Disclosed is a refiner segment (1) that is adapted to be attached to a refining disc (30) of a refiner (100) of lignocellulosic material. The refiner segment (1) comprises a number N, N≥2, of bars (10) and a number, M≥2, of dams (11), said bars (10) and dams (11, 11*) being arranged in a pattern whereby essentially box shaped regions (20) are created in areas defined by neighboring bars (10, 10*) and at least two dams (11, 11*) extending between said neighboring bars (10, 10*) at different locations along the length direction of the bars (10, 10*), wherein at least one of the bars (10, 0*) that defines such an essentially box shaped region (20) comprises a bar weakening section (12), arranged within said essentially box shaped region (20), and provided to enable steam trapped in said essentially box shaped region (20) to escape said essentially box shaped region (20) via said bar weakening section (12). Disclosed is also a refiner disc provided with such a refiner segment and a refiner comprising such a refiner segment.