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This earthy material can withstand earthquakes

ASWATH M.U.

Wood is versatile, with a well-established tradition of being affordable and durable, if properly seasoned, says ASWATH M.U.



Ready for sale: Palm tree wood used for constructing houses.

A peep into history will reveal the use of wood as a construction material and dates back to the Vedic period. Houses were made of wood and bamboo. Roofs were thatched and walls were made of reed bundles in wooden framework. For the construction of palaces, wood was used where stones were scarce.

The use of mud, thatch, bamboo and timber in the valleys of Ganga and Saraswati is well recorded. Use of structural timber in the form of roof trusses, floor joists, rafters, floor panels and columns were also seen in many of our historic buildings. Even some of the temples in Kerala and southern and coastal parts of Karnataka are built with wooden roofs and covered by tiles.

In the foothills of the Himalayas and north-eastern parts of India, people have built their houses in timber and wooden planks.

Types of wood/timber

There are over 150 species of timber which are used for engineering purposes in the construction industry. Basically timber/wood is classified into hardwood and softwood.

Hardwood is obtained from trees that lose their leaves in winter.

The most common hardwoods are rosewood, teak, mahogany, walnut, oak, maple and cherry. Softwood is obtained from evergreen trees such as fir, pine, redwood, cedar and hemlock spruce.

Rosewood and teak are popular.

Rosewood is close grained with dark reddish brown colour with an exclusive fragrance. It is hard to work upon and takes very good polish. It is good for making musical instruments, piano cases, tool handles, art projects, veneers and furniture. Teak is a hard and moisture-resistant wood. It resists warping, cracking and decay and is best used in furniture, panelling, shipbuilding, doors, window framing, and flooring and as a general construction wood.

Panels and wood composites (engineered wood)

Wood composites, also known as man-made wood or manufactured boards, are more popular these days because of their value-added advantages. Some of them are mentioned below:

Plywood: It is made from thin laminates of wood glued together. Each layer is at right angle to the grain of the other. It is very strong but also quite flexible, especially if there are thinner sheets. It is used widely in the building industry and is available in various grades including general purpose plywood, decorative plywood, marine plywood, shuttering plywood, structural plywood, preservative treated plywood, fire-retardant plywood and flush doors.

Wooden sheeting (4 mm to 24 mm), MDF (Medium Density Fibre) board: This is made from powdered wood bonded with glue and compressed to form the sheets. It is quite soft and very easy to work with. It cuts, sands and finishes very easily. It is used widely for interior projects, especially for cupboards and shelving.

Chipboard: It is made like MDF but from actual wood chips. It is used widely for kitchen furniture for which it is covered with a laminate. It is also used widely for low-cost flooring.

Particleboards: They are formed from agricultural residue, jute stick, rice husk, Portland cement bonded particleboard etc.

Standard hardboard: Tempered hardboard, prefinished hardboard.

Laminated veneer: Laminated panel board, glued laminated wood.

Block board, Insulation board and Bamboo mat board etc.

Wood I-Joists and Glu-lam Beams: Designed for a particular structural purpose.

Uses

Wood is a very versatile material with a long and well-established tradition of being affordable and durable for many applications if properly seasoned. It is widely used in artefacts, boards, boxes, building construction, chest making, closet lining, crates, dock planks, doors, fencing, frames, furniture, general millwork, interior finishing, making wooden boxes, masts and spars for ships, aircraft, molding, novelties, outdoor furniture, panelling, planks, plywood, posts, scaffolding, shingles, siding (cladding), staircases & ladders, structural beams and columns, sub-flooring, veneer, venetian blinds windows and many more.

Construction types

As I have stressed in my earlier articles, engineered construction is very important and one should consider the soil conditions, foundation types, land slopes, building dimensions, orientation, configuration and various forces acting on the building including earthquake force. Wood seasoning, preservation and protecting against termites is also very essential. Wood was used as the main construction material in the Himalayan ranges and north-eastern parts as it was locally available.

But the traditional construction methods were not effective even though they evolved over the years.

There is a need for scientific methods as these areas fall under seismic zones 4 and 5. The kat-ki-kunni (timber-cornered building) type of construction found in Himachal Pradesh, Kulu and Kangra valley showed good performance during earthquakes.

Scientists such as A.S. Arya, head of the quake engineering cell of the Bureau of Indian Standards (BIS), have improvised and transformed traditional designs like the kat-ki-kunni into contemporary typology of continuous reinforced tie-bands.

The main construction types used worldwide are:

Thatch construction (traditional construction type)

Post-and-beam frame construction

Walls with bamboo/reed mesh and post (waffle and daub)

Wooden frames with or without infill

Stud-wall frames with plywood/gypsum board sheathing

Wood panel construction and log construction.

The above wood construction methods are mainly used in the United States, Canada, Japan, Russia and other earthquake-prone regions. In India we find traditional methods in northern parts and Himalayan regions for dwellings and in south India mainly for temple architecture. The earthquake resistance of wood-frame structures is relatively high, if the quality of materials and the construction are satisfactory.

The buildings are generally lightweight, compared to brick or stone, which helps reduce earthquake forces on the structures. Many multi-storeyed constructions locally known as Sumer, Chaukhat or Kothi have withstood a number of earthquakes. Forest Research Institute (FRI), Dehradun, and Indian Plywood Industries Research & Training Institute (IPIRTI), an autonomous body of the Union Ministry of Environment & Forests, are carrying out research and development on timber engineering, seasoning of timber, timber mechanics, preservation of timber, wood working and earthquake-resistant timber and bamboo construction technologies.

Even though wooden construction is in practice in some parts of the country, the industry is facing problems due to non-availability of wood at affordable cost. Wood is mainly used for doors, windows, furniture and interior works all over the country.

Gap

In India, the demand for industrial timber and pulpwood is more and there is a gap between demand and supply. We are importing hardwood and logs from tropical countries to meet our requirements.

The price of imported timber is less than indigenous timber and is helping in stabilising the prices of timber in the local markets.

For sustainability we need to manage our forests and forest recourses effectively.

(With inputs from FRI, ICFRE and Ministry of Environment & Forests)

[The author is Professor at Bangalore Institute of Technology and Secretary General-ACCE (I)]

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