LOWE'S MOTOR Speedway engineers have confirmed that they found high levels of calcium chloride in grout used in the construction of bridge beams which collapsed catastrophically in North Carolina on Saturday 20 May.
US prestressed beam manufacturer Tindall Corporation has accepted that the grout caused vital steel strands to corrode at midspan and is investigating how the suspect grout got into its manufacturing process. It insisted that the problem was an isolated incident.
The 25m long span collapsed at 11.15pm just as spectators flooded out of the stadium after the day's races. Hundreds plunged 5m on to the carriageway of the Route 29 highway below. Around 107 people were taken to hospital with broken bones, cuts and bruises, several in a critical condition As the bridge passes over a state highway, North Carolina Department of Transport is responsible for ensuring that the accident investigation and repairs to the bridge are carried out correctly. However, responsibility for the bridge itself is carried by the owner, Lowe's Motor Speedway - despite the fact that the bridge crosses a state highway.
It is understood that to achieve the 25m span required of the beams, the designer - locally-based Tindall - had to deflect the tendons to increase moment capacity at the midspan. To achieve this, special vertical steel rods were used to hold the tendons down during stressing and casting.
Grouting was necessary to fill holes left in the precast beams by the rods and the calcium chloride is likely to have caused thermal cracks in the grout, which, with no waterproof surfacing on the bridge beams, would have provided a direct water path to the steel tendons.
According to North Carolina Department of Transport assistant bridge inspection engineer Don Idol, the bridge that collapsed and the similar structure alongside, would originally have been designed, checked and constructed to state standards. However, once in place it would have been the speedway's responsibility to check the structure.
'The department has no jurisdiction over the bridge as it is owned by the speedway, ' he said. 'The speedway is leading the investigation. It is down to its engineers to ensure it is safe.'
Commenting on the ongoing investigation, Idol said a full review of the design calculations for the structures would have to be carried out. This, he added, would include a check on all of the materials used in the construction.
Idol also pointed out that longitudinal cracks up to 3mm wide were also visible on the underside of at least one of the other beam webs at midspan. Cracks of this nature were not consistent with overloading or of poor beam design, he said. 'Expansion due to corrosion is the first thing I would examine.'
A spokesman for Lowe's Speedway confirmed that the bridge designs were being analysed as part of the investigation but refused to elaborate on the work being carried out. He added that its engineers were satisfied that the bridge was not overloaded before the collapse.
'We have brought specialist outside engineers in to help with the investigation, ' he said. 'We are studying all possible causes of the corrosion but we are happy that the weight load is not a factor.
It was designed to carry 100lb/sqft and even with 150 people on the bridge their weight is well below that.'
The remaining three spans of the footbridge were demolished a week later, just before 170,000 spectators were due to arrive at the track for the NASCAR Coca Cola 600 race.
A second, similar footbridge remains closed while tests continue. Initial indications have shown that the bridge is unaffected. However, the North Carolina Department of Transport said it would not allow the bridge to reopen until it had seen test results from its collapsed neighbour.
And all private owners of pedestrian and vehicle bridges throughout the state have been ordered to make safety checks.
The collapsed bridge is a four span structure made up of eight prestressed double Tbeams spanning between reinforced concrete piers and abutments. Two 2.4m wide beams sit side-by-side between the supports to form the 4.8m wide, 25m span bridge deck - thought to be about as long as it possible to span using this technique. The bridge was constructed by Lowe's Motor Speedway in 1995 to allow fans to access one of the vast car parking areas without having to cross the busy highway. A second, similarly constructed structure is still in place 100m up the road. This was built a year later as part of the same contract. However, this bridge is wider and uses three double T-beams simply supported side by side.