Back to Paper Mill Bridge Rehabilitation
Bennington's Paper Mill Bridge Reconstruction - continued.
The Bridge Engineer Responds to Critics
Phil Pierce, the engineer who led The Vermont Agency of Transportation Covered Bridge
Study responded, by e-mail, to the article by Richard Wilson entitled, "A Sad Sight." Mr. Pierce's
response is recorded below. For Mr. Wilson's article, click "The Bridge is Gone" link above.
Richard Wilson is President of the New York State Covered Bridge Society.
The longer I am involved with covered
bridges, the more I find it fascinating how many people are interested in them. These structures
truly have a strong following.
I want to provide some background on
the situation at Paper Mill to address the issues that you raised in the material on Joe Nelson's site
(and I assume in the Courier). I do not speak on behalf of the Vermont Agency of
Transportation, or for that matter, the Consulting firm of McFarland-Johnson, that conducted the
work in question (I have since left the firm). However, I was the leader of the work and take
great pride in what was done.
I consider myself blessed to have been
chosen to lead the Vermont Statewide Study on behalf of the consulting team. The in-depth
involvement with 75 bridges provided a unique opportunity to carefully examine each bridge and
help establish a plan for each bridge to help it survive indefinitely. There were a host of technical
and non-technical issues that were involved and it obviously took a long time. Although I was
involved in other projects during the 3-year duration of the Study, I did little else but wrestle with
this fun project during any free time that I could find.
The Vermont Agency of Transportation
was sufficiently pleased with our work to select us to provide engineering services under a follow
up, but separate, contract for the work to be performed at the Hopkins Bridge in Enosburg and
the Paper Mill Bridge in Bennington. As you know, the Hopkins work was completed a couple of
years ago and I believe it was considered a successful project and was able to retain a lot of the
original fabric of the bridge, while replacing deteriorated elements and a weak floor system.
Obviously, I learned all about the
situation at the Henry Bridge that had occurred before we got started with the Vermont work. So
when I became more involved with Paper Mill, I was especially cautious that we avoid a repeat of
that situation. While I have not attempted to contact any of the officials in the project (State,
Federal, or Local), I am confident that all would agree that each and every task required by the
Federal government for a project using Federal funding was satisfied. If we had not done so,
the Feds would not have released funding for it.
The project included all standard steps
in an engineering project, complete with an extensive evaluation of the bridge, an evaluation of
various alternatives to achieve the desired reopening of the bridge to vehicular traffic,
consideration of all relevant cultural resource issues, intensive coordination with the Vermont
Division of Historic Preservation, and involvement with the local community leaders. A televised
public meeting was held to discuss the project and the proposed solution. The project took longer
than anticipated and construction was delayed due to funding issues - situations not unusual for
complex projects of this nature.
I'll try to summarize the condition of
the bridge at the time of the Study as follows: The bridge had deteriorated to such an extent that a
bypass structure had been erected a number of years earlier. Local people had used the roof of
the bridge as a diving platform into the adjacent deeper pool of the dam for a long time.
Unfortunately, the access to the top of the roof was provided by kicking off roof boards and
shingles directly above a truss near mid-span of the bridge. Although attempts were regularly
made to repair the roof, the result was a leaking roof above a truss at the worst place possible on
the bridge. The long-term leakage had so badly deteriorated the condition of the truss top chord
that it had collapsed in bearing a total of over 2 inches at the time of our work. While Mr. Cote
(VP of Blow & Cote who are the contractor's selected to rebuild the bridge) noted in this
thoughts as recorded on Mr. Nelson's web page, that he felt the bridge would not collapse as a
result of the condition of the top chord, I urged some sort of action by the either the Town of
Bennington or the State of Vermont to stabilize that deteriorated portion of the top chord until
the contract was let to rebuild the bridge. I had watched the chord gradually crush more and
more, each time I visited the bridge over the course of our work. The AOT agreed with my
assessment and the AOT designed the metal braces that were installed a few years back.
The worst part of the bridge was the
bottom chords. Previous work (I believe in the 50's) had attempted to "strengthen" the chords
during their replacement by the use of split ring connectors (large metal bands similar in nature to
a wedding ring, 4 inches in diameter and about an inch wide by 1/4 inch thick) [good intentions,
yet ill advised]. To install these connectors, the ends of the lattice members that already were
perforated with the usual arrangement of trunnels had been routed to accept the connectors. The
resulting condition of the ends of the lattice resembled Swiss cheese (none of which was directly
visible without careful examination of the bridge). Further, the ends of many of the lattice had
been broken by ice so that they were ineffective in the transfer of floor loads to the truss. The
poor condition of the lattice was addressed previously (again, mostly in the 50's I think) by the
addition of a lot of "sister" lattice members. The result was the loading of the bridge with a
lot of excess weight from the inordinate number of sister members.
A study of the geometry of the bridge
(depth of trusses to their span length) in comparison to the dozens of other similar Town Lattice
bridges in Vermont and surrounding areas clearly indicates that the bridge was inordinately
shallow. It should have been a least 2 feet deeper to be compatible with other surviving Town
Lattice bridges. I have no answer as to why it was built so shallow, yet it was plenty clear that the
bridge was so badly misshapen and distorted due to the overload of the trusses. Indeed, our Study
analysis indicated that the bridge, even if in good condition, was overloaded by its own weight
and had no reserve capacity for live load.
