Engineering Failure Analysis 31 (2013) 236–247

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Engineering Failure Analysis

journal homepage: www.elsevier.com/locate/engfailanal

Assessment on the stress state and the maintenance schemes of the transmission tower above goaf of coal mine ⇑ Fengli Yang , Qinghua Li, Jingbo Yang, Binrong Zhu

China Electric Power Research Institute, Beijing 100192, China article info abstract

Article history: FEA model of a transmission tower was established. According to the monitoring data of Received 23 May 2012 the non-uniform settlement for the temporarily strengthened tower, the bearing capacity Accepted 1 February 2013 analyses were carried out with the wind loads of different velocities and directions. When Available online 20 February 2013 the wind velocity is up to the design value, the stress of the main member exceeds the design stress by 34%. For the frequent velocity, stresses of all the members are lower than Keywords: the design stress. By considering the settlements at the four foots are stochastic, one single Goaf of coal mine installing program of the staying wires cannot decrease the stress of the members for all Transmission tower cases. For the settlement case which the member stress is up to the maximum value, the Foundation deformation Monitoring member stress decreases a little but even higher than the design value. By applying the Assessment large panel foundation, the bearing capacity of the tower can be enhanced signiﬁcantly, and the settlement limit was determined by the structural analysis. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction

According to the restriction of the terrain characteristics and the corridor conditions, transmission lines usually cannot avoid passing the goaf of coal mine. There are some transmission lines with the electrical grade 1000 kV and not higher than 500 kV passing the goaf of coal mine in China. Effected by the surface settlement of the goaf, the leg opening and height difference of the tower legs will be varied. The tower structures will bear high additional loads, and part body or whole body of the tower struc- ture may break down. Deformations of the foundation have brought serious threat to the safe operation of the power grid. Recently, safety of the transmission towers in the goaf of coal mine has been broadly concerned. The studies have mainly focused on the righting or strengthen technology of the towers [1–5], the healthy state assessment for the towers with foundation deformation or in the righting process. Yuan et al. [6–7] studied the inﬂuence of dynamic ground deformation on the additional internal forces and structural deformation of the transmission tower, and the reliability of the transmission towers in the goaf of coal mine was assessed. Sun et al. [8–9] considered the relationship between the tunneling direction of coal face and line direction, and analyzed the inﬂuence on the bearing capacity of the transmission tower. The calculated results were compared with the simulating model test results in this study. Yang et al. [10–11] carried out the structural analysis of the 1000 kV transmission tower in the goaf of coal mine, and the limited values were determined for the foundation deformation of settlement, inclination and slip. One single monitoring value or assumed empirical values of the foundation deformation were applied in the above studies. Even the values based on the analysis of the dynamic ground deformation still have some differences with the real states. In this paper, according to the long-time monitoring data of non-uniform settlements for the temporarily strengthened tower, the bearing capacity analysis were completed under the wind loads with different velocities and directions. The

⇑ Corresponding author. Tel.: +86 10 58386199. E-mail address: yangﬂ[email protected] (F.L. Yang).

1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.02.001 F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247 237 healthy state of the tower was also determined. Two maintenance schemes including installing staying wires and applying the large panel foundation were assessed. The limit value of the settlement was proposed for the large panel foundation.

2. Illustration of the accident line section

On 6th April in 2009, affected by the settlement of the adjacent goaf of coal mine, No. 53 tower of a 500 kV transmission line inclined severely. The inclination slope is 9% which exceeds the maximum slope 5% regulated in the design standard [12]. Deformation of the main members of No. 53 tower is shown in Fig. 1. The settlement states of the ground surfaces near No. 53 and No. 54 tower can be seen in Fig. 2. The maximum elevation difference of the fault movement near No. 54 tower

(a) (b)

Fig. 1. Deformation of main member and integral inclination. (a) Main member deformation. (b) Whole tower inclination.

(a) (b)

Fig. 2. Surface subsidence. (a) South east side of No. 53 tower (55 m). (b) South side of No. 54 tower (18 m).

