What is it and When Does it Make Sense?

By Augustus “Gus” Raymond

Entering the 19th century, the 482’ Strasbourg Cathedral was the tallest structure in the world, horses, and wind sails were the fastest form of travel, almost a billion people were alive, the world’s GDP was barely $1 trillion (in 2019 $US), 80% of people lived in destitution, and the average person lived 35 years. As the new millennium started only two centuries later, the Petronas Towers stood at 1,483’ tall, space shuttles could reach speeds of 17,500 mph, over six billion people were alive, the world’s GDP exceeded $30 trillion, only 20% of people lived in poverty, and the average person lived 65 years. It could reasonably be argued that humanity advanced more in those two centuries than all history prior to 1800. Among the factors contributing to this success was a simple change in building materials, allowing buildings to be built faster, bigger, safer, and more economically. Stone and wood, favorites of old construction, were surpassed by rediscovered concrete and novel steel which boast superior strength-to-weight ratios, structural predictability, production replicability, and various shapes and sizes. However, in recent decades, global recessions, natural disasters on rising populations, increasing sustainability concerns, and escalating manual labor costs are presenting engineers with challenges that call for supplements to concrete and steel. Humanity looks to alternative building solutions to overcome the obstacles of the present and future.

Figure 1: Cross Laminated Timber Diagram.

Wood comprises both the most historically utilized and replenishable building material on earth. Despite being overtaken by concrete and steel as the favorite of engineers, wood continues to be used globally, particularly in vernacular and residential structures. Wood, even given its status as a most ancient building material, is finding ways to be innovated and improved. Over the last half-century, the science of combining wooden members via glue and fasteners to make bigger elements known as Mass Timber has been on the rise around the planet. With the invention of Cross-Laminated Timber (CLT) by Austrians in the mid-90’s, though, Mass Timber has now proven to be a truly viable building solution.

Formed by crisscrossing and gluing laminae of sawn lumber, CLT can manifest as walls, roofs, floors, and other panelized building components. This orientation provides high axial compression and in-plane shear loads as well as reduces swelling and shrinking. CLT’s size and connectivity render construction quicker than even concrete and steel. The panel’s thickness offers insulation, strength against extreme wind activity, and even fire resistance. The fibrous nature also suggests seismic flexibility. As a newer material, CLT’s limits are still being explored with research from skyscrapers to shear diaphragms to vibration responses. While some are praising CLT’s triumphs, people are historically resistant to change.

Due to shared dominance of concrete and steel across the global industry, there has been some pushback against CLT’s approval, and the AEC industry (architects, engineers, and contractors) has few endorsees of CLT’s examination. This trepidation stems from an idea that CLT could potentially replace concrete and steel in certain niches of the market, exacerbated by the increasing tally of Mass Timber exclusive buildings. Aggravating matters further, Mass Timber too often orates how it overcomes deficiencies of concrete and steel through its advantages rather than seeking cooperation with the building material paradigms. If concrete and steel perform better in cooperation than alone, could Mass Timber likewise perform better when included within the mix than unaccompanied?

The duo formed by concrete and steel provide many benefits that either one individually might not be able to achieve, forging a close relationship between them that is regularly selected. Concrete possesses high compression strength, moldability on site, variability in mix designs and applications, and imperviousness to combustion and moisture (provided no cracks form). Steel, almost concrete’s counterpart, likewise boasts compactness, ingredient abundancy, high structural predictability, and high tensile capacity. Concrete covers steel’s weaknesses in compression strength, corrosion, vulnerability to heat, and workability. Steel accommodates concrete’s weaknesses in tensile capacity, weight, and cracking. The two, despite some of their individual deficiencies, integrate well and provide a strong, economical (in material costs), and predictable option for buildings around the globe.

Figure 2: comparison of typical reinforced concrete/steel/combination construction and CLT construction (redstone), one of CLT’s many advantages.

As stellar as concrete and steel in tandem are, adding Mass Timber, particularly CLT, to the team could bring even more benefits. CLT can be built between 25% and 75% faster than similar reinforced concrete and steel buildings on a square footage basis, has a 20% overall faster schedule, and uses 90% less construction traffic. CLT has a much higher strength-to-weight ratio than concrete or steel, providing lighter buildings on foundations if soil conditions are less than opportune. CLT, comprised of renewable resources, contributes significantly to a building’s environmental conscientiousness, particularly in buildings seeking certification, such as LEED. Likewise, joining concrete and steel can mitigate the construction unfamiliarity and high material costs of CLT.

An example of such a building where CLT united harmoniously with concrete is the 18-story University of British Columbia’s Brock Commons Residential Hall, which uses a concrete core, CLT flooring and walls, and glulam columns. This provides both structural rigidity and seismic flexibility, construction familiarity and logistic speed, lighter loads on a foundation and stability in wind events, and many other dichotomies frequently desired when designing a building. Upon completion, it was the tallest Mass Timber building in the world and the tallest wooden building in the hemisphere. Since then, codes have changed and taller Mass Timber exclusive buildings have been built, but Brock Commons proved that CLT and concrete can integrate effectively. Other examples of structures that incorporate some combination of Mass Timber, concrete, and steel are the John W. Oliver Design Building at UMass Amherst (all three), Woodland Trust’s Headquarters (Mass Timber and steel), The Cube in London’s ShoreDitch (Mass Timber and steel), and Hoho Vienna (Mass Timber and concrete). The prototypical building of the future could likely be a combination of the trio: a building where floors, shear walls, and roofs are CLT; long span beams or beams under extreme loads are steel; and foundations and cores are concrete. Such a structure would be able to provide construction speed, strength, seismic flexibility, fireproofing, construction familiarity, and environmental consciousness that display ingenuity and serve humanity.

As with many systems, the combination of parts is more valuable than the parts individually. If concrete and steel’s preeminence led to such astounding progress in the last two centuries, imagine what adding another revolutionary material with its own set of benefits to the assembly can accomplish over the next two centuries.

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Augustus Raymond is EI Project Manager at CE Solutions.

*This article was originally published in Civil + Structural Engineer in November 2019