Testing the Compressive Strength of Timber Lattice Columns for Low Rise Construction
Timber is now acknowledged as a material used to build ‘healthier hospitals’ (TRADA 2017) due to its environmental benefits and therapeutic qualities. Three full-scale column prototypes consisting of diamond, triangular and circular lattice infills were fabricated and tested to determine whether the geometrical arrangement of the internal lattice structure affected the compressive strength of the column. The purpose of this research was to undertake research into lightweight structural timber elements with varied lattice infills for low rise construction, particularly for hospices and palliative care facilities where uplifting spaces are essential for therapy, treatment and end of life care.
The study was highly important to assess whether the columns could satisfy the structural requirements needed to support the CLT roof system. Results found that all columns performed well beyond the minimum structural requirements to support a roof load of kN/m2 which means each column can be considered safe for use in construction. Findings concluded the porosity volume (cellular voids) had a significant impact on the compressive performance, the lattice column with the circular lattice proving to be the highest performing column failing at 31.46kN (3.16 tonnes), due to increased surface area of material. Nevertheless, the diamond lattice infill column was an efficient lightweight alternative.
Compression tests commenced on all three columns until each column reached complete failure with the minimum roof load threshold of 2 kN/m2 considered. Each timber lattice column was positioned horizontally on a large ‘I’ beam testing rig apparatus. The wide cantilevered tip of the column was secured into the loading jack connected to the hydraulically controlled loading cell which applied compressive load. The loading cell controlled the rate of load with two linear displacement transducers used to measure displacement. The transducers were positioned underneath the column to measure the flexural bending behaviour of the column as compressive load was applied whilst a transducer located on the loading cell assessed how each column performed under stress observing for evidence of delamination and deflection in the columns.
The lattice columns were unexpectedly flexible as they showed signs of torsional strength where the column started to twist before rupturing into complete failure. However, the test exposed the performance materials when undertaking material analyses, the modes of failure were the determined by delamination as the buckling force led to the wood veneer to splitting which then transpired to the adhesive failing and causing the layers to separate which led to material failure of the WBP plywood. All prototypes were fabricated using three standard- sized sheets of water boil proof plywood dimensioned at 1220mm x 2440mm, the columns were designed to fit on standard ‘off-the-shelf’ sheets supplied by manufacturers in the UK. The key focus of the test was to assess the correlation between the lattice geometry and compressive strength as opposed to examining the quality of material. Plywood would not be appropriate for palliative care facilities.
The project embraced CAD programmes and construction technologies throughout to aid the fabrication process. The lattice column profiles were drafted as a two-dimensional drawing using AutoCAD software during the preliminary stage to aid the laser cutting manufacturing process of the 1:1 (full-scale) prototypes components before transitioning onto three-dimensional modelling software, Sketch-Up, to gain an understanding of the volume of the column which forms the portal frame system. The use of CAD was an instrumental factor towards the preparation for the fabrication process. This practice was about maximising the use of CAD to design prefabricated elements accurately to allow a faster and more efficient building construction process. Each lattice column consisted of three components that were adhesively bonded together.
The project embraced inclusive design and building performance for end users and the functional design solutions to create prototypes that were manufactured, assembled and tested. In particular, the laboratory testing of the timber lattice columns was conducted to determine the optimum share of the lattice infills. An extremely innovative study embracing the spectrum of Architectural Technology with a particular emphasis on the importance of structural elements not only as the primary function but optimisation including use.