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A Covered Bridge Question--and Some Answers
Which truss lasts longest?
When ever I have a question that I can't seem to find an answer for, I reach out to those who are more knowledgeable about our covered bridges than I am. Thus, the reason for this email.
At a recent Covered Spans of Yesteryear presentation, the following question was asked and I'm hoping you will be willing to share your thoughts in regards to an answer. Here's the question: Was there a particular type of truss that lasted longer in a bridge than others?
Although I'm not sure if there is any way we can actually determine this, I'm hoping some of you might have a scientific answer, or at best, an educated guess. Our uneducated guess is a Town Truss. Our reasoning is based solely on knowing how strong the Town Truss is, and from seeing so many old photographs of Town Lattice bridges that have been removed from their abutments due to flooding. Often times you will see where the entire side of the bridge is still intact even after the trauma from the flooding. We have also seen a few photos of Town Lattice bridges that have been destroyed by fire to some degree, but yet the truss still appears to be pretty much still in one piece. As I said, this is just an uneducated guess.
If any of you would care to express your thoughts on this question, we would be grateful.
Thanks everyone . . . and enjoy your day.
Yours in Bridging, Bob and Trish Kane
Response from Miriam Wood, Hilliard, OH
That is indeed an interesting question. I would say it depended greatly on the maintenance it received through the years. On the all-wood trusses, I would have to say the Long truss, if correctly built according to Col. Long's specs. For the combination trusses, it has to be the Howe.
Response from David Simmons, Columbus, OH
I think the issue would have to do with "tightness" over time that would come from a consequence of the natural tendency of wood to shrink and shorten. The technical term for this is "creep." That's one feature of wooden trusses that made a true Long truss with wedges such an advantage, since it could periodically be re-stiffened by re-driving the wedges (see the HAER documentation for the Eldean CB, Miami County, OH).
For just that same reason, a Howe truss would have had an enviable longevity since the iron verticals would allow the entire truss to be retightened. The fact that the Howe remained a part of standard RR construction throughout the 19th century is testimony to its value and durability. Furthermore, a serious research issue that remains is searching through the literature of railroad maintenance crews to discover if the process for tightening Howe trusses was ever written down. It has obvious value for any local bridge authority that now owns a Howe truss. Your suggestion of a Town truss as a long-lasting truss is reasonable simply because there were so many redundant members which would make shrinkage overall less of an issue.
Response from Robert Durfee, NH
I do not believe you will find a true scientific answer to your question. There are many factors that relate to longevity of a covered bridge and to the trusses, including frequency of traffic, loading, maintenance,extent of protection (siding), timber species, etc. You have already received several different opinions, all are good.
I would say that the Town Lattice Truss lasts longer, but I have no definitive proof. Here is my reasoning:
1. A review of the NSPCB World Guide to Covered Bridges indicates that more Town Lattice Truss Brides are in existence and have survived, over any other truss type. This fact may be misleading, in that it does not prove that the Town Lattice last longer. It may just point out that more Town Lattice Trusses were built over other truss types, thus leading to more of these trusses surviving.
2. A Town Lattice is a very sturdy and rugged structure, having a lot of redundancy in its members. Should one member fail due to rot, bug infestation, collision damage, or overloading, there are several adjoining members to pick up the load. Other truss types, such at Howe, Pratt, Kingpost & Queen Post, etc., do not have much, if any, redundancy. A failure of one member in these trusses would usually lead to a total failure of the truss and a collapse of the bridge. Thus the redundant Town Lattice Truss would have a better survival record.
3. Different truss types were chosen for covered bridges for specific reasons related to a site such as economy, timber availability, and skilled or unskilled labor available. The most economical truss for a given location was usually the deciding factor. The original builders probably did not think much about longevity when selecting a truss type, as all trusses were sheathed or covered to protect them.
Good luck in your quest for an answer.
Response from Phil Pierce, Treadwell, NY
Morning, All - I am not disagreeing with responses to date - but from another perspective, the Burr Arch has been identified as one of the earliest patented configurations and there are a lot more of them extant than other configurations which must mean something. Now that doesn't indicate that they were better perhaps, but at least popular and long lasting in spite of the typical lack of attention that most CBs have endured.
Response from John Weaver, VT
To my knowledge there is no particular type of CB truss that lasted longer than other types. They all require protection and maintenance to function over a long lifetime.
The plank lattice truss is the easiest CB truss to fabricate and repair. However it is subject to racking, due to its slender cross-section, and is vulnerable to lattice member impact damage.
Response from Leola Pierce, VA
In answer to your question about the strongest truss. I believe it is a Burr Truss because the Burr Truss is used on the longest bridges. Therefore it would seem to be the strongest and last the longest.
Response From Sylvain Raymond, Canada
Okay... although not at all an expert, but more like an observer, I will venture an answer and everyone here will be able to see if it makes sense. The Town truss is a pretty good choice for an answer for it has proven easy to build and long lasting. It has been used for the Railroad as well as for little used private roadways. Where wood was plentiful and skilled workers few, the Town truss became all the more popular. Now, can we deduct from this that the Town truss is a longer lasting proposition?
My answer is no. I think that maintenance and overall building material quality determined if a bridge was to last or not. ANY structure will slowly degrade and need repair. The art of bridge building has to do with material, maintenance, structure type and, less we forget... CLIMATE!
The recent events in Laval, Quebec can be brought in here and made to mirror our very own evolutionary curve! When a covered bridge failed in Vermont or Ohio back in the 182X, man and beast where lost. Much to my understanding of structural engineering, a 200 year old covered bridge can still be a solid part of ANY infrastructure. Much to my very own fear... the next highway overpass may kill me!
As I saw in Germany again last week, bridges built around 1700 are still used for everyday use by vehicular traffic. So why can't an overpass built in 1971 stay up on its abutments? Maintenance is the answer.
Blessed where those covered bridges built in areas where their construction was deeply appreciated and well understood. The thrifty elected officials saw to it that the money spent was going to be an investment as well as a way to carry traffic over waterways. Cursed are those who poured concrete everywhere and now tell us that our roads are safe... for they are not! Of the thousands of cement bridges built in the Americas since the 1950s, very few are actually in good shape. Many are in what we can call the gray zone, this is a structure which is in no danger of collapse but, a structure that has lost structural integrity due to ice formation and pealing of the protecting layers, lack of proper maintenance, clogged drains and total lack of basic inspections. And I could go on . . .
So for the covered bridge longer lasting must be taken in account the team of dedicated folks who will see to it that the structures remain safe at all time. By sheer number, the Town truss main win overall. But then... considering other system like Howe, the basic Queen and others . . . I have a hard time giving the credits for long longevity to an assembly on timber alone
Response from Joseph Conwill, ME
On bridge longevity, I agree with those who say the key is proper maintenance, rather than any inherent advantage of one truss type over another.
Another factor is unstable economic conditions, which bring changes in loading demands. We see this now in covered bridges that have been swallowed up by the suburban development. An earlier example was found in Oregon where bridges were designed for relatively high loadings—typically 15 tons—but then suddenly had to carry log trucks which might weigh up to 40 tons. These bridges were among the strongest built yet also among the shortest lived. This problem could happen with any truss type.
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Joe Nelson, P.O Box 267, Jericho, VT 05465-0267
Posted 10/27/06, Revised 11/1/2006