Forensic engineering can provide in-depth understanding of bridge deterioration and failure, says Dr Jonathan Wood, but it is essential for the information to be disseminated rapidly.

Worldwide the risks from deteriorating and substandard bridges, which cannot easily be inspected, provide a major challenge to the engineering profession and to governments who must resource the work required to maintain the national infrastructure. The costs and disruption of investigations and the costs of premature and unnecessary remedial or replacement works also make it essential to improve investigation and assessment procedures.

If risks are to be correctly identified before failures occur and remedial work is to be done effectively, research is needed to develop and disseminate best practice for engineers and material scientists in three areas. The first involves finding hidden defects arising from design, construction or deterioration, while the second is to quantify the effects of deterioration on strength, robustness and serviceability. The third area centres on defining the limits of acceptable deterioration.

The rigorous investigation of deteriorating and collapsed structures using forensic techniques can provide essential information for developing improved inspection and assessment procedures for our aging infrastructure. The processes of deterioration cannot be reliably modelled or accelerated in laboratories. This makes it essential for research to be based upon fieldwork on real structures with rigorous laboratory examination and testing of large removed samples.

More cooperation between owners, consultancy practices, contractors and the research community, with strong international links, is essential if we are to make further progress.

Codes and guidance documents need to be based on the reality revealed by rigorous investigation, rather than oversimplifications from theory.

A fundamental problem with concrete bridges is that it is very difficult to find out exactly where the reinforcement is, its condition and the internal condition of the concrete. Currently available NDT procedures [1] can only give limited near-surface data on selected accessible locations. Many sudden collapses have occurred despite routine inspection of the bridges.

When bridges are considered to be a potential hazard, have been seriously damaged or have collapsed, there is an opportunity to conduct a rigorous and properly funded in-depth forensic investigation of the causes.

This may be necessary because of possible litigation or because the owner needs the information to evaluate the relative merits of remedial works or replacement on this or similar bridges. Only when bridges are opened up can the variability of construction and patterns of deterioration to be clarified. The rigorous questioning of the validity of data, assumptions and calculations essential in litigation is also required for detailed investigations of structural behaviour.

The importance of using this information in improving design guidance and codes has been stressed in previous papers [2]. In particular they identified the need to improve partial factors to reflect actual construction variability, particularly for shear, and the need to improve the treatment of durability in design.

We must ensure that the 'forensic cycle' (see Figure 1) works rapidly. This requires us to relax the commercial, legal, bureaucratic and funding constraints to the development and dissemination of better inspection and remedial works procedures. The UK's Standing Committee on Structural Safety (SCOSS) and Confidential Reporting on Structural Safety (CROSS) make a significant contribution to this process.

There are many examples of substantial revisions to codes and guidance documents following major failures during construction or in service. One example is the introduction of robustness requirements and a major revision of wind loading after the Ronan Point tower block collapse. Similarly the box girder failures in the early 1970s led to introduction of partial factor design based on quantified variability of construction tolerances, and many other major changes in bridge design practice in BS5400. Both these developments resulted from substantial well-funded research programmes.

The swifter deterioration of car parks provides a test bed for bridge inspection and assessment techniques. Data from car parks are valuable for the understanding of bridge deterioration and the development of remedial techniques. Easy access away from traffic facilitates detailed inspection.

In 1994 SCOSS warned about the risks from deteriorating car parks. But there was little action until the collapse of the Pipers Row car park in 1997 led to a comprehensive investigation [3]. While the report was being prepared, the Institution of Structural Engineers (IStructE) set up a committee which made recommendations in 2002 [4] on the design and management of new car parks. The Institution of Civil Engineers (ICE) set up a parallel study and reported on the management of existing car parks [5]. These both drew on a comprehensive research review [6] of 200 car parks where detailed investigations had been carried out on defects and developing deterioration.

Major elements in the IStructE and ICE recommendations - equally valid for all structures - include emphasis that it is the responsibility of owners to maintain full records, including reinforcement drawings and records of repairs. Owners are also responsible for arranging regular inspections and structural assessments. The institutions stressed the necessity of proper integration of structural appraisal, site investigation of the deterioration and repair contracts.

The Pipers Row report included a seven-point check list for assessment, inspection and repair of deteriorating structures. This was based on experience of in-depth investigation of a wide range of building, bridge and tunnel structures.

1 Check 'as-built' drawings and maintenance records.

2 Carry out a structural review to identify the key areas of structural weakness and/or structural sensitivity to deterioration as a basis for inspection procedures. This should cover both strength and risks from spalling.

3 Check for any features:

  • for which factors of safety may be inadequate for actual construction method and quality;
  • where the structural form is not explicitly covered by codes;
  • which may be vulnerable to progressive collapse.

4 Establish by inspection and testing:

  • any departures from as built drawings;
  • any indications of defective or substandard construction;
  • indications of severe local environments from ponding, waterproofing breakdown, seepage etc;
  • the current trends of deterioration and likely long term trends. 5 Identify where and when protection, strengthening and/or repair may become appropriate and cost effective as part of the long term maintenance programme.

6 Ensure that before repairs are carried out that there is:

  • a full specification and procedure for repair, propping and testing;
  • a Structural Engineer's check of the structure:

'as built',

'as deteriorated',

'as cut out for repair, with propping if required',

'as repaired, with propping if required'

'with repair delaminated'.

7 Insist on a full recorded survey of condition before problems are hidden below patch repairs, coatings or waterproofing.

This check list is similar to the recommendations of the Commission of Inquiry [7,8] into the fatal coll