Bamboo Bends

     Bamboo has been, and is, an obvious choice of building material for those in the third world that are truly poor, and living off the land. Bamboo is cheap, renewable, available, and has very desireable structural qualities.  The relatively recent discovery of bamboo as a structural material in the developed, industrial world, is hampered by a quality of bamboo that is beyond the established norms for more established, and traditional building materials.  Bamboo bends.  It can bend alot.
       The  bending of a material is measured as "deflection"-the amount of distortion a material demonstrates with a certain amount of force applied laterally.  This quantified measure dictates construction parameters, and is then established in a uniform building code.  The parameters for steel,concrete, and wood have been well established, and they dictate the design of a structure such that the finished product is rigid.  But rigid to an arbitrary degree, that which a person does not feel or experience the distortion of a structure under normal "loads".  The lack of noticeable distortion or movement has been a fundamental "given" for good, solid construction.
     Although bamboo has been shown to have a tensile strength comparable to steel, a compressive strength of concrete, and a strength to weight ratio greater than wood; it ability to bend under loads has hindered its ready adoption as a building material. The rigidity that is required necessitates heroic efforts in bamboo design.  Either prodigious amounts of bamboo must be used, or, for rigid wall and roof support, an elaborate "truss" system has been developed, with well engineered joinery.  The resulting structure can have the requisite rigidity, but will no longer be cheap, easy to build, or reflective of the most unique and advantageous structural quality of bamboo-its flexibility.
     The requirement of rigidity in structures has several disadvantages. Concrete is rigid, and coupled with its mass, causes its rapid failure in earthquakes.  A buildings' huge inertia tends too keep it in one place, while the ground moves in all directions.  The tremendous sheer resulting quickly crumbles the concrete, and destroys the structure. Cement floors are rigid to the degree of causing discomfort, there is no deflection to the the human body, either in walking or, in hopefully rare instances, falling down.  This may be of little consequence for adults, but young children do fall down.  One of the worst sounds I know of is the sound of a childs' head hitting a tile floor.  The resulting trauma can be serious.
     Wood, like bamboo, has advangages of flex , but has well established building parameters to reduce flex to a degree compatible with the human need for rigidity in a structure.  Wood structures can be made earthquake and hurricane proof, and are much more comfortable in terms of deflection.  A wood floor is comfortable to walk on, and much more forgiving to a childs' head impact.  However, though wood can be a renewable resource, and instrumental to a path towards sustainability, population and economic  pressures often push the harvesting of a countrys' forest inventory beyond sustainability.  Bamboo, as another quickly renwable building resource, can alleviate pressure on forest reserves.
     I doubt very much that a special accomodation for bamboo construction will be established that allows its deflection. But for structures less than 120 square feet of floor area, a flexible design strategy could be adopted, as this is the minimum structure size that the uniform building code applies.  The "flexible flyer" strategy for small bamboo structures is appropriate.  The structure is affordable, uses the minimum of materials, and reflects a bamboo mindset.
     A bamboo structure, with a new tolerance for deflection, is indeed a major departure from our collective experience.  Bamboo bridges bend and sway with the human load, but this flex is to its advantage, distributing the stress caused by the load throughout the structure.  In addition, by designing bamboo flex into the design, less stress is focused on the bamboo joinery, the achilles heel of bamboo structures.   Earthquake resistance of bamboo buildings has been well documented, and is attributable to its flex, coupled with its very low mass.   Hurricane resistance, the other environmental extreme that must be addressed, is also aided by flex of bamboo.  Ultimately, structure failure in hurricanes is attributable to the failure of its fasteners.  Sustained winds can cause lift, pulling the roof up, and off.  Powerful gusts can cause tremendous lateral force upon a structure.  Flex will distribute these forces throughout the structure, making any individual joint less likely to fail, and start a cascading failure of structural integrity.  I was witness to all these forces on my bamboo pole "hooch" through the early morning hours of our rendezvous with hurricane Georges.  The structure was subjected to category three hurricane conditions, and the resulting flex was substantial.  I observed my structure for three hours, as it was buffetted by lateral gusts and aerodynamic lift, and could not believe the contortions  it assumed, while always snapping back to its designed shape.   The open bamboo pole design, with no vertical walls, and a hip roof, all contributed to its resilience.  But the degree the bamboo poles flexed, and returned to their original shape, was no doubt instrumental in the structures' survival, and a testament for the incorporation of bamboo flexibility in the design strategy of bamboo structures.
     The experience of living in a flexible bamboo structure is one of celebration in the "life" that the bamboo imparts.  Its' movement  reflects the natural world, where a tree or, for that matter, bamboo, distributes environmental stress throughout its "body", insuring against failure by moving with the force.  This distribution, or dilution of stress, is not only throughout the structure, but ameliorated by the spreading of the impact through time.  A sudden impact on a rigid structure can cause tremendous force, and consequent failure of even the strongest of materials.  By flexing, the sudden impact is distributed through time to a fraction of its instantaneous level.  The maximum resilience to the forces of nature is achieved with the least amount of material, or energy investment. That is why nature has designed its plants to flex, and move, and flow.  A flexible bamboo structure imitates life, and I cannot think of a more tested and evolved design strategy.

return to homepage