Bridge Structures

The I-435 Bridge over the Missouri River at Kansas City, Missouri, consists of twin two-girder structures. Inspections by the Missouri Department of Transportation (MoDOT) in 2003 detected numerous cracks at the floor beam and lateral member connections throughout the 30-year-old bridge. An initial evaluation suggested that many of the cracks were the result of distortion-induced stresses. No evidence of unstable crack extension was observed. A comprehensive fatigue study was subsequently carried out to investigate the extent and cause of cracking and to evaluate possible long-term retrofit solutions. Based on the study findings, a retrofit plan was implemented that addressed details susceptible to distortion-induced cracking, end restraint cracking, and crack growth from embedded defects. Large-hole and loosening type retrofits were utilized to reduce the possibility of crack extension. The installed retrofits will significantly extend the remaining fatigue life and facilitate ease of inspection of this critical infrastructure link.

The present paper describes a practical method for analysing and estimating the remaining fatigue life of a ‘cut-short’ web stiffener detail widely used in highway steel box girder bridges constructed in Ontario, Canada between 1960 and the early 1980s. The paper also discusses the design used to repair steel box bridges fabricated with this stiffener detail. Since 2000, over 20 bridges in southern Ontario have been retrofitted using these details. Steel box girder bridges with composite concrete decks were commonly fabricated in Ontario with a ‘cut-short’ welded stiffener detail that was used to connect intermediate vertical cross bracing to the girder webs. The bracing members, consisting of single angles, were attached to the ends of the stiffeners with fillet welds. The vertical web stiffener would usually be terminated above the girder bottom flange with a 20 – 50 mm web gap. Often, the fillet welds attaching the stiffener and the web would terminate short of the stiffener end, resulting in an even longer web gap. This detail introduced both in-plane and out-of plane bending into the portion of the web between the end of the stiffener and bottom flange. As a result, several steel box girder bridges have exhibited fatigue cracking problems at the end of the stiffener welds after less than 20 years of service. The cumulative fatigue effect on a bridge is influenced by the volume of truck traffic as well as truck size, configuration and weight. The following approach was used by the authors to estimate the remaining fatigue life of un-cracked stiffener details fabricated in two steel box girder bridges located at the Highway 406 interchange of the Queen Elizabeth Way in Ontario, one of the busiest highways in Canada. A third bridge had already experienced cracking problems and was used to validate the approach: (1) Historical traffic volumes were obtained from available traffic survey records. (2) The volume of truck traffic was determined either from available traffic data or from an assumed percentage based on existing data from other similar Ontario Provincial highways. (3) The cumulative number of truck/axle loads that exceeded the allowable fatigue threshold value for the detail was calculated. (4) The cumulative number of load cycles from trucks that resulted in fatigue damage to the web was estimated with consideration for the load effects caused by multi-lane loadings. (5) A finite element analysis of the fatigue prone transverse stiffener detail using an ‘average’ damaging fatigue truck loading was used to estimate the maximum fatigue life of the web stiffener detail based on Miner’s rule. (6) From the model results, an ‘average’ total stress range was applied to estimate the fatigue life of the detail. The remaining service life was estimated by subtracting the maximum life from the cumulative number of damaging load cycles since construction. The results of the analysis compared closely with the actual observed fatigue life of the stiffener detail for the three bridges that were investigated. The current paper also discusses design details that may be considered for repairing bridges with similar cracking problems as well as those considered to be susceptible to future fatigue cracking.

The Interstate 79 Neville Island Interchange Bridge complex located north of Pittsburgh, Pennsylvania, carries four through lanes of the Interstate over Neville Island and the main and back channels of the Ohio River. The mainline bridge consists of a tied through arch span and 26 dual parallel approach spans. These mainline dual approach spans are continuous units comprised two welded steel plate deck girders, welded steel plate floorbeams and rolled steel wide-flange stringers. In addition, the complex includes eight ramp bridges consisting of both straight and curved continuous dual welded steel plate girders, welded steel plate or rolled wide-flange floorbeams and rolled steel wide-flange stringers. These structures, built and opened to traffic during the early to mid-1970s, are owned and maintained by District 11-0 of the Pennsylvania Department of Transportation. All nine bridges are considered fracture-critical (FCM) owing to the two-girder floorbeam system used on the mainline approach spans and all of the ramp bridges. The steel welded box tie-girder of the main tied-arch span also makes this span fracture-critical. SAI Consulting Engineers, Inc. has performed the required annual NBIS and FCM inspections for the last15 years on most of these structures. There were many brittle fracture and fatigue-prone details that were of concern; some of which were cracked. These details included girder-floorbeam and stringer-diaphragm out-of-plane distortion, floorbeam flange end terminations, floorbeam-to-girder welded kneebrace connections, poor weld details at back-up bar splices, intersecting welds of transverse and longitudinal stiffeners, etc. In 2000 and 2005, SAI developed fatigue retrofits for nine brittle fracture or fatigue sensitive-type details that were part of two major design rehabilitation contracts. In 2003, SAI also designed retrofit-type repairs for an emergency contract to retrofit existing Hoan-like details. This paper discusses the design, fabrication, and construction of the retrofits for these various brittle fracture and fatigue-prone details.

