Guadua angustifolia is a species of bamboo native to South America. The aim of this study was to accomplish experiments and finite-element simulations to determine the stiffness of two-culm G. angustifolia beams, as typically used in the construction of low-rise buildings. Calculations were also accomplished to find the moment of inertia (i.e. effective) to assess beam deflections using a solid mechanics formula that includes bending and shear effects. First, full-scale tests of simple culms were performed with four different spans to find the axial and the circumferential–axial shear moduli. Next, full-scale tests of two-culm beams with two shear connectors located at L/6 from the supports were performed. There was a marginal increase of stiffness compared to that of two-culm beams without connectors. Finite-element simulations confirmed that the stiffness of two-culm beams with connectors is similar to that of two superimposed culms. Only the addition of cement mortar yielded an important increase of stiffness. Results indicate the need to reformulate the calculation of the effective moment of inertia of multiple-culm bamboo beams, as the value calculated using the parallel axis theorem is too large, and yields deflections substantially lower than those obtained experimentally.