The present invention is directed to a steel segment assembly for lining a tunnel. The segment assembly has a frame to which is attached a steel panel, the frame and panel being a segment of a cylinder dimensioned to the desired tunnel diameter. The frame is preferably square, rectangular or trapezoidal in shape and is constructed of suitably dimensioned members having a square or rectangular cross section welded together. The frame has two curved spaced apart side members to form the long sides of the frame, the curved spaced apart side members being joined together at either end by end members to form the short sides of the frame. At least one intermediate member is provided between the two end members to bridge the two curved spaced apart side members to provide transverse structural support to the steel segment assembly.