Nondestructive Evaluation of Pins in Bridge Structures
I. PROBLEM TITLE
Nondestructive Evaluation of Pins in Bridge Structures
II. RESEARCH PROBLEM STATEMENT
Pins are commonly used in steel bridge structures. Pins are found in truss bridges, and they also occur in pin and hanger details used to suspend spans from an adjacent portion of the bridge structure. As envisioned by the designer, the pin and hanger detail provided a means to easily accommodate thermal expansion and contraction of a bridge structure. This feature is particularly desirable in long, continuous structures where the total expansion or contraction can be quite significant. However, problems with this detail are well documented and have developed as a result of corrosion in either the pins or the hangers. When pin & hanger systems were initially installed during the interstate era, most often the joint in the deck was a open joint allowing drainage to freely fall on the pin & hanger systems. This often leads to serious corrosion, especially on the bottom pin of the system. A typical scenario for failure includes three stages; the first stage of pin & hanger failure is visual indication of fretting rust running out the end of the pin, indicating that the pin and hanger are mating, thus restricting the free movement. The next stage is pack rust building up between the girder web and the hanger bar, increasing the restriction on movement. The third and final stage is an completely frozen system form corrosion. The result can be pin failure and/or hanger failure. This leads to extra stresses being developed in the pins and hangers, which can result in the elongation of pins and development and propagation of fatigue cracks in pins. Hanger plates develop fatigue cracks. Pins in link bars on truss bridges typically do not experience drainage from the deck, but do experience similar deterioration due to exposure. Typical deterioration includes fretting corrosion and pack rust between the faying surfaces of the link bars, thus free movement is restricted. (Recent event is Oakland Bay Bridge) The structural condition of the pins in a structure is critically important. Failure of a pin in a pin-connected structure can result in serious consequences. In the Mianus River Bridge collapse that occurred in June 1983, a large section of the bridge collapsed and fell into the river as a result of the failure of two pins. Earlier deck re-surfacing blocked proper drainage of the deck and resulted in excess water being redirected onto the pin and hanger assemblies. Some of the pins developed corrosion, and over time build up of pack rust corrosion that pushed the hangers bars off the ends of the pins resulting in and high bearing stresses (overstress) at the opposite side hanger bar that lead to fatigue cracking in the pins. A couple of the pins eventually fractured and was responsible for the collapse of a section of the bridge on I-95 in Connecticut. Pins are typically inspected using ultrasonic techniques. However, evaluation of the results from the inspection is complicated by the fact that there are no commonly accepted standards available to assess the pin elements. While some advocate the use the AWS D1.5 Bridge Welding Code, the pins are not technically welded elements, the code is not a valid choice. The research should include the following: • A survey and literature review of practicing inspection organizations should be done to establish the current state of the art regarding the inspection of pin elements. • The research should investigate current procedures available for nondestructive inspection to determine which procedures are most effective in detecting irregularities and cracks in pins. Both single transducer ultrasonic inspection and phased array ultrasonic inspection should be included as a minimum. Other nondestructive techniques should also be considered to determine their viability for pin inspection. • The research should develop nondestructive evaluation methodologies to assess the results of an inspection and provide a means for the inspector to make a decision regarding the suitability of the pin. Existing nondestructive methodologies, such as AWS D1.5 and other peripherally related specifications, should be reviewed to assist in developing fitness-for-purpose criteria for pins.
III. LITERATURE SEARCH SUMMARY
A TRID search was performed using the search terms “bridge pin” AND “evaluation”. The most relevant literature found was FHWA Report FHWA-HRT-04-042 “Guidelines for Ultrasonic Inspection of Hanger Pins” from July 2004. This report explained the techniques and issues of testing hanger pins using ultrasonic testing (UT), but did not give acceptance criteria to evaluate a discontinuity. This report stated that no standard scan pattern exists and that a scan pattern must be developed for each pin to ensure thorough inspection capable of detecting reflectors at critical locations. Typically, both straight beam and angle beam testing is used on pins. This report gave general guidelines on the critical locations which must be scrutinized carefully and discussed the typical signal responses of different discontinuity and indication sources, but did not give any recommendation how to evaluate indications or determine critical size of discontinuities. A journal article titled “Ultrasonic Inspection of Bridge Hanger Pins” was published in Public Roads Vol. 64 Issue. 3 from Nov./Dec. 2000 after the collapse of the Mianus River Bridge which investigated the reliability of contact ultrasonic testing in the field to locate defects in a pin. This study compared the results from contact UT in the field to noncontact immersion UT and found that field inspections could identify crack-like defects in pins. This study did not investigate the critical size of discontinuities in pins or acceptance criteria for ultrasonic testing. The Montana DOT Pin (Transverse Girder) and Pin & Hanger Inspection Procedure was reviewed as a typical DOT pin inspection procedure. This procedure stated that signals between the shoulders of the pin are Relevant Indications and that as much data should be taken so that the indication can be evaluated and monitored or the pin replaced. This procedure gives a minimum signal amplitude cut-off above which all signals shall be recorded, but it does not give any guidance on how to evaluate an indication or determine its acceptability.
IV. RESEACRH OBJECTIVE
The objective of the research is to develop procedures that can be used to inspect and evaluate pin connected structures to improve the consistency of pin inspections and improve the overall safety of pin-connected structures.
V. ESTIMATE OF FUNDING AND RESEARCH PERIOD
$700,000 over a three year period
VI. URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION
The recommended research is urgent because considerable variability exists in how pin connected elements are inspected and how the results of those inspections are interpreted and evaluated. The absence of an accepted procedure and protocol for the inspection and corresponding evaluation of the pins in a bridge structure results in considerable variability in the inspection results and inconsistent degrees of safety achieved as a result of the inspections.
VII. PERSON(S) DEVELOPING THE PROBLEM
Philip Fish & Curtis Schroeder, Fish & Associates Jon McGormley, Wiss Janey Elstner & Associates Glenn A. Washer, University of Missouri Mark D. Bowman, Purdue University
VIII. PROBLEM MONITOR
To be completed by NCHRP
IX. DATE AND SUBMITTED BY (State Endorsements)
1. Demers, C.E. and Fisher, J.W. (1990). Fatigue Cracking of Steel Bridge Structures; Volume I: A Survey of Localized Cracking in Steel Bridges – 1981 to 1988, “I-95 Over Mianus River, Connecticut”, Federal Highway Administration, Report FHWA-RD-89-166: McLean, VA. 2. Bridge Welding Code. (2010). American Welding Society, AASHTO/AWS D1.5M/D1.5. 3. Moore, M., Phares, B.M., and Washer, G.A. (2004). Guidelines for Ultrasonic Inspection of Hanger Pins, Federal Highway Administration, Report FHWA-HRT-04-042: McLean,VA. 4. Graybeal, B.A., Walther, R.A., Washer, G.A., and Waters, A.M. (2000). Ultrasonic Inspection of Bridge Hanger Pins. Public Roads, 64 (3), 20-26. 5. Montana Department of Transportation (2007). Bridge Inspection Manual. “Chapter 6: Steel, Pin & Hanger and Fracture Critical”