Compressive load resistance of straw bale assemblies under concentric and eccentric loading

To reduce the significant carbon emissions associated with construction materials, there is growing recognition of the potential for bio-based resources, including materials derived from agricultural crops such as straw, hemp and flax. The biogenic carbon stored with plant based materials can significantly reduce carbon emissions compared to other products. Uses of bio-based materials are however generally limited to non-structural, in particular as insulation, but in contrast to many other solutions straw bales can also be used structurally in lowrise construction. Despite the use of straw bales for over 100 years there is still relatively little known about their compression performance, and there are no recognised guides or standards to support structural engineers. This paper describes an experimental study into the load carrying capacity of straw bale wall assemblies. In keeping with practice in the UK, and different to previous North American studies, the wall plates do not directly bear onto the plaster coats. The aim of the paper is to characterise and compare the vertical load carrying performance of straw bale walls without and with plaster coatings, and study the effect of wall height, plaster use, and load eccentricity on compressive resistance. A series of wall assemblies were built from stacking 1 m long wheat straw bales. Wall assembly heights varied with the number of bale courses: one, two, three and four bales high. In total results from 29 tests are presented. Key findings of the study concern the influence of load eccentricity, use of plaster and wall geometry on compression performance. In the absence of national guidance, the results of this experimental study will support structural engineers designing loadbearing straw bale walls.