11870b5d_oric_Significance_Rating__HSR___Description
[HSR] 9 [Examples] Cast iron bridges. Pre-1800 stone arch bridges with average historic integrity. 1870s Bowstring truss bridges, lenticular truss bridges, and bridges with Phoenix or Keystone columns. Multi-span concrete curved chord through girder bridges. Movable bridges with first-of-kind technology, or extremely rare type. [Description] 
Bridges that are extremely old and rare, and/or also feature extremely rare components fall into this category. With a few exceptions, governments have been recognizing the significance of structures at this level and preserving them.
HistoricBridges.org - Historic Significance Rating (HSR)
Description