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Behavior of bolt-connected steel plate girder attributable to bolt loosening failure in the lower flange
Journal Pre-proofs Behavior of bolt-connected steel plate girder attributable to bolt loosening failure in the lower flange Jun Min You, Yu Chan Hong, Seok Hyeon Jeon, Jungwon Huh, Jin-Hee Ahn PII: DOI: Reference:
S1350-6307(19)30814-3 https://doi.org/10.1016/j.engfailanal.2019.104208 EFA 104208
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
8 June 2019 10 September 2019 23 September 2019
Please cite this article as: Min You, J., Chan Hong, Y., Hyeon Jeon, S., Huh, J., Ahn, J-H., Behavior of boltconnected steel plate girder attributable to bolt loosening failure in the lower flange, Engineering Failure Analysis (2019), doi: https://doi.org/10.1016/j.engfailanal.2019.104208
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Behavior of bolt-connected steel plate girder attributable to bolt loosening failure in the lower flange Jun Min, You1, Yu Chan Hong2, Seok Hyeon Jeon3, Jungwon Huh4, Jin-Hee Ahn5,* 1Ph.D.
Candidate, Department of Civil Engineering, Gyeongnam National University of Science and Technology, 33 Dongjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
2Master
3Ph.D.
course Student, Department of Civil Engineering, Gyeongnam National University of Science and Technolo gy, 33 Dongjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
Student, Department of Civil Engineering, Gyeongnam National University of Science and Technology, 33 Do ngjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
4Professor, 5Associate
Dept. of Ocean & Civil Engineering, Chonnam National University, Yeosu, Jeonnam 59626, Korea
Professor, Department of Civil Engineering, Gyeongnam National University of Science and Technology, 3 3 Dongjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
Abstract To quantitatively evaluate changes in the structural behavior of a high-strength-bolt-connected steel plate girder attributable to loose bolts in the lower flange, this study conducted tests on bolt loosening in the lower flange using a 12m steel plate girder connected by high-strength bolts. Its displacement and strain were compared with those of a steel plate girder affected by loosening of a lower flange bolt. In this study, various loosening processes of the lower flange bolt and different load conditions were considered to compare the effects of bolt loosening. The results demonstrated that the displacement of the loaded steel plate girder increased in accordance with the loosening of the high-strength bolts in its lower flange, and that its behavior was affected by both the bolt loosening position and the process of bolt loosening. From the comparison results for the stiffness change in the steel plate girders caused by the loosening of the lower flange bolt, its stiffness decreased by approximately 15%–17% and its measured neutral axis rose by approximately 15.8% depending on the bolt loosening position and bolt loosening process. Thus, it can be surmised that
the structural behavior of a bolt-connected steel plate girder is highly affected by the bolt connection of the lower flange due to its considerable structural stiffness change. Although these were conducted under limited experimental conditions, the effect of the lower flange bolt connection on the relative structural stiffness of the plate girder can be considered to induce a stiffness decrease exceeding 15%.
Keyword: Steel plate girder, Bolt connection, Bolt loosening, Girder connection strength, Lower flange bolt
*Jin-Hee Ahn (Corresponding author/Associate Professor/Ph.D.) Tel: +82-55-751-3293, Fax: +82-55-751-3209 E-mail: [email protected]
1. Introduction In steel structures such as bridges and frames, welding or high-strength bolts are generally used to connect structural members. In welded connections between steel members, the strength of the joint is determined by the operating conditions and the skill of the operator. The use of high-strength bolts in joints between members of a steel structure allows rapid and secure connections. However, over long periods of use, the tension of bolt connections might decrease and bolt loosening might occur through continuous load, impact, and vibration [1, 2, 3, 4]. Because bolt loosening induces a loss of clamping force acting on the joint in the connected member and further causes stiffness degradation and potential structural failure [5], various studies have been conducted to detect loosened bolts and investigate the cause of bolt loosening for a bolt connected structure or facility [1–24]. For detecting loosened bolts, bolt loosening detection technologies have been suggested or developed in structural health monitoring (SHM) and nondestructive testing (NDT) fields [1–24] as an ultrasonic technique [6], a piezoelectric-based sensor network [7, 8], a radio-frequency identification (RFID)-based sensing method [9], and other detecting methods [10]. To investigate the cause of the bolt loosening, various studies were also conducted using numerical and theoretical analysis models examined from vibration experiments, etc [11–15]. In the construction management field for bolted steel structures, especially, vision-based loosening bolt detection tests have been conducted because of its advantages such as inspection cost, contactless condition, and varied applicability to field inspection situations as unmanned aerial vehicles (UAVs) instead of the established method of visual inspection [4, 16–24]. In contrast to these studies, some studies have examined the strength or behavior of the connection [25–37]. However, there is a scarcity of research on or reviews of the quantitative behavior evaluation or quantitative performance of the members of steel structures in which the
loosening or omission of bolts is discovered. If the clamping force of the bolt connection is reduced or if the bolt is loosened, the displacement may increase owing to the reduced stiffness of the connection between the steel structure members, thereby changing the stress state acting on the member. Accordingly, this study fabricated a steel plate girder specimen with high-strength-bolt joints to evaluate the change in the behavior of the steel structure when the bolts in the connection were loosened or removed. This study sought to analyze the behavior of the steel plate girder when, after connecting the members through high-strength-bolt joints and applying a load, a bolt falls out under the applied load state.
