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A question of mechanics


From John Rastall's response (NCE 1 June) to the article on the heat straightening repair of the M5 bridge (NCE 18 May), it appears that he is unaware of the research that has been conducted into the process in the USA. It seems to me, from his description of the repair he carried out in 1982, that the method used was not heat straightening but hot mechanical straightening.

In hot mechanical straightening external force is applied to straighten the damage. The pliability of the damaged member is increased by significantly reducing the yield strength through heating and forcing it back into its original position. This type of straightening is unpredictable and specific concerns include fracture, adverse effects on material properties and buckling.

Heat straightening involves applying a limited amount of heat in specific patterns to the plastically deformed regions of a damaged steel member in repetitive heating and cooling cycles to produce a gradual straightening of the material.

Heating of the steel is kept below the lower critical temperature.

The process relies on internal and external restraints that produce thickening during the heating phase and in-plane contraction during the cooling phase. Force is not used as the primary instrument of straightening.

Fundamentally, it is an unsymmetrical process of thermal expansion and contraction in which each cycle leads to a gradual straightening trend. Material properties are ostensibly unaltered and the risks of fracture are negligible.

Heat straightening is not new. However, what is new is that the USA's Federal Highway Administration has recently undertaken a comprehensive evaluation of the method.

Details are available through the FHWA's website (

Martin Lynch, Principle Bridge Engineer, Highways Agency, Bristol,

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