INVESTIGATIONS INTO the collapse last weekend of a prestressed concrete footbrige at Lode's Motor Speedway in Concord, North Carolina, US, are centring on the way the precast support beams were manufactured.
Sources close to the investigation have revealed that procedures to deflect the pretensioned prestressing tendons during casting left holes running down from the deck to the tendons themselves.
Failure of the grout used to seal these holes is being blamed for the serious corrosion discovered at midspan.
Samples of grout found in the beams which collapsed last week 'appeared like paste' surrounding the dark, heavily corroded strands, according to the source. Engineers from bridge owner Lode's Motor Speedway have taken these materials away for analysis.
The 25m long span collapsed at 11.15pm on Saturday night 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.
As NCE went to press, 47 of the injured were still in local hospitals recovering from broken bones, cuts and bruises. One spectator remains in a critical condition.
According to North Carolina Department of Transport assistant bridge inspection engineer Don Idol there was 'not much question' that the collapse was caused by corrosion in the prestressing tendons.
'I observed rust on many of the tendons at the midspan point of failure, ' he told NCE. 'From my initial visual inspection of the collapsed section, this appears to be limited to the midspan point. I estimate that, from what I could see in one web, there were probably only three of the 11 strands broken by the collapse as the rest had already corroded.'
As the bridge passes over a state highway, NCDOT is responsible for ensuring that the accident investigation and repairs to the bridge are carried out correctly.
However, responsibility for the bridge falls to the speedway which owns it - despite the fact that it crosses a state highway. The speedway said it had drafted in a team of specialist forensic catastrophic collapse experts to assist its in-house team.
Engineers were also exposing prestressing tendons in the webs of the remaining spans to determine whether similar problems had occurred on other beams. A similarly constructed bridge alongside remains closed.
It is understood that to achieve the 25m span with these 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.
Investigations already under way will examine the grout used to plug similar holes at such a vital location in the remaining beams. Sources have indicated that in some cases an epoxy grout may have been used in preference to a cementitious material.
Idol said 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.
However, he said that from his visual inspection of the remains of the deck, there was no evidence that the bridge had any corrosion or material problems other than in the midspan location.
'The concrete appears sound and the strands are all clean and bright elsewhere, ' he said. 'Investigations must concentrate on what caused that localised corrosion.'
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.'
A decision on whether or not the second prestressed concrete footbridge can reopen will be taken by the end of this week. This weekend is expected to be one of the speedway's busiest of the year with up 170,000 spectators arriving for the NASCAR Coca Cola 500 race.
Special traffic control measures are planned to allow spectators to access the arena across the highway should the bridges - as is expected - remain out of action.
The collapsed bridge is a four span structure made up of eight prestressed double T-beams 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 Lode'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 Tbeams simply supported side by side.
A spokesman for the speedway said that the second bridge would remain closed to the public until engineers identified the cause of the collapse. 'We are studying all possible causes of the collapse. We are hoping to be able to make a decision by the weekend.'