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Federal Highway Administration > Publications > Research > Structures > Covered Bridge Manual

Publication Number: FHWA-HRT-04-098
Date: APRIL 2005

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Appendix H. Replacement of the Mill Covered Bridge, Tunbridge, VT

The picture shows the longitudinal view of the long-span bridge (built circa 1883) with natural siding over a river with boulders.

Figure 233. Original view.

The picture shows the bridge collapsed with snow in the foreground. The roof looks largely intact but the sides have splintered and are leaning outward after thebridge was struck by ice

Figure 234. Destroyed March 4, 1999.

This diagonal view of the newly built bridge in winter shows snow on the roof and the banks, the extended portal with a sign, four windows, and metal side rails.

Figure 235. View in December, 2002.

Mill Bridge at Tunbridge, VT

A covered bridge was constructed over the first branch of the White River in the heart of the Village of Tunbridge, VT, in 1883. The single-lane structure was supported by multiple kingpost trusses and was approximately 22 m (72 ft) long (see figure 236). Unfortunately, the bridge was struck by ice on March 4, 1999, and collapsed the following day.

The picture shows the original 22-meter (72-foot)-long structure built in 1883. While the abutments are concrete, the approach rail is simple post and board fence.

Figure 236. A view of the original bridge

The community wanted to replace the bridge with another covered bridge, and the Vermont Agency of Transportation supported the project by providing the bulk of the funding for the new bridge.

The design criteria for the project included a stipulation for a design vehicle of a single 13.5-MT (15-ton) truck (a convenient bypass exists for heavier vehicles). Further, the trusses were to be built of local native Hemlock, if possible, with similar sizes and configuration as the original bridge. Unfortunately, the design stresses resulting from using the original member sizes would have required select structural Hemlock. Although some local timber sawyers were confident that they could provide acceptable good quality timber, a lumber grader willing to certify that grade could not be found. Accordingly, it was concluded that local Hemlock in the grading necessary was not available. Select structural Douglas Fir was accepted as an appropriate substitute for the critical truss elements.

One unusual detail in the design involved reinforcing the corbels of the vertical posts against shear failure. The details involved inserting four 25.4-mm (1-inch) diameter hardwood pegs in the corbel to provide additional resistance to high shear stresses (see figure 237). According to the analysis of the bridge, the reinforcement was required in only the end three-post elements. Unfortunately, shortly after the bridge was opened to traffic, an overweight vehicle crossed the bridge, contrary to the posted weight restriction, and caused a shear failure in an unreinforced corbel. A steel rod was inserted adjacent to the post as a repair (see figure 238).Figure 239 shows a closeup of the rod connection, and the arrows indicate a slight vertical dislocation along the slip plane. This indicates that it would be wise to reinforce more corbels than required by the design vehicle; the cost was nominal at the time of construction, and the repair was difficult to install with the siding in place and with the bridge located over water.

The picture shows the vertical post configuration with diagonal supports. The white arrow points to the 25-millimeter (1-inch) diameter wooden pegs used as reinforcement in the post corbel to prevent shear failure.

Figure 237. Wooden peg reinforcement of post corbel.

The picture shows the vertical reinforcement rod inserted next to the post after an overweight vehicle caused shear failure in an unreinforced corbel. This accident indicated that reinforcement of more corbels than required by the design vehicle would have been a cost-effective measure.

(photo courtesy of Scott Sabol)

Figure 238. Reinforcement rod.

This picture shows the closeup of the rod connection with dtwo whnte arrows indicating the vertical misalignment along the slip plane. This repair was more difficult once the bridge was reinstalled over the water and the siding was in place.

(photo courtesy of Scott Sabol)

Figure 239. Closeup of the failed corbel after the repair.

The bottom chord tensile splices duplicated the use of bar and rod connections, one of several types of tensile connections commonly used in original covered bridge construction (refer to figure 240). This is a view of the original. The replica bridge used a very similar detail.

This picture of the original underside of the bridge shows the bar and rod connections used as a tensile connection that were duplicated in the rebuilt bridge. Large steel bars were inserted into the bottom chord on either side of the splice, with their ends projecting.. These bars were connected by a square steel rod with threaded ends and nuts for tightening the joint.

Figure 240. Bar-and-rod connector tensile splice.

Another important issue related to the bracing of the original bridge. Although the bridge had managed to survive for more than 100 years, the internal bracing was minimal. The contractor sought approval for installation of heavier internal bracing, and permission was granted. New stout tie beams were installed along with heavy knee braces that were connected above the tie beams to form a substantial transverse frame.

Timber curbs were installed at 3-m (10-ft) spacing along the bridge to prevent vehicles from impacting the knee braces, a common occurrence at the previous bridge (see figure 241). The gap between curb and approach rail allows pedestrian traffic to pass through the bridge outside of the curb. Note the reflector used at the end of the curb-an effective identifier.

This picture shows the timber curbs installed to prevent vehicles from hitting the knee braces, which happened frequently at the old bridge. The gap between the curb and the approach rail lets pedestrians pass safely through the bridge.

(photo courtesy of Scott Sabol)

Figure 241. Transition from approach rail to inside curb.

The bridge was built on the approach roadway and moved into place with steel rollers supported on two steel I-beams spanning between the abutments. Consistent with the traditions of original construction practices, the power for the relocation of the bridge was provided by oxen turning a capstan (see figures 242 and 243). Figure 244 provides a view of the completed bridge. Note that the windows exist specifically to facilitate traffic crossing through the bridge from the far side to help users see the sharp roadway curve on this side.

The picture shows moving the bridge on steel rollers supported on I-beams spanning the abutments. Wires attach to a capstan turned by oxen. The bridge still needs some finish siding and work completed at the portal.

Figure 242. Bridge being moved into position.

The picture shows the oxen turning the capstan that moved the bridge. A crowd stands in the background observing the July fourth event.

Figure 243. Oxen power and capstan.

This diagonal view of the completed bridge in winter shows snow on the roof and the banks, the extended portal with a sign, four windows, and metal side rails.

(photo courtesy of Scott Sabol)

Figure 244. Completed bridge.

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