It spans the East Fork of the San Gabriel River and was meant to be part of a road connecting the San Gabriel Valley with Wrightwood. The East Fork Road was washed out during the great flood of March 1-2, 1938 and the bridge was abondoned.
Approximate latitude, longitude
+34.28319, -117.74696(decimal degrees) 34°16'59" N, 117°44'49" W (degrees°minutes'seconds")
My grandfather was one of the engineers who worked on this bridge. On an unrelated note, you can hike out to it and bungee jump off of it.