A bridge bearing anchor transmits various loads of a superstructure to a substructure. Most anchors are generally designed without consideration of characteristics such as concrete pedestal, grout bedding, and anchor socket. This study investigated the shear behavior of anchors in accordance with the edge distance, embedment depth, compressive strength of concrete, and height of the concrete pedestal in order to simulate the practical characteristics of the bridge bearing anchors. The actual shear capacity of the anchor differs from the shear strengths calculated by the ACI 318 and EN 1992-4; especially, the importance of the embedment depth is underestimated in these codes. An increase in the height of the concrete pedestal has a negative effect on the shear capacity because of the stress concentration. The grout is fractured prior to the occurrence of local damages in concrete, resulting in a secondary moment. As a result, the effect of the level arm is observed. An equation, which can predict the relative cracking degree of concrete, is proposed by analyzing the displacement of grout and concrete. High strain occurs in the stirrups close to the anchor, and the behavior of the strain is more influenced by the embedment depth than the edge distance. The comparison of obtained and analytically evaluated failure loads by calculations according to EN 1992-4, Schmid model and Sharma model was conducted to consider the effect of supplementary reinforcement. Finally, the design equation of concrete breakout strength is modified to predict the more precise shear resistance of a bridge bearing anchor.
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Figure 7 shows the load displacement curve of the edge distance series, which display a positive relationship between an increase in shear capacity and edge distance. For the series of anchors with an embedment depth of 150 mm (Fig. 7a), only specimen LN-6d-15 failed by anchor shaft fracture (Fig. A.1), whereas the specimens with an edge distance of less than 6d failed by concrete breakout (Figs. A.3, A.7, A.11, and A.13). In the figure, SF indicates the steel failure of the anchor bolt. In this study, the anchor bolt was not directly embedded in the concrete, thus the load was transmitted to structure through the anchor socket. The anchor fracture started from a flexural crack of the bolt after reaching a maximum load and then the anchor did not exhibit ductile behavior after the peak load. The failure load exceeded far beyond the code-specified anchor shear capacity because it was mainly caused by the fracture of the anchor shaft largely, as shown in Fig. 8a33. The bearing anchors with edge distances of less than 5d showed slight ductility behavior after the peak load, unlike the LN-5d-15 anchor. In addition, only the measured shear capacity of the LN-5d-15 anchor was smaller than the predicted strengths by ACI 318. This is because the breakout strength and pryout strength calculated by the ACI 318 code are similar due to the sufficient edge distance of 5d. It is considered that this affects the difference in failure load only for LN-5D-15 specimen among duplicated specimens and lead to dissimilar crack pattern (Figs. A.3 and A.4). In addition, the grout bedding of the specimen with a relatively short edge distance than concrete initially resisted most of the load, thus cracks first occurred on the grout bedding. If the mortar is damaged, the capability to prevent the rotation and displacement of the anchor rod is lost34.
Anchors with an embedment depth of 70 mm exhibited different failure behaviors, as shown in Fig. 7b. The slope of the graph changed significantly after the initial cracking, and then rapidly decreased after the peak load. In addition, for the variable with an edge distance of 6d, the decrease in the embedment depth resulted in rear cracks rather than front cracks. The failure mode also changed from bolt failure to pryout failure (Figs. A.1, A.2). Similarly, rear cracks occurred more frequently compared to front cracks in the variable edge distance of 4.5d (Figs. A.7, A.8, A.10). This failure pattern was slightly different from the general pryout failure mode, because the socket was not pulled out by preventing the rotation of the anchor socket through the bearing effect of the reinforcing bar35,36,37. In accordance with the increase of the edge distance, the actual strength compared to the predicted pryout strength increased from 0.73 to 1.09 and from 0.96 to 1.43 when using ACI 318 and EN 1992-4, respectively. The shear capacity significantly decreased by 56.2% for 6d and by 53.0% for 4.5d by embedment depth decrease, since the governing failure mode was changed from breakout to pryout (Figs. A.1, A.2, A.7, A.8, A.10). These results indicate the importance of securing a sufficient embedment depth, as designated failure mode can be pre-planned by designing appropriate embedment depth to prevent sudden failure of anchor bolt or concrete pryout.
