- Email: [email protected]
Remediation of settlements in a steel structure due to adjacent excavations
Engineering Failure Analysis 20 (2012) 156–165
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Remediation of settlements in a steel structure due to adjacent excavations Kadir Ozakgul ⇑, Ozden Caglayan, Ovunc Tezer, Erdogan Uzgider Department of Civil Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 23 July 2011 Received in revised form 23 October 2011 Accepted 7 November 2011 Available online 12 November 2011 Keywords: Marmaray Project Local settlement Steel structure Strengthening Excavation
a b s t r a c t Marmaray Project is a 13.6 km long Bosphorus crossing railway system in Istanbul connecting Asia and Europe by a 1.4 km immersed tube tunnel for the first time and also comprises refurbishment of 63 km long existing railway systems which makes it one of the major transport projects in the world. During excavations of one of three new underground stations of the project at Yenikapi district, an ancient Byzantine port was discovered and the archaeological excavations are still being performed on a plot of land measuring 58,000 square-meters. Excavation-induced local ground settlements occurred in an existing steel structure adjacent to this excavation area. In this study, these settlements and their effects on the steel structure were investigated. To investigate the behavior of the structure under local settlements, 3D computer models using 3D beam elements and gap elements were carried out by using the commercially available software SAP2000. The effects induced on the structure by local settlements were simulated using gap elements at the column bases. As a result, a strengthening scheme was proposed to remediate the negative effects caused by settlements. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Marmaray Project is a major transportation infrastructure project in Istanbul. By this project (see Fig. 1) including the construction of an underwater tunnel, combination and modernization of approximately 76 km of the light rail transport systems currently used on both sides of the Bosporus Strait are of primary purpose. The project commenced in 2002 and continues currently with delays occurred owing to the discovery of an ancient Byzantine port during excavations of a new underground station at the site of Yenikapi. The existing railway systems (red1 color) on each side of the Bosphorus that will be upgraded and the new railway part (white color) that will be constructed underground including the immersed tunnel under the Istanbul Strait are shown in Fig. 1. The main structures and systems of the project comprise of an immersed tube tunnel having 1.4 km length at 56 m depth that will be the deepest immersed tube tunnel in the world when completed, bored tunnels, cut-and-cover tunnels, three new underground stations, 37 surface stations (renovation and upgrading of existing stations, if possible), an operations control center, maintenance facilities, upgrading of existing tracks and addition of a new third track on the ground [1]. An ancient Byzantine port was discovered during excavations of one of three a new underground station of the project at Yenikapi and the archaeological excavations are being carried out at the 58,000 square-meter area. Owing to these excavations, local settlements were observed in a historical building neighboring the excavations. The purpose of this paper is to investigate damages occured in this building consisting of steel frames adjacent to excavation area and to propose a
⇑ Corresponding author. Tel.: +90 212 2856572; fax: +90 212 2856587. 1
E-mail address: [email protected] (K. Ozakgul). For interpretation of color in Figs. 1–13, the reader is referred to the web version of this article.
1350-6307/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2011.11.008
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
157
Fig. 1. Railway track of Marmaray Project [1].
strengthening scheme to prevent structural damages due to settlements. The building response and damages in the nearby structures due to excavation-induced ground movements have been studied by many researchers [2–11]. Finno and Bryson [2] presented correlations among construction activities, measured deformations and distortions, and attendant damage in a three-story school building supported by shallow foundations and affected by an adjacent 12.2-m-deep excavation in soft clay. In a study carried out by Son and Cording [3], building damage due to excavation-in-
Fig. 2. Plan view of steel structure adjacent and archaeological excavation area.
158
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
Fig. 3. A view of the truss girders in the first hall.
Fig. 4. A view of a column in the first hall.
duced ground movement is evaluated using a damage criterion based on the average state of strain in the distorting portion of the structure. A model was proposed by Finno et al. [4] to evaluate adequately the response of the structures which was affected by an adjacent deep excavation. For evaluating the equivalent bending and shear stiffness of masonry structures
Fig. 5. A view of the truss girders in the second hall.
