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The Vermont Covered Bridge Society Preservation Committee Conference

Meeting Summary

  1. The value of an historic covered bridge lies in the workmanship, methods and joinery of the original builders and the timber from Vermont's old forests used in the construction. This historic integrity needs to be preserved.
    • David Wright:
      • Historic fabric is at the heart of preserving a covered bridge. A replica is not an original. Even if you repair a covered bridge in kind, the most sensitive restoration or rehabilitation, it is still not the original structure.
      • The fabric that should be saved is that which is still functional and could continue to serve.
      • The historic fabric is the focus of preservation. Otherwise, it is not preservation at all, it is pretty structures for tourists, it is replication.
      • If you see a stick that looks good from the outside, but has rotted, it is the illusion of the historic fabric, so one shouldn't feel too badly on removing that stick.
      • If you remove pieces of a covered bridge truss that don't need to be removed in order to get greater capacity from it, you're not preserving the bridge, you are not restoring it, you are doing damage to it.
      • Even a very badly deteriorated bridge can be restored somehow. It may mean a lot of replacement material, a lot of splicing, and a lot of doubling up, but we're trying to save the historic fabric.
      • The real monument is the trusses, upper and lower lateral bracing systems and probably the wind bracing. So anything which alters those makes the bridge less bear witness to it's period.
    • Phil Pierce:
      • Our efforts for historic preservation should be more focused on maintaining the skills necessary to rebuild these important structures, than just on maintaining the material of the bridge.
      • If we replace components as necessary, using traditional details and methods, to the extent possible, then we are maintaining that structure for our children and their children to enjoy for the indefinite future. Otherwise, they will eventually decay to the point that they will become useless heaps of rotting timber.
      • I believe the builders of these bridges would much prefer a useful bridge that strongly resembles the one they built to an unsafe object.
      • In naval Architecture, fabric be damned, keep the keel. It is imperative that you keep the integrity of the structure.
    • Jan Lewandoski:
      • The design and the historic engineering of the bridge is as important as the actual structure.
      • It's the historic trusses that are the most important thing we are trying to preserve. Rather than subdivide a bridge into spans just to keep the historic fabric intact, it is better to change more material to keep the bridge as a single span still functioning, rather than save historic fabric but reduce the function of the bridge.
  2. To preserve historic integrity when a covered bridge is repaired, the joinery used by the original craftsman should be duplicated and the timber used in the repair should be of the same species as the original.
    • David Wright:
      • To preserve the historical integrity when a covered bridge is repaired, the joinery used by the original craftsman, 'must' be duplicated and the timber used in the repair should be of the same species as the original.
    • John Weaver:
      • If the desired methods and materials to preserve historic integrity are not practical, then supplemental support systems or splicing with modern materials should be considered to preserve otherwise functional historic members.
    • Jan Lewandoski:
      • If you can, restore in kind; same species, same joinery, same member sizes, same everything. But there are often reasons why you can't, one of them being changing use of the bridge. If you are going to put much heavier loads on it, you may have to beef it up. Most of the bridges in the state had lighter floor systems at one time, for horses and wagons. Heavier trucks required heavier floor systems. The floor systems were changed throughout the 30s, 40s, and 50s.
      • Some bridges were no good the day they were built. They are still here, but they have flaws in their design, so what do you do? Throw the bridge away? No. Stick piers all over the place in it? Well, maybe, but you can't easily do that. You can do a number of other things. From repair in kind; same species; joinery; size; to change of species of some members.
    • Phil Pierce:
      • It is more important to maintain the skills of dealing with a bridge effectively by craftsmen that know what they are doing, rather than necessarily saying that thou shalt keep the fabric at all costs. (Develop craftsmen to keep the tradition of building wooden bridges alive.)
    1. Native timber should be used in covered bridge restoration, other timber to be used as last resort.
    • Phil Pierce:
      • If we can generally duplicate the bridge with stronger species, than at least we are producing a bridge that has a chance of meeting code requirements.
    • Jan Lewandoski:
      • If you go to a different species, sometimes without making a great change in the disposition of the truss you can get quite a bit greater carrying capacity. Sometimes just changing one or two sticks to a different species will help a great deal with whatever the problem is in the bridge.
      • Native species are difficult to find, especially in great quantity or on short notice. You are going to have to get something else. Some early bridges used white pine, but you can't get white pine of that quality anymore.
      • Glue laminated lumber is not needed. It has high design values, but it's no stronger than anything else. You may have more confidence in it. It comes in longer pieces.
  3. Covered bridges should remain in service with historic integrity intact. To achieve this the bridge needs to be maintained on a regular basis to keep deterioration from occurring to avoid large-scale repairs that could compromise historic integrity.
