Contractors closed the final gap in the southbound main span of the replacement I-35W bridge across the Mississippi river in Minneapolis, Minnesota this week with a final concrete pour.
The structures for the northbound and southbound main spans for the new crossings are now complete, less than a year after the original I-35W steel truss bridge collapsed (NCE 9/16 August 2007).
The main spans are formed from 120 precast concrete box girder sections. Over 46 days, the sections were erected incrementally from the piers on either bank, cantilevering until a 2m gap was left. As each section was erected, cables were threaded through ducts in the units and the bridge section was post tensioned.
For the closure pour, frames were inserted in the 2m gap, pushing apart the two sections by 50mm. The concrete for the northbound span was poured on 15 July and left to cure overnight when the thermal effects were at their lowest. Pressure on the jacks was released the next morning, introducing compression into the concrete. The final pour for the southbound structure was due to take place this week.
Work on the bridge is now three months ahead of schedule and is expected to finish in September.
The 139.6m span steel truss predecessor suffered a catastrophic collapse last August, killing 13 people.
A preliminary report by the US National Transportation Safety Board (NTSB) in January revealed that the collapsed structure had under-designed gusset plates (NCE 21 January 2008).
This led to a directive to reanalyse all the gusset plates in Minnesota’s 25 truss bridges in the state. Further onsite inspections to support the analysis were carried out this spring.
Three major bridges had to be closed or traffic limits imposed while repairs were carried out following the analysis, although the Minnesota Department of Transport (MnDOT) said none
of these bridges were in danger of collapse.
"They were carrying traffic and there was no reason that they wouldn’t have carried on carrying the traffic, but we did the computations and after you’ve done the repairs you sleep better at
night," said MnDoT assistant district engineer of operations Duane Hill.
A nationwide recheck of all bridges is not being carried out but US secretary of transportation Mary Peters has recommended that all states calculate how possible changes in bridge weight,
capacity or evolving bridge conditions will aff ect gusset plates. Hill believes the collapse and subsequent findings of the investigations will be picked up by engineers from across the country.
"On a national level, bridge offices are talking to peers and word is getting out there that we did have problems with gusset plates," said Hill.