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
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subjected to excavation-induced ground movements, numerical analyses were conducted by Son and Cording [5] and it was shown that shear deformation is dominant for these buildings. Damages occured in masonry and frame structures on shallow foundations subjected to excavation-induced ground movements were investigated using numerical model tests by Son and Cording [6] and as a result of the numerical studies, it was shown that the structural response to excavation-induced ground movements is highly dependent on both cracking in structures and structural types. To investigate the response of RC frames to adjacent excavation induced settlement, one-tenth scale laboratory study was performed by Laefer et al. [7] and this study was combined with numerical modeling to determine the most appropriate set of input parameters and evaluation criteria. For evaluating damage potential of a building adjacent to a braced excavation, a simplified model was proposed by Schuster et al. [8]. Gayarre et al. [9] presented a forensic analysis of ground subsidence triggered by the collapse of abandoned underground mining. For a fully probabilistic analysis of the potential for damage to buildings adjacent to an excavation, a outline was presented by Juang et al. [10]. Responses of buildings with different structural types including brick-bearing, open-frame, and brick-infilled frame structures on shallow foundations subjected to excavation-induced ground settlements were investigated by Son and Cording [11] with the purpose of a better understanding of the complex soil-structure interaction in building response. 2. Description of the steel structure The steel structure investigated in this study consists of two adjacent halls which will be denoted as first and second, respectively. Plan view of this structure and the archaeological excavation area is shown in Fig. 2. Due to the lack of
Fig. 6. 3D computer models of the halls.
Fig. 7. A schematical view for the proposed repairment method.
160
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constructional drawings of structures, building survey was mandatory to identify and reveal dimensions and properties of the structural members used for each hall of building. The load-carrying system of the first hall has main and secondary truss girders which upper and bottom chords and, diagonals directly welded to chords were produced from circular tubes £48/5 and £33/3.5 (diameter/thickness), respectively (see Fig. 3), and the columns (see Fig. 4) which were made from circular tubes £49/3.5 and £77/3.5. The columns were fully fixed to the foundation and welded directly to the truss girders. The vertical bracing system employed only along the axis 4 was produced from angle section L70 70 7 mm. In the second hall, the structure system was built from truss girders (axes between A and D) and moment-resisting frames (axes E and F). The truss girders were constituted from 2L70 70 7 mm for upper chord, 2L60 60 6 mm for bottom chord and, 2L50 50 5 mm for diagonal and vertical members. In addition, a secondary truss girder was constructed between A–D
Fig. 8. Results of the single base lifting process applied at the supports of the first hall.
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161
and 3–4 axes were produced from upper and bottom chords made of 2L50 50 5 mm and, diagonal and vertical members made of L50 50 5 mm (see Fig. 5). The diagonal and vertical members were connected using fillet welding to the chords. The columns were produced from box sections made of face-to-face channel section 140 mm and 100 mm. The roof stability brace members of the second hall were made from back-to-back angle sections of 2L50 50 5 mm at the roof plane. Purlins were composed from IPN120, as two-span-continuous beams. In the moment-resisting frames, the beams were constituted from IPN140 and joined to columns by welding. The roof of both hall structures was covered by corrugated sheets, having weight of 0.03 kN/m2 and by roof aluminum foil glass wool mattress for heat insulation. Material used is S235 quality structural steel and Young’s modulus E of the steel material was taken as 210,000 MPa. The local settlements occured in the first hall were measured at the each column base in axis 4. The measurement values at the column bases named A4, B4, C4, D4, E4 and F4 were given as 6.2, 7.0, 7.8, 7.0, 6.2 and 5.4 cm, respectively. For the second hall, the measured settlement values at the column bases named B1, C1, D1, E1 and F1 were given as 0.35, 0.80, 1.15, 5.50 and 6.00 cm, respectively. To increase resistance against settlements and to strengthen and stiffen the ground, cement injection has been employed by filling cement into voids in the ground. Cement injection method is one of the methods that are widely used to stabilize problematic grounds [12]. For this structure, another viable solution to this settlement problem could have been to anticipate a certain amount of settlement and by continuously monitoring the structure, to interfere when necessary before excessive settlement occurs. A modified version of this technique might have been to jack up the
Fig. 9. Results of the dual base lifting process applied at the supports of the first hall.
162
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
Fig. 10. Steps of the lifting process applied at the supports of the first hall.
structure beforehand at possible settlement points and then monitor the structure for settlements and control the related damage at low risk before significant damage accumulates. However, for this structure, action has been taken long after the problem has started, and therefore cement injection technique has been employed to stabilize the ground as soon as possible.
