Fish-Mouth (Boca de Pescado) Notching and Mortar-Injecting Joinery in Guadua Bamboo

In structural engineering, joining tubular members has always presented formidable geometric and mechanical challenges. When building with cylindrical timber or steel pipes, welders and carpenters must shape complex saddle copes to achieve full-perimeter contact. With Guadua angustifolia, the problem is intensified: the material is hollow, thin-walled (typically 12 mm to 20 mm thick), anisotropic, and brittle under perpendicular point compression. Simply drilling a hole through a hollow bamboo stalk and torquing a steel bolt through it creates intense point-bearing stresses that inevitably split the culm along its longitudinal cleavage planes.

To overcome this inherent limitation, Colombian master builders and engineers developed the boca de pescado (fish-mouth) connection coupled with internal mortar injection. This hybrid joint transforms the vulnerable hollow bamboo end into a solid, reinforced composite socket. The fish-mouth cut provides 360-degree perimeter contact that distributes compressive gravity loads across the entire circumference of the receiving column, while the internal mortar plug absorbs the concentrated bearing forces of the steel tension rod, transferring stress smoothly into the culm wall through interface friction.

The geometry of an authentic fish-mouth joint is an intersection between two cylinders. When a circular cylinder of diameter D1 intersects another cylinder of diameter D2 at a 90-degree angle, the resulting intersection curve is not circular; it forms a 3D saddle curve characterized by two symmetrical parabolic peaks and two valleys. If the intersection angle is diagonal (such as a 45-degree knee brace), the intersection curve elongates into a complex asymmetric ellipse.

Achieving a zero-tolerance fit requires meticulous craftsmanship. Master carpenters layout the cut by snapping reference centerlines along the culm, transferring the column diameter using curved contour gauges, and cutting the profile with curved gouges, coping saws, or specialized hole-saw jigs. A loose or poorly fitted fish-mouth joint concentrates all compressive loads onto the tips of the saddle cheeks, inducing localized bending stresses that will shear off the bamboo fibers under dynamic live loading.

The true structural breakthrough in modern Colombian bamboo architecture—formalized into standard practice during the reconstruction of the Coffee Axis following the 1999 earthquake—is the injection of a fluid cementitious core into the connection internode. This technique effectively converts the hollow culm chamber into an artificial solid node. Before mortar is pumped, a 1/2-inch or 5/8-inch threaded galvanized steel rod is passed transversely through the joint, fitted with heavy square plate washers that distribute tension across the bamboo exterior.

A fluid, non-shrink micro-concrete (composed of 1 part Portland cement, 2 parts washed fine silica sand, and polycarboxylate superplasticizers) is then injected under low pressure through a port drilled in the upper surface of the culm. The mortar flows effortlessly around the steel rod, filling every microscopic crevice up to the natural nodal septum. Once cured, the mortar plug prevents the bamboo walls from collapsing inward under washer pressure and prevents the steel rod from bending or ovalizing the bolt holes under violent seismic shear cycles.

Extensive full-scale laboratory testing conducted at the Universidad de los Andes in Bogotá and international testing facilities has proven the remarkable structural capacity of mortar-injected fish-mouth joinery. Under monotonic tensile loading, an un-injected bolted bamboo joint fails catastrophically at loads as low as 2 to 4 kN due to longitudinal fiber cleavage. In contrast, an identical joint reinforced with an injected mortar core routinely achieves tensile capacities between 15 and 28 kN—a five- to seven-fold increase in structural capacity.

Furthermore, during cyclic reverse loading simulating severe earthquake ground motion, the mortar plug displays high ductility. Rather than suffering sudden brittle cleavage, the connection dissipates energy through micro-frictional slippage along the interface between the cured mortar plug and the textured internal bamboo wall. The ductile steel threaded rod yields progressively, absorbing tremendous seismic energy and providing the visible and audible warning signs required by modern structural life-safety codes before ultimate structural failure.