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Falls from Heights: Assessment of Wood and Metal Guardrails Under Real Working Conditions


Falls from Heights: Assessment of Wood and Metal Guardrails Under Real Working Conditions

The statistics prove it: Falls from heights are a veritable scourge in Quebec’s construction industry. In 2017, across all industries, 10 workers died from falls, accounting for 16.1% of all workplace fatalities—the third highest cause of death for the reference year. Furthermore, falls rank second as a cause of accidents in terms of costs: $397 million a year for 2010 to 2012.

According to the CNESST, “falls to a lower level, which are defined as a point of contact at the end of the fall lower than the surface where the person was before falling,” cause some 4,000 occupational injuries each year in Quebec.

The best strategy to protect workers against this hazard is simply to eliminate all work at heights. But when that’s just not possible, collective means of protection, such as guardrails, have to be used. Besides ensuring increased safety by preventing falls, guardrails also give workers greater mobility. Temporary guardrails, as opposed to permanent ones, are commonly erected on construction sites. They have to meet the requirements of the Safety Code for the Construction Industry (SCCI) in force in Quebec, however.

While the IRSST had already studied the design of the vertical posts and rails of temporary metal guardrails, it had never examined their method of securement, which means how they are screwed or nailed to the support structure. Wood guardrails, which are generally installed on open-web joists or prefabricated walls, had simply never been assessed at all. It was therefore hard to ensure compliance with the resistance-to-loadings requirements prescribed by the SCCI, noted Bertrand Galy, a researcher in Mechanical and Physical Risk Prevention at the IRSST and author of the research report on the strength of wood and metal guardrails. “Earlier work hadn’t looked at metal guardrails under real working conditions and had completely ignored wood guardrails, even though they’re used frequently,” the researcher said.

Hundreds of tests

To fill this knowledge gap, Bertrand Galy and his collaborators carried out three series of guardrail strength tests. The first, involving 262 tests, was conducted in the lab on wood guardrails made with boards (two-by-fours) and secured to open-web joists. A host of different combinations were tested to determine the influence of the various parameters studied, such as joist height (9.5 in. to 16 in.) and number of spans (one to three). A number of variables, including the number and type of nails (smooth, ring shank, screw shank) or screws (wood, lag), were selected to reflect the reality of work on construction sites. For others, such as guardrail height, the researchers based their choice on SCCI provisions, which stipulate that “any guardrail shall be between 1 m and 1.2 m above the surface on which the worker is working” so that it can effectively prevent a worker from falling.

The second series of strength tests (98 in all) was also conducted in the lab on wood guardrails, but ones that were secured to a prefabricated wall consisting of a structure made of two-by-fours covered with OSB panels. The influence of the same parameters mentioned above was examined. It is worth noting that prefabricated walls were incorporated into the research design following comments received from partners in the construction industry. “To begin with, we were planning to focus on the joists. But then a number of people urged us to consider prefabricated walls, as they are widely used on residential construction sites,” said Bertrand Galy.

The third and last series of tests was conducted in the field, with metal guardrails installed by roofers on two actual buildings in good condition. The purpose of these 36 tests was to assess the strength of the guardrails using the method IRSST researchers had established, but on parapets whose condition was unknown. Three guardrail models were tested and several variables were studied, including the type of screws used (black, self-tapping, lag) and the number of sides (one or two) of the vertical post securement system. “The high number of constraints in the field meant that we had to limit the number of tests,” Galy noted.

Recommendations

Some of the conclusions of the research were expected. Thus, in tests on structures erected in the lab, the strength of a 1.2 m high guardrail was systematically less than a 1 m one, simply because of the leverage effect. Similarly, a guardrail with three spans was naturally stronger than one with one span or two, with force applied directly to the vertical post. Contrary to what the researchers had assumed, however, joist height had a relatively minor effect on strength. On prefabricated walls, the base to be nailed or screwed into was not deep enough to provide maximum strength. “Pull-out occurred pretty early. Longer nails or screws need to be used in this case,” the expert recommends. In the field, the results underscored the importance of choosing the right type of screw to anchor metal guardrails to parapets, even though differences are not that significant.

In the end, the two key variables are the number of screws used to secure the foot of the vertical post of the guardrail—follow the manufacturer’s recommendations—and the system used to secure the posts, which must involve two sides of the parapet. Bertrand Galy advises opting for versatile systems that offer several methods of securing the vertical posts and can be adapted to suit small parapets. “The observations from our study can be applied in the field fairly quickly. They represent a real opportunity to reduce falls from heights,” he noted.

Guardrails: Close to 40 years of research

The IRSST’s experts in Mechanical and Physical Risk Prevention have been conducting research into guardrails and their effectiveness for many years now. Preventing falls from heights is still a component of the Institute’s research program. Retrospective.

By Marjolaine Thibeault

For concrete floors

In the late 1980s, IRSST researcher Jean Arteau and architect Raymond Bertrand developed a guardrail for use on concrete flooring and fly-forms during formworking. The new guardrail was adjustable width-wise and could be installed in a straight line or at an angle, at a floor’s perimeter. Its base could be secured to the concrete slab before or after the concrete was poured.

Built and installed on site

In the early 2000s, another study was conducted by André Lan, Jean Arteau and Renaud Daigle of the IRSST, to develop and validate a method of evaluating guardrails built and installed on construction sites. The team assessed the strength, effectiveness and reliability of wood guardrails and rails supported by metal shoring and connected metal frames. The laboratory tests reproduced the conditions observed in the field. The validated method proved practical and reproducible for assessing the safety and compliance of guardrails of this kind with standards and regulations. The report (R-415), published in 2005, included technical recommendations for construction industry workers and contractors.

Metal prefab

A few years later, André Lan and Renaud Daigle carried out a study of metal prefabricated guardrails (see also the Report Appendix) using the models most commonly found on Quebec construction sites. The guardrails were put through validation testing to see whether they complied with Quebec’s Safety Code for the Construction Industry and to determine what installation and securement methods met the Code’s requirements. The researchers used facilities at Polytechnique Montréal to perform the strength tests. The testing showed that the three guardrails satisfied the Code’s strength requirements and ensured appropriate safety protection for workers.

When this research was completed, the IRSST and the Association sectorielle paritaire pour la santé et la sécurité du travail du secteur de la construction [construction-sector OHS association] worked together on writing and publishing a prevention fact sheet for construction industry workers and contractors. André Lan then teamed up with IRSST knowledge transfer advisor François Ouellet and ASP safety advisor Louise Lessard to produce a summary of the results of the research on the three models of guardrail for flat roofs, used primarily by roofers. The fact sheet includes a table that can be used to check that each component of a guardrail meets the specifications of the Code, as well as a description and illustrations of each guardrail model studied.