety at Work etc Act 1974, the centre owners have a statutory duty of care to ensure, so far as is reasonably practicable, that visitors to the centre are not exposed to risks to their health and safety.

The investigation demonstrated that where the presence or the potential for inclusion expansions is realised in toughened glass in overhead situations, there exists a risk of serious personal injury. This risk arose because the glass could be displaced from its support system due to several causes, one of which was a known inherent defect. Therefore, this did not provide an adequate safe construction to meet the building regulations, which require materials to be suitable for the location in which they are used.^{9, 10}

The construction did not meet the requirements for safety specified in BS5516 (Code of practice for design and installation of sloping and vertical patent glazing). This deals specifically with overhead sloping glazing, and in particular the risks from broken in-fill material falling onto people underneath.

In the US and many European countries, laminated glass must be specified in overhead situations. This retains structural integrity, due to the immediate laminate, in the event of failure. Further, since the early 1990s, it has been recommended practice in the UK not to use toughened glass in overhead glazing situations when it is either a single pane or the inner pane of a double-glazed unit.

In the long term, much needed dialogue should seek to influence the alignment of this country's building regulations with that of the US or those European countries where laminated glass must be specified in overhead situations. However, in the interim, the lack of available literature or guidance in such a technologically complex discipline is of concern for those required either to give advice or to carry out investigations following glass failure incidents. A HELA circular may therefore be the best placed tool to resolve these deficiencies.

The author would like to acknowledge the assistance given by John Willbourne, specialist inspector HSE for construction engineering and Barry Josey, senior associate, Bickerdike Allen Partners.

References

1. HM Government, Health and Safety at Work etc Act 1974. HMSO.
2. BSI, BS 6206: 1981, Impact performance requirements for flat safety glass and safety plastics for use in buildings.
3. Brungs & X Y Sugeng (1995) "Some solutions to the nickel sulphide problem in toughened glass", Glass Technology, vol 36 No4, August 1995.
4. BSI, BS 5516: 1991, Design and installation of sloping and vertical patent glazing.
5. BSI, BS 6262: 1982, Glazing for buildings.
6. BSI, BS 8000: Part 7: 1990, Workmanship on building sites, Part 7. Code of practice for glazing.
7. HSE, HS(G)33, Health & Safety in Roof Work. HMSO.
8. HSE, HS(G)150, Health & Safety in Construction. HMSO.
9. DETR, The Building Regulations 1991, Approved Document "N" Glazing-Safety in relation to impact, opening and cleaning. The Stationary Office.
10. DETR, The Building Regulations 1991, Approved Document "to support regulation 7" Materials and Workmanship. The Stationary Office.