I don’t know why my modeling is so high in poly. I didn’t use and submodifyers. all I did was scale down a cube and enter 6 6sided hexagons and the screws. Should this be happening? I am asking because I can’t finish a scene. my computer starts to lag then Blender crashes when I try to make a 990ft bridge.
Difficult to tell without you supplying your blend file. Upload to http://www.pasteall.org/blend/ and tell us the download link
Have you tried modelling just one plank and then using an Array modifier to place the others in position. As long as you don’t apply the modifier then it shouldn’t really use that much memory.
As davidh7426 said, you could use the array modifier. I think you are making the nuts/bolts and the planks the same object and then trying to connect them while using good topology skills (polygons instead of 4+ sides faces). Probably use array modif and use bump maps or normal maps to fake things if you can
Hope this helps 
Screws? There are screws attached to those bolt heads? What on earth for?
Look, this is CG. You aren’t building a real bridge. Those planks will stay where you put them without all those extra vertices inside them doing nothing but slowing down your computer, while it decides that it doesn’t really have to render them because they are inside a plank. Four planks with inset hex bolt heads should take around 1500 vertices.