Otherwise known as the A Frames. These carry the crosshead guides and support the engine entablature (the cylinder block). On older engines, the A frames were individually erected on the bedplate directly above the transverse girders. When boxed in with plating they formed the crankcase. The trend nowadays is to build the frame box as a separate fabricated construction and then, after stress relieving and machining the mating surfaces, to mount it on the bedplate. This has the advantage of saving weigh. 
Lowering the A frame onto the bedplate. A small amount of jointing compound is used to ensure an oil tight joint
When the frames are aligned on the bed plate they are secured together by drilling and reaming and using fitted bolts.
Cracking in A frames can occur leading to misalignment and excessive wear of the running gear. Cracks can start from welds, sharp changes in section and where strengthening stringers are terminated sharply. Repairs can involve cutting the crack out, grinding and rewelding. The danger is that after repair there may still be misalignment.