Precast concrete solutions for risk management

Photo courtesy Groupe Lépine

By Brian J. Hall
Climate projections by the World Meteorological Organization (WMO) show past and current practices will influence the climate for decades to come. Such projections are usually statements about the likelihood something will happen several decades in the future if certain influential conditions develop in contrast to a prediction. For projections extending well into the future, scenarios are developed over what could happen given various assumptions and judgments. Therefore, in addition to efforts to reduce climate change, design/construction professionals need to prepare for the climate change that cannot be avoided.

After catastrophic flooding in Canada and the United States in recent years, as well as the devastating 2016 wildfires in Fort McMurray, Alberta, people are realizing climate change is one of the biggest challenges facing the planet. For design/construction professionals, part of the task of creating the built environment has now become ensuring structures can safely endure these extreme weather events. In other words, today’s structures need to display resilience.

This term is defined by the globalization-focused Rockefeller Foundation as:

making people, communities, and systems better prepared to withstand catastrophic events—both natural and manmade—and able to bounce back more quickly and emerge stronger from these shocks and stresses.

Increasing resilience is a shared responsibility among citizens, the private sector, and government. It requires bold decisions and investments that often appear to pit short-term thinking against longer-term interests. For example, should we relieve pressure on housing prices by relaxing building codes to allow for cheaper and lighter construction methods at the expense of safety? This article examines the broader implications of resilience in the built environment, and explores how durable materials like precast concrete can help provide the strength and stability needed to weather the coming storm, both literal
and figurative.

Spinning out from the Rockefeller definition, resiliency can be thought of as the adaptability of a system (communities) to maintain functions and structure in the face of turbulent internal and external change. A crucial part of disaster recovery is not only to get essential services back up and running normally, but also to get people back to work. That means buildings must not only resist the damages caused by a disaster, but also remain in a condition that is safe and suitable for occupancy as soon as possible.

Key attributes of enhanced resilience are:

  • longevity (i.e. service life);
  • robustness (i.e. minimized potential for structural progressive collapse);
  • sustainability;
  • life safety;
  • durability;
  • adaptability for reuse; and
  • resistance to disasters.

The Community and Regional Resilience Institute (CARRI) considers ‘community resiliency’ to be the “capability of a community to anticipate risk, limit impact, and recover rapidly through survival, adaptation, evolution and growth in the face of turbulent change.” This last phrase can refer to a range of various natural and manmade calamities that include:

  • extreme weather events (e.g. snowfall, tornadoes, hurricanes, or flooding);
  • geological (e.g. earthquakes, tsunamis, as well as volcanic eruptions);
  • man-made crises (e.g. terrorism, war, forest fires, pandemics, or large-scale industrial accidents); and
  • economic (e.g. company closing, recession, or depression).

CARRI’s Resilience Loss Recovery Curve is the capacity of hazard-affected bodies to resist loss during disaster and to quickly recover afterward. It helps explain how community function is affected by an acute disturbance and depicts response and recovery curves.

Whether cast-in-place or precast, properly designed concrete assemblies offer a robustness making them inherently resistant to wind, hurricanes, flooding, and fire. Plant-cast precast systems offer even greater risk enhancement because of the superior quality control and protection from the environment during fabrication. Since 2004, this has been recognized by CSA A23.3, Design of Concrete Structures, which allows for an increased resistance factor for precast concrete produced in a precast plant that is certified in accordance to CSA A23.4, Precast Concrete−Materials and Construction. The resistance factor is increased from 0.65 for cast-in-place concrete to 0.70 for precast from a certified facility.As a structural material and as a building exterior skin, precast concrete has the ability to withstand nature’s normal deteriorating mechanisms as well as natural disasters.

One example of a resilient structure is Winnipeg’s Grosvenor House, a 33-suite total precast apartment building constructed in 1960 and designed by Libling Michener and Associates. The form of this apartment building resulted from studies of the site, the living patterns of the anticipated market, and most particularly, of the structural system.

Precast concrete can differ from traditional poured-in-place concrete in several important respects. It is possible to obtain a high-quality of finish and colour control in the structural elements, allowing the use of the materials as both structure and finish. The natural method of ‘joinery’ of precast beams to spandrels and to columns produces an articulation of the structural elements unlike the monolithic nature of poured-in-place concrete. Each element of the building is joined in a precise and definitive manner; from this, develops the architectural form. At 57 years old, the building is an attractive showcase of precast concrete resiliency with a life expectancy of well over a century.

Looking further west, the country’s first prestressed, precast concrete bridge can be found in North Vancouver. The Mosquito Creek Bridge, originally completed in 1953 and located near the intersection of Marine Drive and Fell Avenue in North Vancouver, represents the first use of prestressed concrete technology on bridge stringers in Canada and is still in use today.

This significant advancement in civil engineering technology is now one of the most widely used methods of bridge construction worldwide. In the summer of 2017, it was recognized during a historic site dedication ceremony held by the Canadian Society for Civil Engineering (CSCE).

The bridge and the Grosvenor House are two of many examples of Canadian projects showcasing the ability of precast concrete to endure. With the renewed push for resilience, future projects can also benefit from the material’s long-term structural strength and performance.

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