    1. The trusses must be kept dry: Approaches to the bridge must be properly drained, or sloped to keep run-off away from the bridge structure; Roof leaks need to be repaired immediately. Road dirt must not be allowed to accumulate on the truss members; the bridge must be kept clear of weeds, brush, and saplings.
    2. Oversized and overweight vehicles must be prevented from entering the bridge. A smooth transition between approach roadway and the bridge floor must be maintained to prevent shock to the bridge structure by entering vehicles.
    3. In addition to regular maintenance, the bridge should be inspected at least twice annually: Once prior to spring, and once prior to winter with remedial action taken in preparation for the season.
    • John Weaver:
      • All covered bridges should be treated with fire retardent in the course of maintenance. Fire protective systems should be part of the contract when covered bridges are restored or renovated.
      • We need to develop a policy towards rehabilitation, maintenance and reconstruction of abutments, piers and other structures. Sometimes that's all you have to go with if the bridge is burned down or has been destroyed for some other reason, then the only thing that still qualifies that bridge for rehabilitation are the substructure units that are left. That is necessary to getting five percent local participation for the project.
      • Otherwise you'd have to start a new bridge and the local entity would have to pay at least ten percent. We are interested in the substructure units at least being maintained or looking as much as they did originally, even if it means facing the new concrete abutment with the old stone, or underpinning or some other reconstruction to maintain these foundations in an historic sense.
    • Ed Barna:
      • Use of height limiting structures at discrete distances
    • Neil Daniels:
      • Tie the bridge inspections to the federally mandated state inspection which carries with it some teeth. Our Bridge-watch people should be able to work with the state inspections to see that things get done.
      • We need to address the fact that towns neglect all road structures and see if we can change this.
  4. When a covered bridge has deteriorated to the point that it must be repaired or restored, the restoration contract needs to be explicit that if the bridge is found to be beyond restoring to full use to convey motor traffic, it must not be destroyed by replacing all or a significant portion of the original truss, but instead left intact, bypassed or removed from its site.
    1. The bridge, retired on site, can be repaired to safely support foot or cycle traffic while maintaining its historical integrity.
    2. If the retired covered bridge is removed from its site, it should repaired to prevent further deterioration, preserving its historical integrity, and maintained as a visitor's center or placed in a park or museum.
    3. It would be optional to replace the removed bridge with a replica or with a modern bridge.
    • John Weaver:
      • A significant portion of the original truss is 50% to 60%.
    • Phil Pierce:
      • As soon as a bridge is bypassed, taken out of service, its going to lose the probability of funding, in time it's going to fall in the water. I don't think that's what the original builder of that bridge intended to happen. I think our mission, from my standpoint, should be to keep that a functioning bridge to serve its public.
      • I save my most ardent objection to the philosophy of closing bridges to use rather than resort to different details and/or materials. We have seen too many examples of bridges destroyed by vandals that target closed bridges.
    • David Wright:
      • Bypassed bridges can turn into abandoned bridges. Public money is tight. It's a lot easier to justify funds for something that is serving an obvious public good getting people from one side of the river to the other than it is to save something that is on display.
    • Ed Barna:
      • Policy which should advise against methods of historic preservation which isolate bridges and marginalize them from community use.
  5. To keep historic integrity: Native timber, meaning northeast species, should be used in covered bridge restoration, other timber to be used as last resort. To permit the use of native timber in bridge structures, there needs to be an effort by the timber industry to test, characterize, and certify north-eastern species.
    1. A non-destructive test protocol needs to be developed to be used on a covered bridge prior to writing the contract for repair or restoration to ascertain the true condition of its components, this to avoid discovering the structure to be in such a state as only complete replacement of a significant percentage of the truss is required to restore it to full use, resulting in the loss of historic integrity.
    • Phil Pierce:
      • The national leaders in the establishment of timber allowable stresses are centered at the Forest Products Laboratory in Madison, Wisconsin. They would love to do even more than they already have (yet they have led the world in this field). However, additional research carries with it the need for additional funding. If the VCBS wishes to urge more research, you should recognize that no one will voluntarily conduct such work without funding.
      • Regarding non-destructive testing protocols; timber is very difficult to deal with, unlike the more consistent nature of metal or even concrete. To my knowledge, no governmental agency has adopted any existing protocols as consistently effective in this regard. Again, if more development of this field is wanted it will take a lot more funding.
    • David Wright:
      • Non-destructive test protocols must be developed.
  6. To make maintenance of historic covered bridges effective: A practical way to enforce vehicle dimension restrictions needs to be developed. Partnerships between towns, the Vermont Historic Bridge Program should be encouraged. A practical way to enforce vehicle weight restrictions needs to be developed.
