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  1. 1. Sabri Attajkani, Abdellatif Khamlichi, Abdellah Jabbouri / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January -February 2013, pp.1178-1183 Modelling the Effect of Infill Walls on Seismic Performance of Reinforced Concrete Buildings Sabri Attajkani*, Abdellatif Khamlichi*, Abdellah Jabbouri* *Department of Physics, Abdelmalek Essaâdi University, M’hannech 2, Tetouan, MoroccoABSTRACT Infill walls contribute to lateral stiffness mortar used, reinforcements, type of junction withand resistance of buildings they stuff. These the frame members, etc.variations of rigidity and strength are dependenton the mechanical properties of the material used Because of the complexity to take intofor the infill and also on the interaction existing account the infill effect on the frame behavior, manybetween this last and the frame. In this work, researches have attempted at simplifying themasonry like infill walls were modeled by using modelling of the infill effect on the frame responsethe equivalent diagonal strut concept in order to by introducing simple analytical models. Extensiveasses their involvement in seismic resistance of experimental investigations were used to identifyregular reinforced concrete building. Pushover these approximate models. In this context, infilledanalysis was performed by means of ZeusNL steel frames were studied at first. On the basis ofsoftware package. Various scenarios of infilled experimental evidence showing that detachment offrames that include weak story arrangements at the frame from the infill occurs, Holmes [1] hasdifferent storey levels were considered. proposed replacing the panel by an equivalentComparison between complete infilled building, diagonal strut made of the same material as the infillpartially infilled with a weak story and bared and having a width equal to 1/3 of the infill diagonalbuildings was performed. The obtained results length. Based on experimental investigation onhave shown that infill walls have considerable diagonally and laterally loaded square infilled steeleffect on the lateral stiffness and resistance of frames, Stafford Smith [2] has subsequentlyreinforced concrete buildings when subjected to developed furthermore the idea of an equivalent strutthe static equivalent seismic loads. It was found as suggested by Holmes, and provided a numericalalso that infill enhances seismic performance. procedure to evaluate its dimensions.This enhancement is however largely affected by The procedure proposed in [2] for thethe distribution of infill through the building evaluation of the geometrical dimensions of thestories. The soft storey mechanism was found to equivalent strut that represents the stiffening effectbe more severe when the bared storey is located of the infill is nowadays well accepted. It was foundin the inferior part of the building. For non to be sufficient in many situations, in spite ofinfilled higher stories an unusual equilibrium neglecting some mechanical aspects of the infill-state can be reached showing very high lateral frame interaction [3-5]. Other refined models thatresistance. embody the effect of infills walls can be found in the literature [7-10].Keywords - masonry infills, reinforced concrete The equivalent strut characteristics arebuildings, seismic performance, pushover, identified according to Mainstone model [9] andequivalent diagonal strut used after that for pushover analysis of the infilled frames, where all the walls are replaced by their1. INTRODUCTION equivalent diagonal struts. ZeusNL [11] software It is well known that infill walls enhance package is employed in this analysis. The objectivethe lateral behavior of the frames they fill up. In is to assess the influence of infills on seismiccommon situations, the infill stiffens the frame capacity of buildings. A four-storey three-baylaterally by an order of magnitude and increases its reinforced concrete building will be studied and theultimate strength to very high values. These weak-story effect investigated.variations of stiffness and strength are dependent onthe mechanical properties of the material used for 2. EQUIVALENT STRUT MODELS FORthe infill: masonry, concrete blocs, reinforced INFILLED FRAMESconcrete, etc. The interaction between the frame and In FEMA 273 [6], FEMA 306 [7] andthe infill wall is also strongly affected by the FEMA 356 [8] it is suggested that the stiffness of theextension of the infill in the frame. It is also infills is represented in the structural model byinfluenced by the ratio between the horizontal and equivalent diagonal struts based on the work ofvertical applied loads and the infill characteristics: Mainstone [9]. The equivalent strut width is given by 1178 | P a g e
  2. 2. Sabri Attajkani, Abdellatif Khamlichi, Abdellah Jabbouri / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January -February 2013, pp.1178-1183 nonlinear concrete model with constant active H2  L2w  0.201  Ef Ic H0.1 (1) confinement modelling (con2), Fig.2. This enables H0.4  Ed ssin(2)  0.1 accurate uniaxial concrete behaviour descriptionwith where a constant confining pressure is assumed in order to take into account the maximum transverse  H    tan 1   (2) pressure from confining steel. This is introduced on  L  the model through a constant confinement factor, where s is the actual infill thickness that is used to scale up the stress-strain relationshipin contact with the frame, d the diagonal length of throughout the entire strain range. To enter thisthe infill, E d is the Young modulus of the infill concrete model during simulations, four parametersalong the diagonal, E f the Young modulus of the are required: compressive strength f c , tensilereinforced concrete, H and L are the height and the strength f t , crushing strain  co and confinementlength of the frame, and H and L are the height factor k .and the length of the infill as shown in Fig.1, finally Ic is the entire inertia moment of the cross-sectional area of the column. Figure 2. Uniaxial constant confinement concrete model The reinforcement steel behavior was assumed to be a bilinear elastic plastic model with kinematics strain-hardening (stl1), Fig.3. This modelFigure 1. Scheme of the infilled-frame showing the is applied for the uniaxial modelling of mild steel.equivalent strut median fibre To enter this model during simulations, three parameters are required: Young’s