Caroline Bennett

Analysis of Laterally Loaded Piles in Sand Considering Scour Hole Dimensions

AbstractScour is a process of soil erosion, which can occur around the foundations of bridges or offshore structures through the action of flowing water; it can, therefore, reduce the capacity of the foundations and sometimes lead to the failure of structures. During an analysis of scour effects on laterally loaded piles, scour hole geometry is often ignored. Instead, scour effects are considered by simply removing the whole soil layer to the scour depth. However, a scour hole has not only depth, but also width and a slope. At present, the widely used p-y method for analyzing laterally loaded piles cannot consider three-dimensional scour hole dimensions. For this reason, a simplified method was developed herein for the analysis of laterally loaded piles in sand under a scour condition. The p-y curves in this method were based on a wedge type of failure. The effects of the scour hole dimensions on the response of laterally loaded piles in sand were evaluated using this simplified method. The computed resul...

Use of CFRP Overlays to Strengthen Welded Connections under Fatigue Loading

This study evaluates the performance of various methods to prevent and repair fatigue damage in welded connections, a recurring problem that affects a significant number of steel bridges. Experimental tests and analytical simulations were carried out to investigate the fatigue performance of coverplate specimens in which the welded connections were reinforced with carbon-fiber reinforced polymer (CFRP) overlays. Specimens were loaded in three-point bending induced by a cyclic load to evaluate the change in fatigue-crack initiation life of the welded connections caused by the attachment of the CFRP overlays. Test results showed that when bond between the CFRP overlays and the steel was maintained, the reduction in stress demand was sufficient to extend the fatigue life of the welded connections from AASHTO fatigue-design Category E’ in the unreinforced configuration to the infinite fatigue life range. Test results also showed that the fatigue strength of the bond layer was drastically improved by introducing breather-cloth material within the bond layer.

Parametric Analysis of Cross-Frame Layout on Distortion-Induced Fatigue in Skewed Steel Bridges

The effects of skew angle, cross-frame spacing, cross-frame layout, cross-frame stiffness, and load placement on the potential for distortion-induced fatigue damage in steel bridges was investigated by performing a suite of more than 1,000 analysis jobs of high-resolution three-dimensional finite-element models. Susceptibility to fatigue damage was quantified in terms of computed stress demand in the web gap region of the girders. Bridge configurations with three different cross-frame layouts were evaluated, including configurations with cross-frames placed parallel to skew angle (skewed-parallel) and perpendicular to the girder line, both staggered (skewed-staggered) and unstaggered (skewed-unstaggered). Skew angles of configurations evaluated ranged between 0 and 50°, and cross-frame spacing ranged from 2.29 to 9.14 m (7.50 to 30.0 ft). Influence and envelope surfaces were constructed to show the relationship between load placement, location of the maximum web gap stress, and the magnitude of the maximum web gap stress. It was found that maximum web gap stress always occurred when loads were positioned directly above the intersection of a cross-frame and girder web. The parametric study showed that cross-frame stiffness and spacing had a significant effect on the susceptibility to distortion-induced fatigue damage; greater cross-frame stiffness resulted in higher web gap stresses, and increased cross-frame spacing resulted in increased web gap stresses. It was also found that the bridge configuration was key to determining the location of the web gaps where damage is most likely to occur. In skewed-parallel and skewed-unstaggered layouts, maximum web gap stresses were identified in top web gaps, whereas in skewed-staggered configurations, maximum stresses occurred in bottom web gaps. It was found that in configurations with staggered cross-frames, maximum web gap stresses tended to occur in regions of support, where cross-frames are often placed back-to-back along the skewed alignment.

Development of a Technique to Improve Fatigue Lives of Crack-Stop Holes in Steel Bridges

A common technique to prevent the propagation of fatigue cracks in bridge girders is to drill crack-stop holes at crack tips. Stress concentrations at the crack tips are reduced and fatigue life of the bridge is extended. The size of the crack-stop hole needed to prevent further crack growth is determined by using known material properties and relationships developed through experimentation. However, these equations often result in a crack-stop hole diameter larger than can be practically drilled; physical limitations force crack-stop holes to be undersized in the field. To improve effectiveness of undersized holes to that of full-sized holes, a method is needed to strengthen undersized crack-stop holes. This study investigated the potential of a technique to improve the fatigue life of undersized, crack-stop holes. It uses piezoelectric actuators operated at ultrasonic frequencies to convert electrical signals into mechanical work. The technique produced residual compressive stresses of the same order of magnitude as those produced by static cold expansion. A suite of finite element models was created to quantify and characterize the residual stresses surrounding the cold-expanded, undersized, crack-stop holes. Results were compared with analyses in the literature.

Evaluation of Behavior of a Laterally Loaded Bridge Pile Group under Scour Conditions

Scour in the vicinity of bridge piers may result in a reduced capacity of bridge foundations. A complete study of bridge stability under scoured conditions requires integration of water, soils, pile systems, and bridge superstructure analyses. This paper describes an analysis performed in which soil and pile group interaction was evaluated using the Group Equivalent- Pile (GEP) method. A pile foundation supporting Kansas Bridge 45 was studied to evaluate its lateral behavior under scoured conditions. Effects of scour depth and pile head boundary conditions on the total deflections of the pile head and pile deflection profiles were examined. Results showed that scour reduced the lateral capacity of the pile group, especially when the scour depth was below the depth of the pile head. In addition, scour changed the influence depths of the deflection, bending moment, and shear force of the pile group.

