VOL. 5, NO. 4, APRIL 2010 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences © 2006-2010 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com DEVELOPING A NEW COMBINED STRUCTURAL ROOFING SYSTEM OF DOMES AND VAULTS SUPPORTED BY CEMENTITIOUS STRAW BRICKS Gihan L. K. Garas, Hala G. El Kady and Ayman H. El Alfy National Research Centre, Civil Engineering Department, Dokki, El Tahrir St., Cairo, Egypt E-Mail: gihangaras@yahoo.com ABSTRACT The primary objective of this research was to develop a structural technique for roofing rice straw buildings by domes and vaults through encouraging the specification of innovative materials which may be more sustainable than their traditional counterparts. This paper examines the effectiveness of using rice straw cementitious posts and walls as an innovative natural construction method to support domes and vaults in straw bale buildings. Three proposed mix ratios for rice straw cementitious bricks were mechanically tested. A finite elements analysis of the proposed roofing system was performed, to check the safety of the roofing material, and their resultant reactions on the rice straw bricks support. Different cases of loading were applied including wind and settlement. Obtained results helped in the decision making of the most economical and environmental friendly brick mix. Besides, the results emphasized that all stresses were within acceptable limits of the Egyptian code of practice. Keywords: domes, vaults, roofing system, rice straw bricks, building materials, finite element analysis. 1. INTRODUCTION From adobe to straw bales, traditional building materials are being adapted to meet code-required standards for health and safety in contemporary buildings. These natural alternatives match the strength and durability of many mainstream construction materials, as well as being cost effective and environmentally friendly. 1.1 Straw bale structural systems There are different ways of building with straw bales among which are the load bearing, light weight frame and load bearing, infill and timber frame and hybrid design. 1.1.1 Nebraska also called load bearing This is the original method of building, pioneered by the Nebraskan settlers in the USA. In this method, the bales themselves take the weight of the roof - there is no other structural framework. One of the most important design features of a load bearing straw bale house is to distribute the loads as evenly as possible around the whole building. Never use point loads. The bales are pinned to the foundations and to each other with coppiced hazel, and have a wooden roof plate on top. The roof plate is fastened to the foundations and the bales with coppiced hazel and strapping, and the roof is constructed in the usual manner on top of the roof plate. The main advantage of this style of building is its simplicity, ease of design, minimal use of timber, and the opportunity it affords for a modern day wall raising. While it is not favoured due to the constraints that limits the openings for windows and doors so as not to exceed 50% of the wall surface area in any wall. Also the maximum unsupported (un-braced) wall length is 6m (20 ft) [1]. 1.1.2 Infill also called post and beam or timber frame In this method, the weight of the roof is carried by a wood, steel, or concrete framework, and the bales are simply infill insulation blocks between the posts. This has often been the preferred option for architects, as the structural concepts are not innovative and rely on an already established method of construction; therefore the risk associated with an experimental technique is minimized. There is no need to satisfy oneself of the capacity of the bales to take the weight of the roof, since the framework does this. This method requires a high level of carpentry skill and uses substantially more timber than a load bearing design, which has significant cost and environmental implication [1]. 1.1.3 Light weight frame and load bearing This design has been pioneered by Barbara Jones of Amazon Nails as a way to retain the benefits of the load bearing style, yet enabling the roof to be constructed before the straw walls are built, thus giving protection against the weather throughout the wall-raising process. It uses a timber framework that is so light-weight that it cannot stand up alone. It requires temporary bracing to give it stability until the straw is in place. The straw is an essential part of the structural integrity of the building, more so than the timber, and it works together with the timber to carry the load of floors and roof. Timber posts are located at corners and either side of window and door openings only, and are designed such that the timber wall plate at first floor and/or roof level can be slotted down into them once the straw is in place allowing for compression on the bales. Compression of the straw bale infill walls is essential for stability [1]. 1.1.4 Hybrid method Here the bales are used much more like conventional brick walls, with cement mortar holding 44