# Dynamic behaviour of hybrid steel

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Dynamic behaviour of hybrid steel

Southern Cross University [email protected] 23rd Australasian Conference on the Mechanics of Structures and Materials 2014 Dynamic behaviour of hybrid steel-concrete structures beyond conventional stability limits during severe seismic S Salib Ryerson University Publication details Salib, S 2014, 'Dynamic behaviour of hybrid steel-concrete structures beyond conventional stability limits during severe seismic', in ST Smith (ed.), 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), vol. II, Byron Bay, NSW, 9-12 December, Southern Cross University, Lismore, NSW, pp. 949-954. ISBN: 9780994152008. [email protected] is an electronic repository administered by Southern Cross University Library. Its goal is to capture and preserve the intellectual output of Southern Cross University authors and researchers, and to increase visibility and impact through open access to researchers around the world. For further information please contact [email protected] 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23) Byron Bay, Australia, 9-12 December 2014, S.T. Smith (Ed.) DYNAMIC BEHAVIOUR OF HYBRID STEEL-CONCRETE STRUCTURES BEYOND CONVENTIONAL STABILITY LIMITS DURING SEVERE SEISMIC EVENTS S. Salib* Civil Engineering Department, Ryerson University Toronto, ON, Canada. [email protected] (Corresponding Author) ABSTRACT Considering Soil Structure Interaction (SSI) for structures located in high seismic zones directs many structural engineers towards developing rigorous three Dimensional Finite Element Modelling (3DFEM). This modelling usually represents SSI as well as many details of both superstructure and foundations. In addition, structures composed of hybrid skeleton systems and different materials add more complexity to the desired FEM. Realistically, a lot of time, effort and budget are spent on such detailed FEM; not only to develop but also to process. Yet, the accuracy of the obtained results is considerably influenced by the SSI modelling itself. Therefore, the present study introduces a FEM approach for a sophisticated nonlinear SSI. The proposed FEM is capable of predicting the plastic soil deformations, tracking the structure position during seismic excitation and capturing the structure behaviour after experiencing instability conditions. Superstructures with steel columns and concrete walls of various configurations are investigated. Further, the proposed modelling philosophy allows transforming the entire capabilities of the 3D-FEM of the structure into a significantly simplified 2DFEM. The study emphasises that structures can be designed to behave beyond conventional stability limits in a manner that restores stability and optimises both design and construction costs. KEYWORDS Soil-structure interaction, steel, concrete, FEM, seismic, dynamic analysis. INTRODUCTION This paper presents a continuation of the author’s research work in the area of Soil-Structure Interaction (SSI), in general, and regarding SSI in high seismic zones, in particular (Abdel-sayed and Salib 2002; Salib and Abdel-sayed 2012; Salib 2012). Earlier, the SSI of relatively short concrete structures was addressed (Salib 2012) and, herein, taller superstructures of hybrid systems such as concrete walls and steel columns are investigated. In general, most of design codes and common practice consider that SSI is always in favour of the structure and consequently a full fixation at the base of superstructure, i.e. at the ground/foundations level, for seismic analysis is a reasonable assumption. While this approach might be conservative regarding the straining actions induced in relatively tall structures, it underestimates the total structure deformations and entirely neglects the soil deformations (Figure 1, NHERP 2012). On the other hand, simplifying soil nonlinearities, including initiation of soil failure mechanism(s), into equivalent linear parameters in order to predict the plastic soil deformations associated to a seismic event can be quite complicated if not impossible (FEMA-440 2006). Therefore, FEM is believed to be a reasonable tool to represent the nonlinearities of both structure and SSI. However, even through FEM, modelling nonlinear SSI has been usually terminated by the formation of SSI failure mechanism (Hutchinson et al. 2006). Recently, few researchers investigated the SSI of existing structures during seismic events where some structures survived and others collapsed (Gazetas 2006; Gazetas and Apostolous 2004; Gazetas and Makris 2002). They found This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ 949 that the initiation of uplifting, sliding and the mobilization of bearing capacity failure can be quite beneficial for the structure as a way to absorb part of the input seismic energy through experiencing some plastic, i.e. permanent, soil deformations. Herein, the objective of the present study is to develop a FEM that presents the nonlinear behaviour of SSI along with global instability scenarios such as foundation uplifting, sliding and bearing capacity failure (covering both individual and simultaneous scenarios). A 3D-FEM as well as a corresponding simplified 2D-FEM is developed. A comparison between the details and results of both modelling approaches is presented. (a) Fixed base (b) Partially fixed base Figure 1. Structure deformations associated with lateral forces (NHERP 2012). DETAILED FINITE ELEMENT MODELLING General The study focuses on structures with shallow foundations near or on ground surface; both active and passive earth pressures have been eliminated. The footings, slabs and walls of the investigated structures are of concrete and have been modelled as shell elements. Each node on the soffit of the foundation is linked to a multi-linear elastic-plastic member. The member behaviour in the horizontal plane (both longitudinal and transverse directions) represents the soil-structure friction characteristics including the sliding resistance and post-sliding behaviour. The soil-structure bearing behaviour is represented in the member vertical direction where the soil is assumed to be engaged only if the structure is pressing down. The member traces the bearing behaviour up to and beyond the bearing capacity limit and represents any possible rocking/uplift scenarios during the seismic cyclic loading. The own weight of structure has been applied as a primary condition after which the seismic analysis has been carried out. This simulates the initial deformations and straining actions experienced by the soil under such weight prior to the seismic event. The details of the seismic data of the proposed study site were provided in a previous study where the magnitude of the applied earthquake acceleorgram approaches ±0.8g (Salib 2012). Modelling of Structure ST1 The model, as shown in Figure 2, represents 5 storey building (15m height) with a mat foundation of 20.00m length along X (North-South direction), 15.00m width along Y (East-West direction) and 2.00m thickness along Z direction. The structure has a 4-sides core of 0.20m thickness concrete walls while the exterior bays, on the North and South sides of the core, are carried by steel columns (hollow square section of 0.3mx0.3mx13mm). The concrete floors and roof slab are of 0.20m thickness. The first and second Modes Of Vibration (MOV) are shown in Figure 2. The displacement at the centre of foundation and the centre of roof slab in X, Y, and Z directions are shown in Figure 3. There is a permanent displacement in Z direction of approximately 8mm which remains constant/downward and takes place mainly due to the self weight of structure. Also, the permanent displacement in the horizontal plane (along X and Y directions) indicates that the soil has been stressed beyond the sliding resistance and, consequently, has experienced plastic deformations. However, the model results indicate no collapse or resonance pattern (i.e. no displacement magnification towards infinity). ACMSM23 2014 950 N (a) FEM (b) 1st MOV; f=3.84Hz (c) 2nd MOV; f=4.02Hz Figure 2. Isometric view of FEM and MOVs for structure ST1. Displacement (mm) Displacement (mm) 100 100 Footing, Centre-Dx