Structural performance and digital fabrication of low-cost folded sandwich structures

Abstract

This thesis develops a new structural system suitable for low-cost transitional prefabricated housing. The system utilises rotational press-fit (RPF) integral joints, a lightweight hybrid fibre reinforced polymer (FRP) timber material, and a novel folded assembly method. Together, these techniques allow for extremely high system performance, including low cost and streamlined fabrication; high-speed and uncomplicated construction; and strong and predictable structural behaviour. The new system has been developed following systematic improvement to existing features of digitally-fabricated residential housing systems, specifically systems that employ press-fit connections for plywood building component assembly. The key findings of this thesis are in three areas, summarised as follows.First, experimental, numerical, and analytical methods are used to accurately characterise the structural behaviour of press-fit integral connections. Digitally-fabricated thin-walled square hollow sections (SHS), formed by attaching four plates with press-fit integral joints, are tested under uniaxial compressive loading and shown to fail through both conventional crushing and novel pop-off bifurcation failures. Pop-off buckling behaviours are shown to be governed by integral joint transverse stiffness and its magnitude relative to a critical edge stiffness value. Columns with joint transverse stiffness values less than a critical edge stiffness value exhibit pop-off failures and comparative investigations show thin-walled sections with integral joints can match the compressive capacities of glued sections, only when crushing governs.Second, a detailed geometric design-to-fabrication procedure is presented for the new folded timber sandwich structural system. The computational method segments and unfolds a target building profile, with generated segments formed as timber sandwich panels with integral press-fit and RPF joints. Two structures are built to validate the procedure and hypothesised rapid construction speed: a 30m2 house comprising six identical folded timber arches, built in one week; and a 42m2 canopy structure comprising 4 pairs of curved timber wings, built in two weeks. The as-built structures were 3D scanned and a defect analysis was conducted to assess the reliability and precision of assembled geometries. Both structures were highly accurate, with average absolute surface error generally less than the thickness of the timber material.Third, experimental and numerical methods are used to investigate the structural behaviour of the folded sandwich arch structures. Six arches are tested to failure to validate the suppression of press-fit pop-off instability and the structural performance of the assembly method. Experimental testing of the arches show failures occurring through both FRP tensile rupture and core compressive failures. A nonlinear static analysis and simplified 2D frame model is proposed to predict moment distribution and failure load for FRP rupture modes. This model characterises the RPF joint as a nonlinear semi-rigid hinge, with assigned bilinear moment-curvature relation obtained from analysis of joint strain data collected during arch testing. Core compressive failures are shown to occur as an inelastic core buckling behaviour when there is misalignment between assembled core segments.Overall, the new structural system demonstrates a fast, accurate, and consistent built form with strong and robust structural performance. It succeeds in creating a new design solution for low-cost prefabricated system and is likely to be adaptable for construction of many other curved and lightweight building types.