Heat straightening Wellington Bridge

Heat straightening Wellington BridgeHeat straightening Wellington BridgeHeat straightening Wellington Bridge

Wellington Bridge Repairs, Highways Agency, Area 6

Heat Straightening

Regions of both plastic and elastic deformation are likely to co-exist in the steelwork of bridges that suffer severe vehicle strikes, and particularly so in the case of bridges with concrete composite decks.

A fundamental principle of heat-straightening is that heat should only be applied to regions of plastic deformation. Regions of elastic deformation will naturally assume their original alignment as plastic deformation is progressively reduced. This affect can often be used to increase the efficiency of the early heating cycles in most heating plans.

These considerations were particularly important to the repair of Wellington Bridge.

The structure spans the A12 near Chelmsford and was originally constructed in 1965, to a design that enjoyed some popularity at the time.

The bridge suffered a strike from a vehicle traveling south in February of 2010. Before a contractor for the repair could be appointed, the bridge was  struck again in May of 2012. The location of each strike is shown in the sketch above.

Seven lines of girders support the bridge deck. The first strike caused extensive damage to the cantilever portion of the northernmost girder. The second strike was to the suspended-span girder, causing extensive damage to the half-joints of the first two girder lines and causing a significant twist over the length of those girders. The 3rd girder line suffered only local damage to its bottom flange, but in a low stress area.

At an early stage it was decided that repairs to damage from the first strike would be by locally cutting out the damaged steelwork and replacing with new components as required, with the structure being appropriately supported via trestling from below.

 The damage from the second strike would be based on heat-straightening of the distorted main beams of the suspended-span with reinstatement of half-joint stiffeners as required.

WS Atkins, working on behalf of Skanska, initially retained Codorus in 2012 to comment on the repair procedure and review the heat-straightening specification.

The winning tender for the repair work was submitted by Interserve Ltd, and with the permission of Skanska, their specialist steelwork contractor HS Carlsteel Ltd retained Codorus Engineering Ltd to develop a detailed heating plan and oversee that work on site.

Compared to many of the bridge strikes we have experienced the misalignments of the damaged girder lines were not great. The main challenge at Wellington Bridge was to realign the damaged girders such that the half-joints could be reassembled to a tight tolerance, and in so doing occupy as little of the roadway below the bridge as possible.

Careful examination of the deflected shape indicated that the overall twist in the first two beams lines was likely to be predominantly elastic. The heating plans were drafted on this basis, avoiding disruptive traffic management measures that would have been needed to gain access to the central portion of the suspended-span.

This assessment proved to be correct and working closely with Skanska’s and Interserve’s site teams, the heat-straightening element of the work was completed by HS Carlsteel in one day with only three simple patterns of line-heats  being applied to each girder line, close to the damaged half-joints.

The local flange damage to the 2nd beam, above,  could have been repaired by heat straightening techniques, but numerous cycles of heat would be required and the time to apply those heating patterns would have been extensive in the context of the overall operation. Codorus advised a local cut-out and replacement repair as the flange loading at that point was particularly low.

With the twist in the damaged suspended-span beams removed, and their cross sections returned to a condition well within the original manufacturing tolerances,  HS Carlsteel’s site team reconstructed the damaged section of the northernmost cantilever beam and reassembled the half-joints to the first two girder lines impressively quickly, handing the structure back to Interserve to complete local deck repairs.

 

For those more familiar with the process of heat straightening, further details as follows:

  • The extensive damage to the half joints and to the cantilever portion of line 7 made it necessary to carry out heat straightening operations with full support around the half-joints.
  • The heating plans were reviewed and adjusted as necessary after each cycle and agreed with Skanska’s site team.
  • Restraining props were set to a jacking-ratio of zero for all heat-cycles.
  • Heating was restricted to one girder line at a time due to deck loading constraints.
  • Heating temperature was restricted to 600oC as generally required by the Highways Agency.
  • Oxy-acetylene heating equipment was used.
  • Temperature control was via colour observation by the operative, and remotely monitored  by HS Carlsteel’s site supervisor using a digital pyrometer.
  • No form of accelerated cooling was used.
  • The heating patterns were such that only one operative was necessary.
  • Access for heat-straightening was via fixed scaffold.

 Codorus wishes to thank:

Ranjit Mistry, Route Manager for the Highways Agency.

Chris Sadler  Structures Team Leader, Skanska, Area 6.

Jim Kilcar and Dave Day of WS Atkins.

Carlos Alleyne, Interserve Steelwork Manager

Chris Springate, Phil Healy and the team at HS Carlsteel Ltd

Chris Hodgett, James Lewis and Peter Balaam of the Skanska site team.

Adrian Harper, Ian Reed, and Matt Hook of the Interserve site team

 

 

 

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