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The Earthquake Resistance of Cob Structures

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The Earthquake Resistance of Cob Structures University of Technology Sydney Faculty of Engineering THE EARTHQUAKE RESISTANCE OF COB STRUCTURES by Jean-Michel ALBERT-THENET Student Number: 10186397 Project Number S08-101 In partnership with Luke PUNZET Student Number: 10320570 Project Number S08-137 Supervisor: Prof. Bijan Samali A 6 Credit Point Project submitted in partial fulfilment of the requirement of the Degree of Bachelor of Engineering November 2008 i STATEMENT OF ORIGINALITY I certify that the contents of this report is the solely the work of myself, Luke Punzet, and Jean-Michel Albert-Thenet Work relating to the physical experimentation of the project has been co-authored while the authorship of sections detailing background research is as outlined in the introduction. All text, theories or results that are not the result of our own research have been duly acknowledged. Luke PUNZET November 2008 i THE EARTHQUAKE RESISTANCE OF COB STRUCTURES ABSTRACT This project set out to conduct research on cob as a sustainable, low-cost and earthquake-resistant building material for use in developing regions, using the Philippines as a case study. To do this scaled test structures were built and tested under seismic conditions to determine the level of structural integrity that can be expected from cob structures, to provide a comparison of different reinforcing conditions, and to evaluate how this method of construction compares to alternative earth building methods. These structures were designed to utilise Oregon cob, a material consisting of clay, sand and straw to create monolithic walls. The role of our research was to determine the structural suitability of cob in meeting the earthquake conditions of the Philippines and across South East Asia. Conventional building methods typically used in developed nations are generally based on the assumption of high labour costs relative to the cost of materials or transport; however, in developing regions the methods are often not economically viable. The design used in our study has been developed to utilise only readily available materials and to reduce transport and machinery costs. Similarly, the design exploits cob's thermal mass properties to regulate internal temperatures. To determine how Oregon cob reacts under seismic conditions, two 1:2 scale structures were constructed, each weighing over one tonne. These structures were built in-situ in the Structures Libratory of the University of Technology, Sydney and were then subjected to simulated earthquakes. By maintaining dimensional consistency with existing research on reinforced mudbrick methods, we were able to draw comparisons between the structural performances of the two materials. Our findings indicate that cob offers a much higher seismic resistance compared to mudbrick. This, in addition to constructability advantages, makes cob an arguably superior building method for the people of the Philippines and South East Asia. TABLE OF CONTENTS LIST OF FIGURES .......................................................................................................... 2 1 INTRODUCTION ......................................................................................................... 2 2 EARTH BUILDING ...................................................................................................... 2 2.1 The History of Earth Building................................................................................. 2 2.2 Earth Building Materials ......................................................................................... 2 2.2.1 Base Materials .................................................................................................. 2 2.2.2 Additives .......................................................................................................... 2 2.3 Advantages and Disadvantages of Earth Building .................................................. 2 2.3.1 Advantages ....................................................................................................... 2 2.3.2 Disadvantages .................................................................................................. 2 3 EARTH BUILDING IN DEVELOPING COUNTRIES ............................................... 2 3.1 Comparison of Earth Building Methods ............................................................. 2 3.1.1 Mudbrick (Adobe) ............................................................................................ 2 3.1.2 Pressed Mudbrick (Compressed Earth Blocks)................................................ 2 ii THE EARTHQUAKE RESISTANCE OF COB STRUCTURES 3.1.3 Rammed Earth (Pisé) ....................................................................................... 3 3.1.4 Poured Earth (Cast Earth) ................................................................................ 3 3.1.5 English Cob ...................................................................................................... 3 3.1.6 Oregon Cob ...................................................................................................... 3 3.2 The Philippines as a Case Study ............................................................................. 3 3.2.1 Economic Considerations ................................................................................ 3 3.2.2 Social and Environmental Considerations ....................................................... 3 3.3.3 Regions ............................................................................................................. 3 3.4 Available Building Materials .................................................................................. 3 3.4.1 Conventional Building Materials ..................................................................... 3 3.4.2 Vegetation ........................................................................................................ 3 3.4.3 Soil ................................................................................................................... 3 4 EARTHQUAKES .......................................................................................................... 3 4.1 The Nature of Earthquakes...................................................................................... 3 4.2 Earthquake Severity ................................................................................................ 3 4.2.1 Magnitude ........................................................................................................ 3 4.2.2 Intensity ............................................................................................................ 3 4.3 Historical Earthquakes ............................................................................................ 3 M7.8 El Salvador, 13 January 2001 .......................................................................... 3 4.4 Earthquake Effects .................................................................................................. 3 5 DESIGN CONCEPTS .................................................................................................... 3 5.1. Material Suitability................................................................................................. 3 5.2 Testing Limitations ................................................................................................. 3 5.2.1 Comparability ................................................................................................... 3 5.2.2 Project Scope .................................................................................................... 3 5.2.3 Reduced Scale .................................................................................................. 3 5.3 Other Considerations ............................................................................................... 3 5.4 Alternatives not adopted ......................................................................................... 3 6 HYPOTHESIS ............................................................................................................... 3 7 EXPERIMENT PREPARATION .................................................................................. 3 7.1 Specimen Design ..................................................................................................... 3 iii THE EARTHQUAKE RESISTANCE OF COB STRUCTURES 7.1.1 Specimen Specifics .............................................................................................. 4 7.1.2 Support Structure ............................................................................................. 4 7.1.3 Reinforcement .................................................................................................. 4 7.1.4 Wall Restraint................................................................................................... 4 7.2 Earth Building Process ............................................................................................ 4 7.2.1 Mixture Composition ....................................................................................... 4 7.2.2 Mixture Testing ................................................................................................ 4 7.2.3 Mixture Preparation ......................................................................................... 4 7.2.4 Cob Building .................................................................................................... 4 8 EXPERIMENT PROCEDURE ...................................................................................... 4 8.1 UTS Shake
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