Structural Benefits of Hempcrete Infill in Timber Stud Walls

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Mukherjee, Agnita
Hempcrete , Column on Elastic Foundation , Timber Column
Due to an increase in environmental consciousness in the construction industry there has been increasing interest in using natural building materials like rammed earth construction, straw bale construction and hempcrete construction. These materials have very small embodied energy and are carbon negative or carbon neutral. Hempcrete is a bio-composite mix made up of hemp shive, lime, cement and water. There has been extensive research undertaken on the thermal, acoustic, and mechanical properties of hempcrete. It has low density, good thermal and acoustic insulation properties, and can passively regulate humidity in a built environment. However it also has low compressive strength and modulus of elasticity. Thus hempcrete cannot be used as a direct load bearing material but can used as an infill material in timber stud walls. This thesis investigates how hempcrete can have beneficial structural effects by preventing buckling of timber (increasing the buckling load) columns when used as an infill in between columns in a timber stud wall. Eight timber walls were constructed (both half scale -1200mm high; and full scale – up to 2133mm high) with varying column dimensions. Six of the walls were infilled with hempcrete of varying density. Two walls were not infilled and were baseline tests. All walls were tested in compression. It was found that high density hempcrete (715 kg/m3) not only prevented weak axis buckling of columns but also carried some direct load. Low density hempcrete was also successful in preventing weak axis buckling of the infilled walls. In-filled walls failed in strong axis buckling at a load twice (for half scale walls with 38x89mm columns) or 4 times(full scale walls with 38x235mm columns) that of the unfilled walls. An analytical model based on buckling of a strut on an elastic foundation was proposed to predict the maximum strength hempcrete can add to a wall by preventing buckling. The timber column strength equations from CSA –O86-01 closely predicted the buckling load of the walls from the experiments. The results from the model were also put in design equations from CSA –O86-01 to predict practical failure loads if the walls failed by buckling into hempcrete. Although only a single test result was available, it was in good agreement with the model predictions.
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