3. Structural analyses of the existing building In order to investigate the structural behaviors of the structural systems under local settlements, three-dimensional finite element models were generated by using SAP2000 software [13] according to the measurement results of the building survey performed for the each hall of building. These analysis models created for both the hall are shown in Fig. 6. In numerical models, beam and gap elements were used. The structural elements were modelled with 3D beam elements located at the centerline of actual elements. Due to the use welding in the member connections of the truss girders and the truss/frame girder to column connections, these connections were assumed to be fully fixed. In order to model the effects induced by local settlements on the structure, gap elements which act nonlinearly in the column length direction were used at the column bases where the settlements occured. Opening values of gap elements used in nonlinear static analyses were defined as corresponding settlement values at each column base location. Additionally, to determine load-carrying status of the building before local settlements and to compare with the last situation, 3D computer models without gap elements of the structures were carried out. Linear and nonlinear static analyses were performed on the models without and with gap elements, respectively. For estimation of dead and snow loads acting on the steel structures, current national design load code, TS498 [14] was used. Member sections of the load bearing system of the structures were checked according to the current structural steel design code TS648 [15] which is generally very similar to ASD-89 specification [16]. A comparison between the response of the linear and the nonlinear structural models considering member stress values obtained from the results of the structural analysis was carried out. The results of this comparison are given in Fig. 6. Members with over-stress values obtained from the results of the nonlinear static analyses were presented by green lines and also, the results of the static analyses were presented by blue lines (see Fig. 6). To eliminate over-stress values that occur on the structures owing to the local settlements which means to return to the original stress values of initial situation with
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
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no local settlements, an available repairment method was suggested. Since the region where the building is a historically protected area, the repairment solution was proposed taking this status into account.
4. Implementation of the proposed repairment solution In the proposed solution, for each column base where local settlements have occured, vertical displacement as the amount of subsidence was implemented in the column length direction via hydraulic jacks mounted the column base, as presented schematically in Fig. 7. During the application of the solution, the aim is nullifying the stress increments resulting from the local settlements that occured at the supports. This reset process may be conducted with the help of strain transducers mounted for continuous monitoring of strains at the critical structural members. Considering various combinations of the lifting process by means of hydraulic jacks such as from the single point or the dual points, stress analyses were carried out using the present computer models having gap elements. The results of the lifting process from the single point and the dual points applied at the supports of the first hall were given in Figs. 8 and 9, respectively. For each step, the gap opening values for relevant base points were taken as zero and afterwards static analyses were done. Support reaction values were obtained which mean required jack forces for each step at indicated locations.
Fig. 11. Results of the single lifting process applied at the supports of the second hall.
Fig. 12. Results of the dual lifting process applied at the supports of the second hall.
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Fig. 13. Steps of the lifting process applied at the supports of the second hall.
The most appropriate option from above mentioned lifting combinations was selected as the synchronized lifting applied from C and D supports, because this lifting combination induced more stable and lower displacements at other supports. After C and D supports were lifted and fixed, then the second step was started (see Fig. 10). In the second step, E and F supports were lifted at the same time and in synchronized and then were fixed. In the next step, A and B supports were lifted and fixed. Eventually, all supports occured settlements were lifted doubly with the order given in Fig. 10 and stress increases on the structural members induced the local settlements were reseted. Similar operations were repeated for supports in the second hall at settlement points. Firstly, to find the most convenient lifting options, the lifting process was applied from single and dual supports, as given in Figs. 11 and 12, respectively. Because significant differences were not found in stress values obtained from results of the lifting from single or dual supports, the lifting option for the second hall was selected as lifting from single support due to the ease of application. Initially, F support were lifted as single and fixed, then the second step was performed and finally, in the fifth step, this process was repeated for B support, as given in Fig. 13. The stress increases on the structural members induced the local settlements occured in the second hall were eliminated. 5. Conclusions In this study, the structural effects of the excavation-induced local settlements occurred in an existing steel structure adjacent to excavation area of Marmaray Project at Yenikapi quarter of Istanbul city were investigated. For this purpose, 3D computer models using beam and nonlinear gap elements were analysed by using the commercially available finite element analysis software SAP2000. The effects caused by local settlements on the structure were simulated using gap elements at the column base points with settlements. As a result of linear and nonlinear static analyses, a remediation scheme was proposed to eliminate the negative effects on the structural system due to settlements. A systematic and versatile method that was used has been given in detail based on the results of structural analyses. Acknowledgements The authors would like to thank Mr. Suat Emrah Dulger, who was then a research assistant, for his contributions during building surveys. Many thanks are also owed to the anonymous referees for their valuable comments and contributions during manuscript review. References [1] Marmaray. 2004. DLH Marmaray Project Directorate. http://www.marmaray.com. [2] Finno RJ, Bryson LS. Response of building adjacent to stiff excavation support system in soft clay. J Performance Construct Facilities 2002;16(1):10–20. [3] Son M, Cording JE. Estimation of building damage due to excavation-induced ground movements. ASCE J Geotech Geoenviron Eng 2005;131(2):162–77. [4] Finno RJ, Voss FT, Rossow E, Blackburn JT. Evaluating damage potential in buildings affected by excavations. ASCE J Geotech Geoenviron Eng 2005;131(10):1199–210.
