Post Weld Treatment of Tubular Bridge Joints

• Forschungsstelle: ASTRA SBT
• Projektnummer: AGB2002/011
• Projekttitel Englisch: Post Weld Treatment of Tubular Bridge Joints

Schlüsselwörter (Englisch)

Bridges, fatigue; tubular structures; tubular joints, post weld treatment, reliability, probabilistic methods, quality control.

Kurzbeschreibung (Englisch)

Engineers have found the fatigue resistance of the joints to be a critical aspect in the design of tubular bridge structures. This means that larger member sizes must often be specified for these structures than would be required to meet ultimate and serviceability limit state requirements alone.

One potential means of improving the fatigue behaviour of tubular bridge structures involves the use of post weld treatment methods. These methods have proven themselves in other applications. Preliminary tests on treated tubular bridge joints have shown promising results. In order to determine the extent to which post weld treatment methods can be used to improve tubular bridge structures, a better understanding of their effect on the fatigue behaviour of tubular bridge joints is required. In order to ensure that post weld treatments are applied in a manner that is both effective and economical, practical guidelines and quality control procedures must be developed. In addressing these issues, it is believed that a probabilistic approach is needed that considers the effects of post weld treatment and the presence of multiple potential crack initiation sites.

Methoden (Englisch)

Literature review, development of probabilistic approach, validation through testing and numerical comparisons, application of new approach to define strategies evaluation and synthesis.

Weitere Hinweise und Angaben (Englisch)

From the list of conclusions provided in the project report, the following items are considered to represent the most important recommendations of the project work:

• In general, it is shown that the higher the treatment intensity or uniformity, the greater will be the improvement in the fatigue performance of a single potential crack site or an entire tubular bridge structure due to post-weld treatment. This performance improvement is more sensitive to variations in the treatment intensity than the treatment uniformity over the studied ranges for these parameters. However, these ranges assume that a minimum level of treatment uniformity is ensured (i.e. no gross errors). On this basis, it is recommended that the primary goal of the adopted quality control procedures should be to eliminate gross errors. Furthermore, it is concluded that the existing treatment procedures and non-destructive quality control techniques are sufficient for achieving this goal.

• The depending on the target reliability index and whether the lower or upper bound series system reliability model is used.

• Post-weld treatment can result in potential savings in steel weight due to needle peening ranging from 5.6 to 12.9%. But it can also be used to increase the fatigue life of a bridge with a given steel weight. Towards this end, it shown in the project report that fatigue life increases ranging from 120 to 930% are obtainable by needle peening, for a target reliability index between 1.16 and 3.74. This finding is applicable to highway bridges under principal road traffic volumes with planned service lives of 70 years.

• It is shown that the benefits of full treatment can often be achieved with much more concentrated partial treatment strategies. For tubular K-joints such as those found in the studied example bridge, an partial treatment strategy is recommended such as the one given in the report.

• In real tubular truss bridges, the various member load cases acting on a given joint are generally out-of-phase. If these loads are assumed to be in-phase, conservative estimates in the design stress range may result. Although more time consuming, it is recommended to perform calculations of the design stress range that consider phase effects; this can lead to significant material savings, while still ensuring safe bridge designs according to the existing codes.

• The effect of the dead load stress on the improvement in fatigue performance due to post-weld treatment can be significant and often highly detrimental. This is especially true in the case of treatment prior to the dead load application (i.e. shop treatment). Thus, it can be deduced that potential benefits of field as opposed to shop treatment can be significant.

• A preliminary economic evaluation of needle peening in the shop demonstrates that, under favourable circumstances, this treatment approach can result in significant reductions in the construction costs of tubular bridge structures. There are circumstances, however, under which this treatment approach will result in little or no economic benefit.

From a similar economic evaluation of needle peening in the field, it can be deduced that the potential economic benefits of this approach may be much greater.

Stand der eigenen Forschung (Englisch)

The Steel Structures Laboratory (ICOM) at the EPFL has a high level of experience in fatigue and fracture mechanics of steel structures. This experience has included the development of very complete deterministic fracture mechanics models enabling the influences of the material properties, crack closure, residual stress fields from post weld treatment, and overloads to be accounted for. In the domain of tubular bridge structures, along with the work conducted for OFROU AGB 1998/103 (88/98), ICOM currently has a project underway and a collaboration (to start in 2003) to investigate the behaviour of tubular joints with cast nodes. ICOM also has experience recording and analysing field measurements of tubular bridge structures (see Aarwangen, Dättwil, Lully).

Projektziele (Englisch)

To quantify the beneficial effects of post weld treatment on the fatigue behaviour of tubular bridge structures.
To determine the best post weld treatment strategies for these structures.
To propose practical guidelines and quality control procedures for the application of post weld treatment methods to these structures.

Forschungsplan (Englisch)

October 2002: Start of work.
2002: Literature review, development of a probabilistic approach that will consider effects of post weld treatment and the presence of multiple potential crack initiation sites.
2003: Further development of the new approach, design and execution of tests and numerical comparisons in order to validate the approach and obtain data needed for the various input parameters.
2004: Completion of tests, validation of the new approach, implementation of the new approach to develop post weld treatment strategies for tubular bridge structures and completion of a preliminary report.
2005: Completion of the final report.

Zielerreichung (Englisch)

The original objectives of the current project can be summarized as follows:

• To quantify the beneficial effects of post-weld treatment on the fatigue behaviour of tubular bridge structures,

• To determine the best post-weld treatment strategies for these structures, and

• To propose practical guidelines and quality control procedures for the application of post-weld treatment methods to these structures.

