Introductory Presentation: J.B. McCarthy and Mark Colgan apologized for the oversight in not notifying town officials about the previous meeting in September and then discussed the town's concern about the need for its loaded highway maintenance truck, H25 loading, to use the bridge. The proposed alternative achieves only an H17 loading, adequate for school buses but not the maintenance vehicle. Town officials had inquired whether it would be possible to increase the secondary steel support system in order to achieve the H25 load. Subsequent discussion focused on the exact weight of the maintenance vehicle - loaded or partially loaded, and calculated the figure roughly to be between H23 and H25.
Mark Colgan explained that the engineering problem is not inadequate strength of the steel structural system but rather that the steel girders are more flexible than the combined timber truss-arch system and deflect to a greater extent when carrying loads, as much as an inch or more. Because the floor system of the Burr arch-truss is tied to the steel girders, the former flexes as much as the steel girders, creating excess stress on the joints of the arch-truss system. Eventually, that stress will cause the arch-truss system to fail. By limiting the loading to H17, the timber structure would be capable of carrying loads independently of the steel girders, the two structural systems could be separated, and the latter could serve as a secondary safety system functioning only during overloading.
Discussion: The discussion that followed framed the dilemma facing the committee, namely that continued use of the bridge by loaded town maintenance trucks will create excessive stresses on the bridge, ultimately damaging the bridge's historic structural and material integrity. However, should the town's maintenance vehicle be unable to use the bridge, the town would be forced to select an alternative route that is six miles longer (three miles each way), necessitates greater changes in elevation, and hinders the town's ability to clear and sand roads in the vicinity of the bridge in a timely way for school buses when snow or ice collects. Although Route 7 would reduce the extra length of an alternative route, town officials choose not to use that road because of the volume of traffic and speed of that traffic.
Committee members then began a dialogue about the various other alternatives available for consideration.
Strengthening the existing Burr-arch truss / removing steel girders: Eric Gilbertson observed that he had reviewed the project twenty years ago when the steel girders were installed and that the bridge capacity, as originally designed, carried a loaded, single-axle truck (possibly H22 or H23) with only a ¼ inch deflection, suggesting that the steel girders might not be necessary. However, current engineering analysis concerning the capacity and deflection of steel members is well established, and the analysis of the arch-truss, if not precisely accurate due to various uncertainties in quality of material, capacity of joints, and stress distribution between arch and truss, is nevertheless accurate to the point of reliability.
Continuing in that same vein, though, Gilbertson then suggested that the arch-truss might be strengthened by replacing upper (and if necessary, lower) chords with continuous glu-laminated chords or, alternatively, requiring only a single splice (centered) or two quarter splices, the latter leaving the critical central span of the chord continuous. Although this alternative would require substantial change to the bridge's structural and material integrity, the steel girders could be removed and the bridge would function structurally in its originally intended fashion. The glu-laminated chords would probably be larger in dimension that the existing chords. Discussion about the various types of splices followed.
Substituting glu-laminated girders: Priority Treatment No.4 in the Historic Covered Bridge Preservation Plan permits the use of co-functional, reversible glu-laminated girders as a secondary structural system because these girders function more consistently with timber truss systems. Those girders can be placed either below the deck, above the deck, or partially above and below the deck. However, placing girders below the deck in this case would create an obstacle to high water because of the bridge's low clearance; placing the girders above the deck would visually obscure the bridge's structural system.
Constructing a New Bridge on New Alignment/Restoring Historic Bridge: Where the load capacity of historic timber-framed bridges is inadequate to support the traffic that must use the bridge, finding a good location for a new bridge represents an ideal solution. The new bridge can meet all load requirements, and the historic bridge can be restored for use by limited loads. Charlotte officials have not considered possible sites for a new bridge but anticipate public opposition to such an idea. The discussion did not advance far enough to consider the additional costs or other difficulties that would be associated with that option.
Modifying Burr arch-truss / steel girder connections: John Weaver suggested that a fourth option might involve modifying the manner in which the timber arch-truss system is connected to the steel girders, creating a deeper notch that allows the steel girders to deflect under load without causing a corresponding deflection in the timber arch-truss. As loads increase, the arch-truss would deflect under load separately, becoming truly secondary to the steel structural system. It might also be possible to introduce a type of spring mechanism to insure that the two structural systems deflect independently.
Recommendations: Each alternative considered requires engineering analysis, and Scott Newman requested that he be given an opportunity to review the project for compliance with Section 4(f) and Section 106 before additional engineering analysis is undertaken. Specifically, VTRANS and FHWA must consider whether the additional mileage and concerns associated with the available detour makes that alternative imprudent for purposes of satisfying Section 4(f). Whether a site for a new bridge exists, and the comparative costs for that bridge, must also be considered. The meeting concluded with agreement to investigate those issues prior to further engineering analysis, and with plans to schedule a third meeting at a convenient opportunity.
Meeting Adjourned 12:05 P.M.
[This article was originally posted March 21, 2012]