Footing, Centre-Dy Footing, Centre-Dz 75 50 Roof, Centre-Dx Roof, Centre-Dy Roof, Centre-Dz 75 50 25 25 0 0 -25 -25 -50 -50 Time (Sec) Time (Sec) (a) At centre of footing (b) At centre of roof slab Figure 3. Displacements of ST1 during the seismic event. Modelling of Structure ST2 This model has been selected to represent a wide variety of relatively tall structures (e.g. commercial and residential buildings); see Figure 4. It is 10 story high (30.00m height) with hollow concrete core of 10.00m length along X direction, 10.00m width along Y direction and 0.20m walls thickness. Each bay on the North and South sides of the core is supported by steel columns. N (a) FEM (b) 1st MOV; f=1.98Hz (c) 2nd MOV; f=2.27Hz Figure 4. Isometric view of FEM and MOVs for structure ST2. The foundation, columns and both floor and roof slabs are the same as of ST1. The first and second MOVs are shown in Figure 4. The displacement at the centre of foundation and the centre of roof slab in X, Y, and Z directions are shown in Figure 5. Also, the displacements in the Z direction at both North and South edges of the footing are shown in Figure 6. Having positive/negative cycles of displacement in the Z direction at the footing edges (Figure 6) along with permanent structure displacements in X and Y directions (Figure 5) indicate that the structure experiences a simultaneous process of rocking, uplifting and sliding (between 0 and 20 seconds). However, the displacement patterns in all directions eventually have narrowed down to a steady condition (after 20 seconds until ACMSM23 2014 951 the end of seismic event). In other words, the structure has performed beyond stability limits without experiencing an overturning or any other global stability failure. Displacement (mm) Displacement (mm) 200 200 Footing, Centre-Dx Footing, Centre-Dy Footing, Centre-Dz 150 100 50 Roof, Centre-Dx Roof, Centre-Dy Roof, Centre-Dz 150 100 50 0 0 -50 -50 -100 -100 -150 -150 -200 -200 Time (Sec) Time (Sec) (a) At centre of footing (b) At centre of roof slab Figure 5. Displacements of ST2 during the seismic event. Displacement (mm) 15 10 5 0 -5 -10 -15 -20 -25 Footing, N edge-Dz Footing, S edge-Dz Time (Sec) Figure 6. Vertical displacements of the footing edges of ST2 during the seismic event. SIMPLIFIED FINITE ELEMENT MODELLING General The entire 3D-FEM of the structure (superstructure and foundation) is replaced with an equivalent 2DFEM in the form of frame elements; which represents the same distribution of mass and stiffness of the structure. Also, the grid of the SSI link members in the 3D-FEM is replaced with a single link member at each of the few footing nodes in the 2D-FEM. Each of these equivalent link members is specified as a set of 6 mutli-linear elastic-plastic curves. Each curve represents the load-deformation relationship of the soil underneath the footing in each spatial degree of freedom of the footing soffit node. Figure 7 shows the simplified 2D-FEM, ‘ST1S’ and ‘ST2S’ that is corresponding to the original 3D-FEM, ‘ST1 and ST2’, respectively. (a) ST1S (b) ST2S Figure 7. Isometric view of the 2D-FEM (simplified models). ACMSM23 2014 952 Results and Discussion A remarkable compatibility of the dynamic behaviour between the 3D-FEM and 2D-FEM can be seen as the maximum difference in the first five (fundamental) MOV’s frequency is less than 10% (Table 1). Table 1. MOV obtained for both original and simplified models. MOV 1st 2nd 3rd 4th 5th ST1 3.84 4.02 6.06 6.22 6.29 ST1S 3.71 4.25 5.51 6.18 6.19 ST2 1.98 2.27 5.97 5.98 6.00 ST2S 1.99 2.06 6.00 6.01 6.12 Also, the overall structure behaviour and displacement pattern in X, Y and Z directions of the simplified models (Figures 8-10) are quite similar to those of the original models (Figures 3, 5 and 6). Further, the values of the maximum (Max.), minimum (Min.) and permanent (Per.) displacements obtained from the simplified models in each direction does not differ by more than about 10% on average basis from the corresponding values of the original models as listed in Table 2. 