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.

The ‘forensic cycle’ must work rapidly so that information that arises can improve safety on other schemes

This check list is similar to the recommendations of the Commission of Inquiry [7,8] into the fatal collapse of the de la Concorde overpass in Quebec in 2006 (Bd&e issue NO 49). The full report should be read by all concerned with the management, assessment and inspection of deteriorating infrastructure.

The Concorde overpass had an inherently fragile design based on now superseded shear design rules. The detailing of the ‘as-designed’ reinforcement did not properly anchor either the vertical shear steel or the top bars in a deep cantilever adjacent to a half joint. These critical bars were misplaced in the ‘as-built’ construction which further weakened it.

These problem areas were adjacent to an expansion joint which was replaced with associated repairs about 15 years ago. This work revealed more extensive deterioration than was expected and was of poor quality. No detailed structural reassessment was carried out, nor were records kept. A gap in waterproofing adjacent to the joint allowed water and salt ingress and aggravated frost damage to the weak concrete, all leading to the progressive weakening of this detail.

The fall of lumps of concrete onto the road and the development of a shear crack alerted the highways team that an investigation was required. However they were unaware from visual inspection of the hidden defects in design and construction and the developing hidden deterioration. The connection of the vertical shear steel to the top bars failed with the opening of a delamination below the top reinforcing. This developed into a tensile shear failure with fatal consequences for those in the cars trapped below. Five people died.

Thus many of the features of the Pipers Row shear failure of an inherently weak and deteriorating structure were also factors leading to the de la Concorde overpass collapse. The Quebec Commission recommended that a protected budget of at least US$500 million per year should be dedicated for 10 years exclusively to the rehabilitation or reconstruction of existing structures.

One consequence of de la Concorde has been a major Canadian review on shear strength research and design codes with further testing reported by Professor Michael Collins of the University of Toronto [9]. This shows that for one configuration tested a brittle failure occurred at 0.57MPa and the predicted strengths were 0.54, 1.38, 0.98, 0.64MPa for CSA, ACI, EC2, BS8110 standards respectively.

Papers by Collins and others have made clear the serious uncertainties with current shear rules worldwide. Problems are greater with bridges designed to older shear rules like CP114. That EC2 is much less safe than BS8110 is a matter of serious concern. The brittleness of shear fracture may become a governing criterion in setting stress limits and partial factors because of the growing emphasis on robustness requirements in design and assessment and on measures to prevent progressive collapse. The embrittling effects of corrosion and delamination on shear strength need priority in research.

Following the collapse, there have been investigations and assessments of 135 similar structural slab bridges in Quebec, of which 28 will have to be demolished and 25 will need substantial strengthening. This type of deep slab cantilever design is found in many other countries including the UK. More accurate shear assessments following Collins’ research may necessitate checks on many deep flat slab bridge decks and also on thick foundation slabs.

The development of guidance on the structural effects of Alkali Aggregate Reaction (AAR) is another example of combining research data with data from forensic examination and testing of deteriorating structures. The UK guidance for specification to prevent AAR also benefited from a large input from forensic studies.

Detailed petrographic, chemical and structural testing programmes were set up following the identification of the first significant case of AAR in the UK in 1975, with severe cracking in the Charles Cross car park. Mott MacDonald initiated and has reported [10] much about this work, which was carried out with the active support of client organisations.

This work is being further developed [11] within RILEM TC-ACS to provide international guidance on diagnosis, testing, assessment and modelling of AAR. This will incorporate, inter alia, results from forensic investigations of Dutch bridges where shear strength was being markedly reduced and Japanese bridges where brittle reinforcement is fracturing on bends as AAR expansion forces develop.

When deterioration is identified, it is not sufficient to quantify current strength. Trends of rising risk over a decade or more, with and without remedial works, need to be quantified. Managers of structures need to plan, so that the works required to maintain acceptable safety standards are carried out at an appropriate time. Minimising disruption is equally important for road bridges, retail centres and hospital buildings.