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[5] Son M, Cording EJ. Evaluation of building stiffness for building response analysis to excavation-induced ground movements. ASCE J Geotech Geoenviron Eng 2007;133(8):995–1002. [6] Son M, Cording EJ. Numerical model tests of building response to excavation-induced ground movements. Canadian Geotech J 2008;45:1611–21. [7] Laefer DF, Ceribasi S, Long JH, Cording EJ. Predicting RC frame response to excavation-induced settlement. ASCE J Geotech Geoenviron Eng 2009;135(11):1605–19. [8] Schuster M, Kung GTC, Juang CH, Hashash YMA. Simplified model for evaluating damage potential of buildings adjacent to a braced excavation. ASCE J Geotech Geoenviron Eng 2009;135(12):1823–35. [9] Gayarre FL, Alvarez-Fernandez MI, Gonzalez-Nicieza C, Alvarez-Vigil AE, Garcia GH. Forensic analysis of buildings affected by mining subsidence. Eng Failure Anal 2010;17:270–85. [10] Juang CH, Schuster M, Ou CY, Phoon KK. Fully probabilistic framework for evaluating excavation-induced damage potential of adjacent buildings. ASCE J Geotech Geoenviron Eng 2011;137(2):130–9. [11] Son M, Cording EJ. Responses of buildings with different structural types to excavation-induced ground settlements. ASCE J Geotech Geoenviron Eng 2011;137(4):323–33. [12] Kazemian S, Huat BBK. Assessment and comparison of grouting and injection methods in geotechnical engineering. European J Sci Res 2009;27(2):234–47. [13] SAP2000. Structural analysis program. Advanced 10.0.1. Berkeley, California: Computers and Structures Inc.; 2005. [14] TS498. Design loads for buildings. Ankara (Turkey): Turkish Standards Institute; 1987. [15] TS648. Building code for steel structures. Ankara (Turkey): Turkish Standards Institute; 1980. [16] American Institute of Steel Construction. Manual of steel construction, allowable stress design, 9th Edition. Chicago (USA): AISC; 1989.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Remediation of settlements in a steel structure due to adjacent excavations Kadir Ozakgul ⇑, Ozden Caglayan, Ovunc Tezer, Erdogan Uzgider Department of Civil Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 23 July 2011 Received in revised form 23 October 2011 Accepted 7 November 2011 Available online 12 November 2011 Keywords: Marmaray Project Local settlement Steel structure Strengthening Excavation
a b s t r a c t Marmaray Project is a 13.6 km long Bosphorus crossing railway system in Istanbul connecting Asia and Europe by a 1.4 km immersed tube tunnel for the first time and also comprises refurbishment of 63 km long existing railway systems which makes it one of the major transport projects in the world. During excavations of one of three new underground stations of the project at Yenikapi district, an ancient Byzantine port was discovered and the archaeological excavations are still being performed on a plot of land measuring 58,000 square-meters. Excavation-induced local ground settlements occurred in an existing steel structure adjacent to this excavation area. In this study, these settlements and their effects on the steel structure were investigated. To investigate the behavior of the structure under local settlements, 3D computer models using 3D beam elements and gap elements were carried out by using the commercially available software SAP2000. The effects induced on the structure by local settlements were simulated using gap elements at the column bases. As a result, a strengthening scheme was proposed to remediate the negative effects caused by settlements. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Marmaray Project is a major transportation infrastructure project in Istanbul. By this project (see Fig. 1) including the construction of an underwater tunnel, combination and modernization of approximately 76 km of the light rail transport systems currently used on both sides of the Bosporus Strait are of primary purpose. The project commenced in 2002 and continues currently with delays occurred owing to the discovery of an ancient Byzantine port during excavations of a new underground station at the site of Yenikapi. The existing railway systems (red1 color) on each side of the Bosphorus that will be upgraded and the new railway part (white color) that will be constructed underground including the immersed tunnel under the Istanbul Strait are shown in Fig. 1. The main structures and systems of the project comprise of an immersed tube tunnel having 1.4 km length at 56 m