A Cross brace is the cross-beam connecting between arches that is meant to add rigidity to the construction of mining tunnels. It is often tubular, having a diameter (generally made of lengths of pipes), and modified on the ends with clips or apertures for screw fastening; it serves as a stiffening element for the stability of a design. This brace could also be an ordinary cross-section wooden cross-beam, or even a board.
References
Cross brace Wikipedia(Text) CC BY-SA