2. Bolt-connected steel plate girder: test specimen and method 2.1 High-strength bolt-connected steel plate girder specimen This study evaluated the change in the behavior of a steel structure member connected with a high-strength-bolt joint when a lower flange bolt becomes loose and falls out. The lower flange of a steel plate girder member has the largest effect on the bending moment, and its effect on member behavior associated with reduction in connection strength might be the largest. In an actual steel plate girder of a steel bridge, where many large bolts are used to form the bolt-connection joint, it is difficult to confirm directly the effect of bolt loosening on the behavior of the steel structure. Therefore, this study assembled a test specimen steel plate girder with a high-strength-bolt joint, confirmed the clamping force of the bolt joint in the connection process, and analyzed the behavior of the girder associated with the decrease in the strength of the connection caused by loosening of a lower flange bolt. For this study, H-beams (588 × 300 × 12 × 20 mm) with height of 588 mm and flange width of 300 mm were used to fabricate the high-strength-bolt-connected steel plate girder test specimen. 4
The test specimen used SM355A steel with yield stress and tensile stress of at least 345 and 490 MPa, respectively. Figure 1 shows the specifications of the steel plate girder specimen used in this study. To evaluate the effect of bolt loosening on the high-strength-bolt connection under limited conditions, during fabrication of the H-beam steel plate girder specimen, the H-beam members were cut into 6000-mm lengths and connected at the center using F10T M22 bolts. The inner and outer joint plates of the flange comprised 20-mm-thick steel plates and 12-mm-thick steel plates were used for the web joint plates. A vertical stiffener was installed to prevent local deformation that might have occurred in response to loading. A steel block was installed on the lower support of the girder specimen, and a roller and hinge were installed for simple support conditions.
2.2 Test method and measurement of test specimen To analyze the change in behavior of the steel plate girder associated with loosening of the lower flange bolt, a load was applied to the connection of the girder specimen. For bolt loosening in real structures, loosening a bolt body can affect the structural behavior of a plate girder. However, neglecting the bolt body effect due to the incomplete loosening of bolts, the clamping force of each bolt in the loosening position was removed completely under the state of applied load used in this study. The loosening effects of all bolts were not accounted for in the loosening direction or the loosening position because bolt connection of the girder specimen is symmetrical. Four different cases were assumed for bolt loosening occurring in various situations during the life of a steel structure, and the change in the behavior of the steel plate girder was confirmed according to the order of assumed loosening of the lower flange bolts. Additionally, bolt loosening in other connection parts such as the web and the upper flange was not considered during the bolt loosening experiment for the lower flange. In addition, as the behavior of the connection might change 5
depending on the load condition, the application of the load to the girder test specimen was divided into three stages. A steel frame was used to apply the load. For the bolt-loosening process, different locations of the applied load were considered in two cases (load case 1 and load case 2). The loads applied to the girder specimen according to the self-weight of the steel frame were 9.837 kN for load level 1, 19.673 kN for load level 2, and 29.858 kN for load level 3. A small steel block was installed on the steel plate girder and the steel frame was mounted on this block to prevent direct application of the load to the connection. For the bolt-loosening test, the effect of loosening a lower flange bolt under the state of the applied load was confirmed. Here, the load level was limited when the bolt was loosened for safety considerations. To evaluate the behavior of the steel plate girder associated with bolt loosening, the lower flange bolt installed on the girder specimen was defined as BF. In the lower flange bolt loosening process, after assigning a number to each bolt, the loosening bolt test was conducted. A linear variable differential transformer (LVDT) was installed on the lower part of the steel plate girder connection to measure the change in behavior of the steel plate girder resulting from the applied load and bolt loosening. Strain gages were installed on the upper and lower flanges and on the web plate near the girder connection. A data logger was used to verify the displacement and strain of the steel plate girder under load. Figure 2 shows the loading and measurement locations on the girder specimen. Figure 3 shows the location numbers of the bolts for the lower flange bolt-loosening test. To fasten the connection of the girder specimen, the bolt connections of the webs and flanges of the separately fabricated H-beams were first fastened using the high-strength bolts, after which a bolt clamping force was introduced. This clamping force was based on the order of bolt fastening proposed in the Standard Specifications for Building Works (2016) [38] and the design axial force 6
of F10T M22 specified in the Korean Highway Bridge Design Code, Limit State Design [39]. A bolt strain gage was installed at the center part of the high-strength bolt, which confirmed the tensile strain introduced to the bolt and thereby identified the design bolt clamping force. Figure 4 shows load case 1 applied to the girder specimen.
3. Bolt loosening in lower flange of girder specimen: test results In this study, two types of load conditions were selected to consider the effect of loading position on the bolt connected section and bolt-loosening. For load case 1, the maximum load effect on the bolted connection was examined for loading on the bolt connected section, which is the center of the plate girder specimen. To examine the effects of the loading position, the other loading position (load case 2) was set away from the bolt connected section. 3.1 Load applied to center part of connection (load case 1) To confirm the change in behavior of the steel plate girder due to loosening of the lower flange bolts of the high-strength-bolt connection, for the case in which the maximum moment acted on the connection of the steel plate girder (load case 1), three loads were applied: 9.837 kN for load level 1, 19.673 kN for load level 2, and 29.858 kN for load level 3. Then, the effect of bolt loosening was confirmed by removing the clamping force of the lower flange bolts under the loaded state. As loosening of the lower flange might occur under various situations, the bolt-loosening conditions were classified into the following four types. (1) Loosening that occurs continuously for all bolts from the lower flange right-side girder (GRside) left connection (SL) bolt (loosening bolt case I, in the order BF1, BF3, BF5, BF7, BF8, BF6, BF4, BF2) 7
(2) Loosening that occurs only in the lower flange right-side girder (GR-side) connection bolts (loosening bolt case II, in the order BF1, BF2, BF3, BF4) (3) Loosening that occurs first in the lower flange inner connection bolts (loosening bolt case III, in the order BF6, BF3, BF5, BF4, BF8, BF7, BF2, BF1) (4) Loosening that occurs first in the lower flange outer connection bolts (loosening bolt case IV, in the order BF8, BF1, BF7, BF2, BF6, BF5, BF4, BF3) Load levels 1–3 were applied for the case in which loosening occurred continuously in the outer side of the lower flange joint side area, and the case in which loosening occurred only in the bolts of the connection joint plate. For the cases in which inner and outer loosening occurred first, only load level 3 was applied. Then, the results of each case were compared. Figure 5 shows a comparison of the changes in displacement of the steel plate girder associated with bolt loosening for each load level. Figure 5(a) shows the case in which bolt loosening was introduced sequentially from the outer bolt of the lower flange connection. After the outermost bolt BF1 was loosened, the displacement of the steel plate girder was shown to increase continuously as the other bolts were loosened sequentially. The increase in displacement more than doubled after BF1, BF3, BF5, and BF7 lost their clamping force completely and the outer bolts of the lower flange connection were completely loosened. This demonstrates that the bolts of the lower flange connection affect the girder connection stiffness depending on the bolt-loosening sequence. Figure 5(b) shows the case in which bolt loosening was introduced sequentially in only the rightside bolts of the lower flange connection. As in Fig. 5(a), displacement increased as the outer bolts BF1, BF2, BF3, and BF4 were loosened in order. Nevertheless, even when the outermost bolts (BF1 and BF2) were loosened, the displacement of the girder did not increase greatly; however, the displacement showed a relatively larger increase when the inner connection bolts (BF3 and BF4) 8
