*SPOILER* *ALERT*
phew... completed it... kinda scary as i thought like 1000000 bug thingys will come out and pwn me... but no... opps, sry for the spoiler...
actually, your problem, kinetics, is that you nid "symetrical" triangles to support your bridge, try to branch out from the support as much as possible as well. ( Finished last mission without steel and no sandbox )