Fig. 3. Demonstration of the lengthen legs. 238 F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247 reaches 1.1 m. From 3rd to 7th May in 2009, emergently recovering construction was carried out by the electric operation department. The additional main members were welded with the original main members on the settled locations (Fig. 3). Four tower foots of No. 53 tower were connected with one steel plate. Then the monitoring of the foundation settlement was proceeded.

3. Analysis model

The tower type of No. 53 tower is SZ1 and the height of the tower is 60 m. The design wind velocity is 32 m/s at 15 m reference height and the design ice thickness is 10 mm. Height of the tower is 58.5 m and the leg opening is 12 m. The real span of conductors and ground wires is 477 m. The type of the conductor and the ground wire is 4⁄LGJ-400/35 and GJ-50 respectively. The general FEA software ANSYS was applied for the tower structural analysis with foundation deformation. The leg members and the main members of the tower are simulated by BEAM4 element. The diagonal members are simulated by LINK8 element. The secondary members are ignored in the FEA model. The connection between the main members is rigid, and the connection of the diagonal members is hinged. The FEA model of No. 53 tower is shown in Fig. 4. The numbers of the tower foots and the wind directions are illustrated in Fig. 5. Number A, B, C and D refers to four nodes of the tower foots. Deformation along A–B side is in the reverse direction of wind, and deformation along C–D side is in the down direction of wind. The lengths of the additional leg members at four foots (A, B, C and D) are 0 mm, 180 mm, 460 mm and 270 mm respectively. The foundation deformation of settlement, inclination or slip can be simulated by different constrained types at the tower foots. If the large panel foundation is applied and the foundation is not destroyed, the foundation appears inclination in the transversal or longitudinal direction. Inclination along the longitudinal direction is demonstrated in Fig. 6. When the tower foundation is inclining, the whole deformation of the tower is large and the additional moment is produced. The large deformation option should be opened in ANSYS, and effect of the large deformation of the tower is considered. The constraints of the tower foots for different cases are listed in Table 1. UZ and UX is the translational freedom degree of the vertical direction and the transversal direction, and ROTY is the rotational freedom degree around the longitudinal direction of

Z direction

X direction(Transversal direction)

Y direction(Longitudinal direction)

Fig. 4. FEA model of the transmission tower.

10.400m 90-degree wind Longitudinal direction 60-degree wind B C

2503 8.538m 2502

45-degree wind A D 0-degree wind 2501 2500 Foundation

Fig. 5. Distribution of the tower foots and description of the wind directions. F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247 239

Fig. 6. Analysis model for the inclination of the large panel foundation.

Table 1 Constrained conditions of tower foots.

Case Constrained types Settlement Release UZ of settled foots, fixed at the foots without settlement Inclination Release UZ and ROTY of settled foots, release ROTY at the foots without inclination Slip Release UX of settled foots, fixed at the foots without slip the transmission line. The displacements will be applied on the tower foots [13–15] and the foundation deformation can be simulated in ANSYS. For the slip case, one side of the tower foots is fixed and the other side is slipped along the horizontal direction. For the non-uniform settlement case, one of the four tower foots is fixed, and the other three foots are settled according to the relative deformation value. When the large panel foundation occurs inclination, one side of the tower foots is fixed, the other side is settled uniformly.

4. Illustration of the analysis cases

According to the demonstration in Section 2, the tower legs attached to the tower foots of B, C and D have been length- ened in the temporarily strengthening process. For the locations of the tower foots are not changed, the endpoints of the tower legs at B, C and D foots have been bared initial horizontal displacements. Explanation of the initial displacements can be seen in Fig. 7. The calculated values of the initial horizontal displacements are presented in Table 2.

Location of the tower foot Extension line Real location after of the tower leg strengthening

Initial horizontal displacement

Fig. 7. Explanation of the initial displacements. 240 F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247

Table 2 Horizontal displacements of the tower legs.