were loosened. When the bolts of the right-side girder connection were completely loosened, the level of displacement was equal that of the case in which all the bolts of the lower flange connection were loosened. Figure 5(c) shows the case in which bolt loosening was introduced first in the inner bolts (BF6, BF3, BF5, BF4) of the lower flange connection. Even when these inner bolts were loosened, the girder displacement did not change greatly; however, when the outer bolts (BF8, BF7, BF2, BF1) were loosened, girder displacement increased dramatically. Figure 5(d) shows the case in which bolt loosening was introduced first in the outer bolts (BF8, BF7, BF2, BF1) of the lower flange connection. The steel plate girder showed negligible increase in displacement when these outer bolts were loosened; however, with loosening of the inner bolts (BF6, BF3, BF5, BF4), girder displacement increased dramatically. Figure 6 compares the change in strain of the steel plate girder due to bolt loosening depending on load level, together with the change in displacement depending on bolt-loosening case. As shown in the displacement variations in Fig. 5, the strain gages attached to the upper and lower flanges and the web plate indicated that the strain had changed. It can be seen from Fig. 6 that the strain increased or decreased according to the bolt-loosening process. The strain gage attached to the upper flange showed that the strain decreased as the bolts were loosened; the strain was even found to decrease in the strain gage attached to the lower flange. Conversely, the strain gage attached to the web plate showed that the strain increased with bolt loosening. As shown in Fig. 6, the stiffness of the steel plate girder changed as the lower flange bolts were loosened. Therefore, a change in the neutral axis of the steel plate girder can be confirmed through the strain gage attached to the web plate. As can be observed, the neutral axis of the steel plate girder rose as the bolts were loosened; consequently, the compressive strain increased as the 9
compressive force of the upper flange increased. For the lower flange, it could be expected that the strain would increase because of to the reduced stiffness of the lower flange. However, because of the characteristics of the connection member, the tensile strain was reduced because of the lowered stiffness and connection strength of the lower flange. These results are evident in the strain distribution by height of the steel plate girder (Fig. 7). Figure 7 shows the strain distribution of a cross section of the steel plate girder using the mean value of the changes in strain associated with the bolt loosening of Fig. 6. The distribution shows that as the bolts were loosened, the neutral axis changed because of the change in stiffness of the steel plate girder. As shown in Fig. 8, based on the positions of the attached strain gages, the measured neutral axis rose by about 18 ~ 50 mm because of the reduced stiffness of the bolt connection of the lower flange. The results of the initial strain demonstrate that the change in the neutral axis is larger than the measured change.
3.2 Connection side loading (load case 2) A loading test for load case 2 was conducted and the results compared with load case 1, in which the maximum moment acted on the connection of the steel plate girder. Under load condition 2, the bolt-loosening effect was confirmed only for load level 3 with an applied load of 29.858 kN. The lower flange bolt loosening was varied as shown in load case 1. From the bolt-loosening cases of load case 1, case (1), in which bolt loosening occurred continuously in the outer bolts of the lower flange, and case (2), in which loosening occurred first in the lower flange inner bolts, were applied to evaluate the change in displacement and strain. Figure 8 compares the change in displacement of the steel plate girder depending on the boltloosening cases and load level. Figure 8(a) shows the case in which bolt loosening was introduced sequentially from the outer bolt of the lower flange connection. As in load case 1, after the 10
outermost bolt BF1 was loosened, the displacement of the steel plate girder was shown to increase continuously as the other bolts were loosened sequentially. Figure 8(b) shows the case in which bolt loosening was introduced sequentially in only the right-side bolts of the lower flange connection. As in Fig. 8(a), the displacement increased as the outer bolts BF1, BF2, BF3, and BF4 were loosened in order. Based on the results of the analysis of load cases 1 and 2, Figs. 9 and 10 show comparisons of the strain variation and cross-section strain variation associated with the change in the stiffness of the girder connection resulting from the loosening of the lower flange bolts. In each load case test, the steel frame was installed on the upper part of the steel plate girder to apply the load, and the position of the neutral axis remained constant as the stiffness of the steel plate girder changed. However, as shown in the results of load case 1, the measured neutral axis of the steel plate girder rose up to about 93mm following bolt loosening, thereby increasing the compressive strain as the compressive force of the upper flange increased. While the strain should increase because of the reduced stiffness of the lower flange associated with the reduced stiffness of the steel plate girder, the tensile stress decreased because of the reduced stiffness and connection strength of the lower flange because of the characteristics of the connection member.
3.3 Change in connection stiffness due to bolt loosening Figure. 11 shows the behavior of plate girder associated with loosening of bolted joint. A plate girder connected using high-strength bolts resists the axial force, shear force, and moment acting on the girder through the bolt connection. However, when the high-strength-bolt joint of the lower flange is loosened, as in this study, the stiffness of the lower flange joint is reduced, thereby reducing the cross-section stiffness resisting the flexural load and increasing the displacement. In 11
this study, to quantitatively compare the change in the stiffness of the connection joint associated with bolt loosening, the change in girder displacement obtained from load cases 1 and 2 was evaluated. The ratio of the increase in displacement based on the displacement results of the load cases was calculated. Figure 12 shows the change in stiffness of the steel plate girder connection. As shown in Fig. 12, the stiffness of the steel plate girder decreased to 85% or less relatively because of the increase in displacement caused by the reduced stiffness of the girder connection associated with bolt loosening. In this study, four high-strength bolts were used at either side of the lower flange and a loosening test was conducted to evaluate the stiffness of the lower flange connection. In this regard, loosening of the bolts of the lower flange was found to have substantial effect. In an actual steel plate girder bridge, as a greater number of bolts are applied than used in this study and web bolt connections are also used extensively, the stiffness of the web connection could be expected to be very high. Therefore, even if loosening of the lower flange bolts occurs, the reduction in connection stiffness observed in this study was not expected to occur. In addition, the level of the load applied to the steel plate girder was not large; thus, the effect of the working load on the displacement of the steel plate girder was reasonably small. Therefore, even if the load increased, the displacement of the steel plate girder would not increase significantly, and it would depend on the ground pressure or shear resistance of the bolt and joint plate. However, the stiffness of the steel plate girder can be reduced substantially by the decrease in the connection stiffness due to bolt loosening in the lower flange; thus, it can be highly related to the structural behavior of the bolt connected steel plate girder.
