The Industrial Accident Resulted from the Failure of Bolt

Transcription

1 The Industrial Accident Resulted from the Failure of Bolt Hyun Wook YEO 1, Jae Min LEE 2 and Sang Won CHOI 1, * 1 Occupational Safety and Health Research Institute, Korea Occupational Safety and Health Agency, Ulsan, South Korea. 2 Department of Mechanical Design Engineering, Andong National University, Andong, South Korea. Abstract This study was to perform an investigation of the cause of industrial accident, the cause of failure of anchor bolt to collapse a steel frame structure on a construction site for two workers on the steel frame structure to fall down and die. Through the investigation of the accident site, the process of accident and the situation of accident site were evaluated. The finite element analysis on the steel frame structure was used for an analysis, and the occurrence process of accident and the circumstances of accident site when the accident occurred were applied as analytic conditions. According to the investigation of accident site and the finite element analysis, it is evaluated that the asymmetric load by the weight of steel frame structure and the external load under insufficient fixture and support of steel frame structure led the failure of anchor bolt to cause the collapse of steel frame structure. Also, it is confirmed that two anchor bolts at the back side to receive the tensile stress among anchor bolts installed at the pillar on the opposite of external force were broken in the first place to lead the collapse of steel frame structure by the feature of anchor bolt failure evaluated through the Von Mises stress distribution. Figure 1: Feature of steel frame structure before accident Keywords: Industrial accident; Bolt; Failure; Finite element analysis; Investigation of accident; INTRODUCTION In the industrial site, when bolts to be used to fasten construction materials or machinery parts are failed due to overload or fatigue, a serious industrial accident to cause a severe damage may occur frequently. However, while there are several studies about an analysis of engineering cause of bolt failure in the industrial site [1, 2, 3], the analysis of cause of bolt failure with respect to the investigation of cause of industrial accident has not been studied much. In this study, the investigation was performed on the cause of anchor bolt failure to cause the serious industrial accident letting two workers on a steel frame structure to fall and die. The feature of steel frame structure at which the accident occurred is shown in Figure 1 and Figure 2. The root diameter of bolt was 20mm, and the nominal ultimate tensile strength was 444N/mm 2. For reference, the investigation of accident site and the testimony by witness confirmed that the steel frame structure was collapsed during the main assembly (fastening of upper bolt connection) from the temporary assembly. Figure 2: Feature of steel frame structure fell after accident METHODS Through the site investigation of industrial accident, the process of accident and the situation of accident site were evaluated, and the collapsed steel frame structure was modeled to be analyzed by using the finite element analysis including the process of disaster and the situation of accident site as analytic conditions. For concrete determination of a possibility of anchor bolt failure, EN :, Eurocode 3: Design of steel structures. Part 1-8: Design of joints was referred [4]. RESULTS Anchor bolt failure process Steel frame structure modeling To predict the process of steel frame structure falling, the steel frame structure modeled into the same dimension and the same weight (9921.9kg) using the density and the external 7426

2 load (5000N of load each by two wires connected to the steel frame structure) during the accident were applied. The steel frame structure was modeled as the 3-D beam, the complete fixture at both ends of lower part of steel frame structure was applied as the boundary condition, and the modulus of elasticity of N/m 2, Poisson ratio of 0.285, and density of 7850kg/m 3 provided by the manufacturer of steel were applied on the steel frame structure. two anchor bolts 5 and 6 (refer to Figure 6) located on the opposite side of falling direction of steel frame structure out of four anchor bolts on the right fixture receive the tensile stress by asymmetric load from the weight of steel frame structure [6], it was evaluated that they were damaged in the first place among eight anchor bolts. Review of the feature of anchor bolt failure Figure 5: Feature of left anchor bolts failed Figure 3: Modeling of steel frame structure (Load: weight of steel frame structure and external load, Boundary: location of anchor bolts) Figure 6: Feature of right anchor bolts failed Figure 4: Von Mises stress distribution (Load: weight of steel frame structure and external load, Boundary: location of anchor bolts) As a result of Von Mises stress distribution review [5], on the right fixture at the bottom section of steel frame structure (refer to Figure 4), the maximum stress was formed, and since The characteristics of anchor bolt failure evaluated through the site investigation of accident are the followings: Among four anchor bolts on the right side, the threads of anchor bolt 5 and 6 located at the back side (opposite of falling direction) were crushed by tensile stress [6], but they were not broken. Since this section receives the strongest tensile load by the asymmetric load of the weight of steel frame structure and by the pulling load of two wire ropes connected to the steel frame structure among anchor bolts, this section shall be deformed or failed in the first place. 7427