Therefore, after an exhaustive analytical
evaluation and consideration of alternatives, it became clear that the previous misuse of the bridge
provided no chance to retain much of it for use in a replacement structure. However, the plans
were prepared to retain as much of the overhead bracing and some of the short lattice members at
the end of the bridge, (which were not brutalized by the installation of the split ring
The capacity of the bridge was required
to support 20-ton trucks. One should keep in mind the situation that this is the middle of three
covered bridges in a row along the stream and the bridge would be used by traffic, often
disregarding any load posting signage.
Being sensitive to the desire to avoid
use of supplemental steel support members or large glue-laminated members which have been
used at other locations over the years (often to the abhorrence of preservationists), we spent a lot
of effort to develop plans to rebuild the bridge with all sawn truss members. Although I
recommended the use of a deeper truss to give it a better chance to serve well into the next
century, we were directed to maintain the geometry of the existing structure.
The floor beams of the rebuilt bridge are
of glue-laminated construction due to the inability to design them using currently established
practices and allowable stresses for sawn members in a size that could be installed in a town
lattice bridge. Glue-laminated floor beams have been used regularly around the United States just
for this reason.
I hope this summary provides the meat
to address the concerns of preservationists. As an engineer challenged to help prolong the lives of
these unique structures, I pride myself on the care and attention to detail that they deserve, while
at the same time, insisting that the public be protected by the use of conventionally accepted
engineering practices. While we can debate the issue that these fine old structures have served
well for a long time, the fact remains that the engineering evaluations of them often find serious
distress and overload. So what's wrong with the engineering, one might question. We cannot,
and I will not, disregard the history of my profession in its commitment to the paramount interest
in public safety. If I chose to go my own way and develop my own standards and practices
without regard to the guidance of earlier engineers, I would be subject to immediate attack by
other engineers and the lawsuits of people wronged by the collapse of unsafe structures. I take
my responsibility as a Professional Engineer very seriously.
There is always room for disagreement
or differences of opinion among experienced Professional Engineers, just like all professions.
There may be a Professional Engineer out there who would propose that a different result was
possible. In my 27 years of service, I have attained a lot of confidence in my engineering
judgement and I have rarely had anyone challenge it. And, in my 8 years of obsessive involvement
with covered bridges, I have found that the small number of covered bridges which obviously
require few engineers to be involved with them, coupled with the relatively small number of
experienced timber engineers, leads to a relatively small number of engineers with actual
experience with them. It is my opinion, that covered bridges represent an especially difficult
engineering challenge and deserve unusual care.
I have devoted an inordinate amount of
energy to the challenge of finding the "apparent" reserve capacity of these structures.
Unfortunately, the small number of covered bridges in the United States, in relation to the total
number of bridges, have not previously fostered much attention to research of the engineering
issues related to them. Hence, my current work on the preparation of a textbook devoted to the
engineering and construction of covered bridges. I am continuing my quest to help preserve
them using conventional means and methods and avoid the use of unnecessary steel and
The work on the Paper Mill Bridge
included the involvement of a large team of professionals. Each person involved fulfilled a role
and the result met all requirements in place by the Feds and State. I am disappointed in the
perception that the engineering of this bridge let down some that cherish these structures. And I
am hurt personally since I have put so much of my heart into this work.
You are certainly entitled to your
opinions. I ask that you consider the challenges of engineering related to these structures. If it
was easy to handle, everyone would be a covered bridge engineering expert.
Sincerely, Phil Pierce
Date: Sun, 30 Jan 2000
Hi Phil, thank you for taking the time to write up the problems with the Paper Mill Bridge.
We needed this a long time ago. I now have something I can take to the meeting with me. I was
going to ask you if I could share this with other people, but I guess I can because I got an e-mail
from Joe Nelson and he is going to put it on his page. Good.
In no way did I attack your
professional ability. I know you are an expert and you like covered bridges. That's why I asked
your opinion in the first place. I worked for the Department of Transportation for 36 years. I was
an electronic Tech. with the Federal Aviation Administration and I considered myself a
My biggest beef is with the people that
made the decision to destroy the bridge. Covered bridge people seemed to have been left out.
You say it was on television, but not many people saw it. Did it make the papers? I don't think
There are other ways to keep a bridge
that was as bad as you determined this one was. How about putting steel I-beams under it? It has
been done in lots of other places in Vermont. Many of Lamoille Counties covered bridges have
I-beams under them. Then they can take the traffic and the problem parts of the covered bridge
can be repaired.
Did you see the Newfield, N.Y.
Covered Bridge before it was repaired in 1972? It had such a sag that you might not have driven
over it. You know what it looks like now after Mr. Graton fixed it.
I documented the building of the replica
Henry Bridge. They glued the tree-nails in place and then cut them off flush with the planks!! It
doesn't look good and I don't think this is a good way to put a Town Truss together.