Descriptions of the models and the numbers X direction Y direction Coordinates of the tower foots before strengthening (m) D 2500 10.400 0 A 2501 0 0 C 2502 10.400 À8.538 B 2503 0 À8.538 Coordinates of the tower foots after strengthening (m) D 2500 10.427 0.019 A 2501 0 0 C 2502 10.446 À8.571 B 2503 À0.018 À8.551 Initial horizontal displacement (m) D 2500 À0.027 À0.019 A 2501 0 0 C 2502 À0.046 0.033 B 2503 0.018 0.013

90-degree wind 60-degree Design wind wind Initial horizontal Non-uniform velocity(32m/s) 45-degree slip settlement wind 0-degree wind Structural Analysis Design ice Initial horizontal Non-uniform Plan thickness(10mm) slip settlement

90-degree 9 cases wind 60-degree Frequent wind wind Initial horizontal Non-uniform velocity(10m/s) 45-degree slip settlement wind 0-degree wind

Fig. 8. Illustration of the analysis cases.

120

100 Tower foot A Tower foot B 80 Tower foot C Tower foot D 60

20 Relative settlements (mm) 0

0 50 100 150 200 Numbers of the monitoring data

Fig. 9. Curves of the relative settlements.

Wind loads and accreted ice loads are frequent load types for the transmission lines. The total number of the analysis cases is 9, and the illustration of the cases is shown in Fig. 8. The analyses are mainly focused on the foundation deformation of settlement, inclination or slip, which are combined with the working load cases of 90-degree wind, 60-degree wind, 45- degree wind, 0-degree wind and 10 mm accreted ice load. F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247 241

The loads are applied by three steps for every load case. Firstly, the self-weight and the wind loads or the accreted ice loads are applied on the nodes. Secondly, the initial horizontal displacements in the temporarily strengthening process were applied at the endpoints of the tower legs. Lastly, the non-uniform settlements are applied on the tower foots. This analysis process can be realized in ANSYS by transient analysis. The TIMINT option for considering the dynamic effect is in OFF state. Based on the monitoring data (from 12th May 2009 to 17th June 2011) of the absolute elevations at the tower foots, relative settlements of the tower foots were calculated and the settlement curves are presented in Fig. 9. There are 223 groups of monitoring data. They are numbered according to the monitoring time in turn.

5. Structural analysis of the transmission tower

According to on the analysis results, the stress state of the main members is controlled by the load case of 60-degree wind in Fig. 8. Structural analysis of the transmission tower with initial slip and non-uniform settlement under 60-degree wind were major analyzed. Distribution of the main members for analysis is shown in Fig. 10. The stress state of the towers is estimated according to the FEA results. Generally, the stress of the main member is controlled by the compressed force which induces buckling failure. Design bearing capacities of the members can be calculated according to the formulas in [16]. The design force is assumed as the critical value of the member force.

5.1. Design velocity

Under the wind load of design velocity (32 m/s), time histories of the stress ratios for the main members in Fig. 10 are shown as Fig. 11. Stress ratio is the ratio of real stress to the design stress. When the member force is up to the design force, the stress ratio is 1.0. It can be seen that stresses of the main members under some settlement cases exceed the design stresses. The maximum stress ratio occurs at the 29th settlement case. In this case, the absolute settlement is not up to the maximum value, and the relative deformation is very signiﬁcant. The relative settlement of tower foot A, B, C and D is 50 mm, 10 mm, 10 mm and 0 mm respectively. Axial forces and stress ratios of the main members under the 29th case are shown in Table 3. Stress state of the main member is controlled by compression force. Stresses of the main members exceed the design stress by 11.2–39.6%. Stress ratio of the main member t is the maximum.

5.2. Frequent velocity

Under the wind load of design velocity (10 m/s), the maximum stress ratio occurs at the 29th settlement case. Axial forces and stress ratios of the main members under the 29th case are shown in Table 4. Contours of the axial forces and the displacements with 10 m/s wind are presented in Fig. 12. The maximum displacement at the top cross arm is 0.202 m. Stresses of the main members are lower than the design stress and the transmission tower is in a safe state.

