CROSS Safety Report
Glazing design and horizontal barrier loading
This report is over 2 years old
A reporter shares problems they have encountered with barrier loading requirements on a curtain walling glazing system.
Key Learning Outcomes
For civil and structural design engineers:
Glazing design is often carried out by a specialist contractor. If the glazing is to withstand crowd loading this requirement should be conveyed to the contractor
Where specialist systems are used, it is beneficial to have a close working relationship with the supplier from the earliest opportunity to ensure design requirements are met
A useful reference is the Institution of Structural Engineers guide on Structural use of glass in buildings (Second edition)
For construction professionals and clients:
The need for collaboration over boundaries should be recognised where multiple parties are involved, in particular for safety critical elements such as glass barriers
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A reporter has experienced problems in relation to evidence of compliance with barrier loading requirements on a curtain walling glazing system. The system in question is in an entertainment building which is subject to crowd loading.
Inadequate glazing units
Enquiries were made into the design of the glazed units. It was found that the glass installed on the inside of the panels was unable to withstand the required horizontal loading, whereas the glazing on the outside of the panels could. The reporter found it difficult to obtain comprehensive design information justifying the system that was to be installed.
The deficiencies were addressed by the introduction of additional transoms at barrier height. This issue calls into question how glazing is designed and the level of scrutiny that systems are being subjected to.
It is clear that the way in which critical elements are being procured and the responsibilities that are being put in place in relation to design are not being properly defined, leaving grey areas or gaps.
It is clear that the way in which critical elements are being procured and the responsibilities that are being put in place in relation to design are not being properly defined, leaving grey areas or gaps
Crowd loading in escape corridors
On the project in question, extensive use of glazing has been used along escape corridors. These corridors would be required to convey large numbers of people to safety in the event of an emergency yet, says the reporter, the capacity for the glazing to withstand the critical load case was found to be in question. Some of the corridors and escape routes are on upper floors of the building, and on the perimeter, meaning that any failure could result in a fall from height.
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There are numerous facade engineers working on the consultancy side of the construction industry who can advise on glass, glazing system, curtain walling systems that need to resist lateral loads, and reliance does not need to be made on the sub-contractor installing the element in question. Not only is the strength of the glass an issue, but consideration needs to be given on how the glass will fail, will it fail spontaneously due to other causes, does it comply with building regulations in terms of safety glass and so on. CWCT (Centre for Window and Cladding Technology) Technical Note TN 99 - Design of glazed barriers is a good place to start for guidance on the matter.
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This is another example of problems with complex interfaces where management of the design, procurement, and construction processes must be linked. Particularly important are the boundaries of responsibilities for connection design and detailing between specialist subcontractors and the design team.
Elsewhere, there is some evidence that adequate design and detailing of critical safety structures such as balustrades is given secondary attention during the design process. It is fortunate that the reporter identified the issues before the units were subject to live crowd loading.
Design considerations for glass safety barriers
In the past, there have been devastating failures with multiple fatalities due to inadequate barriers in sporting venues. Glazing is quite capable of taking crowd loading if designed correctly. Glass design is however a specialist area and often the detailed design is left to the specialist contractors.
The type and thickness of glass are important, but so are details such as glazing beads, sealants, and gaskets. It must always be recognised that glass is a brittle material, and where it is in a safety critical location, this should be explicitly taken account of. This is normally done by the use of laminated glass.
A useful reference is the Institution of Structural Engineers guide on Structural use of glass in buildings (Second edition).