depth that will be the deepest immersed tube tunnel in the world when completed, bored tunnels, cut-and-cover tunnels, three new underground stations, 37 surface stations (renovation and upgrading of existing stations, if possible), an operations control center, maintenance facilities, upgrading of existing tracks and addition of a new third track on the ground [1]. An ancient Byzantine port was discovered during excavations of one of three a new underground station of the project at Yenikapi and the archaeological excavations are being carried out at the 58,000 square-meter area. Owing to these excavations, local settlements were observed in a historical building neighboring the excavations. The purpose of this paper is to investigate damages occured in this building consisting of steel frames adjacent to excavation area and to propose a
⇑ Corresponding author. Tel.: +90 212 2856572; fax: +90 212 2856587. 1
E-mail address: [email protected] (K. Ozakgul). For interpretation of color in Figs. 1–13, the reader is referred to the web version of this article.
1350-6307/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2011.11.008
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
157
Fig. 1. Railway track of Marmaray Project [1].
strengthening scheme to prevent structural damages due to settlements. The building response and damages in the nearby structures due to excavation-induced ground movements have been studied by many researchers [2–11]. Finno and Bryson [2] presented correlations among construction activities, measured deformations and distortions, and attendant damage in a three-story school building supported by shallow foundations and affected by an adjacent 12.2-m-deep excavation in soft clay. In a study carried out by Son and Cording [3], building damage due to excavation-in-
Fig. 2. Plan view of steel structure adjacent and archaeological excavation area.
158
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
Fig. 3. A view of the truss girders in the first hall.
Fig. 4. A view of a column in the first hall.
duced ground movement is evaluated using a damage criterion based on the average state of strain in the distorting portion of the structure. A model was proposed by Finno et al. [4] to evaluate adequately the response of the structures which was affected by an adjacent deep excavation. For evaluating the equivalent bending and shear stiffness of masonry structures
Fig. 5. A view of the truss girders in the second hall.
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
159
subjected to excavation-induced ground movements, numerical analyses were conducted by Son and Cording [5] and it was shown that shear deformation is dominant for these buildings. Damages occured in masonry and frame structures on shallow foundations subjected to excavation-induced ground movements were investigated using numerical model tests by Son and Cording [6] and as a result of the numerical studies, it was shown that the structural response to excavation-induced ground movements is highly dependent on both cracking in structures and structural types. To investigate the response of RC frames to adjacent excavation induced settlement, one-tenth scale laboratory study was performed by Laefer et al. [7] and this study was combined with numerical modeling to determine the most appropriate set of input parameters and evaluation criteria. For evaluating damage potential of a building adjacent to a braced excavation, a simplified model was proposed by Schuster et al. [8]. Gayarre et al. [9] presented a forensic analysis of ground subsidence triggered by the collapse of abandoned underground mining. For a fully probabilistic analysis of the potential for damage to buildings adjacent to an excavation, a outline was presented by Juang et al. [10]. Responses of buildings with different structural types including brick-bearing, open-frame, and brick-infilled frame structures on shallow foundations subjected to excavation-induced ground settlements were investigated by Son and Cording [11] with the purpose of a better understanding of the complex soil-structure interaction in building response. 2. Description of the steel structure The steel structure investigated in this study consists of two adjacent halls which will be denoted as first and second, respectively. Plan view of this structure and the archaeological excavation area is shown in Fig. 2. Due to the lack of
Fig. 6. 3D computer models of the halls.
