Parametric Study and Early-Stage Structural Design for Tall Timber Buildings

Abstract

Buildings today account for a substantial portion of global greenhouse gas emissions due to the usage of carbon intensive materials such as steel and concrete. An alternative to these materials such as timber, a low embodied carbon material, has been gaining traction in the tall building industry. However, timber structures today have been limited in height due to code restrictions, lack of research and design guidance, cost, and fireproofing. Research has proved that tall timber buildings are feasible but are highly susceptible to large overturning moments and drift due to timber’s light weight and lower stiffness. A solution and trend to create better performing tall buildings with timber has been to design a hybrid structure using a mixture of timber, reinforced concrete, and/or steel in the structural system. Nonetheless, there is a lack of research and guidance taking a wholistic and comparative view into the efficiencies of these different timber structural systems at taller heights. With material quantities determining the economics and efficiency of a tall building, and embodied carbon determining its carbon footprint, this thesis conducts a parametric study to evaluate the efficiencies of multiple tall timber structural systems ranging from 10-50 stories and ultimately creates the first timber premium for height graph. Results show that core and winged wall systems, as well as braced systems, are consistently efficient up to 50 stories for material quantities and embodied carbon. The timber premium for height curve shows that the material quantity of timber required for a safe building design increases linearly when designing for gravity loads, increases linearly when designing for lateral strength, and increases exponentially when designing for lateral serviceability. For gravity loads, this quantity of timber needed is 0.65 cu.ft/sf for 10 stories, linearly increasing to 0.80 cu.ft/sf for 50 stories. For lateral loads, the quantity of timber needed is 0.68 cu.ft/sf for 10 stories, exponentially increasing to 1.15 cu.ft/sf for 50 stories. The premium for height curve also proves that all-timber building designs are controlled by lateral strength up to 20 stories, whereas from 20-50 stories, designs are controlled by lateral drift, meaning a stiffness-controlled design. Timber-hybrid systems can be used for more stiffness, but result in a 3-12% increase in embodied carbon when compared to all-timber options and excluding timber sequestration of carbon. Ultimately, these results can be useful for early-stage structural design considerations for tall timber buildings and help promote a sustainable future.