HN39, I'm not an engineer and know nothing formal about structural work. That made clear, my sense of the triangles thus formed is that they receive, then transfer and distribute mostly-vertical loads of the horizontal stabilizer to the larger aft-fuselage structure. The very beefy forged/milled joins between the top and bottom of the aft section where the THS is mounted appear to mean business in load-bearing. The aft frame (APU tailcone structure) is, IIRC, similar in the bracing seen here but of course, smaller. Perhaps too, these twin-braced major frames counter "twist" (as viewed from the rear), which would come from loads borne by the vertical stabilizer hoops shown earlier, which would be transferred to the lower structures through the hoops.
All this of course a complete guess and could be wrong, but it "feels right"...I would welcome corrections.