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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.

Hello Dick,
        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 connectors).
        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 glue-laminated components.
        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 professional.
        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 so!
        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 instances.
        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 options.
        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 components);
        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

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Text Copyright © 2000, Phil Pierce
Text Copyright © 2000, Richard Wilson
This file updated 3-3-00