Anyway the bridge is gone. We can't
bring it back. I and many others had many questions as to why? I now have something to show
them. Thanks again for answering my questions. Dick
Date: Mon, 31 Jan 2000
Hi Dick, I am pleased if my ramblings have offered any help. Your latest note included a
couple of thoughts that I have attempted to address herein.
Regarding the notification of the locals,
I can only note that when the Contract was underway to provide engineering services to the
Vermont Agency of Transportation for the work at Bennington, the local officials were apprized
of our work and they obviously attended the Public Meeting, again with local television and press.
Now that the design engineering contract is completed, the follow up is being handled directly by
the Agency who will be providing construction observation services (I think), since
McFarland-Johnson was not contracted to provide such services. If no special notification was
given in the local press, it was unfortunate, but not the responsibility of the design engineer.
Regarding other means of rehabilitating
a covered bridge, yes there have been many instances of the addition of supplemental steel or
glue-laminated components. And yes, there have been instances where the original timber floor
has been removed from within the covered bridge with a replacement floor system to support
vehicular traffic, independent of the original timber trusses. However, with the passage of the
National Historic Preservation Act in 1966, there are fewer acceptances of these actions. The
Vermont Agency of Transportation, in conjunction with the Vermont Division of Historic
Preservation, directed us to avoid these actions. Supplemental systems would have been tolerated,
if absolutely necessary, but removal of the floor and substitution of an independent floor would
not have been accepted.
The historic preservation issues related
to this are complex and I do not profess to be an expert with respect thereto; however, I am
strongly opposed to the removal of floor systems with installation of an independent floor for
structural and practical reasons. First, the self weight of a bridge, in combination with the
substantial snow loading in Vermont represent a loading on the bridge that is often substantially
heavier than the loading from vehicles. Therefore, removal of the loading of vehicles from the
trusses does not necessarily relieve them in any substantive way. The trusses, if expected to still
support their own weight and the roof, would still require major rehabilitation in many
Second, the regularly scheduled bridge
inspections, mandated by the Federal government, are focused primarily on the components that
support vehicular loading. If an independent system is installed that relieves the trusses and
overhead structure from involvement with vehicular loading, it might lead to reduced attention by
bridge inspections. If that was the case (and I admit that there are differences of opinion in this
regard), the trusses might suffer from increased neglect. If so, this action risks elimination of a
regular reminder of the need to maintain the entire bridge.
Considering the potential of the
installation of supplemental members beneath the existing structure (including a timber floor)
raises another critical issue: that is hydraulic opening, i.e., the area beneath the bridge for passage
of flood waters. The Paper Mill Bridge has a very small area beneath it for water -- in fact, one
has to stoop to get under it and a large portion of the span is over a sandbar. To install new
members of sufficient size to offer any real support, they would have to be quite deep. Therefore,
the bridge would have to be raised so that the bottom of the new beams would be no lower than
the existing bridge components. However, the east approach is already quite steep and increasing
its grade was considered unacceptable. The close proximity to the intersection limits the
So, if we cannot add supplemental
members beneath the bridge, how about adding them inside? There are a few bridges around
wherein supplemental girders have been added and connected to the trusses so that they hold up
the bridge from within. There are structural disadvantages related to this action by limiting the
accessibility to the trusses, which aggravates the ability to continue to maintain and repair them.
But more importantly, in my opinion, the affect on the look of the inside of the bridge is awful.
When I cross through a covered bridge, I am most interested in seeing the trusses that hold it up,
not some other add-on system (again -- my opinion).
Again, I state with conviction, that all
rational options were explored for this bridge, before the final solution was selected.
I commend the Agency of
Transportation for their willingness to expend fairly substantial sums in support of the covered
bridges in the state. The Statewide Study was the most extensive and exhaustive study of its type
ever undertaken in the world. Further, the follow up investments in an attempt to find the "hidden
reserves" that some of us believe exist, are without equal. In addition to the more routine aspects
of this work, the VAOT authorized and funded the following actions:
The removal of a few of the original
lattice members which were tested at Virginia Tech to ascertain their actual strength for
comparison with values obtained from national timber specifications for average members of the
same species and grade (I am unaware of any other similar tests of covered bridge
The most extensive testing (at MIT)
ever undertaken in the world, (to my knowledge), of full size trunnel connections of typical
configurations used in Town Lattice trusses, and the most extensive computer simulation of the
behavior of a covered bridge supported by Town Lattice trusses, using complete
three-dimensional modeling, ever undertaken in the world (again, to my knowledge).
My eight plus years of involvement with
the covered bridges in Vermont, working for the Agency of Transportation, has been inspiring.
Yes, we all have regrets for past actions, and no one professes to be perfect, but I think the
Agency has demonstrated remarkable national leadership in their support of the covered bridges in
Vermont. I am proud of them and the part that I have played in this work.
Thanks for your consideration of these
remarks, sincerely made. Phil
Return to top
Joe Nelson, P.O Box 267, Jericho, VT 05465-0267, firstname.lastname@example.org
No part of this web site may be reproduced without the written permission of Joseph C.
Text Copyright © 2000, Phil Pierce
Text Copyright © 2000, Richard Wilson
This file updated 3-3-00