Bamboo roof structure
The main advantages of using bamboo for roof constructions are:
• It is a traditional technology, which is well understood by local artisans. No special tools are required.
• The large-scale utilization of bamboo has no disastrous environmental consequences (as in the case of timber), on account of its quick replacement within 4 or 5 years.
• The physical properties of bamboo make it an ideal construction material for seismic areas.
• Compared with most other building materials, bamboo is cheap to buy, process and maintain.
There are, however, drawbacks that need to be overcome, for example:
• limited durability, mainly on account of excessive wetting and drying, insect and fungal attack, physical impact, and wear and tear;
• limited social acceptability, as a result of the limited durability of bamboo.
Further information: Bibl. 13.05, 13.06, 13.07.
Barrel Vault (Bibl. 13.05)
• This construction system was developed at the Research Laboratory for Experimental Construction, Kassel College of Technology, Federal Republic of Germany, headed by Prof. Gernot Minke.
• It demonstrates an unusual use of bamboo, in which the construction obtains its stability by compressive forces, acting perpendicularly to the bamboo's axis.
• On the principle of masonry barrel vaults, full-section bamboo culms are laid horizontally, one on top of the other following a curve, defined by an inverted catenary. (This is a curve formed by hanging a uniform chain freely between two points. The tensile forces induced by gravitation run along the line connecting the points of contact of each chain link. Since the curve remains stable when reversing the direction of forces, an inverted catenary is the ideal shape of a barrel vault.)
• Split bamboo strips of equal length are hung such that their ends are exactly the same distance apart as the ultimate roof span. The full-section bamboo culms are laid horizontally forming an inverted vault. Split bamboo strips are then laid on the inside, exactly opposite the outer ones. Holes are drilled through the split and whole bamboo and fixed by bolts or rivets.
• The whole structure is then turned over and fixed on the top of the walls, which preferably should have a timber or concrete ring beam, onto which the roof is connected.
• The roof should be covered with a waterproof membrane for rain protection. This can be covered by a suitable local thatching material, or more appropriately by a 10 cm layer of soil on which grass can grow. For initial reinforcement (to prevent slipping) the soil should be held down by a strong net (as used for fishing). The dense structure of the grass roofs will give the soil cover its ultimate stability.
Small Geodesic Dome (Bibl. 13.05)
• This construction system was also developed and tested by Prof. Minke and his team.
• The supporting structure of the dome is made up of approx. 1.5 m long pieces of full-section bamboo culms, connected in a series of triangles, making it rigid. The lengths of the bamboo members are determined by a geometrical design, which requires fairly accurate cutting to achieve a uniform shape. However, the simple connection system allows for adjustments during assembly. For a tighter fit at the connecting points, at which in alternate succession six and five members meet, the bamboo ends are bevelled (slanted).
• In the example described, the span of the dome was 5 m, a size that is easy to prefabricate and transport manually with 5 people.
• Sand filled tin cans served as footings, providing simple adjustment to differing loads. These were placed in foundations made of old steels drums, which were filled with building rubble and lean concrete.
• A strong waterproof membrane is needed to cover the dome, on which several roofing materials may be used, eg palm leaf or soft stem grass thatch, or wooden shingles on lathing. The structure erected at the Kassel College of Technology had a grass roof.
Grid Shell on a Square Base (Bibl. 13.05)
• The aim of this project, carried out by the Aachen Technical College, Federal Republic of Germany, was to develop a low-cost, earthquake resistant roof structure for developing countries, using only local materials and tools. The result was a bamboo grid shell, which is prefabricated on a flat surface and later lifted in the centre to give it its ultimate shape.
• The bamboo cane used had an average diameter of 30 mm and length of approximately 4 m. For the required length 7.2 m, each grid bar required the joining of two canes. Tests showed that the strongest joints were obtained by inserting thinner bamboo pieces in the cavities at the connecting ends and fixing them by means of short dowels.
• With these lengthened bars, a grid is laid out on the ground forming grid sectors of 50 x 50 cm. Each cross point had a dowel connection which was tied with string to prevent slipping, but to allow a scissor-like movement. After lifting the centre of the grid to the required height, l m cane pieces are placed approximately diagonally to the rhombic grid sections, in the direction of slope, and firmly tied to the grid, giving it stability.
• The edges of the grid form a square of 6 x 6 m, corresponding to the wall dimensions. A vertical bamboo piece is embedded in each corner of the walls and a kind of bamboo ring tie beam is fixed to them. This in turn holds the grid shell roof in place. The roof is covered by a waterproof membrane and a suitable local thatching material, other than stiff-stem grass. A possible alternative to thatch is a ferrocement cover, which would remain in place even if the bamboo grid shell should cease to support it.
Irregularly Shaped Grid Shells (Bibl. 13.05)
• In order to construct spatially curved load-bearing structures using relatively thin bars, the same principle of inverting catenary lines, as described under "Barrel Vault", is applied. The shape of such grid shells is, therefore, not designed, but determined by using suspended models (eg with chain nets). Several such structures using split bamboo have been developed and erected on a joint project of the Institute of Lightweight Structures, Stuttgart, Federal Republic of Germany, and the School of Architecture, Ahmedabad, India.
• Corresponding to the chain net, the grid is assembled on the ground and tied at each cross point. For irregular base plans, each bar will have a different length, which is measured off the suspended model. Since the split bamboo gets more twisted, the steeper the slope of the grid shell, dowel joints cannot be used, while rope tie joints maintain a harmonious curvature of the structure.
Bamboo Trusses (Bibl. 13.06, 13.07)
• In many regions, bamboo is traditionally used for truss constructions, but often use more bamboo than necessary and are not always structurally sound.
• A research project, conducted by Dr. Jules Janssen of the Eindhoven University of Technology, Netherlands, developed and tested four types of bamboo joints and an improved truss design.
• Joint 1: plywood on both sides of the bamboo and held by steel bolts.
• Joint 2: the diagonal member rests against pins inserted through the upper member, whereby the pins support both the purlin and the diagonal member. An intermediate layer (a kind of washer) considerably improves the strength.
• Joint 3: two "horns" at the end of the diagonal fit into two holes in the upper member. (Disadvantage: requires craftsmanship, time and excludes prefabrication).
• Joint 4: bamboo pin passing through three bamboo members, the outer two being parallel.
• The improved bamboo truss, built with joint 2 and a free span of 8 m, was tested in the laboratory by placing it on the floor and simulating vertical roof loads, by a system of hydraulic jacks acting horizontally.
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