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4. Conclusions In this study, the structural behavior of a high-strength-bolt connected steel plate girder with the loosening of the bolts in the lower flange was experimentally examined using a plate girder specimen with a total length of 12 m, comprising 6 m H-beams connected by a high-strength-boltconnection. To consider different bolt loosening cases in the lower flange due to various situations occurring during the life of a steel structure, four different cases of bolt loosening in the lower flange were assumed for two loading cases with three load levels on the steel plate girder specimen. From the measured displacement and strain results for each load case and load level, the structural behavior of a plate girder was evaluated depending on the bolt loosening position and loosening process. In all load cases and levels for bolt loosening in the lower flange, the displacements of the plate girder continuously increased and the strain on the girder section was changed as the loosening process and position of bolt in the lower flange. The increase in its displacement was relatively high when all the bolts on one side of the lower flange connection were completely loosened or when additional bolt loosening after loosening all the inner or outer bolts in the lower flange connection occurred. Its neutral axis also correspondingly changed with the increase in the displacement due to the loosening of the lower flange bolt; therefore, the tensile strain in the lower flange decreased because of the reduced lower flange stiffness and girder connection strength. From these lower flange bolt loosening test results, the relative stiffness change of the steel plate girders caused by the loosening of the lower flange bolt was evaluated using its measured displacement response. The relative stiffness of the connected girder decreased by approximately 15%–17% and its measured neutral axis also increased by approximately 15.8% from the decrease in its relative stiffness. Therefore, the effect of the lower flange bolt connection on the relative structural stiffness of the plate girder can cause a decrease in stiffness exceeding 15% of its structural stiffness. In this 13
study, only a limited bolt connected condition of a single girder was considered; however, the effects of various bolt installation conditions in steel plate girders and multi girder systems with bolt connections on bolt loosening in the bolt connection must be evaluated.
Acknowledgments This work was supported by Gyeongnam National University of Science and Technology Grant in 2018~2019.
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(a) Side view of H-beam
(b) Side view of bolt
(c) Front view of bolt connection
connection
(d) Front view Figure 1 Dimensions of bolt-connected girder specimen (mm)
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Figure 2 Load condition and instrumentation (mm)
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Figure 3 Lower flange bolt connection
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Figure 4 Test set-up (load case 1, load level 3)
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(a) Loosening bolt case I
(b) Loosening bolt case II
(c) Loosening bolt case III
(d) Loosening bolt case IV
(e) Loosening bolt case I
(f) Loosening bolt case II
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(g) Loosening bolt case III
(h) Loosening bolt case IV
Figure 5 Relationship between bolt-loosening process and girder displacement; load case 1
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(a) Loosening bolt case I
(b) Loosening bolt case II
(c) Loosening bolt case III
(d) Loosening bolt case IV
Figure 6 Relationship between bolt-loosening process and strain; load case 1
25
(a) Loosening bolt case I (load level 1)
(b) Loosening bolt case I (load level 2)
(c) Loosening bolt case I (load level 3)
(d) Loosening bolt case II (load level 1)
26
(e) Loosening bolt case II (load level 2)
(f) Loosening bolt case II (load level 3)
(g) Loosening bolt case III (load level 3)
(h) Loosening bolt case IV (load level 3)
Figure 7 Relationship between bolt-loosening process and sectional strain distribution
27
(a) Loosening bolt case I
(b) Loosening bolt case II
(c) Loosening bolt case I
(d) Loosening bolt case II
Figure 8 Relationship between bolt-loosening process and displacement; load case 2
28
(a) Loosening bolt case I
(b) Loosening bolt case II
Figure 9 Relationship between bolt-loosening process and strain; load case 2
29
(a) Loosening bolt case I (load level 3)
(b) Loosening bolt case I (load level 2)
(c) Loosening bolt case I (load level 3)
(d) Loosening bolt case II (load level 1)
30
(e) Loosening bolt case II (load level 2)
(f) Loosening bolt case II (load level 3)
Figure 10 Relationship between bolt-loosening process and sectional strain distribution
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Figure 11 Behavior of plate girder associated with loosening of bolted joint
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a) LVDT 1 for load case 1
(b) LVDT 2 for load case 1
33
(c) LVDT 1 for load case 2
(d) LVDT 2 for load case 2 Figure 12 Ratio of bolt-loosening process to displacement 34
Research highlight
Behavior of bolt-connected steel plate girder attributable to bolt loosening failure in the lower flange - Experiments for examining the behaviors of a high strength bolt connected steel plate girder attributable to loose bolts - Effects of bolt loosening in the lower flange in a steel plate girder - Comparisons of structural responses of a steel plate girder affected by loosening of lower flange bolts.
35
Conflicts of Interest: The authors declare no conflicts of interest.
36
S1350-6307(19)30814-3 https://doi.org/10.1016/j.engfailanal.2019.104208 EFA 104208
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
8 June 2019 10 September 2019 23 September 2019
Please cite this article as: Min You, J., Chan Hong, Y., Hyeon Jeon, S., Huh, J., Ahn, J-H., Behavior of boltconnected steel plate girder attributable to bolt loosening failure in the lower flange, Engineering Failure Analysis (2019), doi: https://doi.org/10.1016/j.engfailanal.2019.104208
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© 2019 Published by Elsevier Ltd.
Behavior of bolt-connected steel plate girder attributable to bolt loosening failure in the lower flange Jun Min, You1, Yu Chan Hong2, Seok Hyeon Jeon3, Jungwon Huh4, Jin-Hee Ahn5,* 1Ph.D.
