Literature Review
Client Information
The community of Las Malvinas is located in Santo Domingo Norte. It is a poverty -stricken barrio near a large city consisting of about 800 families. The community of Las Malvinas came together and decided what projects they thought were most urgent and important to their community. Among these projects was a new shade structure for their existing community center. The current shade structure was built with green bamboo, and is deteriorating quickly because of this.
Weather
In designing a shade structure in Las Malvinas, it is necessary that the structure will remain physically sound and not deteriorate in a short amount of time. One of the main causes of deterioration is the harsh weather environment that is present in the Dominican Republic. From 1900-2009, the Dominican Republic has seen approximately 5 inches of average rainfall every month (worldbank). This value is the most varying weather condition, ranging from 2 inches per month in the dry season (December through April), to 10 inches per month in the rainy season (May through November) (worldbank). On average, the country encounters approximately 50-60 inches of rain each year (worldbank). While precipitation amounts vary, the temperature and wind conditions are relatively constant year round. The Dominican Republic experiences an average of 80 degree Fahrenheit temperatures and an average wind speed of 5 mph monthly each year (weather-and-climate). While there is prevalent sun, rain, and wind exposure, there is also exposure to strong tropical storms and hurricanes. From 1944 to 1999, the probability of a tropical storm occurring and producing wind speeds greater than 40 mph is approximately 30% (noaa2). Once wind speeds increase over 74 mph, the Saffir-Simpson Wind Scale classifies the storm as a hurricane, the probability of which is 10% in the Dominican Republic from 1944 to 1999 (noaa1). Finally, once wind speeds reach 111 mph the storm is classified as a Category 3-5 hurricane, the probability of which is 3-4% over that time frame (noaa2). These intense seasonal weather conditions need to be considered when designing any architectural structures in the Dominican Republic.
Bamboo
Because of its rapid growth, abundance, and ability to contribute to both environmental protection and economic growth, the use of bamboo as an alternative building material is ideal in most tropical Latin American countries (Londoño). In the Caribbean, Bambusa vulgaris is the most cultivated exotic species of woody bamboo (Londoño). Timing is an important first step when considering when to harvest bamboo for use as a sound building material. In order to ensure optimum strength and to avoid pest invasion, it is advised to harvest bamboo during the dry season when starch content in the bamboo sap is low (Environmental Bamboo Foundation). In addition, bamboo that is 3-5 years old is ideal as a construction material (Environmental Bamboo Foundation), although other sources argue that bamboo must be between 4-7 years before harvesting (Schröder). Nevertheless, bamboo older than five years (or seven years according to Schröder), becomes harder, drier, less permeable, and begins to deteriorate faster (Environmental Bamboo Foundation, and Schröder). One can identify a mature bamboo stalk by its color and absence of rings around the stalk. An immature stalk will be shiny, bright green, and will have many white bands around the stalk; a mature stalk ready for harvest will be dark green to gray and the bands will be harder to see; an overly mature bamboo stalk will be white in color and the foliage will be less green than mature bamboos (Schröder). Harvesting bamboo mindfully will ensure both a better building product and will secure bamboo growth in the future. When felling bamboo, it is important to cut above the first or second node above ground level using a machete or saw; this way, rainwater is not able to collect in culm, which could cause rot and damage the bamboo plant system (Schröder). After harvesting, storage is the next most important step. The bamboo culms should be stood upright and placed on a stone, making sure that the bamboo stalks do not come in contact with the ground, as this can impact the drying process (Schröder).
Bamboo Treatment
Pests/Rotting
Coconut Palm Fronds
Palm fronds have been used as a traditional building material in tropical locations around the world. The time for which coconut frond thatch will last as a construction material depends on the palm species and how the thatch is harvested and woven (Sokial). It is most ideal to choose a larger palm frond with long leaflets and it is advised to use only fresh palm fronds as opposed to dead palm fronds since fresh fronds will be less brittle and have more strength as a building material (Sokial). The basic process to create a coconut frond panel is as follows: the coconut frond will first need to be evenly divided from the spine of the palm frond with a machete (Sokial). The fifth leaflet is bent in the opposite direction of its natural growth and woven over the fourth leaflet, third leaflet, and so on using the formula (5+2+2) (Sokial). ‘Pegs’ should be created at intervals of 8-10 woven leaflets in order to create a hook to ensure the woven leaflets will be secured together (Sokial). To prevent water leakage, the leaflets must be well opened while weaving, and the woven palm frond at the end of the process must be tied together to make a complete woven panel of thatch (Sokial).
