(your file contains 2 same overlapping mesh objects so delete1)
1. Run MeshRepair
2. Press Check Mesh and Repair Mesh, then Next.
3. Check (48) Disjoint pieces box, click on Repair and Finish.
4. Click on open beam mesh and run MeshToNURB command.
5. Select created polysurface and run Cap command.
Hope this helped
There should be a CapMesh command for such situations.