Will Lindquist

Distortion-Induced Fatigue Cracking in a Seismically Retrofitted Steel Bridge

This study examines the effect of a seismic retrofit on distortion-induced fatigue cracking in a steel girder bridge in the Midwestern United States. The seismic retrofit of the bridge was deemed necessary following a structural review initiated by the Federal Highway Administration (FHWA) in response to the 1989 Loma Prieta Earthquake. Following the review, the bridge was modified using a conventional seismic retrofit strategy to meet the 1995 FHWA standards for survivability and seismic performance. Upon completion of the retrofit, subsequent inspections identified new cracks (at the location of the retrofit) in the webs of longitudinal girders at transverse stiffener locations in an area known as the web gap. This study investigates the influence of the seismic retrofit strategy on crack formation by comparing principal stresses in the web-gap region of the original and retrofitted bridge under dead, live and thermal loading. A three-dimensional linear-elastic finite element analysis of the bridge before and after the retrofit is presented with results obtained from a commercial finite element software package. Comparisons of stresses before and after the retrofit indicate a measurable increase in stress near the web-gap region attributable to thermal and live loads following the retrofit. In addition, several viable repair strategies to limit these web-gap stresses and subsequent distortion-induced fatigue are presented. The results to date clearly demonstrate the importance of considering fatigue-sensitive details in seismic-retrofit strategies.

Investigation of Distortion-Induced Web-Gap Cracking in a Seismically Retrofitted Steel Bridge: Repair Measures

AbstractThis paper studies distortion-induced fatigue cracks in a 1960s-era welded plate girder bridge that developed cracks in the web-gap region shortly after completion of a comprehensive seismic retrofit. Finite-element analysis (FEA) and field testing were conducted to investigate the cause of cracking and then to recommend appropriate repair measures for the bridge. The field test results validated the high-stress concentrations in the web-gap region of the bridge that led to fatigue cracking after only a limited number of cycles. FEA results indicated that the high-stress concentrations were principally a result of replacing K-type diaphragms with stiffer cross diaphragms as part of the seismic retrofit. Two conventional repair measures in addition to two innovative repair measures were evaluated. The proposed innovative repair measures focused on ease of installation compared with more conventional options and were found to reduce the distortion-induced stresses well below the constant amplitude f...

Comparison of Spacing Factors as Measured by the Air-Void Analyzer and ASTM C457

ABSTRACT The Kansas Department of Transportation (KDOT) began using the Air-Void Analyzer (AVA) in 2001 and first incorporated an AVA spacing factor requirement into paving specifications beginning in late 2002. In 2005, a statewide investigation to evaluate the AVA and specifications began with the collection of 100-mm diameter hardened concrete samples taken at or near locations where the spacing factor was determined with the AVA. The hardened concrete samples were tested to determine the spacing factor in accordance with ASTM C457, a well-established test method used to determine spacing factors in hardened concrete. A total of 270 data pairs were collected to evaluate KDOT’s current use of the AVA and ASTM C457 tests and to determine if a correlation (either direct or pass-fail criteria) exists between spacing factors obtained with the two methods. Results of the study indicate that average spacing factors obtained with the AVA were 1.67 times higher than average spacing factors determined using ASTM C457. A strong direct correlation was not identified between the two test methods although pass-fail criterion that limits agency risk of accepting concrete with an inadequate air-void system was identified.

Repair Measures for a Seismically Retrofitted Steel Bridge Experiencing Distortion-Induced Fatigue Cracks: Numerical Study

This paper studies distortion-induced fatigue cracks in a 1960’s era design Missouri Department of Transportation (MoDOT) welded plate girder bridge that developed cracks in the web-gap region shortly after completion of a comprehensive seismic retrofit. Finite Element analysis (FEA) was conducted in order to investigate the cause of cracking and to recommend appropriate repair measures for the bridge. The results indicated that the high stress concentrations were principally a result of replacing K-type diaphragms with stiffer cross-diaphragms as part of the seismic retrofit. Two conventional repair measures in addition to two innovative repair measures were evaluated. The first repair strategy provided a positive connection between the connection plate and the girder flange (referred to as the top-angle repair). The second repair created a 203-mm (8-in.) slot where one-sixth of the connection plate depth is cut from the web (slot-repair) resulting in a softening of the web gap.

Mitigation of Exterior Beam Rotation in Bridge Construction Through Experimental Investigation of Different Bracing Systems

Bridge contractors use different bracing systems to prevent exterior girder rotation in bridge construction. The reason behind this rotation is the unbalanced eccentric loads induced from loading the deck overhang, which can cause excessive twisting in the exterior girders and excessive overhang deflections. This situation can lead to several problems during construction and over the life of the bridge. Contractors traditionally use block-and-tie systems, which are formed by a combination of tie bars and timber blocks, to mitigate exterior girder rotation. However, block-and-tie systems do not always perform as expected and their efficiency to prevent rotation not yet been evaluated. This paper investigates current block-and-tie systems and presents an analysis of improved rotation prevention systems that can be implemented in the field. Both the block-and-tie systems and the new proposed bracing alternatives were tested in a twin steel girder system [1.8 m × 4.6 m (6 ft. × 15 ft.)] that simulated a steel girder bridge bay. The prototype was eccentrically loaded at mid span through the application of a vertical load in a bracket which was mounted in one of the girders, simulating the loading from the bridge overhang. Forty-five bracing combinations were tested by varying the number of bracing elements and the spacing between them and obtaining girder rotation corresponding to the mid span of the loaded girder. The forty-five cases were divided into nine different groups to allow a clear and easy comparison between bracing alternatives. The results show very promising potential replacements of the current temporary bracing systems which, in many cases, offer minimal protection against exterior girder rotation.