Candidate, Department of Civil Engineering, Gyeongnam National University of Science and Technology, 33 Dongjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
2Master
3Ph.D.
course Student, Department of Civil Engineering, Gyeongnam National University of Science and Technolo gy, 33 Dongjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
Student, Department of Civil Engineering, Gyeongnam National University of Science and Technology, 33 Do ngjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
4Professor, 5Associate
Dept. of Ocean & Civil Engineering, Chonnam National University, Yeosu, Jeonnam 59626, Korea
Professor, Department of Civil Engineering, Gyeongnam National University of Science and Technology, 3 3 Dongjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
Abstract To quantitatively evaluate changes in the structural behavior of a high-strength-bolt-connected steel plate girder attributable to loose bolts in the lower flange, this study conducted tests on bolt loosening in the lower flange using a 12m steel plate girder connected by high-strength bolts. Its displacement and strain were compared with those of a steel plate girder affected by loosening of a lower flange bolt. In this study, various loosening processes of the lower flange bolt and different load conditions were considered to compare the effects of bolt loosening. The results demonstrated that the displacement of the loaded steel plate girder increased in accordance with the loosening of the high-strength bolts in its lower flange, and that its behavior was affected by both the bolt loosening position and the process of bolt loosening. From the comparison results for the stiffness change in the steel plate girders caused by the loosening of the lower flange bolt, its stiffness decreased by approximately 15%–17% and its measured neutral axis rose by approximately 15.8% depending on the bolt loosening position and bolt loosening process. Thus, it can be surmised that
the structural behavior of a bolt-connected steel plate girder is highly affected by the bolt connection of the lower flange due to its considerable structural stiffness change. Although these were conducted under limited experimental conditions, the effect of the lower flange bolt connection on the relative structural stiffness of the plate girder can be considered to induce a stiffness decrease exceeding 15%.
Keyword: Steel plate girder, Bolt connection, Bolt loosening, Girder connection strength, Lower flange bolt
*Jin-Hee Ahn (Corresponding author/Associate Professor/Ph.D.) Tel: +82-55-751-3293, Fax: +82-55-751-3209 E-mail: [email protected]
1. Introduction In steel structures such as bridges and frames, welding or high-strength bolts are generally used to connect structural members. In welded connections between steel members, the strength of the joint is determined by the operating conditions and the skill of the operator. The use of high-strength bolts in joints between members of a steel structure allows rapid and secure connections. However, over long periods of use, the tension of bolt connections might decrease and bolt loosening might occur through continuous load, impact, and vibration [1, 2, 3, 4]. Because bolt loosening induces a loss of clamping force acting on the joint in the connected member and further causes stiffness degradation and potential structural failure [5], various studies have been conducted to detect loosened bolts and investigate the cause of bolt loosening for a bolt connected structure or facility [1–24]. For detecting loosened bolts, bolt loosening detection technologies have been suggested or developed in structural health monitoring (SHM) and nondestructive testing (NDT) fields [1–24] as an ultrasonic technique [6], a piezoelectric-based sensor network [7, 8], a radio-frequency identification (RFID)-based sensing method [9], and other detecting methods [10]. To investigate the cause of the bolt loosening, various studies were also conducted using numerical and theoretical analysis models examined from vibration experiments, etc [11–15]. In the construction management field for bolted steel structures, especially, vision-based loosening bolt detection tests have been conducted because of its advantages such as inspection cost, contactless condition, and varied applicability to field inspection situations as unmanned aerial vehicles (UAVs) instead of the established method of visual inspection [4, 16–24]. In contrast to these studies, some studies have examined the strength or behavior of the connection [25–37]. However, there is a scarcity of research on or reviews of the quantitative behavior evaluation or quantitative performance of the members of steel structures in which the
loosening or omission of bolts is discovered. If the clamping force of the bolt connection is reduced or if the bolt is loosened, the displacement may increase owing to the reduced stiffness of the connection between the steel structure members, thereby changing the stress state acting on the member. Accordingly, this study fabricated a steel plate girder specimen with high-strength-bolt joints to evaluate the change in the behavior of the steel structure when the bolts in the connection were loosened or removed. This study sought to analyze the behavior of the steel plate girder when, after connecting the members through high-strength-bolt joints and applying a load, a bolt falls out under the applied load state.
2. Bolt-connected steel plate girder: test specimen and method 2.1 High-strength bolt-connected steel plate girder specimen This study evaluated the change in the behavior of a steel structure member connected with a high-strength-bolt joint when a lower flange bolt becomes loose and falls out. The lower flange of a steel plate girder member has the largest effect on the bending moment, and its effect on member behavior associated with reduction in connection strength might be the largest. In an actual steel plate girder of a steel bridge, where many large bolts are used to form the bolt-connection joint, it is difficult to confirm directly the effect of bolt loosening on the behavior of the steel structure. Therefore, this study assembled a test specimen steel plate girder with a high-strength-bolt joint, confirmed the clamping force of the bolt joint in the connection process, and analyzed the behavior of the girder associated with the decrease in the strength of the connection caused by loosening of a lower flange bolt. For this study, H-beams (588 × 300 × 12 × 20 mm) with height of 588 mm and flange width of 300 mm were used to fabricate the high-strength-bolt-connected steel plate girder test specimen. 4
The test specimen used SM355A steel with yield stress and tensile stress of at least 345 and 490 MPa, respectively. Figure 1 shows the specifications of the steel plate girder specimen used in this study. To evaluate the effect of bolt loosening on the high-strength-bolt connection under limited conditions, during fabrication of the H-beam steel plate girder specimen, the H-beam members were cut into 6000-mm lengths and connected at the center using F10T M22 bolts. The inner and outer joint plates of the flange comprised 20-mm-thick steel plates and 12-mm-thick steel plates were used for the web joint plates. A vertical stiffener was installed to prevent local deformation that might have occurred in response to loading. A steel block was installed on the lower support of the girder specimen, and a roller and hinge were installed for simple support conditions.