Effects of Cross-Frame Placement and Skew on Distortion-Induced Fatigue in Steel Bridges

Because of detailing practices common before the mid-1980s, many bridges are highly susceptible to distortion-induced fatigue. This research explored the influence of cross-frame placement and skew angle in bridges subject to distortion-induced fatigue. Forty high-resolution, three-dimensional finite element analyses of a bridge with multiple cross-frame and skew configurations were performed to examine the relationships between skew angle, cross-frame placement, and stresses on distortion-induced fatigue susceptibility. Bridges with skew angles of 0°, 20°, and 40° and cross-frames spaced at 4.58 m (15 ft) and 9.15 m (30 ft) were investigated. Cross-frame configurations examined included staggered perpendicular to the girder line as well as parallel to the support skew. The analyses found maximum stresses in the web-gap occurred in positive moment regions but not necessarily in regions of highest differential deflection. In configurations with cross-frames placed parallel to the skew angle, maximum stress demand was in the top web-gap but it was in the bottom web-gap region when cross-frames were staggered. Increased spacing between cross-frames correlated with slightly increased maximum web-gap stresses for parallel-to-skew cross-frame arrangements and decreased maximum web-gap stresses for perpendicular-to-girder line cross-frame arrangements. Skew angle had a minimal effect on web-gap stresses in the bridge studied.

Fatigue Enhancement of Welded Details in Steel Bridges Using CFRP Overlay Elements

AbstractCarbon-fiber reinforced polymer (CFRP)-overlay elements were developed with the purpose of enhancing the fatigue performance of welded connections in steel bridge girders. Fatigue tests of seven specimens, including four CFRP-strengthened specimens and three control specimens, were performed to quantify the effect of the CFRP overlays on the fatigue crack initiation lives of the welded connections. Results showed that bonding of CFRP overlays significantly reduced the stress demand on welded connections tested at high stress ranges, leading to a large increase in fatigue crack initiation life. The level of effectiveness of the CFRP-overlay elements in extending the fatigue crack initiation lives of the tested connections was found to be affected primarily by bond strength under cyclic loading; bond strength was found to be dependent on the composition and thickness of the resin layer used to bond the CFRP to the steel. With the AASHTO fatigue design curves as a frame of reference, it was found tha...

A Literature Review on Behavior of Scoured Piles under Bridges

Scour is a natural phenomenon caused by erosion or removal of streambed or bank material from bridge foundations due to flowing water. Literature has shown that most bridge failures are caused by scour. While extensive research has been done on the behavior of scour, only limited research has been done on how to evaluate effects of scour on piles. This paper presents a literature review on scoured piles including description and evaluation of scour at bridges, analysis methods for laterally loaded piles, and behavior of scoured piles. The review clearly shows that the behavior of scoured piles is important to the safety of bridges and requires more systematic investigation in future research. A suitable model to simulate scoured piles and a practical design method should be developed to evaluate the safety of bridges subjected to flood.

Numerical simulation and experimental validation of a large-area capacitive strain sensor for fatigue crack monitoring

A large-area electronics in the form of a soft elastomeric capacitor (SEC) has shown great promise as a strain sensor for fatigue crack monitoring in steel structures. The SEC sensors are inexpensive, easy to fabricate, highly stretchable, and mechanically robust. It is a highly scalable technology, capable of monitoring deformations on mesoscale systems. Preliminary experiments verified the SEC sensors capability in detecting, localizing, and monitoring crack growth in a compact specimen. Here, a numerical simulation method is proposed to simulate accurately the sensors performance under fatigue cracks. Such a method would provide a direct link between the SECs signal and fatigue crack geometry, extending the SECs capability to dense network applications on mesoscale structural components. The proposed numerical procedure consists of two parts: (1) a finite element (FE) analysis for the target structure to simulate crack growth based on an element deletion method; (2) an algorithm to compute the sensors capacitance response using the FE analysis results. The proposed simulation method is validated based on test data from a compact specimen. Results from the numerical simulation show good agreement with the SECs response from the laboratory tests as a function of the crack size. Using these findings, a parametric study is performed to investigate how the SEC would perform under different geometries. Results from the parametric study can be used to optimize the design of a dense sensor network of SECs for fatigue crack detection and localization.

Behavior of Laterally Loaded Piles under Scour Conditions Considering the Stress History of Undrained Soft Clay

AbstractScour can remove soils around bridge foundations and result in reduced bridge capacity. Because most bridge failures have been associated with scour events, it is important to evaluate scour effects on bridge safety. In practice, an evaluation of the performance of bridge piles under scoured conditions is usually achieved by simply removing soils from around the piles while possible changes in stress history of the remaining soils are ignored. In reality, the remaining soils undergo an unloading process when scour removes soils from above the remaining soils. This process increases the overconsolidation ratios of the remaining soils. In this study, the stress history of soft clay was considered in the analysis of laterally loaded piles using the p-y method. To account for the stress history effect, the conventional p-y curves for the soft clay were modified to evaluate the undrained responses of the laterally loaded piles at different scour depths. A case study was used to compare the computed res...