http://gizmodo.com/the-plan-to-3d-print-a-steel-bridge-in-mid-air-1710882593?utm_campaign=socialflow_gizmodo_facebook&utm_source=gizmodo_facebook&utm_medium=socialflow
Pretty nifty.
The first question that comes to mind is - how does the structural strength of 3D printed steel compare to that of steel formed by more traditional means? For a low-load application like a pedestrian bridge iy isn't necessarily a huge concern if it's less, but for larger projects it would be.
Of course, even if the strength is a bit lower, necessitating more material to achieve the same load capacity, it could still be worthwhile if the savings on labor costs make up for it.
Using nanotech to create materials much stronger, combined with using nanites for self-repair will enable us to make maintenance-free structures. In case of massive stress caused by earthquakes, the nanites can change the rigidity factor to one that lets the structure go limp as needed to survive, then it can re-harden to allow for use after the quake. That is how I see 3-D evolving for the building of structures.
The real advance will be when we can come up with Star Trek-style replicators for much faster construction. 22nd century tech would be nice to have when trying to settle Mars!
Rick
The same idea was used five years earlier for the I-35W bridge in Minneapolis (not the collapsed one). It kinda can self repair and doesn't involve humans at all, but maybe just to check how it's doing.