2.2 Test method and measurement of test specimen To analyze the change in behavior of the steel plate girder associated with loosening of the lower flange bolt, a load was applied to the connection of the girder specimen. For bolt loosening in real structures, loosening a bolt body can affect the structural behavior of a plate girder. However, neglecting the bolt body effect due to the incomplete loosening of bolts, the clamping force of each bolt in the loosening position was removed completely under the state of applied load used in this study. The loosening effects of all bolts were not accounted for in the loosening direction or the loosening position because bolt connection of the girder specimen is symmetrical. Four different cases were assumed for bolt loosening occurring in various situations during the life of a steel structure, and the change in the behavior of the steel plate girder was confirmed according to the order of assumed loosening of the lower flange bolts. Additionally, bolt loosening in other connection parts such as the web and the upper flange was not considered during the bolt loosening experiment for the lower flange. In addition, as the behavior of the connection might change 5
depending on the load condition, the application of the load to the girder test specimen was divided into three stages. A steel frame was used to apply the load. For the bolt-loosening process, different locations of the applied load were considered in two cases (load case 1 and load case 2). The loads applied to the girder specimen according to the self-weight of the steel frame were 9.837 kN for load level 1, 19.673 kN for load level 2, and 29.858 kN for load level 3. A small steel block was installed on the steel plate girder and the steel frame was mounted on this block to prevent direct application of the load to the connection. For the bolt-loosening test, the effect of loosening a lower flange bolt under the state of the applied load was confirmed. Here, the load level was limited when the bolt was loosened for safety considerations. To evaluate the behavior of the steel plate girder associated with bolt loosening, the lower flange bolt installed on the girder specimen was defined as BF. In the lower flange bolt loosening process, after assigning a number to each bolt, the loosening bolt test was conducted. A linear variable differential transformer (LVDT) was installed on the lower part of the steel plate girder connection to measure the change in behavior of the steel plate girder resulting from the applied load and bolt loosening. Strain gages were installed on the upper and lower flanges and on the web plate near the girder connection. A data logger was used to verify the displacement and strain of the steel plate girder under load. Figure 2 shows the loading and measurement locations on the girder specimen. Figure 3 shows the location numbers of the bolts for the lower flange bolt-loosening test. To fasten the connection of the girder specimen, the bolt connections of the webs and flanges of the separately fabricated H-beams were first fastened using the high-strength bolts, after which a bolt clamping force was introduced. This clamping force was based on the order of bolt fastening proposed in the Standard Specifications for Building Works (2016) [38] and the design axial force 6
of F10T M22 specified in the Korean Highway Bridge Design Code, Limit State Design [39]. A bolt strain gage was installed at the center part of the high-strength bolt, which confirmed the tensile strain introduced to the bolt and thereby identified the design bolt clamping force. Figure 4 shows load case 1 applied to the girder specimen.
3. Bolt loosening in lower flange of girder specimen: test results In this study, two types of load conditions were selected to consider the effect of loading position on the bolt connected section and bolt-loosening. For load case 1, the maximum load effect on the bolted connection was examined for loading on the bolt connected section, which is the center of the plate girder specimen. To examine the effects of the loading position, the other loading position (load case 2) was set away from the bolt connected section. 3.1 Load applied to center part of connection (load case 1) To confirm the change in behavior of the steel plate girder due to loosening of the lower flange bolts of the high-strength-bolt connection, for the case in which the maximum moment acted on the connection of the steel plate girder (load case 1), three loads were applied: 9.837 kN for load level 1, 19.673 kN for load level 2, and 29.858 kN for load level 3. Then, the effect of bolt loosening was confirmed by removing the clamping force of the lower flange bolts under the loaded state. As loosening of the lower flange might occur under various situations, the bolt-loosening conditions were classified into the following four types. (1) Loosening that occurs continuously for all bolts from the lower flange right-side girder (GRside) left connection (SL) bolt (loosening bolt case I, in the order BF1, BF3, BF5, BF7, BF8, BF6, BF4, BF2) 7
(2) Loosening that occurs only in the lower flange right-side girder (GR-side) connection bolts (loosening bolt case II, in the order BF1, BF2, BF3, BF4) (3) Loosening that occurs first in the lower flange inner connection bolts (loosening bolt case III, in the order BF6, BF3, BF5, BF4, BF8, BF7, BF2, BF1) (4) Loosening that occurs first in the lower flange outer connection bolts (loosening bolt case IV, in the order BF8, BF1, BF7, BF2, BF6, BF5, BF4, BF3) Load levels 1–3 were applied for the case in which loosening occurred continuously in the outer side of the lower flange joint side area, and the case in which loosening occurred only in the bolts of the connection joint plate. For the cases in which inner and outer loosening occurred first, only load level 3 was applied. Then, the results of each case were compared. Figure 5 shows a comparison of the changes in displacement of the steel plate girder associated with bolt loosening for each load level. Figure 5(a) shows the case in which bolt loosening was introduced sequentially from the outer bolt of the lower flange connection. After the outermost bolt BF1 was loosened, the displacement of the steel plate girder was shown to increase continuously as the other bolts were loosened sequentially. The increase in displacement more than doubled after BF1, BF3, BF5, and BF7 lost their clamping force completely and the outer bolts of the lower flange connection were completely loosened. This demonstrates that the bolts of the lower flange connection affect the girder connection stiffness depending on the bolt-loosening sequence. Figure 5(b) shows the case in which bolt loosening was introduced sequentially in only the rightside bolts of the lower flange connection. As in Fig. 5(a), displacement increased as the outer bolts BF1, BF2, BF3, and BF4 were loosened in order. Nevertheless, even when the outermost bolts (BF1 and BF2) were loosened, the displacement of the girder did not increase greatly; however, the displacement showed a relatively larger increase when the inner connection bolts (BF3 and BF4) 8