Fig. 7. A schematical view for the proposed repairment method.
160
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
constructional drawings of structures, building survey was mandatory to identify and reveal dimensions and properties of the structural members used for each hall of building. The load-carrying system of the first hall has main and secondary truss girders which upper and bottom chords and, diagonals directly welded to chords were produced from circular tubes £48/5 and £33/3.5 (diameter/thickness), respectively (see Fig. 3), and the columns (see Fig. 4) which were made from circular tubes £49/3.5 and £77/3.5. The columns were fully fixed to the foundation and welded directly to the truss girders. The vertical bracing system employed only along the axis 4 was produced from angle section L70 70 7 mm. In the second hall, the structure system was built from truss girders (axes between A and D) and moment-resisting frames (axes E and F). The truss girders were constituted from 2L70 70 7 mm for upper chord, 2L60 60 6 mm for bottom chord and, 2L50 50 5 mm for diagonal and vertical members. In addition, a secondary truss girder was constructed between A–D
Fig. 8. Results of the single base lifting process applied at the supports of the first hall.
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
161
and 3–4 axes were produced from upper and bottom chords made of 2L50 50 5 mm and, diagonal and vertical members made of L50 50 5 mm (see Fig. 5). The diagonal and vertical members were connected using fillet welding to the chords. The columns were produced from box sections made of face-to-face channel section 140 mm and 100 mm. The roof stability brace members of the second hall were made from back-to-back angle sections of 2L50 50 5 mm at the roof plane. Purlins were composed from IPN120, as two-span-continuous beams. In the moment-resisting frames, the beams were constituted from IPN140 and joined to columns by welding. The roof of both hall structures was covered by corrugated sheets, having weight of 0.03 kN/m2 and by roof aluminum foil glass wool mattress for heat insulation. Material used is S235 quality structural steel and Young’s modulus E of the steel material was taken as 210,000 MPa. The local settlements occured in the first hall were measured at the each column base in axis 4. The measurement values at the column bases named A4, B4, C4, D4, E4 and F4 were given as 6.2, 7.0, 7.8, 7.0, 6.2 and 5.4 cm, respectively. For the second hall, the measured settlement values at the column bases named B1, C1, D1, E1 and F1 were given as 0.35, 0.80, 1.15, 5.50 and 6.00 cm, respectively. To increase resistance against settlements and to strengthen and stiffen the ground, cement injection has been employed by filling cement into voids in the ground. Cement injection method is one of the methods that are widely used to stabilize problematic grounds [12]. For this structure, another viable solution to this settlement problem could have been to anticipate a certain amount of settlement and by continuously monitoring the structure, to interfere when necessary before excessive settlement occurs. A modified version of this technique might have been to jack up the
Fig. 9. Results of the dual base lifting process applied at the supports of the first hall.
162
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
Fig. 10. Steps of the lifting process applied at the supports of the first hall.
structure beforehand at possible settlement points and then monitor the structure for settlements and control the related damage at low risk before significant damage accumulates. However, for this structure, action has been taken long after the problem has started, and therefore cement injection technique has been employed to stabilize the ground as soon as possible.
3. Structural analyses of the existing building In order to investigate the structural behaviors of the structural systems under local settlements, three-dimensional finite element models were generated by using SAP2000 software [13] according to the measurement results of the building survey performed for the each hall of building. These analysis models created for both the hall are shown in Fig. 6. In numerical models, beam and gap elements were used. The structural elements were modelled with 3D beam elements located at the centerline of actual elements. Due to the use welding in the member connections of the truss girders and the truss/frame girder to column connections, these connections were assumed to be fully fixed. In order to model the effects induced by local settlements on the structure, gap elements which act nonlinearly in the column length direction were used at the column bases where the settlements occured. Opening values of gap elements used in nonlinear static analyses were defined as corresponding settlement values at each column base location. Additionally, to determine load-carrying status of the building before local settlements and to compare with the last situation, 3D computer models without gap elements of the structures were carried out. Linear and nonlinear static analyses were performed on the models without and with gap elements, respectively. For estimation of dead and snow loads acting on the steel structures, current national design load code, TS498 [14] was used. Member sections of the load bearing system of the structures were checked according to the current structural steel design code TS648 [15] which is generally very similar to ASD-89 specification [16]. A comparison between the response of the linear and the nonlinear structural models considering member stress values obtained from the results of the structural analysis was carried out. The results of this comparison are given in Fig. 6. Members with over-stress values obtained from the results of the nonlinear static analyses were presented by green lines and also, the results of the static analyses were presented by blue lines (see Fig. 6). To eliminate over-stress values that occur on the structures owing to the local settlements which means to return to the original stress values of initial situation with
K. Ozakgul et al. / Engineering Failure Analysis 20 (2012) 156–165
163
no local settlements, an available repairment method was suggested. Since the region where the building is a historically protected area, the repairment solution was proposed taking this status into account.
