A gas turbine engine has a core frame separable from a vane frame for removably attaching the vane frame to a core structure through a mid ring. The mid ring includes an annular forward panel and an annular aft panel, each of which has a radially outer flange and a radially inner flange. The inner flanges are welded to the core frame and the outer flanges are bolted to the vane frame. A plurality of circumferentially spaced stiffeners extend between the forward and aft panels, with each of the stiffeners including a pair of stringers extending diagonally across the stiffeners. Panels may be incorporated with the stringers between the triangular shaped areas adjacent the forward and aft panels of the stiffener. The bolted connection between the mid ring and the vane frame includes a plurality of tongue and groove joints formed in the flanges of the mid ring and vane frame. The tongue and groove joints provide additional radial support for the core engine within the vane frame. Preferably, the tongue and groove joints are formed with angular sides so that conformance between the joints can be achieved by simply tightening the bolts attaching the joints. In one embodiment, the tongue and groove joints may only be used at preselected circumferential positions with the remainder of the flange connections using overlapping lap joints.