Abstract: In order to manufacture joint-adapters for doubly curved and preferably single-layer lattice girders with improved strength and lesser weight while at the same time also allowing relatively high curvatures of the lattice girders, the invention suggests to make the joint-adapters 11 predominantly frusto-conical. Their plane rest surfaces 22 for the butt ends of the bars 10 of the lattice girder subtend the same or substantially the same slope angle .alpha. with the extended joint-adapter main axis 16 as the residual part of the joint-adapter wall. The slope angle .alpha. of the rest surfaces 12 relative to the extended joint-adapter main axis 16 determines the curvature of the lattice girder. In the preferred embodiment mode of the invention, the wall thickness of the frusto-conical joint-adapter 11 is of constant thickness over the entire height H at its thinnest sites in the vicinity of the rest surfaces 12.