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Fenestration Forum: April 2012

A compound variable also presents itself when designers begin using new materials and systems

A compound variable also presents itself when designers begin using new materials and systems, or start using traditional materials in new ways. A case in point would be the increasing popularity of using glass for non-structural infill for balcony guardrail systems in residential highrises. In the past, these systems typically employed metal pickets or plates, pre-cast concrete, masonry, or combinations of these materials, which were fully supported by various means to the top of the balcony slab. Over the past 15 years designers gradually began to incorporate glass into their guardrail designs, as glass offers an esthetically pleasing appearance and provides for more outdoor light into the living space. Because of recent changes in how the glass panels are mounted and supported, the glass often falls from the guardrail assembly to other balconies and the street below when it breaks. These design changes have led to increased reports of balcony guardrail failures in the past year, most of which were primarily related to material failures. The majority of these failures involved breaking glass infill panels in residential buildings in downtown Toronto. In the past, breakage of this type was not usually an issue because the glazing infill was smaller, mounted inboard of the edge of the slab, captured on all edges and therefore rarely resulting in glass falling from the building.

These recent failures have attracted a considerable amount of attention from the media, regulatory authorities and the architectural and engineering communities. Engineers involved with the subsequent investigations uncovered a number of fundamental issues associated with the design of glass balustrades and infill panels.

The first of these is the lack of substantive guidance regarding the design and use of guardrail systems, aside from the information given in the building code. There is no applicable reference standard.

Investigating engineers have found that designers follow different design protocols to determine structural loads for guardrails. No data relating to actual wind loads acting on these systems is currently available. There are legitimate arguments that involve interpretation of the building code, and clarification is required as to whether guard loads and wind loads should be determined independently or simultaneously.

There is no specific guidance given in the existing codes and standards regarding the use of tempered and laminated safety glass for use in guardrails. Neither is there mandated guidance regarding post-breakage retention for glass infill or balustrades. There are no mandated test procedures to evaluate guardrail systems for guard load, wind load and impact testing, and there is only limited guidance regarding the materials and design of balcony guardrail assemblies.

The various professionals involved in the investigations of these incidents quickly concluded that development of a formal guardrail standard was required to ensure adequate quality control and safety.

As a result, the Canadian Standards Association has been asked to proceed with development of a standard for balcony guard assemblies after consultation with various stakeholders. This process would involve bringing the appropriate industry representatives and authorities and engineering experts together as a technical committee to develop a reference standard that could then be adopted into existing building codes. The finished standard is expected to provide an introduction discussing applicability, specification guidance, performance parameters and a list of terminology; a list of referenced publications (for instance, ASTM Standards) for materials and testing; definitions providing for loads (guard and wind), assembly types and applications; general requirements for specific applications, assembly types and loading requirements; test requirements including sequence, specimen sizes, details and methodologies; materials requirements setting out prescriptive requirements for the guardrail system components and component requirements outlining how specific guard load components are to be used in situ.

Brian Burton is the author of Building Science Forum and is serving on CSA’s Fenestration Installation Technician Certification Committee. Brian is a research and development specialist for Exp (The new identity of Trow Associates). He can be reached at brian.burton@exp.com or through www.exp.com .