The global push toward sustainable construction practices has brought renewed attention to the benefits of lightweight, materially-efficient structural systems. Within the domain of building floors, thin concrete shell geometries present an opportunity to significantly reduce the embodied carbon footprint compared to conventional bending-active systems. However, the complexity of fabrication and assembly for these funicular forms has historically posed barriers to widespread industry adoption.
Recent advancements in digital fabrication, including robotic concrete spraying techniques, have started to unlock the potential for practical implementation of concrete thin shells. One particular geometry that shows promise is the segmented fan vault, which leverages the symmetry of revolution to enable future design flexibility through increased reconfigurability. By carefully dividing the shell into discrete precast components, the assembly can be simplified while preserving the structural efficiency of the funicular form.
In this article, we will explore the design, prototyping, and testing of a reconfigurable segmented fan concrete shell system tailored for raised access flooring applications in the UK market. We will cover the form-finding and optimization process, key fabrication considerations, structural performance evaluation, and a comparative embodied carbon assessment. Finally, we will discuss the limitations, ongoing challenges, and potential pathways for further development of this novel flooring solution.
Design of the Segmented Fan Concrete Shell
The geometry of the segmented fan concrete shell is derived from the classical Gothic fan vault form, which exhibits a unique shell curvature resulting from the local radial symmetry of each conoid. This symmetry allows for a highly repetitive and modular segmentation plan, where the shell can be divided into standardized conoid segments and flat spandrels.
By aligning the segment interfaces with the hoop and strip directions of the conoid geometry, the load transfer primarily occurs through compressive membrane action, minimizing the need for tensile reinforcement. The use of dry-jointed interfaces between segments further enhances the potential for disassembly and reuse, as no permanent bonding agents like mortar or grout are required.
To enable reconfigurability, the segmented fan shell design incorporates the ability to selectively remove and replace individual conoid segments. This allows the overall span of the flooring system to be adjusted by adding or subtracting components, providing flexibility for future reuse and adaptation to changing spatial requirements. The spandrel regions can also be modified to accommodate different span distances.
http://raised-flooring.co.uk/The form-finding and optimization of the segmented fan concrete shell geometry was carried out using an evolutionary algorithm-based approach. The objectives were to minimize the overall mass while maximizing the buckling load factor, ensuring structural stability under a combination of self-weight, imposed dead, and live loads typical for office environments.
A key factor in the optimization process was the need to account for the non-linear behavior arising from the dry-jointed interfaces between segments. To address this, a novel finite element analysis methodology was developed that combines linear elastic analysis with a custom joint modeling technique. This allowed for the simulation of hinging and contact behavior at the segment interfaces, providing a balance between computational efficiency and accuracy.
The optimized design features a variable thickness profile, with the conoid segments ranging from 100 mm at the base to 75 mm at the top, and a spandrel thickness of 86 mm. This geometry was selected to achieve a minimum acceptable buckling load factor of 10, accommodating potential fabrication imperfections and ensuring a robust structural performance.
Fabrication and Assembly of Scale Prototypes
To validate the structural behavior of the segmented fan concrete shell and identify any practical limitations, quarter-scale prototypes were fabricated and tested. The conoid segments were produced using an Automated Robotic Concrete Spraying (ARCS) process, which deposits glass fiber-reinforced concrete (GFRC) onto a curved formwork surface.
The flat spandrel regions were cast using a conventional C30/37 concrete mix, taking care to incorporate the necessary shear keys for alignment and load transfer between the segments. Assembly of the prototypes was carried out using discrete wooden props to support the segments, which were then tightened using external steel tie rods to engage the arch action.
Two variations of the prototype were constructed, differing in the segmentation pattern of the conoid components. This allowed for an investigation into the effects of the number of interfaces on the overall structural behavior and assembly process.
During the assembly, some challenges were encountered due to fabrication tolerances, resulting in the need for fill material (such as sand or mortar) to close gaps between the segments. This added complexity to the disassembly process, highlighting an area for further refinement in the design and manufacturing stages.
Structural Load Testing and Numerical Analysis
The assembled prototypes were subjected to asymmetric point load testing to assess their structural performance and failure mechanisms. The first set of shells (MN1 and MN1M) exhibited a relatively low load capacity, with a maximum recorded load of 1.4 kN, equivalent to only 35% of the self-weight.
This reduced performance was attributed to the significant rotation and spreading of the corner supports, which allowed the shell to deform excessively and lose its structural integrity. In contrast, the second shell prototype (MN2M) with larger and more stable corner supports demonstrated a much higher load capacity of 9.17 kN, or 231% of the self-weight.
To further investigate the structural behavior, nonlinear finite element analysis (NLFEA) was carried out using the LS-DYNA software. The custom joint modeling approach developed during the design stage was incorporated to capture the complex interactions at the dry-jointed interfaces.
The NLFEA results showed good agreement with the experimental data for the MN2M prototype, where the material properties and support conditions were well represented. However, for the MN1 and MN1M tests, the numerical model struggled to fully capture the effects of the unstable support conditions, highlighting the sensitivity of the segmented shell system to its boundary conditions.
Embodied Carbon Comparison
To assess the sustainability potential of the segmented fan concrete shell, a cradle-to-gate embodied carbon analysis was performed, comparing it to conventional flooring systems such as reinforced concrete slabs, voided hollow decks, and the previously developed ACORN thin-shell concrete floor.
The results indicate that the segmented fan concrete shell achieves a 13% reduction in embodied carbon compared to a flat reinforced concrete slab, while the ACORN shell demonstrates a 27% reduction. However, the added constraints of designing for reconfigurability result in a 19% embodied carbon premium for the segmented fan shell compared to the non-reconfigurable ACORN system.
Importantly, the analysis demonstrates that the potential for reuse and reconfiguration of the segmented fan shell can offset this premium, provided the components are effectively reused over the building’s lifetime. Further improvements in the sustainability of the GFRC material composition could also yield additional embodied carbon savings for the segmented fan system.
Conclusions and Future Work
The segmented fan concrete shell presents a promising approach to addressing the sustainability challenges of building floors through the use of lightweight, funicular shell geometries. The modular design and dry-jointed interfaces enable disassembly, reuse, and reconfiguration, aligning with the principles of a circular economy for the built environment.
The prototyping and testing of scale models have provided valuable insights into the structural performance and practical limitations of the system. The sensitivity to support conditions and fabrication tolerances emerged as key factors influencing the load-bearing capacity, highlighting the need for further refinement of the design, assembly, and manufacturing processes.
Moving forward, additional research is required to investigate alternative joint interface details, such as the incorporation of mechanical fasteners or unbonded post-tensioning, to improve the reliability and predictability of the force transfer between segments. Tuning the embodied carbon of the GFRC material composition should also be a priority to fully realize the sustainability benefits of the thin-shell geometry.
Beyond the structural and material aspects, further work is needed to address the integration of building services, fire safety, acoustics, and overall constructability to ensure the segmented fan concrete shell is a viable and attractive option for the UK’s raised access flooring market. Addressing these multifaceted design challenges will be crucial in translating this novel flooring system from the laboratory to real-world applications.
Overall, the segmented fan concrete shell represents a promising step toward more sustainable, adaptable, and materially-efficient building floors. By leveraging the structural efficiency of funicular forms and the flexibility of modular, reconfigurable design, this system has the potential to contribute to a future of circular construction in the built environment.
