Bridge Description

In 2013, a steel through truss span of the I-5 Skagit River Bridge collapsed, as a result of a high-load impact to the sway frames. A temporary Acrow bridge was erected to reopen the roadway. The permanent span of precast prestressed deck-bulb tee girders was constructed adjacent to the temporary span. Within a single day of roadway closure, a horizontal skid track system was used to relocate the temporary span and install the permanent span.

Interstate 5 across the Skagit River between the cities of Mount Vernon and Burlington in northwestern Washington State

Tier 1 (within 1 day)

The interstate remained operational for 208 days with the temporary span in place, during the design and construction of the permanent span. Therefore, the expected time to construct the bridge conventionally would at least have required the 208 days.

(1) Emergency replacement. (2) Maintain existing alignment. (3) Reduced traffic impacts – interstate traffic, including daily commuter, local tourism and interstate trucking; a significant ABC benefit was to minimize the detour traffic in the local communities, which caused significant impact to local retail.

The existing bridge was four steel through truss main spans (simple spans), eight concrete girder spans. The collapsed span was one of the steel through truss spans. The replacement span is eight W65DG lightweight prestressed concrete deck bulb tee girders with 1.5" concrete overlay, spaced at 7.25'. The replacement span was 60' wide and 163' long.

71000

After the span collapsed, the interstate traffic was detoured through city streets to a nearby bridge, crossing the river. The detour was 3 miles long. A temporary Acrow bridge was constructed as a temporary span while the permanent span was constructed adjacent to the Acrow bridge. The interstate was detoured for 27 days. The interstate was detoured for an additional day to slide the temporary bridge and slide the permanent span. Conventional construction would have required an additional 200+ days.

The existing bridge was four steel through truss main spans (each 160' simple spans), eight concrete girder spans (48' to 84'). The collapsed span was one of the steel through truss spans. The bridge carries two lanes in each direction.

The new permanent span was constructed adjacent to the temporary span (east side), on temporary steel pile supported bents. The permanent span consisted of eight W65DG prestressed deck bulb tee girders using lightweight concrete. The temporary span was slid off to the west, onto temporary steel pile supported bents. Using the same track alignment, the permanent span was slid into the permanent location, to the west.

The temporary and permanent spans were slid in one day using Teflon surface skid tracks and horizontal jacks. The skid distance for each move was approximately 75′.

Lightweight girders can be used to limit weight replaced on existing foundations. Headed bars used for flange connection between adjacent deck bulb tees can cause interference if sufficient placement tolerance is not provided. Using full depth flange connection (instead of welded ties) provided a shear – flexure transfer joint, permitting the use of distribution factors that eliminated one line of girder.

Lightweight concrete in prestressed girders.