Further to this, it's a pretty interesting technical problem to think about why the bridge collapsed. A compression member or node at the top of the truss that

Further to this, it's a pretty interesting technical problem to think about why the bridge collapsed. A compression member or node at the top of the truss that forms the main span seems to have failed for some reason. The news story linked in another comment below says the truck hit something. This[0] secondary(tertiary?) truss looks like it might have taken a hit? It forms part of the horizontal truss above the carriageways which resists shear in the horizontal plane and therefore provides torsional stability; the bridge is effectively a braced tube, I think. Maybe the hit on that pulled the main compression member out of alignment enough for it to buckle? I'm not an engineer, but I'm pretty interested in engineering, I'd be interested to know if anyone with better knowledge can spot an obvious reason for the collapse. [0] http://www.seattlepi.com/local/article/Interstate-5-bridge-collapses-over-Skagit-River-4544826.php#next http://www.seattlepi.com/local/article/Interstate-5-bridge-c...