Although the field and much of its infrastructure is
called “preservation” the reality is that a large percentage of what is reviewed by the State
Historic PRESERVATION Office, or NPS’s Technical PRESERVATION Services
division, is better described as “rehabilitation.” Rehab, in this world, involves transforming a
building or structure to a new use or updating a continuing use in a manner that is
respectful of history.
Determining where the line is between respect and remuddling
is often subjective and it can even be a source of disagreement among “preservation”
professionals. I recall a project
from years ago in which a pioneer era structure was to be rebuilt (from the
ground up, it had been entirely lost) and there was considerable and heated debate
about whether the sills and floor beams had to be hand-hewn timbers or if they
could be the less-expensive and likely more durable, pressure-treated
lumber. On the one hand nobody was ever
going to see the beams, beneath the floor boards, and on the other the original did
have hand-hewn timbers. I thought the
former was nice but unnecessary and pushed for pressure-treated. Others
thought the presence of PT amounted to sacrilege.
EDIT.jpg)
“Preservation” v. “Rehabilitation” has reared its head in on-going discussions surrounding covered bridges. The small number of covered bridges that
survive in Oregon (about 50 out of more than 400 we once had) are beloved in
their communities (and Oregon, despite the losses, still
has more covered bridges than any state west of the Mississippi). The issue, simply put, stems from the undeniable fact that there are good reasons why Oregon’s public works and state transportation officials have replaced
covered bridges over the years. Wood
over water, in western Oregon, isn’t a great match. Wood rots, and it needs to be replaced. Concrete may not have as much character, but
in times of diminishing budgets, it’s a heck of a lot easier to maintain.
To compensate for that fact, there are special sources of money to keep covered bridges
standing, and that makes sense not just to keep them as functional elements of the transportation
system and our history, but from an economic development and tourism
standpoint. For example, I was at a covered bridge a
few weeks ago, the McKee Bridge, way out in the Applegate Valley. They have a register for visitors to
sign. There were people from all over
North America that had driven 30-40 miles off the Interstate to see that
bridge. That’s a good thing, and
justifies the expense of retaining these iconic structures. It's rare to visit a covered bridge that somebody doesn't stop by to take a photo, check it out, or lay out a picnic or something. Oregonians, Americans, love covered bridges.
But keeping them standing requires regular maintenance. And, at the same time that public money is drying up, the
quality of timbers is declining from the old growth that bridge-builders used
in the early 20th century.
That means that even if you can get the large timber members needed to
build certain elements in a covered bridge, they don’t have the same carrying
capacity of the old growth, because the wood isn’t as good. And then, of course, modern vehicles,
especially trucks, are much much larger than they were in 1930 or 1940, so if a
bridge is to carry legal loads as part of the transportation system, the new
timbers ought be carrying MORE weight than the original design.
All of this has led bridge designers and public works
officials to look for new ways to upgrade, to rehab, Oregon’s covered bridges
while meeting the requirements of the traffic system AND honoring history. That’s a fine balance. One method, an easy method, would be to
replace the floor system of a covered bridge with steel or even concrete,
transforming the truss into what would amount to a decorative feature. Nobody wants to do that (though there are
many many covered bridges with concrete approach spans in Oregon…. There
was a time when the approaches weren’t considered historic at all). Another method is to replace the original
timber elements with new timbers, up-sized to increase capacity, but that too changes
the character of the bridge, even where such material is available (YOU try to
find a 20” x 18” timber beam 60’ feet long!).
In some cases original timbers can be strengthened by post-tensioning,
the installation of small steel cables or rods that tighten the truss and increase
its carrying capacity.
But the most typical method, at least of late, is to replace
damaged members (typically floorbeams and stringers that are below the roadway) with glu-lams, laminated
wood that is matched in dimension to the original feature. Glu-lams are just little pieces of wood, usually 2x, stacked, or glued (laminated) to create a single, larger, member. Because of the
stacked lamination and the multiple grain patterns, glu-lams actually carry more load than a single, solid, timber of the same size. Of course replacing original, solid, timber
elements with glu-lams is a change to the original design, justified to keep
the bridge standing perhaps, but still a change.
There are differing opinions, from a technical standpoint, about whether
that change is appropriate or not. I don’t
think most people even notice, but maybe that’s just wishful thinking.
