|Solid isotropic material with thickness penalisation – an additive manufacturing-oriented structural topology optimisation method with a 2.5d approach
|Usmani, Asif (BEEE)
|Construction industry -- Environmental aspects
Hong Kong Polytechnic University -- Dissertations
|Department of Building Environment and Energy Engineering
|xviii, 188 pages : color illustrations
|Climate change is a pressing global issue, and there is a growing demand for carbon-neutral manufacturing processes and sustainable products. However, the construction industry has been slow to adapt its traditional, polluting, and wasteful practices. Compared to other manufacturing industries, the construction industry has been resistant to change and has not embraced the use of templates, prototypes, and economies of scale. This has resulted in a predominantly one-off construction process that is environmentally damaging.
Architects and designers often propose highly personalised and ambitious designs that can pose challenges for engineers responsible for structural and building systems design. To address this issue, engineers have proposed precast, prefabricated, or modular construction, but this severely limits architectural expression. As a result, the construction industry has not fully embraced this approach, and it remains a fringe segment of the industry.
To provide a novel and alternative solution to the traditional structural design process, this thesis proposes an additive manufacturing-oriented optimisation strategy. This approach aims to optimise material usage, provide greater freedom for architectural creativity and expression, and reduce the environmental impact of construction. The optimisation tool used in this thesis is inspired by the widely accepted SIMP method.
SIMP is a commonly used topology optimisation method for minimising material utilisation in structural components using element densities as a design variable. However, this approach has limitations when it comes to additive manufacturing, as it requires voxels instead of pixels. The density design variable restricts the shape outcomes to pixels, making it difficult to use SIMP for additive manufacturing. Additionally, the use of density as a design variable can cause issues with the stiffness matrix's positive definiteness. A possible alternative to the density design variable is a geometric parameter (such as web thickness in beams), which could avoid the limitations of the former. Accordingly, a new optimisation methodology called Solid Isotropic Material with Thickness Penalisation “SIMTP” is developed using thickness as a design variable. To explore the designs with finite element analysis, a 2.5D element has been introduced. The 2.5D element is based on a 2D planar transformation with varying nodal thicknesses, which allows 2D strain energy to be projected onto a 3D space.
The 2.5D SIMTP approach includes a 2.5D element that allows exploration of a variety of design models, including cantilever, MBB, and L-beams, without experiencing issues related to checkerboarding or other topology-related problems. An adaptive refinement strategy has also been implemented to refine elements with high-thickness gradients and negative energies. However, it was found that this alone can lead to the islanding phenomenon, which was not observed until after filtering. Overall, 2.5D SIMTP provides a more efficient and cost-effective way to achieve desired design outcomes with fewer elements, reducing computational costs. Additionally, it can bridge the design coordination gap between architects and structural engineers by using the architect's vision as the foundation for the design process.
This thesis draws inspiration from the renowned architecture of Antoni Gaudi in Barcelona to create a framework that allows for greater freedom in raw architectural expression. The framework involves optimising the structural component shapes imagined by architects for cost, weight, structural function, sustainability, and aesthetic appearance. By doing so, buildings can be constructed with structural components of distinct non-prismatic and even organic shapes, resulting in a spectacular range of architectural styles that can be fabricated using additive manufacturing. In this thesis, an application of this process is demonstrated using 2.5D SIMTP, where an optimised MBB beam prototype is 3D printed at PolyU's U3DP laboratory using ABS M30i material. Additionally, in an effort to explore practical aspects of 3D printed concrete structures, 2.5D SIMTP has been extended to optimise prestressed beams, where the cable and concrete shapes are simultaneously optimised. Several prestressed problems have been explored using 2.5D SIMTP, including single, two, and three-span beams.
To aid in education and future research, MATLAB codes developed throughout the project are presented at the end of this thesis. Overall, this framework provides a new way to approach architectural design and construction, pushing the boundaries of what is possible and allowing for greater creativity and expression in building design.
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