In the final version of the project report, the following detailed objectives are given:

• To model the effects of post-weld treatment on the fatigue behaviour of single crack sites in tubular bridges using a probabilistic approach, and to establish distributions for the statistical variables needed to implement such an approach.

• To develop a model for predicting the overall fatigue performance of tubular bridge structures with multiple potential crack sites using a systems reliability approach.

• To predict the potential improvement in fatigue performance due to the post-weld treatment of tubular bridges, and to quantify this improvement in terms of the adopted treatment method, extent of coverage, and quality (i.e. uniformity) of treatment.

• To assess the suitability of post-weld treatment methods for tubular bridge applications, in view of the results of the work carried out to meet the other objectives.

The second list of project objectives evidently focuses attention on the first two of the three original objectives. A number of the conclusions in the final project report do, however, provide guidance related specifically to the application of post-weld treatment methods in practice and the implementation of appropriate quality control procedures.

Abstract (Englisch)

The following items are considered to represent the most important results of the project work:

• Significant fatigue performance improvements are obtainable with the application of residual stress-based post-weld treatment methods to single potential crack sites in tubular bridge structures.

• This performance improvement is more sensitive to variations in the treatment intensity than the treatment uniformity over the studied ranges for these parameters. However, these ranges assume that a minimum level of treatment uniformity is ensured (i.e. no gross errors).

• Post-weld treatment can be used to increase the fatigue life of a bridge with a given steel weight. Towards this end, it shown in the project report that fatigue life increases ranging from 120 to 930% are obtainable by needle peening, for a target reliability index between 1.16 and 3.74. This finding is applicable to highway bridges under principal road traffic volumes with planned service lives of 70 years.

It is shown that the benefits of full treatment can often be achieved with much more concentrated partial treatment strategies. For tubular K-joints such as those found in the studied example bridge, an optimal partial treatment strategy is proposed

Umsetzung und Anwendungen (Englisch)

The practical outcome of this work will be a tool to determine the best post weld treatment strategies in order to maximize the overall reliability of the structure, considering such aspects as extent of treatment, system redundancy and truss configuration. Practical guidelines and original quality control procedures will also be developed in order to facilitate the effective and economical application of post weld treatment methods to tubular bridge structures.

Berichtsnummer (Deutsch)

592

Literatur (Englisch)

WALBRIDGE, Scott, NUSSBAUMER, Alain, and HIRT, Manfred, A Probabilistic Study of Fatigue in Post-Weld Treated Tubular Bridge Structures, Report VSS 592, February 2006.

WALBRIDGE, Scott, A probabilistic study of fatigue in post-weld treated tubular bridge structures. Doctoral Thesis No. 3330, EPFL, Lausanne, 2005.

WALBRIDGE, Scott and NUSSBAUMER, Alain, Probabilistic Fatigue Analysis of a Post-Weld Treated Tubular Bridge. 11th International Symposium on Tubular Structures (ISTS 11), Quebec City, Canada, August 31 to September 2, 2006 (accepted).

WALBRIDGE, Scott and NUSSBAUMER, Alain, Post-Weld Treatment in the Field to Improve Tubular Bridge Fatigue Performance. International Conference on Fatigue and Fracture in the Infrastructure, Lehigh, USA, August 6 to 9, 2006 (pending acceptance).

WALBRIDGE, Scott and NUSSBAUMER, Alain, Probabilistic model for determining the effect of post-weld treatment on the fatigue performance of tubular bridge joints, International Journal of Fatigue, 2006 (accepted for publication)

NUSSBAUMER, Alain, LES POSSIBILITÉS OFFERTES PAR LES PONTS MIXTES TUBULAIRES, In : Documentation SIA N° D 0212. Société suisse des ingénieurs et des architectes, Zurich, 2005, p. 77-89.

WALBRIDGE, Scott and NUSSBAUMER, Alain, POST WELD TREATMENT TO IMPROVE THE FATIGUE PERFORMANCE OF TUBULAR BRIDGE STRUCTURES, In : Proceedings of the Conference Eurosteel 2005, 4th European Conference on Steel and Composite Structures, Maastricht, June 8-10, 2005, ed. by B. Hoffmeister and O. Hechler, Volume B, paper 167, p. 1.12-25 – 1.12-32.

WALBRIDGE, S. and NUSSBAUMER, A., PROBABILISTIC FATIGUE ANALYSIS OF IMPROVED TUBULAR BRIDGE JOINTS CONSIDERING MULTIPLE-CRACK INITIATION SITES, PSAM 7-ESREL 04, 14-18 June 2004, Berlin, CD rom, paper 0886.

WALBRIDGE, Scott, NUSSBAUMER, Alain and HIRT, Manfred, FATIGUE BEHAVIOUR OF IMPROVED TUBULAR BRIDGE JOINTS, In : Proceedings of the 10th International Symposium on Tubular Structures, 18-20 September 2003, Madrid, “Tubular Structures X”, A. A. Balkema Publishers, p. 339-345.

WALBRIDGE, Scott and NUSSBAUMER, Alain, A PROBABILISTIC STUDY OF THE FATIGUE BEHAVIOUR OF IMPROVED TUBULAR BRIDGE JOINTS, In : Proceedings of the 3rd IABMAS Workshop on Life-Cycle Cost Analysis and_Design of Civil Infrastucture Systems, Lausanne, 2003, Life-Cycle Performance of Deteriorating Structures, edited by Dan M. Frangopol, Eugen Brühwiler, Michael H. Faber and Bryan Adey.