For HN-4.5d-15 and HN-6d-15 specimens in Fig. 9c, the slope of the load displacement after the first crack rapidly decreased, and the maximum shear capacity also decreased in comparison with the low and high concrete pedestals. This is because the structural response of a concrete bearing depends not only on the surface area available to resist loading, but also on its height. Yahya reported that an increase in the pedestal height can improve the ductility of the pedestal under a low load; however, the overall stiffness is reduced23. This result is also attributed to the load not being well-transferred to the substructure but concentrated on the pedestal, as reported by the Korea Expressway Corporation43. Therefore, to minimize the stress concentration in the pedestal and to transfer the load well to the substructure, the edge distance needs to be increased with the increase of the height for proper bridge bearing anchor systems30.
The shear load beyond resistance capacity formed inclined failure surfaces both on the grout bedding and concrete pedestal. In some test specimens, the inclination of the main failure surface in the grout did not match that of the concrete pedestal. In addition, cracks not found in the grout appeared on the concrete pedestal. This result is attributed to the level arm caused by grout failure ahead of failure in the concrete. Eligehausen et al. reported that the spalling of thick grout pads in front of the anchor results in bending of the anchor to transfer the shear load44. Despite the higher strength of grout than that of concrete, the grout with a relatively small edge distance starts failing first. Fuchs et al. reported that grout failure ahead of any other type of failure reduces the load transfer capacity5. Randle explained this complex interaction through the load-bearing behavior of shear dowels46. If the grout can be no longer resistant to the load, a level arm is formed, which results in a complex interaction of tension, shear, and bending stresses developed in the anchor, as shown in Fig. 12. Paschen and Schönhoff investigated the effect of secondary overturning moments in the connection and predicted the shear force corresponding to the concrete breakout of a single anchor, as follows47:
Inspection and maintenance of bridge bearing anchor are generally performed on visible destructions, such as failure of the anchor bolt, concrete pedestal, and large cracks both in grout bedding and concrete pedestal53. However, it is not easy to identify internal cracks that do not extend to the free surface under certain loading, because the concrete around the anchor is located under the grout bedding. These small internal cracks in the concrete may not yet have a significant effect on structural performance; however, they might be vulnerable to chemical penetration, such as de-icing materials coming down from the bridge girder by affecting the durability of concrete54. ACI 224R-01 recommends a maximum allowable crack width of 0.18 mm for concrete exposed to de-icing chemicals55. Meanwhile, non-destructive testing (NDT) of concrete cracks is normally used for the maintenance of these concrete structures, and numerous methods are changing with the development of detection devices56. However, Titman emphasized the need for care and experience despite the advantages of the NDT methods, and indicated that inaccurate interpretation can be made in the absence of a specialized technique57.
Few studies have been conducted on anchors reinforced by a straight reinforcement commonly used in construction, while there are some experimental studies on the hairpin58. A bridge bearing anchor adopts the method of stirrup reinforcement, which extends to the substructure reinforcement. The ACI 318 simply increases the strength through a breakout cracking factor ψc,V for an anchor with reinforcement based on the breakout strength, rather than considering the resistance by the reinforcement2,38. Whereas, EN 1992-4 considers the effect of supplementary reinforcement in the form of stirrups and edge reinforcement on shear performance by presenting equation3. Eligehausen et al. reported that a narrow stirrup spacing with large edge distance increases the restraint effect and evaluated the resistance strength of the stirrup reinforcement by applying the concept of the strut-tie model44. Sharma et al. developed a model for single and multiple row anchorage with supplementary reinforcement by improving the formula proposed by Schmid59,60,61,62. 2ff7e9595c
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