"where's the language in ss. 106 stating that replacing 90% of a historical structure is somehow still considered a repair vs. a replacement as long as you make sure to re-use a few visible pieces of the old one in your new copy?"
I apologize, if you are impatient with my slow responses, please understand I do not have the kind of time necessary to answer your every request on this Easter Sunday morning. But I think I know what you you getting at so here's the way it worked.
The bridge is a historic resource listed on the National Register (kept by the NPS) and has "characteristics" that are unique to it and it alone (for example: the towers, draw span, steel arched girders, piers) Avoid diminishing the "integrity" of those "characteristics" and you have got a winner (expressed in an MOA).
So the basis or standard WAS
to retain the characteristics that makes the thing historic, i.e. "avoids adverse effects" and is still able to convey its significance. The basis WAS NOT
if it keeps a set percentage of original materials.
Below is what I use because I am a layman, not a lawyer, so I hope it doesn't offend you that it is a "Citizen Guide" instead of the actual citation and link to the law. http://www.achp.gov/docs/CitizenGuide.pdf
(see page 7)
SECTION 106: WHAT IS AN ADVERSE EFFECT?
If a project may alter characteristics that qualify a specific property for inclusion in the National Register
in a manner that would diminish the integrity of the property, that project is considered to have an
adverse effect. Integrity is the ability of a property to convey its significance, based on its location, design,
setting, materials, workmanship, feeling, and association. Adverse effects can be direct or indirect and
include the following:
physical destruction or damage
alteration inconsistent with the Secretary of the
Interiorís Standards for the Treatment of Historic
relocation of the property
change in the character of the propertyís use or
introduction of incompatible visual, atmospheric,
or audible elements
neglect and deterioration
transfer, lease, or sale of a historic property
out of federal control without adequate
I hope this helps Chris. ACHP did consult, it did sign the MOA, it was not ignored on this bridge, so argue if you will about it relevance in general.