N ew Road Overbridge, crossing the M5 near Weston-super-Mare, was struck by a northbound truck carrying a shipping container in early 1999.

The outer main girder, an 838 x 292 x 226 universal beam spanning 18m with composite concrete deck, sustained severe damage. There were no bracings connecting the outer girder to inner girders, and consequently the outer girder absorbed the full impact, resulting in the formation of 5 substantial plastic hinges.

The Highways Agency had decided to attempt a repair of the bridge using heat-straightening procedures based on recommendations published by the US Federal Highways Administration. If heat-straightening proved unsuccessful, the damaged girder was to be cut out and replaced.

There are many more highway bridges in the USA than in the UK, and the proportion of steel bridges on US highways is far greater than in the UK. In addition bridge clearances in the USA are generally lower than in the UK. Consequently impact damage to steel bridges is far more common in the USA.

In some States it had been the practice for many years to heat-straighten impact damage to steel bridges. However, the heating procedures were generally left solely to the practitioners. Other States would not contemplate heat-straightening because it wasn’t regulated. The Federal Highways Authority decided that it was time put some solid science behind what had been “black art” for many years, and published “Heat-Straightening Repairs of Damaged Bridges – A Technical Guide and Manual of Practice”, published in 1998.

It is undoubtedly the most comprehensive document yet to be published on the heat-straightening. But we must remember that it isn’t a formal specification.

Back to New Road Overbridge. The Highways Agency had decided that selection of the contractor to undertake the work would be based on interview. At that time I was the Engineering Director of Fairfield-Mabey and the company’s board were interested in the project. They knew that it wasn’t going to generate large revenues, but considered that the technology was important to the reputation of steel bridging, and that it would be worthwhile becoming involved. I was given the task of winning the contract.

I knew that there wasn’t going to be much to choose between the various companies approached and it was clear that we needed an edge to be sure of being selected. I called Pat Loftus.  Pat was the CEO of Highsteel , one of the most respected steel bridge fabricators in the USA. From visits I had made to their factory in Lancaster PA in the Late 1990’s I knew that they had a team that, from time to time, undertook heat-straightening of impact damaged bridges.

We quickly agreed to collaborate on the New Road project.  A few weeks later Bob Urban, one of their site superintendents experienced in this work, arrived in the UK. We took Bob straight to the site and viewed the damaged bridge from the verge of the motorway. His immediate reaction was that we had chosen an extreme case to start with, but nevertheless, it could be done.

We went from site to the interview with the HA and WS Atkins, which went very well. Shortly after we heard that we had been successful.

A few weeks later, Bob flew back into Heathrow with his colleague Chuck, and joined the Fairfield-Mabey team on the site.

Damaged beam prior to heat treatment

The photograph, left, shows the damaged beam before the application of any heat. The energy of the impact had been totally absorbed by the formation of 5 plastic hinges on the outer girder. Two in the bottom flange near mid-span

coinciding with the corner posts of the container, two more in the bottom flange at each end of the span due to the beam being cast into heavy concrete diaphragm beams, and one in the web about 100mm below the top flange, extending for most of the 18m span and centred on the impact points.

The web of the damaged girder was inclined at an angle in excess of 30° at the points of impact.

All of the plastic hinges had formed simultaneously on impact.

The repair process required an incremental reversal of the impact event, working all of the plastic hinges together.

The existence of 5 inter-related plastic hinges of such magnitude certainly complicated the repair process, however, if the girder ends had not been cast-in, and the whole impact had been at a single point, partial demolition of the structure, rather than heat-straightening, would have been the likely remedy.

Horizontal offset (mm)

The chart (left) shows the record of the 24 cycles of heat that were applied to straighten the damaged girder. The heating took place over 5 days of 8 hours. Some cycles of heat delivered more movement than others.

All but the last few cycles involved multiple heating patterns at each plastic hinge.

The first cycle of heat each day generally proved to be the most effective.

Temperature was controlled by colour observation and checked to be in the range 550°C to 600° C.

Passive restraining props were used at the two impact points. Inter cycle cooling was by compressed air.

Damaged beam post treatment

The image (right) shows the outer girder after the repair was completed, and a coat of paint applied. Local buckles in the bottom flange were treated separately after the girder was straightened. All points along the bottom flange lay within a tolerance of span/1000 on completion.

As mentioned above some cycles of heat can result in significantly less movement than expected. This is the time for the site supervisor to have confidence in his plan and exercise patience and discipline. Increasing temperatures and/or restraining forces to gain more movement is to be avoided as it is likely to greatly increase the risk of adversely altering the properties of the material.

Selection of the contractor by interview on the New Road project was adopted because it was the first use of the technology in the UK. However, I think it’s the right way to go on all projects involving significant amounts of heat-straightening. Price certainly has to be factored in, but confidence in the contractor’s ability to plan and execute the work is paramount given that traffic management costs are likely to greatly outweigh the costs of undertaking the repair.

Trials before work starts on site, to verify the rotations on which the contractors heating plan is based, can assist greatly in establishing confidence on all sides and are recommended. This is particularly so for v-heats used for flange straightening.

Local buckles at impact points can be difficult to totally eliminate, and pre-site trials to prove the methodology for removing such defects can also be useful.

Beware of the contractor who insists that the steel surface must be heated to “cherry red” for the heat-straightening process to work. You can be sure that any area under the torch, which glows to any shade of red in normal daylight, is well over 650°C, the recommended maximum temperature for all but quenched and tempered high-strength steels. In addition the assertion that heat-straightening will not work unless the surface is glowing red is frankly rubbish. The process works perfectly well in the range 550°C to 600°C with minimal risk of impairing the mechanical properties of the material.

For those readers who may have wondered, the working relationships amongst our Anglo-American team on the New Road project were excellent. There is no doubt that the experience that Bob and Chuck brought to the project increased the confidence of all parties involved given the extreme level of damage.

A repair involving only heat–straightening, as at New Road, is quite rare. More commonly stiffeners and bracings exist which modify the way in which the energy of any impact is absorbed. In such circumstances it is common for heat-straightening to be accompanied by some degree of cutting and re-welding. Next month I will look at some of these issues.

In the meantime if you need help to:

  • Assess the degree of impact damage on a steel bridge
  • Select a contractor to carry out a heat-straightening repair
  • Review a heat-straightening repair plan
  • Conduct pre-site heat-straightening trials
  • Supervise heat-straightening and other associated repair work on site

Contact    Geoff@codorus.co.uk

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