We need to be able to predict future rates of deterioration. Most models for predicting lifetime performance are based on probabilistic studies of risk of design limit states being reached. However both failures and detailed studies of the condition of older structures show that service life performance is actually dominated by a few local details where low strength and the most severe deterioration interact. The investigation [12] of 1905 Tuckton Bridge, the first major Hennebique bridge in the UK, and the 1930 Montrose Bridge [13] provide excellent data sets for developing realistic models and for calibrating them.

In conclusion, it is time to improve our procedures for investigation and assessment of existing structures against the yardsticks provided by the forensic investigations of failures and during remedial works.

Methods for assessing the limits of acceptable risk for deteriorating structures need to be established. Shear is of particular concern.

Detailed information from historic failures and from structures which have demonstrated robustness when subject to extreme events needs to be analysed and publicised, as well as the steady international supply of new collapses during construction and in service.

In the interest of public safety, detailed information from deteriorating and ‘at risk’ structures, as well as from failures, must be speedily and accurately made available through best practice guidance.

Failure investigations frequently highlight shortcomings in funding and in the availability of specialist skills for the inspection, maintenance and upgrading of substandard and deteriorating structures.

Jonathan G M Wood is director of Chiddingfold-based Structural Studies & Design Ltd and a member of the organising committee for the ICE’s conference on forensic engineering, which takes place from 2 to 4 December (www.forensicengineering2008.com).

References

1. ‘Testing and monitoring the durability of concrete structures.’ CBDG Guide No. 2, Concrete Society, 2000.

2. Wood J G M ‘Forensic Engineering: the true calibrator of design limit states’. pp.3 – 12, ‘Forensic Engineering: the investigation of failures’. B. S Neale Ed., Taylor & Francis 2005.

3. Quebec (2007). ‘Report of the Commission of inquiry into the collapse of the de la Concorde overpass’. Gouvernement du Québec, 15/10/07.

www.cevc.gouv.qc.ca/UserFiles/File/Rapport/report_eng.pdf4. Wood, J.G.M. ‘Implications of collapse of de la Concorde Overpass’ The Structural Engineer 86/1, pp16-18, 8th January 2008.

5. Wood, J.G.M., ‘Pipers Row Car Park, Wolverhampton: Quantitative Study of the Causes of the Partial Collapse on 20th March 1997’ SS&D Contract Report to HSE, http:/www.hse.gov.uk/research/misc/pipersrow.htm.

6. IStructE, ‘Design recommendations for multi-storey and underground car parks’. SETO, 2002.

7. ICE, ‘Inspection, maintenance and management of car park structures.’ Inst Civil Engineers, 2002.

8. Henderson N. A. et al. (2002). ‘Enhancement of whole life cycle performance of existing and future car parks’, Mott MacDonald PII Report. www.planningportal.gov.uk/uploads/odpm/4000000009277.pdf

9. Collins M.P. et al. ‘Shear Design of concrete structures’ The Structural Engineer 86/10 20th May 2008.

10. Wood, J G M, and Johnson, R A, "The appraisal and maintenance of structures with alkali silica reaction." The Structural Engineer Vol.71, No. 2 pp 19-23. 1993.

11. Wood J. G. M., “Improving guidance for engineering assessment and management of structures with Alkali Aggregate Reaction”, Proc. 13th ICAAR Trondheim, M. Broekmans M., Ed. June 2008.

12. Wood J G M, Grantham M G and Wait S, ‘Tuckton Bridge, Bournemouth. An Investigation of Condition after 102 years’. Proc. Concrete Platform 2007. (www.qub.ac.uk/)

13. Wood J.G.M. & E C Angus E.C. ‘Montrose Bridge’ Proc. Structural Faults & Repair Conf. -95. Vol. 1, 103-108. . 1995.

Structures such as Tuckton Bridge, built in 1905 and still in service in 2008, provide excellent data sets for developing realistic life cycle morels and for calibrating them

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