Experimental Study on Traditional Assam-Type Wooden House for Seismic Assessment

Abstract eng:
Assam-type wooden houses are one of the very few housing systems in India that have performed exceptionally well during past earthquake shakings. Due to the historical high seismicity of the north-eastern region of India, the local people developed a unique construction methodology using locally available materials to construct their dwellings that are highly earthquake resistant. Such houses are commonly known as Assam-type houses or Ikra houses. These houses are easy to construct, maintain, and economical. The walls of Assam-type house are the most highlighting feature of this typology, as they are made of light-weight Ikra or bamboo mesh due to which such houses do not require extensive foundations, and therefore, can be used in varied geological conditions. The housing is known to have a number of features that contribute to the safety of the house during an earthquake. These houses have regular and symmetric configuration in plan, smaller openings, and small projections and overhangs. The structural features include very light mass of walls and roof, good wallto-wall connection (in case of formal construction), and good quality and strength of materials used. Another unique feature of these houses is flexible connections (nut-bolts, nails, grooves, etc.) used for different wooden elements at different levels. In spite of all these exceptionally good features these houses have not received due attention and their performance under seismic action has not been studied so far. The uniqueness of this type of house from other traditional wooden houses lies in its construction methodology, framing members, special type of connection between framing members, light-weight walls, and foundation. A description of the material used, typical construction methodology adopted, and the lateral load performance of the Assam-type house are discussed in this paper. In the current study, individual full-scale frames of a typical single-story Assam-type house were tested under monotonic and slow cyclic lateral loads to gain an insight into the lateral load behaviour of such houses. The experimental results showed excellent behaviour in terms of drift and ductility. Both wooden frames, without ikra walls and with ikra walls, exhibited very high lateral displacements without significant drop in lateral load carrying capacity. Failure of the frames was primarily due to separation of various secondary members at connections and flexural failure of vertical posts at high lateral drift levels. Interestingly, the connections between the main posts and the foundation and between the main posts and the top beam remained intact during the tests. Hysteretic behaviour of the frames showed stable and more or less symmetric loops with low energy dissipation due to pinching. The excellent performance observed can be attributed to the unique type of construction with flexible connections and light-weight material used in construction. Experimental data thus obtained can be used to develop an analytical model for seismic assessment of such houses.

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