Back to Chronicles Tunbridge's Mill Bridge (as it was) Tunbridge's Mill Bridge Lost to Ice.
Mill Bridge Inspection Report Mill Bridge Reconstruction in Photos by Dick Wilson


The Mill Bridge Reconstruction -- Tunbridge, Vt.

Engineer Confident Bridge Will Be Ready For July 4
Purity of Construction in Replica Bridge Not Always Possible, Not Always Desirable
The Professional Engineer's Obligation

Mill Bridge as it was. Photo by J. Nelson, June 1993

Friday, December 24, 1999 07:48 AM
Mr. Pierce: How is the Mill covered bridge replacement project going? Will the bridge be ready for the July 4 celebration? I assume the design is complete. Please send me some of the specifications; bridge dimensions (length, width, roadway), timber dimensions, timber species, and similar data for the web page. Maybe you have some insights on the challenges of working with a 19th century design. Hoping to hear from you soon, your friend in bridging, Joe Nelson

Friday, December 24, 1999 11:25 AM
Hi Joe
        We finished the design in the summer. Was reviewed by the Agency of Transportation and they did not find much to change. In fact they sent a very complimentary letter to the Town of Tunbridge about good plans and design. I was delighted with that.
        My initial challenge was to design a bridge to basically duplicate the original with native hemlock. Unfortunately, hemlock allowable stresses are lower than needed to generally duplicate the bridge sizes. While a few timber sawyers were confident that they could find good quality timber, I was unsuccessful at finding any grading organization willing to certify that we could get enough Select Structural or Number 1 grade material to build the bridge with. This finding is not new and many criticize the engineering for not being able to duplicate original work. However, as a Professional Engineer, I must abide by certain standards that are nationally accepted and cannot set my own. If I did, and there was a problem, my professional reputation and licensure would never overcome the liability involved with such a flouting of accepted standards. I know of no other Professional Engineer willing to ignore such standards.
        Therefore, after a complete design was prepared, we had to do it again with better quality materials. We settled on Southern Pine because it is available and a typical good quality product.
        The contractor [Neil Daniels] ordered the material and has been waiting for it to dry before commencing work. I think he may be ready to go soon next year. He is intending to complete the work well before the July deadline, and I am quite confident he will.
        As to the basic dimensions of the Multiple King post trusses, the basic total length is 72'-0" center to center of end posts. The trusses are 17'-0" center to center. We raised the entrance clearance to about 10'-0" to better accommodate higher traffic that routinely has hit the top of the bridge. Timber curbs are going to be installed at 10'-0" to channel traffic to the center of the bridge, thereby allowing the introduction of better knee braces. The previous bridge had virtually no overhead bracing and suffered for its lack. Stout tie beams and a good x top bracing system will help keep the bridge square.
        The bridge is designed for a single 15 ton vehicle since a detour is readily available for heavier and bigger vehicles.
        As I have noted on other occasions to lay people, I continue to look for the "hidden" sources of the reserve capacity that these structures obviously have. I have identified a topic that I am exploring with some of the leaders of the timber specifications industry. I have little confidence of being successful in making changes in the specs, but am trying.
        Regarding other work, I hope to be underway shortly with design work for the rehabilitation of the Fuller Covered Bridge in Montgomery, VT. Still no notice to proceed.
        Am completing the design and contract documents for work on the Hamden Covered Bridge in Delaware County, NY. This work was started by another engineer - I am picking up the pieces and completing it. Have found a special tidbit regarding Colonel Long's use of wedges that apparently hasn't been discussed by others. Will be good fodder for my book.
        Am continuing to chase work with other covered bridges wherever it can be found. Will appreciate any plug you or others might offer.
Best wishes, Phil

Friday, December 25, 1999 3:42 PM
Thank you Phil. I have been getting questions from people concerned with covered bridge reconstruction not using native timber and with using modern materials like glulam -- Your response answers that question, i.e. "professional standards," "national standards," and "reserve capacity." Joe N.

Saturday, December 25, 1999 07:05 AM
        The one aspect to this topic that bothers me is the tendency of lay people to ridicule the engineering profession. While there are innumerable examples of bad engineering judgement, especially so with covered bridge work, I believe it is usually a function of a lack of experience and not an intentional lack of support for historical preservation. I have become very enthralled with this topic, starting with the challenges of the Vermont Statewide Study. Warren Tripp provided a lot of insight from their many years of dealing with a lot of covered bridges and he gave me the challenge of finding that hidden reserve. While we have spent a lot of time chasing all identified avenues, I cannot yet say I have succeeded. But I remain committed to continuing the search.
        Just as I accuse lay people of ridiculing the engineering, I am sure that there have been many examples of engineers acting overly defensive - I may be subject to the same sensitivity. I try to react with honest explanations in an attempt to share my responsibilities which I take very seriously. Via my professional engineering registration, I have committed to serving society with mybest engineering judgement based on my initial education and my years of on-the-job training and research. If I take my responsibilities too lightly, I know I can kill people with poor judgement and engineering, by allowing my structures to prematurely fail.
        I hope it becomes obvious that this topic is quite dear to me and will be a major feature in my book. I also hope that you are able to use this information and presentation to our mutual benefit in the further education of those interested in the preservation of these special structures.
Phil Pierce

Return to top

Joe Nelson
P.O Box 267, Jericho, VT 05465-0267

No part of this web site may be reproduced without the written permission of Joseph C. Nelson
Text Copyright © 2000, Phil Pierce
Photographs Copyright ©, 1997, Joseph C. Nelson
This file updated July 29, 2000