Proof of Concept: A Relocatable Panelized Mass Timber Modular Building System Designed for Disassembly and Reassembly

Canada has been experiencing a housing crisis for several years now with specifically Ontario having the lowest housing supply in the country. To combat this crisis, several rapid housing initiatives and projects have been created; as a result, bringing modular construction back into the mainstream. Modular construction has been the method of choice for a significant portion of rapid housing developments due to its shortened on-site construction timeline and potential for repeatability. Developments in several locations are all designed and prefabricated off-site by a manufacturer and distributed to these locations streamlining the construction process. These projects thus far have focused on the need for rapid affordable and supportive housing in city centres; however it should be noted that with the progression of climate change and its inevitable effects (e.g., fires, flooding, etc.), particularly in more remote regions with limited access to resources, the focus on providing rapid housing should be expanded to include remote regions in addition to meeting the needs of those in cities. Further, the existing rapid housing and modular construction market has gravitated towards the use of traditional light-frame construction methods, steel frame, and in some cases (mostly in literature thus far) cross-laminated timber (CLT) as well as other mass timber products (e.g., glulam). Thus, a relocatable modular building, made using sustainable methods and materials, and designed to meet the conditions in urban, rural, and remote regions in Ontario could provide a solution for rapid housing that can be assembled, disassembled, and relocated to meet the varying housing demands across the province. To focus on the environmental sustainability aspect of the concept, a prototype was designed by implementing CLT wall and floor assemblies and compared to an equivalent light-frame wood solution to assess the feasibility of using mass timber of in this relatively small-scale application. \n \nThe aim of this study is to design a panelized modular building prototype that can be disassembled, relocated, and reassembled to meet the housing demand (or demand for any other small-scale buildings) all over Ontario. A complete prototype design is conceptualized including a full panel set with associated assembly information to create three different configurations of the building. The structure consists of CLT panels and structural insulated panels (SIPs) and is designed to withstand the worst-case structural loading conditions in Ontario. A preliminary prefabricated building enclosure that would be pre-installed onto the structural panels is designed. Finally, novel connections that ensure the prototype can be disassembled and reassembled with ease are conceptually designed. \n \nAn experimental testing program was developed to evaluate the durability of the CLT assembly and compare it to a light-frame equivalent system by loading wall-to-floor assemblies using the designed connection for the CLT system and a typical hold-down for the light-frame system. The testing included two phases, the first phase consisted of a monotonic test to failure to establish the actual capacity of the system, and the second phase consisting of a round of cyclic testing to the design load to simulate a service life, a series of drop tests to induce any damage that might occur during disassembly or transportation, and finally a monotonic test to establish the new capacity of the panel when compared to the capacity found in the phase 1 testing. Ultimately, the light-frame panel lived up to its reputation as the residential structural material of choice in Ontario and was able to be reassembled for the final monotonic test through which a reduction in lateral strength of approximately 12% was observed. The CLT system, when initially monotonically tested achieved a higher maximum load than the light-frame despite being designed for the same design load. Upon conducting phase 2 testing on the CLT system it was observed that no visible damage was caused by the cyclic testing and despite incurring some damage during the drop testing, the system was still easily reassembled for the final monotonic test. Overall, the CLT system saw a reduction in lateral strength of about 20% with a different mode of failure observed between phases 1 and 2. \n \nFinally, a preliminary life cycle assessment (LCA) was conducted on the CLT and equivalent light-frame building systems to investigate specifically the embodied carbon impacts of both systems. The LCA took into account the floor and wall panels of the structure itself, the fasteners between these components, and the insulation required for each system type (as this varies notably between a light-frame and CLT system). The roof panels were omitted from the investigation as the CLT prototype considered a SIP roof which is primarily made up of the same assembly as a light-frame roof and would yield similar LCA results. Further, the foundation system is omitted but would be consistent between the two systems thus also yielding similar LCA results. The initial LCA of the building considering each structure type indicated that the light-frame equivalent had less than half the associated embodied carbon emissions as the CLT prototype. Upon applying reuse parameters in a model to assess if the CLT becomes feasible in a scenario in which it is more durable (can withstand more reuses than the light-frame), it was concluded that the embodied carbon emissions of each system do not significant vary and are therefore comparable. \n \nIt is ultimately concluded that the CLT system is a reasonable solution for a durable relocatable small building. The durability of the CLT; however, is only moderately improved over the light-frame equivalent based on the experimental testing conducted. The LCA has shown that while the system is comparable, several factors are to be considered and the outcome would ultimately depend on the duration of each use, the number of uses expected, and the carefulness with which the building is disassembled, relocated, and reassembled. Thus, while CLT provides a feasible solution to small scale relocatable buildings in certain conditions, it does not necessarily provide a clearcut improvement upon traditional light-frame construction.

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