Structural Performance of Prefabricated Straw Bale Panels

Abstract

The public and construction industry professionals are becoming increasingly aware of the environmental damage buildings can cause. As a result the use of more sustainable and environmentally friendly building materials and techniques is increasing. Straw bale construction is one such technique. ModCell panels are an innovative way of using straw bales in a prefabricated wall system. As ModCell panels are prefabricated the normal risks associated with straw bale construction (fire, straw getting wet during construction, unfamiliar material) are removed from the building site. The panels consist of a timber frame, braced with steel rods, in-filled with straw bales and the surfaces are then rendered with a lime render. To date ModCell panels have been used for cladding on framed buildings. However, they are now being developed for low rise domestic load-bearing applications and in order to achieve this sufficient racking shear resistance is required. Previous testing has shown that the corner joints in the panel’s timber frame are a weak point. This first aim of this thesis is to improve the racking shear resistance of ModCell panels through a series of experimental laboratory based tests so that they can be used for load-bearing applications. This thesis also aims to develop a computer model of load-bearing ModCell panels in order to further understand their structural behaviour and assist with future design and development. Different designs of frame corner joints were tested in order to find the most suitable type of joint. Screw connected joints proved to be the strongest and were twice as stiff as the other joint designs tested. Full scale panel racking shear tests were undertaken to assess the overall performance of the panels when using the screw connected joints and establish if they are suitable for load-bearing construction. Both solid panels and panels with window openings were tested. Two types of bracing were used in the test panels, either corner bracing or cross bracing. The corner braced panels were over twice as stiff as panels tested during a previous research project. The corner braced panels were also stiffer than the cross braced panels at the serviceability deflection limit of the panels height/500, and was sufficiently stiff to resist the required in plane loading under serviceability conditions. The computer model was developed alongside the structural testing. The model was developed using the Robot Millennium software package and modelled all elements of the panels apart from the straw. It was verified against the laboratory test results and found to be accurate within the serviceability deflection limit of height/500. Once material failure occurred in the actual panels the model’s accuracy was reduced as it does not include material failure criteria. The model was then used to undertake a parametric analysis. From this analysis it was found that all the elements modelled have an effect on the stiffness of ModCell panels. Changing the render thickness was found to make the greatest difference on the overall stiffness of the panels. Gusset plate and glued and screwed joint designs were also modelled to find their effect on panel stiffness. From all of the results of this analysis an improved panel was designed and when modelled was 20% stiffer than the original panel.

Date of Award	1 Jul 2009
Original language	English
Awarding Institution	University of Bath
Supervisor	Pete Walker (Supervisor)