C+S January 2020 Vol. 6 Issue 1 (web)

Cross-Laminated Timber

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 suc- cess 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, struc- tural predictability, production replicability, and various shapes and sizes. However, in recent decades, global recessions, natural disasters on rising populations, increasing sustainability concerns, and escalat- ing manual labor costs are presenting engineers with challenges that call for supplements to concrete and steel. Humanity looks to alterna- tive building solutions to overcome the obstacles of the present and future. 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

Figure 1: Cross Laminated Timber Diagram.

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 compo- nents. 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 vibra- tion 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 in- dustry, there has been some pushback against CLT’s approval, and the AEC industry (architects, engineers, and contractors) has few endors- ees of CLT’s examination. This trepidation stems from an idea that CLT

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


january 2020


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