Section type

Numbers of the nodes

Numbers of the members

Fig. 10. Distribution of the main members for the axial force analysis. 242 F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247

0.60

0.75 0.75 0.90 0.90 1.05

1.05 1.20 The 29th data which the peak stress ratio occurs The 29th data which the peak stress ratio occurs 1.20 1.35 Stress ratio of the main member Stress ratio of the main member 0 50 100 150 200 0 50 100 150 200 Numbers of the monitoring data Numbers of the monitoring data (a) (b)

0.45

0.75 0.60

0.90 0.75

1.05 0.90 1.20 1.05 The 29th data which the peak stress ratio occurs 1.35 The 29th data which the peak stress ratio occurs 1.20 Stress ratio of the main member Stress ratio of the main member 0 50 100 150 200 0 50 100 150 200 Numbers of the monitoring data Numbers of the monitoring data (c) (d)

0.45

0.60

0.75

0.90

1.05 The 29th data which the peak stress ratio occurs 1.20 Stress ratio of the leg member 0 50 100 150 200 Numbers of the monitoring data (e)

r s t Fig. 11. Time histories of the stress ratios for main members with non-uniform settlement. (a) Main member . (b) Main member . (c) Main member . u v (d) Main member . (e) Main member .

Table 3 Calculated results with design wind velocity.

Numbers of the main members Axial force (kN) Stress ratio r À1875.7 1.165 s À1972.4 1.253 t À2198.3 1.396 u À2187.4 1.112 v À2223.3 1.138 F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247 243

Table 4 Calculated results with frequent wind velocity.

Numbers of the main members Axial force (kN) Stress ratio r À818.1 0.508 s À892.0 0.567 t À1057.9 0.672 u À1065.5 0.542 v À1070.6 0.548

(a) (b)

Fig. 12. Contours of the axial forces and the displacements with 10 m/s wind. (a) Axial force (N). (b) Displacement (m).

Fig. 13. FEA model of the transmission tower with the staying wires.

6. Assessment on the maintenance schemes

In order to ensure the safety of the transmission tower with foundation settlement, two maintenance schemes were considered. The ﬁrst is installing staying wires at the tower body. The second is rebuilding a new tower and applying the large panel foundation. Assessment analysis on these two schemes was carried out as follows.

6.1. Installing staying wires

According to the design experiences of the design department, four 2⁄GL-100 staying wires were installed. The staying wire was simulated by LINK10 cable element. FEA model of the transmission tower with staying wires is shown in Fig. 13. 244 F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247

Firstly, comparison on the axial forces of the main members with and without the staying wires was completed. In order to study the effect of the staying wires on the transmission tower under different settlement cases, curves of the axial forces ratios for the main members with and without the staying wires are presented in Fig. 14. For most of the settlement cases, the axial forces were decreased by installing the staying wires. Decreased effect of the 49th case is the most signiﬁcant. In this case, the axial forces of the main member r–v are decreased by 21.7%, 29.7%, 32.6%, 45.0% and 44.6% respectively. But in the 29th case which the axial force of the main member is the highest, the axial forces are only decreased by 3.3%, 4.1%, 4.4%, 5.1% and 5.1% respectively. Even for the 38th case, the axial force of the main member r is increased by 0.6%. By considering the settlements at the four foots are stochastic, one single installing program of the staying wires cannot decrease the stress of the members for all cases. Sometimes the reverse effect will be obtained.

1.05

1.00 1.0

0.95 0.9 0.90

0.85 0.8

0.80 0.7 0.75

Axial force ratio of the main member 0 50 100 150 200 0 50 100 150 200 Axial force ratio of the main member Numbers of the monitoring data Numbers of the monitoring data (a) (b)

1.0 1.0

0.9 0.9 0.8

0.8 0.7

0.6 0.7 0.5

0 50 100 150 200Axial force ratio of the main member 0 50 100 150 200 Axial force ratio of the main member Numbers of the monitoring data Numbers of the monitoring data (c) (d)

1.0

0.9

0.8

0.7

0.6

Axial force ratio of the main member 0 50 100 150 200 Numbers of the monitoring data (e)

r s Fig. 14. Time histories of the axial force ratios for main members with non-uniform settlement. (a) Main member . (b) Main member . (c) Main member t u v . (d) Main member . (e) Main member . F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247 245

Table 5 Calculated results with design wind velocity.

