Consider a piece of metal bar tightly fitted in a very stiff clamp as shown in Figure 1.

If intense heat is applied to a small area of the bar as shown, the heated volume of metal will tend to expand, but the clamp restrains longitudinal expansion.

Compressive stress then builds up in the bar as temperature increases and concurrent with that the yield stress local to the heating position reduces. When the actual stress and the yield stress coincide the bar starts to deform plastically in the heated area and the material generally thickens.

When the heat is removed and the bar cools. The heated volume of metal shrinks, but the clamp offers no restraint to shortening in the longitudinal direction and the bar permanently shortens.

The same general principle applies to heat straightening flat plates. If intense heat is applied at a single location on the surface of a plate, the metal under the torch will tend to thicken due to the restraint of the surrounding material, and any externally applied restraint.

By using specific heating patterns areas of the plate can be slightly thickened causing the plate to change shape.    This can be achieved using only the restraint from the surrounding metal, or the effect can be enhanced by the addition of external restraints, as is generally the case in heat-straightening repairs to damaged bridges.

Some complications

Figure 2

The manner in which energy is absorbed by a bridge structure during an impact  can be modified significantly by the existence of stiffeners, bracings and bolted joints. The examples below look at some of these issues.

There did not appear to be much wrong with this structure when viewed from the hard shoulder Figure 2. The girder in the centre of the image had been hit by a truck travelling right to left. The impact point was on the bottom flange edge close to the first channel bracing beyond the bolted splice.

 

 

 

Figure 3

Viewed close to the impact point on the outside of the girder, things looked a little different Figure 3. The force of impact on the flange, together with the stiffness of the bracing system behind tore the web to flange fillet welds for around 200mm on either side of the bracing. The imprint of the web stiffener, which stopped short of the bottom flange and to which the channel bracing was bolted, can be seen in the web.

Figure 4

The solution adopted was to cut out the web stiffener and the damaged web local to the impact point Figure 4. V heats were then applied around the impact point to straighten the bottom flange. Note the passive restraining props between the inner and outer girders. A new web insert was fitted and welded in place using full penetration butt welds. The web stiffener and channel bracing were replaced after successful NDT to the butt welds.

Figure 5

Figure 5 shows typical damage to a web stiffener on a plate girder composite bridge following an impact. The buckle in the web stiffener coincides with the long horizontal plastic hinge that commonly forms a short distance below the top flange during impact. In this case the damage was caused by a blast from an adjacent demolition operation on the other side of the girder. This was one of several web stiffeners that required repair.

The stiffeners could probably have been straightened with heat, but it would have complicated the web repair and often its quicker and easier to cut out damaged stiffeners and replace them once the web is brought back to vertical, as was the case here.

In Figure 6  the impact event caused slippage of a bolted splice in the bottom flange of a composite bridge girder as the flange to the right of the joint suffered a bend in plan. The web splice did not slip, and the top flange remained securely fixed in the composite deck slab. The bent bottom flange could be brought back into line easily with a relatively small number of V heats, but rotation would occur in the joint during the process.

Figure 6

The photograph shows one half of the arrangement used to relieve the splice plates of load. Exactly the same arrangement was fixed to the flanges on the far side of the joint. The two rams were connected in a manner that allowed them to be pressurised and then isolated from the pump as a pair, allowing oil to pass from one ram to the other as rotation occurred during the heat-straightening of the flange. The bottom flange bolts were released after the rams were pressurised but remained in place, and the web bolts remained fully tightened throughout.

Clearly this approach is only going to be feasible for relatively light joints such as this. However, if the splice had been designed to mobilise the full strength of the section it is unlikely that it would have slipped, as was the case in the second photograph shown in this article.

Above I have considered some common deformations that can exist around the impact point on a damaged bridge and how they might be remedied. However, there are other issues that should be checked out, including:

  • Whether any surface tears or cracks exists around the main plastic hinges. If they do heat straightening is unlikely to be successful
  • Whether there is any damage to bearings, tie-back systems or expansion joints.
  • Whether there is any elongation of holes on any bolted joint which has slipped, or where bolts have broken.
  • The extent of damage transfer to other girders.
  • That all shear studs remain firmly embedded in concrete deck slabs or concrete diaphragms and the like.

The FHWA’s technical guide refers to research data that shows that heat straightening can be successful on steel with plastic strains up to 100 times the yield strain.  The statement was silent on steel grade, which would obviously be a factor. I have no reason to doubt the statement, but I think that in most cases if damage to that extent had been sustained it would take a very long time under site conditions to make such a repair by heat straightening.  In such cases I think that it would be generally quicker to repair by cut-out.

I hope my three articles on this subject have been of interest.

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
  • Supervising heat-straightening and other associated repair work on site

Contact    Geoff@codorus.co.uk

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