    • Phil Pierce:
      • We can force dimensional limitations.
  7. To pursue its policies the VCBS and its members will: Organize area chapters (Bridge Watch Areas) for the purpose of establishing a working relationship with the local municipal governments, historical societies and Chambers of Commerce to promote bridge maintenance and tourism, and to guard against vandalism; Educate the public on the value of Vermont's historic bridges; Work with the VAOT Historic Bridge Program.
  8. On preservation discipline:
    • David Wright:
      • One [of the questions] was the dating of the material. New material gets put in should be recorded. There are drawings of record. What's new and what's old should be clearly indicated on them. The pieces could be marked discretely in various places. When one of these bridges is to be restored or rehabilitated or beefed up, there should be drawings of record made of the bridge as it was found at the start of the project. Everything ought to be photographed, everything ought to be drawn. I know it's a cost, but never the less we are talking about principles of preservation here. These steps ought to be part of the preservation process before the bridge is touched.
      • The carrying capacity for some bridges can be upgraded using traditional methods, at the same time taking care that the changes are reversible.
    • Phil Pierce:
      • There should/must be a requirement to distinguish old fabric from new, both in the drawings and on the bridge itself.
      • The bridges were not designed for the heavy kinds of vehicles we ask them to handle today. Push for less design weight vehicles.
      • Is it not imperative to maintain a specific type of roofing material.
    • Ed Barna:
      • It is important to preserve the original timbers as much as possible. I think maybe what the policy ought to do is say, if they can't be preserved in the bridge, they ought to be preserved, or reused in some way out of the bridge to leave something to the future.
      • Save bridges by building higher capacity bridges near them, Preserving covered bridges as secondary rather than primary transportation structures.
      • Grow plots of timber for the express purpose of creating and restoring wooden bridges.
      • OK to put steel beams underneath bridges to allow the wood to remain intact (as fail safe).
    • Neil Daniels
      • We should [build for] passenger car use with these structures because that's closer to the weights the were built for initially, But many of the old ones without any work on them will carry 30,000. They won't calculate, but they'll carry it day after day. That's some of that "Access Carrying Capacity."
    • Jan Lewandoski:
      • Only fix things that are broken on covered bridges.
  9. Improve the process of covered bridge oversight:
    • Jan Lewandoski:
      • The biggest problem with species is the engineering drawings. Many engineers specify Douglass Fir and southern yellow pine for everything. It's easy to get, it comes graded, comes in big pieces and it's got high design values.
      • The contractor is usually late in the planning/design process. He's told to get the contract and start working in thirty to sixty days. Where the plans come specify certain species, the contractor hasn't much control over it.
      • The problem with using native timber and the problem with a lot of joinery is, as soon as you go to quantitative analysis, you go to design values, which are extremely conservative and being very general, are not sub-species specific. That's why live oak has very low values although it's a very strong wood. Because the white oak group has forty members, some of them weak, the whole group is rated low. Avoid quantitative analysis of these bridges, do qualitative analysis.
      • I advocate qualitative analysis, need better quantitative analysis.
      • If a bridge is not showing any signs of failure under live or dead load, there's probably no reason to re-engineer the bridge.
      • There is need to pay more attention to abutments. Stone abutments superior to concrete. Concrete is a very temporary material. Need more people who can do good stone work.
    • John Weaver:
      • I think the answer to covered bridge evaluation is better quantitative analysis. This may involve some evaluation of secondary support members and reserve capacity of the bridge as well for live loads. It certainly involves some investigation and evaluation for dead load which is long term and can be more severe in some cases than live loads in some structures. To enhance the quantitative analysis I think that when a project is started, if possible, a wood scientist should be employed to give an evaluation of just what the lumber is in the bridge so you know what your starting point is, so you don't just run to the NDS tables and look for the most conservative values to work with. If you know what values you really had to begin with, then you know where to start.
    • David Wright:
      • General contractors are one of the problems we have with some covered bridge restoration projects. Covered bridge repair, covered bridge restoration, seasoning of timbers, selecting timbers, finding where to order a special timber is not routine for them. They may win the bidding process, and then issue change orders until projects get out of control and bridges end up being seriously adulterated.
    • Phil Pierce:
      • Advocate design-build contracts rather than design-bid-build so that contractors and engineers can work together as a team. Each has viewpoints and expertise that work well together.
      • Extant covered bridges were not designed to today's standards and we cannot expect them to meet today's requirements.
    • Neil Daniels
      • I want to urge that we work with representative Don Davis, possibly next year, that would be the second year of his two year term, in our preservation policy and the State preservation policy paper that had been circulated today.

[This file originally posted March 25, 2002]