Submitted by Albion Bridge Stewards, a working group of ACAB
Two independent professional engineers—world-class experts in timber bridges—have prepared a report on the condition of the Albion River Bridge.
We strongly recommend anyone interested in this bridge download and read their report. To do so, click this link: Albion River Bridge Engineering Report.
Some key highlights:
The bridge is not in need of immediate or near-term replacement. The bridge is in remarkably good condition, thanks to its well-conceived structural design, high-quality timber materials, and effective connection detailing. While there certainly are locations on the bridge that require maintenance and repair, overall the bridge appears structurally sound and safe for continued use, as per the “Safe Load Capacity and Ratings” reflected in Caltrans’ January 5, 2018 Bridge Inspection Report.
The Albion River Bridge is in remarkably good condition, and is not posted with any load capacity/weight limits. The design of the bridge involves a highly redundant structural system with multiple load paths. Features of the design that lead to its robust and resilient character include the following:
- The timber members are of uniformly high-quality Douglas fir salt-treated by the Wolman method.
- The timber trusses of the superstructure and support trestles consist of members primarily subjected to axial compression forces, for which heavy timber members are well suited.
- The dimensional integrity of the bents in the trestles is impressive. Column lines are straight (to the eye) and the bents are planar. (Figure 1 below.)
- The battered columns provide a base that is wider than the bridge deck, thus enhancing the ability of the bridge to resist lateral loads, such as those from wind.
- In August 2002, a loaded logging truck plunged down the side of a trestle, destroying the cross-bracing members between bents. The fact that the trestle did not collapse due to the loss of bracing is an indication of the resilient and redundant nature of the trestles and demonstrates how the internal shear connectors might be revealed for inspection (as discussed later in this report) without endangering the structural integrity or safety of the bridge.
- There is little or no audible rattle below the bridge deck when traffic passes overhead. This observation suggests that connections are tight and members are sound.
- Generally, access to the connections within the trestles is good, which will enhance the efficiency of inspection, maintenance, and repair activities. Removing bracing members temporarily to inspect the inner connection hardware could be performed relatively easily without requiring closure of the bridge to traffic.
- Each of the outer-most columns of each bent is heavily bolstered at its base to help distribute compressive load into side-grain bearing on the lower sill plates. The next, inboard line of columns is lightly bolstered, because those columns carry less compression under lateral load. The bolsters appear to be effective in controlling overstress of the sill plates, as we observed no indications of localized crushing due to compression perpendicular to grain.
- Many of the large timber members have checks – radial cracks on the surface of the timber. The checks tend to be aligned with the longitudinal axes of the members, which suggests that the members have remarkably straight grain, another indication of high- quality material. Checks are not generally regarded as strength-reducing defects and do not require repair. Checks on the sides or bottoms of horizontal members and on any face over vertical or diagonally oriented members are able to drain freely when subjected to rain water.
- The bridge deck consists of two crossing diagonal layers of timber planks. The upper layer uses tongue and groove planks that lock together and provide uniform support for the asphalt pavement. The lower layer has planks that are spaced to minimize trapping any water that might migrate through the asphalt wearing course and the upper layer. The two layers of planks, properly coupled to the longitudinal stringers along the edges of the deck, have the potential to form an effective diaphragm (a deep, flat beam) to resist out-of-plane loading. With proper drainage detailing, the bridge deck also acts as a roof to protect the superstructure and trestle towers from stormwater runoff.
- Concrete piers supporting the timber trestles are in good condition. We observed no spalling or cracking. The piers provide uniform bearing support for the sill beams of the trestles.