Since conducting fatigue life studies more than 25 years ago of several major railroad bridges, management at Canadian National (CN) has taken constructive steps to introduce measures to mitigate the effects of fatigue distress and ensure the safety of operations over its many ageing steel bridge structures despite increasing stress from the introduction of heavier and more efficient use of rolling stock, greater volumes of tonnage and higher operating speeds. Besides developing an in-house strain gauge testing program and specific inspection practices over the last 16 years, acoustic emission monitoring was utilized on a considerable number of bridges and continues as an effective and reliable method of crack detection and monitoring of critical fatigue prone details. Recently, investigation into developments in ultrasonic impact treatment (UIT) has prompted the use of this promising technology on fatigue susceptible welded details on CN bridge structures. UIT was first applied to one of CN’s larger bridges in 2001 with the expectation that this treatment would considerably extend its fatigue life expectations. The present paper reviews fatigue related issues afflicting CN steel bridge structures and the measures taken over the last 20 years to address effectively these concerns and ensure the continued safe movement of trains across the network.

In-service experience has clearly demonstrated that the base welded connection of the high mast tower or pole luminaire structure is extremely fatigue sensitive. In addition, laboratory research has shown that the commonly used base welded connection details provide a fatigue resistance aligned with either Category E for the full penetration weld type connection or category E′ for the fillet welded socket type connection. These base type connection details limit the service stress range levels from 2.6 ksi (17.9 MPa) (E′)to 4.5 ksi (31.0 MPa) (E). State Departments of Transportation with these common transportation structures for roadway and bridge lighting have experienced significant base connection cracking and total failure after short service lives. One State Department of Transportation had installed eighteen 140 ft (42.7 m) tall high mast towers in 2000. In late 2003, one of these towers failed, and 17 others exhibited base cracks. All eighteen towers were removed with less than 3 years of service. Based on the extent of the problem, a field-installed, mechanically fastened steel strengthening jacket has been designed and installed for several State Departments of Transportation. Base retrofit criteria were established for the design and installation. These criteria included the following: (a) significantly increase the service life of the shell to base plate connection; (b) field install without tower or pole removal; (c) no damage to existing electrical wiring or internal mechanisms; (d) installation accomplished using a standard bridge maintenance or ironworker crew; and (e) retrofit cost limited to about 15% of the cost of a new installed replacement pole or tower. Six high mast tower base retrofits and 800 pole luminaire base retrofits have been completed. Instrumentation and field testing measurements of in-service stress ranges have been recorded over a continuous 77 day interval to access the performance of the retrofitted tower bases. Base retrofits of this type are a practical and cost-effective repair solution that should be considered when base cracking problems occur or if a redundant load path is warranted. The implementation of these base retrofits will significantly increase the service life of these structures.

The primary objective of this work was to perform fatigue life assessment for two types of mounted bridge aluminum light poles, which can lead to a better understanding of the structural behavior of both types of details, currently used as secondary structural elements mounted on various bridges and highways. Included in this study are shoe base and through plate socket connection details. Fatigue tests showed a relatively low life for through plate details as compared with the shoe base connections at the same stress range. Fracture mechanics studies indicated that lower bound design curves could be predicted using higher stress ratio fatigue crack growth data superimposed with the effect of compressive residual stresses at the surface of the tubes. Parametric studies demonstrated the importance of base plate thickness on the through thickness bending of the tube wall, which can be extremely effective on mounted bridge poles due to open terrain wind dynamic loads. A research study revealed that shoe base socket connection fatigue life is on average seven times better than a 25.4 mm (1 inch) plate socket connection details and when 76 mm (3 inch) plate thickness is used, fatigue life for shoe base socket connections is 1.5 times the 25.4 mm through plate socket connection. It is recommended that whenever a through plate socket connection light pole is chosen for bridge luminary, 63 – 76 mm (2.5 – 3 inch) plate thickness should be used to ensure an infinite fatigue life.