An Investigation of the Effectiveness of the Fish-Mouth Joint in Bamboo Structures

Abstract

Sustainability has become a major societal challenge of the 21st century. An important sector that would benefit from sustainable solutions is the construction industry. The negative environmental impact of the production of common construction materials and their associated high economic costs present a need to investigate the potential of alternative sustainable, low-cost building materials. This is especially valid for communities that cannot afford to produce large quantities of concrete and steel for every structure they erect. A useful alternative to these construction materials is bamboo. Bamboo has a myriad of species that grow worldwide. Due to the favorable strength properties of some of these species, it is considered to be one of nature’s most useful grasses to build with. For a structure to be able to channel load to the foundations, the connections or joints are crucial, as they determine whether forces from different loading combinations are transferred properly throughout the structure. This thesis investigates the effectiveness of a fairly commonly used bamboo-bamboo connection type; the fishmouth joint through the structural analysis and physical strength tests. The case study is a 9m by 9m by 9m tall hyperbolic paraboloid bamboo grid shell which is currently under construction in Cali, Colombia. The structural analysis was carried out through analytical hand calculations that determined the maximum allowable loads for the joints connecting to the canopy roof. Laboratory strength tests sought to determine the material properties of the bamboo species used during the tests, Guadua angustifolia, as well as the maximum load capacities of a perpendicular T-shaped fish-mouth joint. In particular, these tests include an experiment to assess the bamboo’s compressive strength parallel and perpendicular to its grain. Another simulated the joint being pulled apart and examined the fish-mouth’s ability to resist tensile loads. An important fourth test analyzed the effect of inserting mortar to the horizontal member of the fish-mouth joint. Due to bamboo’s weak properties perpendicular to the grain, mortar is typically inserted in this section of the joint to provide additional strength. The results of this test seek to provide reference loads for when mortar should be inserted into the fish-mouth joint which could help reduce costs associated with mortar use and give structures additional lightness. Two final tests were conducted to assess the bamboo’s shear strength perpendicular and parallel to the grain, as tensile loads on the joint can induce shear failure at the top of the horizontal bamboo member, where the eye bolt crosses the culm wall, and also in the wall area above the cross bolt. The results of this analysis provided an overall basis for determining the effectiveness of the fish-mouth joint. Though the structural analysis and reference allowable loads were calculated from a specific bamboo canopy structure, the results of these experiments can be related to the fish-mouth joints use in other bamboo structures.