4. Implementation of the proposed repairment solution In the proposed solution, for each column base where local settlements have occured, vertical displacement as the amount of subsidence was implemented in the column length direction via hydraulic jacks mounted the column base, as presented schematically in Fig. 7. During the application of the solution, the aim is nullifying the stress increments resulting from the local settlements that occured at the supports. This reset process may be conducted with the help of strain transducers mounted for continuous monitoring of strains at the critical structural members. Considering various combinations of the lifting process by means of hydraulic jacks such as from the single point or the dual points, stress analyses were carried out using the present computer models having gap elements. The results of the lifting process from the single point and the dual points applied at the supports of the first hall were given in Figs. 8 and 9, respectively. For each step, the gap opening values for relevant base points were taken as zero and afterwards static analyses were done. Support reaction values were obtained which mean required jack forces for each step at indicated locations.
Fig. 11. Results of the single lifting process applied at the supports of the second hall.
Fig. 12. Results of the dual lifting process applied at the supports of the second hall.
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Fig. 13. Steps of the lifting process applied at the supports of the second hall.
The most appropriate option from above mentioned lifting combinations was selected as the synchronized lifting applied from C and D supports, because this lifting combination induced more stable and lower displacements at other supports. After C and D supports were lifted and fixed, then the second step was started (see Fig. 10). In the second step, E and F supports were lifted at the same time and in synchronized and then were fixed. In the next step, A and B supports were lifted and fixed. Eventually, all supports occured settlements were lifted doubly with the order given in Fig. 10 and stress increases on the structural members induced the local settlements were reseted. Similar operations were repeated for supports in the second hall at settlement points. Firstly, to find the most convenient lifting options, the lifting process was applied from single and dual supports, as given in Figs. 11 and 12, respectively. Because significant differences were not found in stress values obtained from results of the lifting from single or dual supports, the lifting option for the second hall was selected as lifting from single support due to the ease of application. Initially, F support were lifted as single and fixed, then the second step was performed and finally, in the fifth step, this process was repeated for B support, as given in Fig. 13. The stress increases on the structural members induced the local settlements occured in the second hall were eliminated. 5. Conclusions In this study, the structural effects of the excavation-induced local settlements occurred in an existing steel structure adjacent to excavation area of Marmaray Project at Yenikapi quarter of Istanbul city were investigated. For this purpose, 3D computer models using beam and nonlinear gap elements were analysed by using the commercially available finite element analysis software SAP2000. The effects caused by local settlements on the structure were simulated using gap elements at the column base points with settlements. As a result of linear and nonlinear static analyses, a remediation scheme was proposed to eliminate the negative effects on the structural system due to settlements. A systematic and versatile method that was used has been given in detail based on the results of structural analyses. Acknowledgements The authors would like to thank Mr. Suat Emrah Dulger, who was then a research assistant, for his contributions during building surveys. Many thanks are also owed to the anonymous referees for their valuable comments and contributions during manuscript review. References [1] Marmaray. 2004. DLH Marmaray Project Directorate. http://www.marmaray.com. [2] Finno RJ, Bryson LS. Response of building adjacent to stiff excavation support system in soft clay. J Performance Construct Facilities 2002;16(1):10–20. [3] Son M, Cording JE. Estimation of building damage due to excavation-induced ground movements. ASCE J Geotech Geoenviron Eng 2005;131(2):162–77. [4] Finno RJ, Voss FT, Rossow E, Blackburn JT. Evaluating damage potential in buildings affected by excavations. ASCE J Geotech Geoenviron Eng 2005;131(10):1199–210.
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