Framework for the Strategic Management of Dimensional Variability of Structures in Modular Construction
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Challenges in construction related to dimensional variability exist because producing components and assemblies that have perfect compliance to dimensions and geometry specified in a design is simply not feasible. The construction industry has traditionally adopted tolerances as a way of mitigating these challenges. But what happens when tolerances are not appropriate for managing dimensional variability? In applications requiring very precise dimensional coordination, such as in modular construction, the use of conventional tolerances is frequently insufficient for managing the impacts of dimensional variability. This is evident from the literature and numerous industry examples. Often, there is a lack of properly understanding the rationale behind tolerances and about how to derive case specific allowances. Literature surrounding the use of tolerances in construction indicates that dimensional variability is often approached in a trial and error manner, waiting for conflicts and challenges to first arise, before developing appropriate solutions. While this is time consuming, non-risk averse, prone to extensive rework and very costly in conventional construction, these issues only intensify in modular construction due to the accumulation of dimensional variability, the geometric complexity of modules, and discrepancy between module production precision and project site dimensional precision. This all points to a need for a systematic and strategic approach for managing dimensional variability in modular construction. This thesis explores dimensional variability management from a holistic construction life cycle viewpoint, examining key project stages (manufacture, fabrication, aggregation, handling, transportation and erection) to identify critical variability sources and proposing adequate strategies to control dimensional variability. The scope of this work relates primarily to the structural system of commercial building modules, based on the assumption that the sequence of production and dimensional variability of building subsystems (mechanical, electrical, plumbing, architectural) hinge upon the dimensional variability of the structure. A novel method for quantifying dimensional variability is developed, which uses 3D imaging by way of laser scanning and building information models to compute deviations between the intent of a geometric design and the actual as-built construction. Novel strategies for managing dimensional variability are also developed, and include adaptation of manufacturing-based principles and practices for use in construction systems. The inspiration and foundation of these new strategies is derived from the original research of Dr. Colin Milberg, who explored how to apply tolerance theory used in manufacturing into civil construction systems. The new techniques developed in this thesis, along with other previous research, demonstrate that there is a clear correlation between manufacturing industries such as aerospace and automotive assembly production, and that of modular construction assembly production. In light of this, there is an opportunity to improve modular construction processes if these manufacturing-based methods can be appropriately implemented. This is the basis for the proposed methodology presented in this thesis. Application of the proposed methodology using case study examples demonstrates that dimensional variability in modular construction should be approached from a holistic viewpoint. Furthermore, it needs to incorporate much more consideration into the key factors and critical sources of variability rather than pursuing the traditional construction approach of developing inefficient trial and error solutions.