3 Except for 5 and 6, other anchor bolts were failed or pulled out by instantaneous impact. Comparing with the surface of failure of other anchor bolts, the surface of failure of anchor bolt 2 at the back side among four anchor bolts at the left side was tilted to left. Estimation of anchor bolt failure The process of anchor bolt failure estimated based on the feature of anchor bolt failure evaluated through the Von Mises stress distribution [5] by the finite element analysis and the site investigation of accident is the following: The asymmetric load by the weight of steel frame structure and the external load (pulling load by a wire rope) crushed the screw threads of anchor bolt 5 and 6 at the back of right side for the steel frame structure to be away from anchor bolt 5 and 6 Instantaneous failure of anchor bolt 7 and 8 at the front of right side Anchor bolt 2 was failed while it was tilted to the left side by the collapsing steel frame structure -Center of compression axis [refer to EN :2005 Figure 6-15] (a) EN :2005 Figure 6-15 c) Review of anchor bolt safety To analyze the failure of anchor bolt, the process of accident and the situation of accident site were concerned to perform the finite element analysis for two cases, and the EN :, Eurocode 3: Design of steel structures. Part 1-8: Design of joints [4] was referred to review the safety of anchor bolt. Case1. Support of the weight of steel frame structure only by anchor bolts without another support To confirm whether the asymmetric load by the weight of steel frame structure may be supported only with anchor bolts without another support, the density was applied on the steel frame structure modeled with 3-D Beam to apply the weight of steel frame structure, and as the boundary condition, the both ends of bottom of steel frame structure were completely fixed to perform the review of anchor bolt safety. (b) Dimension of end-plate installed anchor Figure 8: Center of compression axis of anchor bolt -Review of anchor bolt safety F t,ed = F t,ed,m + F t,ed,n F t,ed,m = (M Ed h)/(n v h r 2 ) F t,ed,n = N Ed /n t F t,rd = (K 2 f μb A s )/(γ M2 ) (1) (2) (3) (4) Figure 7: Von Mises stress distribution (Load: weight of steel frame structure, Boundary: location of anchor bolts) Review of anchor bolt safety [4] Where F t,ed,m = The tension force from bending moment for one bolt from bolt-row1 F t,ed,n = The axial force for one bolt from bolt-row1 M Ed = The bending moment h = The vertical distance from bolt-row1 to the center of rotation axis (h = 23.8mm) n v = The number of vertical bolt-rows (n v = 2) 7428

4 h r 2 = The sum of the squares of the vertical distances between bolt-row r and the center of compression axis N Ed = The axial force n t = The number of bolts (n t = 4) F t,rd = The design tensile resistance per bolt K 2 = 0.9 [refer to EN :2005 Table 3.4] f μb = The nominal ultimate tensile strength of the bolt (f μb = 444N/mm 2 ) A s = The tensile stress area of the bolt (A s = mm 2 ) γ M2 = Partial safety factor for resistance of bolts (γ M2 = 1.25) [refer to EN :2005 Table 2.1] Table 1: Review of anchor bolt safety (Load: weight of steel frame structure, Boundary: Location of anchor bolts) M Ed N Ed F t,ed,m F t,ed,n F t,ed F t,rd F t,rd/f t,ed (N mm) (N) (N) (N) (N) (N) For M Ed and N Ed,, the values at the location of anchor bolts under higher stress in Figure 7 were applied. Since the comparison value (F t,rd/f t,ed) between the design tensile resistance (F t,rd) and the tensile force (F t,ed) by bending moment and axial force is , anchor bolts may not hold the load by themselves. Case2. Support of external load/weight of steel frame structure/weight of two workers by braces/pipe support/anchor bolts To review the safety of anchor bolt supposing two braces and four pipe supports which were installed at the site when the accident occurred hold the steel frame structure without their own failure or deformation, the analysis was performed by applying external load (5,000N of pulling load each by two wire ropes connected on the steel frame structure), the weight of steel frame structure, and the weight of two workers as the load on the steel frame structure modeled in 3-D beam, and as the boundary condition, two braces and four pipe support and the both ends of bottom of steel frame structure were completely fixed. Figure 10: Von Mises stress distribution (Load: external load, weight of steel frame structure, and weight of workers, Table 2: Review of anchor bolt safety (Load: external load, weight of steel frame structure, and weight of workers, M Ed N Ed F t,ed,m F t,ed,n F t,ed F t,rd F t,rd/f t,ed (N mm) (N) (N) (N) (N) (N) For M Ed and N Ed,, the values at the location of anchor bolts under higher stress in Figure 7 were applied. Under the assumption that two braces and four pipe supports hold the steel frame structure without their own deformation or failure, since the comparison value (F t,rd/f t,ed) between the design tensile resistance (F t,rd) and the tensile force (F t,ed) by bending moment and axial force is , if braces and pipe supports are not failed or deformed, the anchor bolst may not fail. Figure 9: Modeling of steel frame structure (Load: external load, weight of steel frame structure, and weight of workers, CONCLUSION Even though anchor bolts without other supports can t hold the asymmetric load only by the weight of steel frame structure, if pipe supports and braces support the steel frame structure without their own deformation or failure, the anchor bolt may not be failed during the accident according to the results of analysis of the failure of anchor bolt. Therefore, it is evaluated that the asymmetric load by the weight of steel frame structure and the external load led the failure of pipe supports and braces temporarily installed at the accident site for the steel frame structure to collapse. Also, it is identified that the two anchor bolts at the back side to receive the tensile stress among four anchor bolts installed at the right fixture to be opposite of the external load were broken in the first place 7429

5 to lead the collapse of steel frame structure by the feature of anchor bolt failure evaluated through the Von Mises stress distribution. CONFLICTS OF INTEREST No potential conflicts of interest relevant to this article were reported REFERENCES [1] M.T. Milan, D. Spinelli, W.W. Bose Filho, M.F.V. Montezuma, V. Tita, Failure analysis of a SAE 4340 steel locking bolt, Engineering Failure Analysis 11 (2004) [2] Z. Zhenqian,, T. Zhiling, Y. Chun, L. Shuangping, Failure analysis of vessel propeller bolts under fastening stress and cathode protection environment, Engineering failure analysis 57 (2015) [3] W. Tizani, N.A. Rahman, T. Pitrakkos, Fatigue life of an anchored blind-bolt loaded in tension, Journal of Constructional Steel Research 93 (2014) 1-8. [4] EN :, Eurocode 3: Design of steel structures. Part 1-8: Design of joints, European Committee for Standardization (CEN), Brussels, Belgium, [5] W.F. Hosford, A Generalized Isotropic Yield Criterion, J. Appl. Mech 39(2) (1972), [6] W.C. Young, R.G. Budynas, Roark s Formulas for Stress and Strain, Seventh ed. the McGraw-Hill Companies, Inc., New York, 2002, pp