Robert J. Connor

Identifying Effective and Ineffective Retrofits for Fatigue Cracking in Steel Bridges using Field Instrumentation

It is estimated that nearly 90% of all fatigue cracking is the result of out-of-plane distortion or other unanticipated secondary stresses at fatigue sensitive details. Interestingly, neither design nor evaluation specifications provide any guidance on how to evaluate the in-service potential for fatigue cracking at these details. Furthermore, the causes and driving forces producing these cracks are typically difficult to accurately characterize using simple design tools. Often, as a result, the effectiveness of various retrofit procedures is often questionable and ill fated. There are many examples where implemented retrofit procedures did not work and fatigue cracking continued. In such cases, details must be re-retrofitted and additional corrective actions taken to ensure a safe structure. Implementation of one or two prototype retrofits is an attractive alternative approach to ensuring effective retrofits are developed. In situations where several dozen or even hundreds of details have to be retrofitted due to a common problem, this approach is most practical. The prototypes should be instrumented and the performance evaluated. Field instrumentation and testing has consistently been demonstrated as the most effective means to confidently determining the effectiveness of a given retrofit strategy. Unanticipated effects, due to modification of the detail, are also revealed thereby reducing the chance for future problems. Monitoring for a few weeks to months can also be conducted at little cost to help ensure the longevity of the retrofit and to identify how the retrofitted detail performs when subjected to the variable-amplitude load spectrum. This paper examines examples of where effective prototype retrofit strategies were shown to be effective as demonstrated through performance and field instrumentation.

Fatigue Design of Modular Bridge Expansion Joints

Experimental and analytical research was recently performed to develop performance-based specifications and commentary for the fatigue design of modular bridge expansion joints (MBEJ). The resulting proposed fatigue-design and test specifications resulting from this research are discussed. These specifications are intended for integration into the present AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications. For consistency with the bridge design specifications, the proposed MBEJ fatigue-design specifications used the same fatigue-design truck as was used by the present AASHTO bridge specifications. However, one important modification of the AASHTO truck loading for MBEJ is the explicit recognition that the rear axles of the HS series trucks actually represent tandem axles. A simplified method to estimate the distribution factor (i.e., the fraction of the range of the design-wheel load assigned to a single “centerbeam”) is recommended. Dynamic impact on both the vertical and horizontal direction is discussed. Also, guidance on structural analysis of MBEJ, good detailing practice, and fatigue resistance and testing are presented. MBEJ designed under the proposed specifications should be resistant to fatigue cracking. The additional material and the improved details, which will be required to comply with the proposed specifications, are not expected to increase the initial first cost of MBEJ substantially.

Unified Approach for LRFD Live Load Moments in Bridge Decks

Current AASHTO-LRFD specifications use many disparate design provisions to establish live load demands in bridge decks. As an example, approximately 17% of Chapter 4 addresses the analysis of decks. One of the AASHTO-LRFD analysis methods for decks uses an orthotropic plate model. The present AASHTO-LRFD orthotropic plate model has a single formulation for the plate torsional stiffness, and this is not generally applicable to all deck types. In this paper, new analytical expressions are developed for moment in bridge decks subjected to arbitrary patch loading considering each of the three cases of orthotropy: (1)xa0relatively torsionally stiff, flexurally soft decks; (2)xa0relatively uniformly thick decks (such as a reinforced concrete deck); and (3)xa0relatively torsionally soft, flexurally stiff decks. Using these newly developed expressions, the AASHTO-LRFD notional live load models were combined with impact, multiple presence, and live load factors to determine maximum strong direction live load moments for...

Review of steel bridges with fracture-critical elements

This paper presents findings of NCHRP Synthesis Project 35-08 that gathered available information on bridges with fracture-critical members (FCM) from the literature, from a survey of bridge owners and consultant inspectors, and from targeted interviews. In the 1970s, material, design, fabrication, shop inspection, and in-service inspection requirements were improved for steel bridges in general. Special provisions for FCM were then implemented, mainly in reaction to bridge collapses. These requirements transformed the industry and the design of modern bridges so that fatigue and fracture are rare in bridges built in the past 20 years. There is a hidden initial cost in some cases because more expensive superstructure designs are being used than necessary to maintain an acceptable reliability level because of restrictions or more subtle prejudice against bridges with FCM. The major impact on life-cycle costs is the additional mandate for hands-on in-service inspection of FCM. There are also varying definitions of “fracture critical,” and consequently there is wide disagreement in classifying different types of superstructures as fracture critical. Numerous bridges have had a full-depth fracture of a fracture-critical girder and did not collapse, usually because of the alternative load-carrying mechanism of catenary action of the deck under large rotations at the fracture. The capacity of damaged superstructures (with FCM removed from the analysis) may be predicted with refined three-dimensional analysis. However, there is a strong need to clarify the assumptions, load cases and factors, and dynamic effects in these analyses. This paper reports on results of this study.

Influence of Flexibility on the Fatigue Performance of the Base Plate Connection in High-Mast Lighting Towers

This study investigates the effect of base connection geometry on base plate flexibility and the fatigue performance of multisided high-mast lighting towers. Three parametric studies investigating the effect of base plate thickness, tube wall thickness, and anchor rod configuration are discussed in this paper. The results of this study show that of the geometric parameters considered, base plate thickness has the largest influence on the stresses in the tube wall adjacent to the weld toe. By increasing the base plate thickness of the tower, significant improvement to the fatigue life is observed because the maximum stress, and hence stress range, at the base plate-to-tube wall thickness is reduced due to a reduction in local bending in the tube wall.

Extending the service life of steel bridges through field instrumentation

Publisher Summary Damage assessment requires an accurate knowledge of live-load-stress cycles so that reasonable estimates of remaining safe life can be made. This chapter presents a paper that examines the behavior of a plate-girder bridge in which web-gap cracking has developed. Retrofits were introduced to limit crack extension and prevent undesirable fractures. In many structures, field instrumentation offers necessary insight into in-situ behavior and live-load stresses. Preemptive retrofits provide attractive solutions to assure continued performance and reliability of existing structures. Together, these methods offer economical solutions to extend the service life of our aging infrastructure. To perform a rational damage assessment, an accurate knowledge of the live-load stress cycles produced in service is essential so that reasonable estimates of remaining safe life can be made. It has repeatedly been demonstrated that field instrumentation and in-service monitoring is a useful tool in estimating the remaining fatigue life in a bridge. These efforts help ensure that the likelihood of unexpected fractures are minimized.