Having successfully undertaken several heat straightening bridge strike repairs in the UK HS Carlsteel was contracted to undertake remedial work on a four spanned simply supported bridge over the A500. The existing structure is of composite construction with 16 No steel 762x267 universal beams and reinforced concrete deck.
The project scope was to carry out repairs to 11 beams that were damaged by high vehicular impact, most of the damage occurred above lane 1 of the A500, In addition to the heat straightening repairs, a number of the existing steel sections were required to be removed & new sections welded in place, these intrusive repairs were carried out where the plastic deformation ratio was in excess of 50mm due to localised impact. Full road closures were implemented during the night with Access to the underside of the bridge gained with the use of MEWPs.
The first phase of the operation required the removal of the existing surface coating by gritblasting back to bare metal. Each beam was subjected to NDT crack detection inspection, with the use of magnetic particle inspection techniques. The crack detection was carried out 1m either side of the impact points. In addition, visible checks of up to 2m each side of the impact points were undertaken. We also carried out hardness testing to determine base line readings prior to any work being carried out.
Prior to commencing the heat straightening, trial heats were carried out on test plates of similar thickness to the existing to demonstrate the suitably of the equipment, techniques and operative competence.To facilitate the repairs to the bottom flange where the existing material had been deformed beyond the strain ratio of 50mm, the damaged flange sections were removed with the use of oxy-acetylene thermal cutting equipment.
The heat straightening process comprised a series of V-Heats, Line-Heats & Web patch heats, the positions of which were marked on the existing beams. The heat-straightening plan for each girder was based on varying heat cycles. The main equipment used comprised various specialist equipment including BOC Lindoflamm equipment. Average temperatures during the straightening process varied around 600°C up to a maximum of XXXXX°C. which was monitored using infrared thermometers. Heat cycles averaged between xxxxx and xxxx degrees C on the webs and xxxxx degrees C on the flanges.
Temporary restraints & jacks were used in the locations indicated on the heat straightening plan. These Restraint systems were designed by HS Carlsteel & were required to give an effective restraint to obtain satisfactory correctional rotation in each heat-cycle, all temporary restraint propping was removed after each working shift as a safety measure.Each existing beam required new additional web stiffeners including new RSA cross bracings which were installed to strict tolerances and HS Carlsteel Engineering are approved to Execution class 3 for the fabrication of structural steelwork
On the completion of heat straightening, flange repairs & installation of the new bracings, all welds were subjected to 100% visual inspection & 40% MPI & 100% UT for butt welds in addition to hardness testing and all results were positive. Finally, the surface coatings were re applied to a full HA type 2 specification. The contract works were carried out over a period of 13 weeks and delivered ahead of schedule.