From foundation to roof
Between bamboo and structural steel, which one has more strength? Which one will perform better in case of deep soil pile foundation? If left open without any protective coat, which one will last longer without rusting or withering? Which one has more options of application and is versatile enough for wall, column, beam and floors?
The surprise answer for all the above is bamboo. At an average tensile strength of 12,500 kg/sq. in., it is 2,000 kg units more than that of steel. The stems of bamboo, popularly called poles and technically as culms, have been traditionally used for pile foundation in water-logged and loose top soil sites. Bamboo mats can be laid as foundation bed in case of soils with low load-bearing capacity. As a versatile material, it is good for both temporary and medium term usages, besides for long term, if designed and built properly.
While bamboo can be used as a single pole, it works equally well in groups and as a composite material. For transferring heavy loads, multiple numbers of culms, say 3 or 4, can be joined by nuts and bolts to act like a single column. Likewise, it is possible to form beam trusses in groups or in lattice formation. Of course, all the bamboo poles in such composite forms need to belong to the same species, of similar girth and be of comparable maturity of age.
In Kerala, lime concrete bamboo reinforced beams have been attempted since the late 1990s and in regular use since then at the Laurie Baker Centre. While stand alone use is feasible, bamboo gains more potential as part of a composite structure and reinforcement material.
The traditional bamboo walls were made with woven bamboo mat with mortar coats on both sides, so that bamboo is not exposed to any adverse weather condition. While lime and mud mortar goes better with the natural material, cement mortar can also be used. Subsequently, protective painting coats can be applied. Being 2 to 3 inch thick, they are light weight, hence save on structures. If split bamboo culms are used, keep them to the outside with inside finished as desired. External silica-based skin of bamboo resists fire for a while, but can be improved by chemical coatings. Bamboo boards with split lengths and panel are very popular as visual barrier in semi-open spaces like verandahs and pavilions.
Roof supports by bamboo rafters are very commonly found even today. Thicker girth culms are used as ridge beams or wall beams, while the smaller diameter ones can go for principal rafters in sloping and cross beams in flat roofs. Their spacing may vary from 1 to 2 metres depending upon context. Floor finish can be achieved by 1:6 ratio lime mud or cement mortar with topping by floor tiles and any other item of choice. There are many valid reasons to use bamboo, which need to be explored further.