Restoration of Covered Bridge Updated |

Restoration of Covered Bridge Updated

Herb Lindberg

David I. Wood’s 1862 covered bridge at Bridgeport in the South Yuba River State Park has been closed for repair since 2011.

The people of western Nevada County, mainly of Penn Valley, Grass Valley and Nevada City, rallied together to petition our state legislators to fund a program to bring the bridge back to life.

The legislator’s response was to support a two-stage project: $1.5M to stabilize the bridge against further damage, and $6M to restore the bridge. Stage 1 is complete and Stage 2 has been underway since mid-June, 2019.

Seven videos totaling 90 minutes were created to show in detail the amazing step-by-step restoration process to date.

Recently, these were compressed to 8 minutes to make the BridgeProgressSummary.html easily accessible. The objective of this article is to give TWI readers an opportunity to see the importance of the project and an understanding these fundamentals. See:

Cradle tower and roof being removed

The product of Stage 1 is the “cradle” you see in the photo. It consists of a tower and cables (and their twins at the opposite end of the bridge) which support four cross beams at critical locations under the bridge that can support the bridge if it becomes dislodged and would otherwise fall into the river bed.

Photo 1 also shows the first step of Stage 2 – restoration: removing the roof. This, and other deconstruction, was done to allow detailed inspection of every timber, steel rod, bolt, plank and so on to determine which can be re-used during reconstruction of the bridge.

Deconstruction is also a necessary part of restoration.

Exposed bridge structure

You can see the key elements of the Burr arch – Howe truss design of David I. Wood’s bridge. The main load-carrying parts of the truss are the thick, doubled, tilting, timbers that repeat to the center of the bridge. Analogous timbers tilt in from the opposite end. The truss is completed by top and bottom bridge chords and pairs of wrought iron tension bars that connect from the bottom of one timber pair to the top of the next, 108 bars in all. The much thicker “chords” also complete the bending strength of the bridge. The bridge center and top of the arch, which also extends below the roadway level to abutments at each end, can be seen in the distance.

Shoring tower for independent support of Mabey bridge

The fundamental idea behind restoring the bridge is to first build an independent steel bridge inside it that can be used to support scaffolding and also raise the wood bridge and attach it to higher abutments. The bridge used was designed by the Mabey Bridge Co. and still bears the name after selling to another company. It is bolted together much like an erector set for quick assembly and disassembly.

Construction of supplied Mabey bridge panels

They are being completed in the work yard (a.k.a. SYRSP south parking lot).

Side panels of the Mabey bridge were completed in the work yard and then further assembled into continuous side panels at the bridge. When assembled they were pushed across the bridge on rollers atop cribs and the shoring towers .

Pushing the Mabey Bridge into Wood’s Bridge

… over rollers atop outside cribs and the shoring towers.

Hanging scaffolding/ lifting beams beneath Wood’s bridge

The main function of the Mabey Bridge is to support scaffolding and lifting beams, hung from it and beneath Wood’s Bridge.

East upper work platform resting on the scaffolding beams

A similar west upper work platform is on the opposite side. There are also east and west lower work platforms beneath Wood’s bridge

The other function of the beams hanging beneath Wood’s bridge is as part of the jacking system to lift the bridge and place it on higher abutments, farther from threatening floods that destroyed Wood’s original 1850 bridge and caused serious damage to the present bridge in 1997.

The project was scheduled for completion on May 29, 2020 but has been delayed to Sept.29, 2020 to give time for final analysis of the crucial and delicate jacking system.

This article, and associated video, are based on photos taken by a team of 10 photographers under the supervision of producer John Field: Dave Arstein, Steve Berry, John Field, Shirley Moon, John Rebenstorff, Janet Peters, John Thompson, Marc Wetherbee, Denise Jaffke and Tim Andrews.

I’m deeply indebted to them for these photos, which include all those in this article.

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