Living Shade
Also called pergolas, living shade structures combine a structural frame made from wood or other strong material and climbing plants to create shade for an area. In the Dominican Republic, climbing plants such as Passionfruit Vine (P. edulis flavicarpa) naturally need the support of a wall or another plant for climbing in order to receive more sunlight (Global Foundation for Democracy and Development). Therefore, climbing plants could be an appropriate option to include in this shade structure.
Upcycled Industrial Materials (Plastic, Metal Roof Sheeting)
Materials such as plastic bottles and plastic containers flood the waste stream in Santo Domingo, most of which end up polluting nearby rivers and the ocean. In order to reduce the amount of plastic waste entering nearby waterways, the use of plastic bottles for a shade structure could serve as an appropriate technology. South African designer Heath Nash created shade structures from upcycled plastic bottles, with frames for the structure made from scrap metal and wood (Inhabitat). Metal roof sheeting is commonly found as scrap pieces in Santo Domingo. Durability, low maintenance, and high UV protection make metal roof sheeting a viable option as a material used in a shade structure (Cancer Council).
Bamboo Mechanical Properties
The cellulose fibers in bamboo act as reinforcement similar to reinforcing steel bars in concrete or glass-fiber in polyester-resin (Jansen). A bending force causes compression stresses on the upper part of the bamboo beam, parallel to the fibers, which does not pose a problem for the material. However, this compression causes strain perpendicular to the fibers, which occurs in between the fibers of the material, which is weak in taking strain. This, then, is the weak point in designing and building with bamboo for a beam; however, if the load is removed, the specimen will return to its original straight form, a definite advantage in the case of a hurricane or earthquake (Jansen). Compression force is another factor to take into consideration when designing the frame and foundation of a bamboo based structure, but is not easily evaluated. During longitudinal compression on bamboo, the bamboo becomes thicker in the middle and gives a false impression of compressive strength, when in reality, the friction at both ends of the culm are holding the material together, similar to a beer barrel (Jansen). Creep, or the increasing deformation under long-term loading, is negligible in bamboo. As stated earlier, bamboo structures are highly advantageous in the presence of an earthquake, it is lightweight and the hollow form gives much stiffness. While the material is great at reforming to its original state after a load is removed, such as a high wind load during a hurricane, it is very lightweight and easily blown away if not properly secured and jointed together.
Structural Integrity
“Next to the foundation and the roof covering, the basic frame is the part of a house most often made partly or wholly of materials other than bamboo” (eric). If the difference in cost can be accommodated, the use of more durable hardwood is preferred for the frame. This is because hardwoods make stiffer joints and more rigid construction than bamboo, partly because a greater load is placed on the frame and partly because certain hardwoods are naturally more resistant to rot fungi and wood-eating insects than untreated bamboo (eric). However, in locations such as the Dominican Republic where earthquakes occur frequently, a bamboo-framed structure may remain intact longer than any other type. Regardless of frame material, a cement foundation separating the frame and earth is desired in order to inhibit rot and insect infestation, as well as increase the strength of the design. The structural elements that compose the frame of a conventional all-bamboo structure are similar to those found in all timber frames: Corner posts, girders or plates, joists, studs, struts, tie beams, rafters, sheathing, etc. Because of their high strength-weight ratio, bamboo roofs carry excellent advantages for the structural integrity of the design (eric). Most bamboos have a tendency to split easily, a tendency the precludes the use of nails, while also limiting the range of techniques suitable for the framing and joining of structural units (eric). A remedy is to make end cuts just beyond a node where feasible as the nodes have a higher shear coefficient than the internodes and therefore have fewer tendencies to split, or by fastening joints using thongs or other lashing materials as opposed to nails or screws (eric).
Lean-to Roof
Lean-to roofs a very common design in forming small canopies or ground floor extensions. Traditional or ‘cut’ roofs usually comprise a series of sloping timbers (rafters) fixed to a wall plate or support posts at their feet, and a ridge board or wall plate at their head (uwe). “The wall plate is a strip of timber which is bedded on the top of the wall, and which evenly distributes the load from the roof and provides good fixing for the rafters” (buildingregs). The depth of the rafter depends on its span and loading, and the width is primarily to prevent twisting and to provide a sufficiently wide surface on which to nail the battens supporting the roof (buildingregs). A good practice is to notch the bottom of the rafter where it sits on the plate or posts as this gives a good bearing and aids alignment of the rafters (buildingregs).