Shake Table Analysis of Concrete Structural Models. Keywords: Earthquake, Fiber Reinforced Polymer, shake table, tape confinement, wire mesh.

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1 Paper ID: SE-009 International Conference on Recent Innovation in Civil Engineering for Sustainable Development () Department of Civil Engineering DUET - Gazipur, Bangladesh 510 Shake Table Analysis of Concrete Structural Models M.Z. Hossain 1, M.N. Hasan 2 and I. Anam 3 Abstract The recent massive earthquake in Nepal has raised cautionary signals for Bangladesh about possible colossal earthquake in the near future, whose impact can be devastating. So, prediction of seismic resistance of existing structures is important in order to assess their vulnerability. Since columns are the most critical and often the most vulnerable of all structural elements, the main objective of this paper is to investigate the seismic response and failure modes of different concrete column models. Another purpose is to assess the suitability of various retrofit measures to improve the structural responses using earthquake shake table tests. Twelve concrete column models without and with two types of confinement; i.e., wire mesh and Fiber Reinforced Polymer (FRP) are tested to investigate their seismic behavior. Experimental results from shake table, calibrated from the El Centro earthquake (1940) ground data, show that the confined structural models have greater capacity in terms of strength and ductility than the unconfined models. And among all the models, the FRP-confined models show the best seismic resistance capacity. Comparison between experimental and numerical results show very good agreement in terms of maximum displacement of structural models. Keywords: Earthquake, Fiber Reinforced Polymer, shake table, tape confinement, wire mesh. 1. Introduction Urbanization in Bangladesh has been rapidly taking place over the last few decades and a large number of buildings have been constructed throughout the country without maintaining standard construction practice and proper planning. The concrete buildings are more susceptible to damage and collapse, which may cause havoc in the densely populated areas mainly in cities, even due to a moderate earthquake originating from nearby faults. So, evaluating the seismic adequacy of the existing structure and strengthening to desired level of seismic resistance is important to mitigate the devastation. New structures can be built sufficiently earthquake resistant by adopting proper design methodology and construction quality control. But the existing old structures which have mostly been planned without considering this important aspect, pose enormous seismic risk. Realizing the possible distress of structures during major earthquakes, the rational numerical approach would be to perform structural dynamic analysis considering material and geometric nonlinarity. On the other hand, the usual experimental technique for earthquake engineering is shake table testing. A prototype structure on a shake table should obviously give better evaluation. But to make the procedure simple and on account of the capacity of shake table, scaled down models of existing building concrete structures are used more frequently. These approaches are used in this work to evaluate the seismic response of concrete column models. 1 M.Z. Hossain, University of Asia Pacific, zakir07h@gmail.com 2 M.N. Hasan, University of Asia Pacific, hnazmul68@yahoo.com 3 I. Anam, University of Asia Pacific, iftekhar@uap-bd.edu

2 Theoretical Background 2.1. Scaling laws Scaled modeling is common in engineering, including scaling for structural elements like columns only (e.g., Islam [1]) to multi-storied buildings (e.g., Lam et al. [2]). In this work, concrete column models are used in earthquake simulation tests and similitude laws can be used to predict nonlinear seismic response of prototype structures. A similitude scale of 1:3.68 is used, because the El Centro earthquake plate used here was calibrated from the original El Centro Earthquake (1940) ground vibration motion data divided by a scaling factor 3.68 (as shown in Fig. 1). Fig. 1. Ground motion for El Centro earthquake and laboratory shake table 2.2. Confinement One disadvantage of concrete is that it is brittle, breaking suddenly without warning. If ductility is increased, it concrete would give more warning before failure. It has been determined that confining concrete by exterior jacketing of steel and plastic increases ductility and strength. Steel jacketing or meshing is an established method of increasing the ductility of concrete. Several researchers have also investigated the feasibility of Fiber Reinforced Polymer (FRP) composite jackets for strengthening of columns winding them with high strength carbon fibers. Among the advantages of FRP are very high strength to weight ratio, enhanced stiffness, shear and tensile capacities, chemical resistance and flexibility Moment-curvature analysis The load-deformation behavior of a structural element using nonlinear material stress-strain relationships can be predicted using a moment-curvature analysis [3]. A moment-curvature diagram plots the curvatures against corresponding moments, thus establishing the ductile capacity of a crosssection. In this work, the moment-curvature of the concrete columns was determined using a computer program, from which some important parameters obtained include the initial flexural rigidity, yield curvature and ultimate curvature values, which are then used to calculate the lateral stiffness, yield deflection and ultimate deflection. 3. Experimental Setup 3.1. Preparation of models In this work, simple concrete structure model is used. For laboratory test, twelve models (of two different sizes) are constructed. Both types have foundation plates of 1 feet by 1 feet. But one type has a 24 length and 1.5 diameter whereas another has a 18 length and 1.25 diameter (therefore, the corresponding prototypes would be of lengths 88.32, and diameters 5.52, 4.60 ). Forms for all concrete structures must be tight, rigid and strong. So steel molds are used for model construction

3 512 and placed in such a way for convenient casting. Fig. 2 shows the steps in model preparation. Structural models are bolted to the base of the shake table. A steel cap is used on top to carry mass easily. A volume ratio of 1:1.5:3 for cement, sand and aggregate is maintained to make the concrete, with brick chips passing #4 and retaining on #8 sieve as aggregate. The w/c ratio of 0.55 is used. Fig. 2. (a) Steel mold, (b) placing concrete, (c) after concrete casting Confinement of models As mentioned, confinement by wire mesh and Fiber Reinforcement polymer (FRP) are used in this work (Fig. 3) to improve strength and ductility of concrete column models. Fig. 3. Confinement by (a) wire mesh, (b) FRP Addition of superstructure mass Structural molds are constructed excluding the mass at the top. So mass is added subsequently in the system (shown in Fig. 4) before the testing of model with and weight blocks. Fig. 4. Model set up for experiment

4 Shake Table test Shake table provides an experimental platform that simulates earthquake motion to verify seismic performance of building structure. It is used to test structural models and components (e.g., [4]), usually to the point of failure. The shake table set up in the Structural Mechanics and Strength of Materials Lab in the Department of CE at UAP consists of structure molds, shaking plate, El Centro earthquake plate, motion regulating motor, etc (as described in [1], [5]). There is a rotating shaft which is fixed to a motor and rotates under the plate, moving it forward. The motor s speed can be regulated to run at three durations; i.e., 15, 20 and 25 sec. 4. Numerical and Experimental Results 4.1. Shake Table test The cantilever columns were tested in the earthquake shake table described, superposing additional masses at the column tips and running the structural models for 15 seconds. The additional masses were increased until the columns failed (as shown in Fig. 5); i.e., initiated by the concrete failing in tension, causing sectional failure and collapse of the columns. Fig. 5. Failure of (a) Unconfined, (b) wire confined, (c) FRP-confined columns 4.2. Numerical analysis Nonlinear dynamic analysis is conducted in order to numerically evaluate the seismic performance of the structures. Structures subjected to earthquake motion are expected to show inelastic behavior. Moment-curvature relation of the column section is taken as the basic starting point of numerical analysis, from which the other parameters of dynamic analysis are derived. Dynamic analyses are performed using a computer program on elastic fully plastic single-degree-offreedom systems, while Figs. 6(a) and 6(b) show the relative tip displacements of unconfined and FRP-confined columns at their respective collapse loads (32 and 58 lbs) (sec) Fig. 6. Result from nonlinear seismic analysis of (a) Unconfined, (b) FRP-confined column

5 (sec) Fig. 7. Results for FRP-confined sample at (a) collapse load of unconfined sample, (b) simulation time of 25 seconds Figs. 7(a) and 7(b) show results from parametric studies for FRP-confined samples. Fig. 7(a) shows reduced relative deflections (8 compared to 0.73 ) if the sample is loaded with failure loads of unconfined sample; i.e., 32 lb instead of 58 lb. Fig. 7(b) shows the effect of earthquake simulation time, as the sample undergoes much less deformation (0.16 ) when subjected to same ground motions slowly (i.e., for 25 seconds) even at its collapse load Comparison of experimental and numerical results Fig. 8 and Fig. 9 demonstrate the reasonable match between time histories of experimental and numerical tip deflections of the unconfined and FRP-confined sample models. Table-1 and Table-2 summarize all the results (for four column samples each with three confinement modes), including comparison of peak experimental and numerical deflections (showing excellent match overall), as well as superimposed loads and relative deflections at failure (showing improved performance of confined models). Fig. 8. (a) Experimental, and (b) Numerical displacement for unconfined model Fig. 9. (a) Experimental, and (b) Numerical displacement for FRP-confined model

6 515 Confinement Table-1. Comparative experimental and numerical results Deflections max(exp) max(num) ] for various samples Sample 1 Sample 2 Sample 3 Sample 4 Unconfined [1.50, 1.57] [1.60, 1.61] [1.20, 1.47] [1.20, 1.71] Mesh-confined [1.65, 1.60] [1.90, 1.55] [1.15, 1.63] [1.30, 1.59] FRP-confined [1.40, 1.60] [1.40, 1.60] [1.60, 1.62] [1.50, 1.60] Confinement Table-2. Performance of different confinements (Weight W) Relative Deflections max(exp) ] for various samples Sample 1 Sample 2 Sample 3 Sample 4 Unconfined (43) [9] (32) [9] (15) [0.37] (22) [0.59] Mesh-confined (45) [0.71] (52) [0.71] (32) [0.70] (37) [0.71] FRP-confined (58) [0.73] (58) [0.74] (42) [6] (45) [7] 5. Conclusions Experimental results obtained from shake table test of different confined structural models are found to be quite similar to numerical simulation results for displacement. Seismic performances of confined models are found to be better than unconfined models; i.e., by providing external confinement using wire mesh and particularly FRP to the structural model the strength and ductility are improved significantly. Parametric studies show marked decrease of dynamic responses with decreased mass and slower ground motion (run for longer durations). Acknowledgement The generous cooperation of Fosroc Inc. in confinement by FRP is gratefully acknowledged. References [1] Islam, M.S., Seismic Shake Table analysis using similitude laws, B.Sc. Engg. Thesis, Dept. of Civil Engg., University of Asia Pacific, Dhaka, Bangladesh, [2] Lam, S.S.E., Zhang, M.Z., Wong, Y.L., and Li, C.S., Shaking Table tests of a 1:20 Scale High- Rise Residential Building, Proceedings Int. Conference on Advances and New Challenges in Earthquake Engg. Research, Volume 1, Harbin, China, pp , [3] Ward, A., and Valencia, T., Investigating the behavior of Reinforced Concrete columns through Static and Dynamic Analysis, Research Experience for Undergraduates (REU), Georgia Institute of Technology, USA, [4] Barnes, J.E., Seismic modeling with an earthquake Shake Table, B.Sc. Engg. Thesis, Linfield College, McMinnville, Oregon, USA, [5] Roy, H., Numerical and experimental study of structures with soft stories, B.Sc. Engg. Thesis, Dept. of Civil Engg., University of Asia Pacific, Dhaka, Bangladesh, 2007.