100 Displacement (mm) Displacement (mm) 100 Footing, Centre-Dx Footing, Centre-Dy Footing, Centre-Dz 75 50 Roof, Centre-Dx Roof, Centre-Dy Roof, Centre-Dz 75 50 25 25 0 0 -25 -25 -50 -50 Time (Sec) Time (Sec) (a) At centre of footing (b) At centre of roof slab Figure 8. Displacements of ST1S during the seismic event. Displacement (mm) Displacement (mm) 200 200 Footing, Centre-Dx Footing, Centre-Dy Footing, Centre-Dz 150 100 50 Roof, Centre-Dx Roof, Centre-Dy Roof, Centre-Dz 150 100 50 0 0 -50 -50 -100 -100 -150 -150 -200 -200 Time (Sec) Time (Sec) (a) At centre of footing (b) At centre of roof slab Figure 9. Displacements of ST2S during the seismic event. Displacement (mm) 15 10 5 0 -5 -10 -15 -20 -25 Footing, N edge-Dz Footing, S edge-Dz Time (Sec) Figure 10. Vertical displacements of the footing edges of ST2S during the seismic event. ACMSM23 2014 953 Table 2. Displacement obtained for both original and simplified models. Displacement DX DY DZ (mm) Min. Max. Per. Min. Max. Per. Min. Max. ST1, Footing -36 66 -25 -33 64 -22 -7 -7 ST1S, Footing -29 70 -23 -27 68 -21 -7 -7 ST1, Roof -46 73 -25 -41 72 -22 -8 -8 ST1S, Roof -47 79 -23 -41 75 -21 -8 -8 ST2, Footing -89 60 -73 -100 52 -85 -8 -8 ST2S, Footing -98 61 -77 -104 57 -83 -8 -8 ST2, Roof -133 177 -75 -180 166 -87 -9 -9 ST2S, Roof -158 173 -77 -177 172 -83 -9 -9 Per. -7 -7 -8 -8 -8 -8 -9 -9 CONCLUSIONS A 3D-FEM is presented which can track the nonlinear behaviour of SSI up to and beyond the mobilization of the failure mechanisms of bearing, uplifting, sliding and overturning. Based on the obtained results, the following conclusions can be achieved: The initiation of such mechanisms, combined, may not cause a global failure of structures even under strong ground motions. In fact, they introduce a dissipation system for the input seismic energy; The developed model can predict the structure position during and after a specific seismic event including permanent structure displacements due to plastic soil deformations. This prediction helps fulfilling the performance-based design requirements for post disaster structures; The 3D-FEM can be transformed into much simpler 2D-FEM that provides the same degree of accuracy. This significantly reduces the time, effort and budget spent over FEM especially during conceptual and preliminary design process; and The study emphasises that structures can be designed to behave beyond conventional stability limits in a manner that restores stability and optimises both design and construction costs. REFERENCES Abdel-sayed, G. & Salib, S. (2002) “Minimum Depth of Soil Cover for Soil-Steel Bridges”, ASCEJournal of Geotechnical and Geoenvironmental Engineering, Vol. 128, No. 8, pp. 672-681. FEMA-440 (2006) Improvement of Nonlinear Static Seismic Analysis Procedures, Federal Emergency Management Agency, Washington, DC, U.S.A. Gazetas, G. (2006) “Seismic Design of Foundations and Soil-Structure Interaction”, First European Conference on Earthquake Engineering and Seismology, Geneva, Switzerland. Gazetas, G. & Apostolous, M. (2004) “Nonlinear Soil–Structure Interaction: Foundation Uplifting and Soil Yielding”, The Third UJNR Workshop on Soil-Structure Interaction, California, U.S.A. Gazetas, G. & Makris, N. (2002) “Uplifting and Overturning of Simple Structures in light of the Athens, Kocaeli, and Düzce Earthquakes in 1999”, The 4th Forum on Implications of Recent Earthquakes on Seismic Risk, Tokyo, Japan. Hutchinson, T., Raychowdhury, P., & Chang, B. (2006) “Nonlinear structure and foundation response during seismic loading: dual lateral load resisting systems”, 8th U.S. National conference on earthquake engineering, San Francisco, CA, U.S.A. NEHRP (2012) Soil-Structure Interaction for Building Structures, National Earthquake Hazards Reduction Program, Washington, DC, U.S.A. Salib, S. (2012) “Dynamic Analysis of Structures with Nonlinear Soil-Structure Interaction in High Seismic Zones”, 5th European Conference on Structural Control, EACS 2012, Genoa, Italy. Salib, S. and Abdel-sayed, G. (2012) “Dynamic Behaviour and Seismic Response of FRP Light Poles in High Seismic Zones”, The 6th International Conference on FRP Composites in Civil Engineering, CICE 2012, Rome, Italy. ACMSM23 2014 954