were loosened. When the bolts of the right-side girder connection were completely loosened, the level of displacement was equal that of the case in which all the bolts of the lower flange connection were loosened. Figure 5(c) shows the case in which bolt loosening was introduced first in the inner bolts (BF6, BF3, BF5, BF4) of the lower flange connection. Even when these inner bolts were loosened, the girder displacement did not change greatly; however, when the outer bolts (BF8, BF7, BF2, BF1) were loosened, girder displacement increased dramatically. Figure 5(d) shows the case in which bolt loosening was introduced first in the outer bolts (BF8, BF7, BF2, BF1) of the lower flange connection. The steel plate girder showed negligible increase in displacement when these outer bolts were loosened; however, with loosening of the inner bolts (BF6, BF3, BF5, BF4), girder displacement increased dramatically. Figure 6 compares the change in strain of the steel plate girder due to bolt loosening depending on load level, together with the change in displacement depending on bolt-loosening case. As shown in the displacement variations in Fig. 5, the strain gages attached to the upper and lower flanges and the web plate indicated that the strain had changed. It can be seen from Fig. 6 that the strain increased or decreased according to the bolt-loosening process. The strain gage attached to the upper flange showed that the strain decreased as the bolts were loosened; the strain was even found to decrease in the strain gage attached to the lower flange. Conversely, the strain gage attached to the web plate showed that the strain increased with bolt loosening. As shown in Fig. 6, the stiffness of the steel plate girder changed as the lower flange bolts were loosened. Therefore, a change in the neutral axis of the steel plate girder can be confirmed through the strain gage attached to the web plate. As can be observed, the neutral axis of the steel plate girder rose as the bolts were loosened; consequently, the compressive strain increased as the 9
compressive force of the upper flange increased. For the lower flange, it could be expected that the strain would increase because of to the reduced stiffness of the lower flange. However, because of the characteristics of the connection member, the tensile strain was reduced because of the lowered stiffness and connection strength of the lower flange. These results are evident in the strain distribution by height of the steel plate girder (Fig. 7). Figure 7 shows the strain distribution of a cross section of the steel plate girder using the mean value of the changes in strain associated with the bolt loosening of Fig. 6. The distribution shows that as the bolts were loosened, the neutral axis changed because of the change in stiffness of the steel plate girder. As shown in Fig. 8, based on the positions of the attached strain gages, the measured neutral axis rose by about 18 ~ 50 mm because of the reduced stiffness of the bolt connection of the lower flange. The results of the initial strain demonstrate that the change in the neutral axis is larger than the measured change.
3.2 Connection side loading (load case 2) A loading test for load case 2 was conducted and the results compared with load case 1, in which the maximum moment acted on the connection of the steel plate girder. Under load condition 2, the bolt-loosening effect was confirmed only for load level 3 with an applied load of 29.858 kN. The lower flange bolt loosening was varied as shown in load case 1. From the bolt-loosening cases of load case 1, case (1), in which bolt loosening occurred continuously in the outer bolts of the lower flange, and case (2), in which loosening occurred first in the lower flange inner bolts, were applied to evaluate the change in displacement and strain. Figure 8 compares the change in displacement of the steel plate girder depending on the boltloosening cases and load level. Figure 8(a) shows the case in which bolt loosening was introduced sequentially from the outer bolt of the lower flange connection. As in load case 1, after the 10
outermost bolt BF1 was loosened, the displacement of the steel plate girder was shown to increase continuously as the other bolts were loosened sequentially. Figure 8(b) shows the case in which bolt loosening was introduced sequentially in only the right-side bolts of the lower flange connection. As in Fig. 8(a), the displacement increased as the outer bolts BF1, BF2, BF3, and BF4 were loosened in order. Based on the results of the analysis of load cases 1 and 2, Figs. 9 and 10 show comparisons of the strain variation and cross-section strain variation associated with the change in the stiffness of the girder connection resulting from the loosening of the lower flange bolts. In each load case test, the steel frame was installed on the upper part of the steel plate girder to apply the load, and the position of the neutral axis remained constant as the stiffness of the steel plate girder changed. However, as shown in the results of load case 1, the measured neutral axis of the steel plate girder rose up to about 93mm following bolt loosening, thereby increasing the compressive strain as the compressive force of the upper flange increased. While the strain should increase because of the reduced stiffness of the lower flange associated with the reduced stiffness of the steel plate girder, the tensile stress decreased because of the reduced stiffness and connection strength of the lower flange because of the characteristics of the connection member.
3.3 Change in connection stiffness due to bolt loosening Figure. 11 shows the behavior of plate girder associated with loosening of bolted joint. A plate girder connected using high-strength bolts resists the axial force, shear force, and moment acting on the girder through the bolt connection. However, when the high-strength-bolt joint of the lower flange is loosened, as in this study, the stiffness of the lower flange joint is reduced, thereby reducing the cross-section stiffness resisting the flexural load and increasing the displacement. In 11
this study, to quantitatively compare the change in the stiffness of the connection joint associated with bolt loosening, the change in girder displacement obtained from load cases 1 and 2 was evaluated. The ratio of the increase in displacement based on the displacement results of the load cases was calculated. Figure 12 shows the change in stiffness of the steel plate girder connection. As shown in Fig. 12, the stiffness of the steel plate girder decreased to 85% or less relatively because of the increase in displacement caused by the reduced stiffness of the girder connection associated with bolt loosening. In this study, four high-strength bolts were used at either side of the lower flange and a loosening test was conducted to evaluate the stiffness of the lower flange connection. In this regard, loosening of the bolts of the lower flange was found to have substantial effect. In an actual steel plate girder bridge, as a greater number of bolts are applied than used in this study and web bolt connections are also used extensively, the stiffness of the web connection could be expected to be very high. Therefore, even if loosening of the lower flange bolts occurs, the reduction in connection stiffness observed in this study was not expected to occur. In addition, the level of the load applied to the steel plate girder was not large; thus, the effect of the working load on the displacement of the steel plate girder was reasonably small. Therefore, even if the load increased, the displacement of the steel plate girder would not increase significantly, and it would depend on the ground pressure or shear resistance of the bolt and joint plate. However, the stiffness of the steel plate girder can be reduced substantially by the decrease in the connection stiffness due to bolt loosening in the lower flange; thus, it can be highly related to the structural behavior of the bolt connected steel plate girder.
