...As is the case with many important examples of modern architecture, the preservation of twentieth-century materials and structural designs has proven challenging. The unusual shape and extreme height of the Arch compound these difficulties, as do economic trends that have led to decreased government funding for the stewardship of national monuments. The complexity of preserving both the structural integrity and aesthetic qualities of the Arch compels cooperation between the public and private sectors to ensure that adequate expertise and resources are brought to bear in the preservation of this important landmark and national symbol.
In an interview with PBS' News Hour the WMF's Bonnie Burnham discussed the list and what it means for monuments to be included (discussion of the Arch at the 5:00 mark):
The 2014 list includes 5 monuments in the United States: