Structural Response Analysis of Reinforced Concrete Frame with Unreinforced Masonry Infill Walls

Transcription

1 Paper ID: SE International Conference on Recent Innovation in Civil Engineering for Sustainable Development () Department of Civil Engineering DUET - Gazipur, Bangladesh Structural Response Analysis of Reinforced Concrete Frame with Unreinforced Masonry Infill Walls Ram Krishna Mazumder 1, Rajen Dey 2, Muhammad Saif Uddin 3 and Md. Abdur Rahman Bhuiyan 4 Abstract In the conventional practice Unreinforced Masonry (URM)walls are often considered as non-structural element and its load is considered on the corresponding elements. Effect of infill is mostly ignored during analysis of the structure. To obtain realistic behavior of a building all the primary components are required including infill responses. This study aims to simulate structural behavior of Reinforced Concrete (RC) frame with URM infill walls using software package Seismostruct v7.0 where equivalent diagonal strut elements were used to idealize the effect of infill walls. A six-storied ordinary moment resisting RC frame with URM masonry infill walls was modeled and analyzed to obtain the responses of the structure. Static pushover analysis was performed to get nonlinear response of the index building, the results also compared with bare frame model to understand the phenomena associated to URM infill interaction to RC frame. It is observed from pushover analysis that the bare frame comprises lesser stiffness when compared to the RC frame model having URM infill walls within a range of displacement. Keywords: Diagonal strut, infill wall, pushover analysis, reinforced concrete, unreinforced masonry 1. Introduction The Reinforced Concrete frame building with URM masonry infill walls are very common in Bangladesh and many other countries. Easy and low-cost constructing is known as a main reason for uses of the brick masonry in the developing countries. The purpose of masonry is mostly to protect inside of the structure from the environment and to separate internal spaces. In most of the cases of seismic resistant design, particularly in Bangladesh, the brick masonry infill walls in RC frame building is typically considered as nonstructural elements. Therefore, this consideration may result inaccurate prediction of the lateral stiffness, strength, and ductility of the structure. Reluctance of numerous engineers to take into account the contribution of brick masonry infill is due to the inadequate knowledge in structural modeling and uncertainty involved in interaction between infill and RC frame. In recent times several researchers (eg. Decanni et al. 2004, Baran and Sevil 2010, etc) have compared experimental and analytical results of interaction between RC frame and URM infill walls. Such experimental results revealed that performance of URM infill walls inside RC frame varied with lateral loads applied on the structure [1], [2]. URM infill remains in contact with RC frame under very low lateral loads and hence there is composite action between RC frame and URM infill walls. Initial lateral stiffness increased for the URM infill model in compare to bare frame model. A number of research works have been done in past decades to generate acceptable model for structural analysis in order to account interaction between URM infill and RC frames. Among several models, equivalent 1 Institute of Earthquake Engineering Research, Chittagong University of Engineering and Technology, Chittagong 4349, Bangladesh. rkmazumder@gmail.com 2 Institute of Earthquake Engineering Research, Chittagong University of Engineering and Technology, Chittago ng 4349, Bangladesh. rajendey445@gmail.com 3 Institute of Earthquake Engineering Research, Chittagong University of Engineering and Technology, Chittago ng 4349, Bangladesh. saif08cuet@gmail.com 4 Institute of Earthquake Engineering Research, Chittagong University of Engineering and Technology, Chittago ng 4349, Bangladesh. arbhuiyan@yahoo.com

2 565 diagonal strut model for infill panels is preferred due to its simplification in URM behaviors. In this study, the structural model was developed in a software package Seismostruct v7.0 to perform structural analyses for the index building. The objective of this work is proper modeling of infill walls and to identify their effects on RC frames. Nonlinear static pushover analysis was performed to obtained structural responses and results were compared between RC frame with infill frame model and bare frame model. 2. Modeling of Infill Wall The most critical part of modeling of a RC frame with URM infill wall is to model the URM infill properly. In general practice, the infill walls are commonly made of masonry bricks of various strength and brittleness. With a view to improving the simulation of actual performance of the infill frame, various infill panel models have been suggested in the literature. There have been several research conducted in past studies to develop micro model for the numerical simulation of infill panels using two dimensional finite element [3], however, the diagonal strut model (see Figure 1) is still the most widely used and accepted by the researchers as its simplified approach for bulk analysis, and has been advocated in many documents and guidelines [4][5]. Fig. 1. Diagonal strut for masonry infill panel modeling (a) Equivalent diagonal strut representation of an infill panel; (b) Variation of the equivalent strut width as function of the axial strain; (c) Envelope curve in compression Diagonal strut model utilizes a four-node masonry panel element for the modeling of infill panel. This concept was developed and primarily programmed by Crisafulli [6] and further implemented in SeismoStruct by Blandon [7].Six strut members are used to illustrate each panel. Every diagonal direction characterizes two parallel struts to carry axial loads across two opposite diagonal corners and a third one to carry the shear from the top to the bottom of the panel. The operation of fifth and sixth strut members activate on deformation of the panel as they only act across the diagonal that is on compression. The axial load struts use the masonry strut hysteresis model, and the shear strut uses a dedicated bilinear hysteresis rule. Also as can be observed in Figure 2 four internal nodes are employed to account for the actual points of contact between the frame and the infill panel, whilst four dummy nodes are introduced with the objective of accounting for the contact length between the frame and the infill panel. All the internal forces are transformed to the exterior four nodes. Stiffness and strength of an infill panel is calculated from width of equivalent strut using formula proposed by Mainstone and Weeks [8] and Mainstone [9].. (1) Where, [ ].....(2)

3 566 Where λ is the coefficient used to determine equivalent width of infill strut; h col is column height between centerlines of beam; h inf is height of infill panel; E c is expected modulus of elasticity of frame material; E m is expected modulus of elasticity of frame material; I col is moment of inertia of column; r inf is diagonal length of infill panel; t inf is thickness of infill panel and equivalent strut; and θ is angle whose tangent is the infill height-to-length aspect ratio. Fig. 2. Equivalent strut model for infill panel (Crisafulli, 1997) 3. Building Description The selected building prototype is a six-storey masonry building located in Lalkhan Bazar, Chittagong. This building can be considered as typical representative structure for RC frame with URM infill wall type. The building has a trapezoidal identical plan having 10 ft story height in each floor. The plan sketch and dimensions are given in Figure 3b. Values for the structural characteristicsrelated parameters associated to the building class represented by the index building are shown in the Table 1 and Table 2. Parameter Table 1. Material properties Value Compressive strength of concrete (f c ) 2900 psi Tensile strength of steel (f y ) psi Unit weight of brick masonry 120 lb/ft 3 Compressive strength of infill (f w ) 145 psi Parameter Table 2. Structural Details Shorter length (L1) Larger length (L2) Width (W) Floor to Floor Height (H) Thickness of infill walls (t w ) Column 1 Column 2 Beam 1 Beam 2 Beam 3 Value 44 ft ft 30 ft 10 ft 6.3 in 12 x12 15 x12 14 x x x10 4. Building Model The structural models for both RC frame with URM infill and bare frame were developed in the SeismoStruct v7.0 framework. The concrete model is based on the Madas uniaxial model, which follows the constitutive law proposed by Mander et al. [10]. The cyclic rules included in the model for the confined and unconfined concrete were proposed by Martinez-Rueda [11] and Elnashai [12]. The confinement effects provided by the transverse reinforcement were considered through the rules proposed by Mander et al. A uni-axial bilinear stress-strain model with kinematic strain hardening was adopted for the representation of steel reinforcement in these analyses. This simple model is also characterized by easily identifiable calibrating parameters and by its computational efficiency. It can be used in the modeling of both steel structures as well as reinforced concrete models. Both models (RC frame with URM infill model and bare frame) are shown in the Figure 4.

4 567 Fig. 3. (a) 3D view of the selected building, (b) Typical floor plan 5. Analysis and Result (a) (b) Fig. 4. (a) Bare frame model; (b) URM Infill frame model The relative performance of both configurations was compared by means of the equivalent static analysis and pushover analysis. Distribution of base shear and floor shear of both bare and infill frame corresponding to the height of the structure is shown in figure 5 and figure 6 respectively. Results obtained from equivalent static method (as per BNBC 1993) showed that base shear for bare frame structure was kips whereas base shear of structure incorporated with URM infill wall was kips. Pushover analysis was performed by applying a controlled displacement (Response control) at the top of a particular frame. Comparative results from pushover analysis for the case study building are shown in Figure 7.It has been observed that pushover curve for RC frame with URM infill structure has larger gradient for initial values of displacement than bare frame structure which substantiate higher stiffness of the structure. However, stiffness drops reasonably after a particular value of displacement and the same trend is observed for further displacement value. Comparative response of bare frame and infill frame is summarized in the Table 3.

5 Force (Kips) Height (ft) Height (ft) Bare Frame Infill Frame Bare Frame Infill Frame Base Shear (kips) Floor Shear (Kips) Fig. 5.Comparison of base shear distribution Fig. 6. Comparison of floor shear distribution bare frame infill frame Displaement (in.) Fig. 7.Resulted pushover curves for both bare frame and RC with URM infill models Table-3. Summary results Parameter Bare frame Infill frame structure structure Base Shear (kips) Fundamental period (sec) Conclusions A six-storied ordinary moment resisting RC frame with URM infill walls was chosen for this study which represents typical residential building in Chittagong. Finite element package software Seismostruct v7.0 is used to develop structural model and perform structural analyses in order to

6 569 obtain responses of URM infill walls and their effects on the structure. It was observed that inclusion of masonry wall in bare frame structures increases the lateral stiffness and resistance of RC frame building significantly. Furthermore, Structural period of the building with infill wall extenuate to about two-third compared to building without infill wall. Presence of infill walls also increases the base shear of the structure about 50 percent. Finally lesser peak loading capacity as evident from the pushover analysis substantiates the poor seismic performance of bare frame compared to infill frame. 7. Acknowledgement The authors would like to acknowledge Seismosoft for providing SeismoStructv7 academic license which was used in this study. 8. References [1] Decanini L.,Mollaloli F., Mura A. and SaragoniR.,Seismic performance of masonryinfilledr/c Frames, Proceeding of 13th World conference on Earthquake Engineering. Vancouver, B.C., Canada, [2] Baran M., and Sevil T., Ana1ytical and experimental studies on infilled RC frames. int. J. of the Physical Sciences, 5(13), , [3] Ellul, F.L. and D Ayala, D., Realistic FE models to enable push-over nonlinear analysis of masonry infilled frames, The Open Construction and building Technology Journal, 6: , [4] Canadian Standards Association (CSA), Design of masonry structures (S304.1), Ontario, Canada, [5] New Zealand Society for Earthquake Engineering (NZSEE), Assessment and Improvement of the Structural Performance of Buildings in Earthquakes, [6] Crisafulli F.J. Seismic Behaviour of Reinforced Concrete Structures with Masonry Infills, PhD Thesis, University of Canterbury, New Zealand, [7] Blandon, C.A., Implementation of an Infill Masonry Model for Seismic Assessment of Existing Buildings, Individual Study, European School for Advanced Studies in Reduction of Seismic Risk (ROSE School), Pavia, Italy, [8] Mainstone, R. J. and Weeks, G. A.,The influence of bounding frame on the racking stiffness and strength of brick walls.2nd International Brick Masonry Conference, Stoke-on-Trent, UK, [9] Mainstone, R. J., On the stiffness and strengths of infilled frame. Proceedings, Institution of Civil Engineers, Supplement IV, 57 90, [10] Mander J.B., Priestley MJN, Park R. Theoretical stress-strain model for confined concrete. Journal of Structural Engineering, Vol. 114, No. 8, , [11] Martinez-Rueda JE. Energy Dissipation Devices for Seismic Upgrading of RC Structures. PhD Thesis, Imperial College, University of London, London, UK, [12] Elnashai A.S, Elghazouli AY. Performance of composite steel/concrete members under earthquake loading, Part I: Analytical model. Earthquake Engineering and Structural Dynamics, Vol. 22, pp , 1993.