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4. Conclusions In this study, the structural behavior of a high-strength-bolt connected steel plate girder with the loosening of the bolts in the lower flange was experimentally examined using a plate girder specimen with a total length of 12 m, comprising 6 m H-beams connected by a high-strength-boltconnection. To consider different bolt loosening cases in the lower flange due to various situations occurring during the life of a steel structure, four different cases of bolt loosening in the lower flange were assumed for two loading cases with three load levels on the steel plate girder specimen. From the measured displacement and strain results for each load case and load level, the structural behavior of a plate girder was evaluated depending on the bolt loosening position and loosening process. In all load cases and levels for bolt loosening in the lower flange, the displacements of the plate girder continuously increased and the strain on the girder section was changed as the loosening process and position of bolt in the lower flange. The increase in its displacement was relatively high when all the bolts on one side of the lower flange connection were completely loosened or when additional bolt loosening after loosening all the inner or outer bolts in the lower flange connection occurred. Its neutral axis also correspondingly changed with the increase in the displacement due to the loosening of the lower flange bolt; therefore, the tensile strain in the lower flange decreased because of the reduced lower flange stiffness and girder connection strength. From these lower flange bolt loosening test results, the relative stiffness change of the steel plate girders caused by the loosening of the lower flange bolt was evaluated using its measured displacement response. The relative stiffness of the connected girder decreased by approximately 15%–17% and its measured neutral axis also increased by approximately 15.8% from the decrease in its relative stiffness. Therefore, the effect of the lower flange bolt connection on the relative structural stiffness of the plate girder can cause a decrease in stiffness exceeding 15% of its structural stiffness. In this 13
study, only a limited bolt connected condition of a single girder was considered; however, the effects of various bolt installation conditions in steel plate girders and multi girder systems with bolt connections on bolt loosening in the bolt connection must be evaluated.
Acknowledgments This work was supported by Gyeongnam National University of Science and Technology Grant in 2018~2019.
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investigation of the effect of clamping force on the fatigue behavior of bolted plates, Engineering Failure Analysis, Vol. 15, pp. 563-574 [31] T. N. Chakherlou, R. H. Oskouei, J. Vogwell (2009), The effect of bolt clamping force on the fracture strength and the stress intensity factor of a plate containing a fastener hole with edge cracks, Engineering Failure Analysis, Vol. 16, pp. 242-253 [32] R. Puthli, O. Fleischer (2001), Investigations on bolted connections for high strength steel members, Journal of Constructional Steel Research, Vol. 57, pp.313-326 [33] P. Moze, D. Beg (2010), High strength steel tension splices with one or two bolts, Journal of Constructional Steel Research, Vol. 66, pp. 1000-1010 [34] P. Moze, D. Beg (2011), Investigation of high strength steel connections with several bolts in double shear, Journal of Constructional Steel Research, Vol. 67, pp. 333-347 [35] X. Guo, Y. Zhang, Z. Xiong, Y. Xiang (2016), Load-bearing capacity of occlusive high17
strength bolt connections, Journal of Constructional Steel Research, Vol. 127, pp. 1-14 [36] J. H. Ahn, J. M. Lee, J. H. Cheung, I. T. Kim (2016), Clamping force loss of high-strength bolts as a result of bolt head corrosion damage: Experimental research A, Engineering Failure Analysis, Vol. 59, pp. 509-525 [37] I. T. Kim, J. M. Lee, J .W. Huh, J. H. Ahn (2016), Tensile behaviors of friction bolt connection with bolt head corrosion damage: Experimental research B, Engineering Failure Analysis, Vol. 59, pp. 526-543 [38] Korean Construction Specification KCS 41 10 00: 2016 (in Korean). [39] Ministry of Land, Infrastructure and Transport (MOLIT) (2016). Korean Highway Bridg e Design Code (Limit State Design), MOLIT (in Korean).
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(a) Side view of H-beam
(b) Side view of bolt
(c) Front view of bolt connection
connection
(d) Front view Figure 1 Dimensions of bolt-connected girder specimen (mm)
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Figure 2 Load condition and instrumentation (mm)
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Figure 3 Lower flange bolt connection
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Figure 4 Test set-up (load case 1, load level 3)
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(a) Loosening bolt case I
(b) Loosening bolt case II
(c) Loosening bolt case III
(d) Loosening bolt case IV
(e) Loosening bolt case I
(f) Loosening bolt case II
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(g) Loosening bolt case III
(h) Loosening bolt case IV
Figure 5 Relationship between bolt-loosening process and girder displacement; load case 1
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(a) Loosening bolt case I
(b) Loosening bolt case II
(c) Loosening bolt case III
(d) Loosening bolt case IV
Figure 6 Relationship between bolt-loosening process and strain; load case 1
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(a) Loosening bolt case I (load level 1)
(b) Loosening bolt case I (load level 2)
(c) Loosening bolt case I (load level 3)
(d) Loosening bolt case II (load level 1)
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(e) Loosening bolt case II (load level 2)
(f) Loosening bolt case II (load level 3)
(g) Loosening bolt case III (load level 3)
(h) Loosening bolt case IV (load level 3)
Figure 7 Relationship between bolt-loosening process and sectional strain distribution
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(a) Loosening bolt case I
(b) Loosening bolt case II
(c) Loosening bolt case I
(d) Loosening bolt case II
Figure 8 Relationship between bolt-loosening process and displacement; load case 2
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(a) Loosening bolt case I
(b) Loosening bolt case II
Figure 9 Relationship between bolt-loosening process and strain; load case 2
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(a) Loosening bolt case I (load level 3)
(b) Loosening bolt case I (load level 2)
(c) Loosening bolt case I (load level 3)
(d) Loosening bolt case II (load level 1)
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(e) Loosening bolt case II (load level 2)
(f) Loosening bolt case II (load level 3)
Figure 10 Relationship between bolt-loosening process and sectional strain distribution
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Figure 11 Behavior of plate girder associated with loosening of bolted joint
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a) LVDT 1 for load case 1
(b) LVDT 2 for load case 1
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(c) LVDT 1 for load case 2
(d) LVDT 2 for load case 2 Figure 12 Ratio of bolt-loosening process to displacement 34
Research highlight
Behavior of bolt-connected steel plate girder attributable to bolt loosening failure in the lower flange - Experiments for examining the behaviors of a high strength bolt connected steel plate girder attributable to loose bolts - Effects of bolt loosening in the lower flange in a steel plate girder - Comparisons of structural responses of a steel plate girder affected by loosening of lower flange bolts.
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Conflicts of Interest: The authors declare no conflicts of interest.
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