Numbers of the main members Axial force (kN) Stress ratio r À1815.6 1.127 s À1894.1 1.203 t À2105.5 1.337 u À2110.3 1.073 v À2114.9 1.082

Secondly, analysis on the bearing capacity of the transmission tower after installing the staying wires was carried out. The maximum stress ratio occurs at the 29th settlement case. Axial forces and stress ratios of the main members under the 29th case are shown in Table 5. Contours of the axial forces of the tower members and the staying wires are presented in Fig. 15. For the 29th case, stresses of the main members exceed the design stress by 8.2–33.7%. The stress ratio of the main member t is the maximum. Comparing with the results in Table 3, it can be seen that the axial forces decrease a little but the stresses of the main members are still higher than the design stress. The transmission tower is still in a dangerous state.

6.2. Applying the large panel foundation

According to the analyzed results in Section 6.1, the scheme of installing staying wires cannot ensure the tower safety for all the settlement cases. So the second scheme of building a new tower and applying large panel foundation was considered. In the analysis process, the load steps were set by a reasonable value, and the relationship curves between the foundation deformation and the bearing capacities of the key members were obtained. The limited values for the settlement of the large panel foundation were determined. When the foundation inclination is along the transversal direction of the transmission line, the tower foot C and D is settled uniformly. The settlement direction is in correspondence with the horizontal component of the 60-degree wind. The wind velocity is equal to the design value. It is assumed that the settlement varies from 0 m to 3 m. Variations of the axial forces for the main members were analyzed. It can be concluded that axial forces of the members marked in Fig. 16 vary in more large amplitudes. The axial forces of the main member with and without large deformation effect are shown in Fig. 17. If the large deformation effect is not considered, the tower body is rotating as a rigid body. The member force varies little with the increasing of foundation inclination. With the consideration of large deformation effect, the additional moment by the deformation of the tower body can be considered. The member force increases nonlinearly with the increasing of the inclination value. Axial force curves of the typical members with the increasing of the settlement are shown in Fig. 18. The critical settlements for different type of members were listed in Table 6. The failure order of the members is the transverse separator member, the main member and the diagonal member in turn. The critical settlement of the transmission tower is 654 mm.

Fig. 15. Contours of the axial forces (unit: N). 246 F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247

Main member

Transverse separator member Diagonal member

Fig. 16. Distribution of the main members.

-1300

-1400

-1500

-1600

-1700

-1800

-1900 With large deformation effect -2000 Without large deformation effect

Axial forces of the main member (kN) -2100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Settlemet (m)

Fig. 17. Comparison of the axial forces.

-1300 400 -1400 -1500 300

-1600 Critical value 200 -1700 -1800 100 Critical value member (kN) member (kN) -1900 0 Axial force of the main

-2000 Axial force of the diagonal -2100 -100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0123 Settlement (m) Settlement (m) (a) (b)

-20

-40 Critical value

-60

-80

separator member (kN) -100 Axial force of the transverse -120 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Settlement (m) (c)

Fig. 18. Variation of the axial forces with the foundation inclination. (a) Main member. (b) Diagonal member. (c) Transverse separator member. F.L. Yang et al. / Engineering Failure Analysis 31 (2013) 236–247 247

Table 6 Calculated results with the inclination of the large panel foundation.

Type of the members Section Critical axial force (kN) Critical settlement (mm)

Main member L200 Â 16H À1574.2 1803 Diagonal member L80 Â 6S 122.0 1947 Transverse separator member L70 Â 5S À40.8 654

7. Conclusions

Deformations of the foundation have brought serious threat to the safe operation of the transmission tower. Based on the long-term monitoring data, FEA analyses and assessment on two maintenance schemes were carried out for a typical transmission tower with foundation deformation. Some main conclusions are as follows:

(1) When the wind velocity is up to the design value, the stress of the main member exceeds the design stress by 34%. For the frequent velocity, stresses of all the members are lower than the design stress. (2) One single installing program of the staying wires cannot decrease the stress of the members for all settlement cases. Sometimes the reverse effect will be obtained. For the settlement case which the member stress is up to the maximum value, the member stress decreases a little but even higher than the design value. (3) By applying the large panel foundation, the bearing capacity of the tower can be enhanced signiﬁcantly, and the settlement limit was determined by the structural analysis. It is proposed as a better method for the maintenance of the transmission towers in the goaf of coal mine.

Acknowledgement

This work has been funded by the State Grid Corporation of China (SGCC), and the authors would like to thank the sponsor of SGCC.

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