Modulus E , yield3. STATIC NONLINEAR PUSHOVER ANALYSIS strength  y and kinematic strain-hardening  .BY MEANS OF ZEUSNL SOFTWARE ZeusNL is an open source software package[11] which provides an efficient way to runstructural analyses such as conventional andadaptive pushover and nonlinear dynamic time-history. The modelling takes into account bothgeometric and material nonlinear behaviour.Common concrete and steel material models areavailable, together with a large library of elementsthat can be used with a wide choice of typical pre-defined steel, concrete and composite sectionconfigurations. The applied loading can includeconstant or variable forces, displacements andaccelerations. In the conventional pushover analysiswhich is used in the following, the applied loadsvary proportionally according to a predefined Figure 3. Uniaxial bilinear elastic-plastic law withpattern. The post-peak response is obtained with a kinematic strain-hardening modelling mild steeldisplacement control procedure. Modelling static pushover under ZeusNL Static pushover analysis was conducted bysoftware requires entering configuration of members taking the most adverse seismic direction when thesections, material properties, applied loadings and building structure is assumed to be a plane gatewayanalysis protocol. frame. Response control protocol was chosen to In the present analysis, the concrete monitor the nonlinear analysis. This refers to thebehaviour was chosen to be described by the situation where the displacement of the building roof 1179 | P a g e
  3. 3. Sabri Attajkani, Abdellatif Khamlichi, Abdellah Jabbouri / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January -February 2013, pp.1178-1183is specified by the user and is incrementally confined part of the beam Hc  600mm , effectiveincreased. The loading applied as well as the width of the compressed span B  1250mm , widthdeformations of the other nodes are determined by of confined part of the compressed spanthe solution of the program. Bc  1200mm , width of the beam b  300 mm and4. PRESENTATION OF THE CASE STUDY width of the confined part of the beam bc  250mm . A reinforced concrete building consisting Table 2 gives the steel reinforcements bar sectionsof a regular framed structure having four stories and and their positions on the transverse beam sections.three bays is considered. The inter-storey height is 3m , the bay length is 4m . Fig.4 shows the portalframe which is equivalent to this building whensubjected to static lateral equivalent loading alongthe most adverse seismic direction.Figure 4. Vertical elevation of the four-storeyreinforced concrete structure in the seismic direction Figure 6. Beams reinforced section; pushover is considered along the horizontal y-axis while x-axis All the columns are assumed to be identical is the other horizontal directionand all the beams equal. Fig. 5 and Fig.6 show Table 1. Steel reinforcements section and theirrespectively the columns and beams sections. locations in columns transverse sectionsColumns characteristics are: section height Section Distance d x Distance d y h  400 mm , height of the confined part mm2 mm mm h c  350mm , section width b  300 mm and width 255 125 175of the confined part bc  250mm . Table 1 gives the 127.5 0 175reinforcements sections and their locations. 127.5 125 0 Table 2. Steel reinforcements section and their locations in beams transverse sections Section Distance d x Distance d z mm2 mm mm 255 25 125 255 775 125 127.5 25 0 127.5 775 0 Material behavior for steel reinforcement bars is chosen to be such that E  2.11011 Pa ,Figure 5. Columns reinforced section; pushover is y  500 106 Pa and   0.05 . For confinedconsidered along the y-axis while x-axis is the other concrete, the following characteristics are assumedhorizontal direction to hold: fc  20 106 Pa , f t  2.2  106 Pa , Beam characteristics are as follows: co  0.002 and k  1.2 . The unconfined concrete iscompressed span height h  200 mm , height of the assumed to have the same properties as for confinedconfined part of compressed span h c  200mm , concrete except that k  1.02 . Material of struts that are equivalent to infills is assumed to be like that ofheight of the beam H  600mm , height of the concrete with the following properties: 1180 | P a g e
  4. 4. Sabri Attajkani, Abdellatif Khamlichi, Abdellah Jabbouri / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January -February 2013, pp.1178-1183fc  10 106 Pa , f t  1.1 106 Pa , co  0.001 andk  1.02 .5. RESULTS AND DISCUSSION The infill section is considered to beuniform over all the infilled stories of the building.Fig.7 gives the different configurations of infills thatare considered. These include the bare frame (a),variable level weak storey (b)-(e) and the completeinfilled frame (f). (7e)(7a) (7f) Figure 7: Considered frame infilled configurations; (a) bared frame; (b) bared first storey; (c) bared second storey; (d) bared third storey; (e) bared fourth storey and (f) completely infilled frame Fig.8 gives pushover curves, as they were computed by ZeusNL for the different infilled frame configurations and given infill section. These last have been varied in the set(7b) 100 100; 200  200; 300  300; 400  400 . This parametric study is intended to determine the effect of infill section for a given infilled configuration. It enables to answer the question about which weak storey will have a minor effect on seismic performance? Fig.9 gives pushover curves, but this time as function of the infilled configuration for the different infill section taken in the set 100 100; 200  200; 300  300; 400  400 . This is to determine for a given infilled section which configuration performs the best?(7c) 3.5E+05 3.0E+05 Base shear (N) 2.5E+05 2.0E+05 1.5E+05 1.0E+05 5.0E+04 0.0E+00 0 0.2 0.4 0.6 0.8 Roof drift (m) (8a)(7d) 1181 | P a g e
  5. 5. Sabri Attajkani, Abdellatif Khamlichi, Abdellah Jabbouri / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January -February 2013, pp.1178-1183 3.5E+05 3.5E+05 3.0E+05 3.0E+05 Base shear (N) Base shear (N) 2.5E+05 2.5E+05 100x100 100x100 2.0E+05 2.0E+05 200x200 200x200 300x300 300x300 1.5E+05 1.5E+05 400x400 400x400 1.0E+05 1.0E+05 5.0E+04 5.0E+04 0.0E+00 0.0E+00 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 Roof drift (m) Roof drift (m)(8b) (8f) 3.5E+05 Figure 8: Pushover curves as function of infill 3.0E+05 sections for the different infilled configurations; (a) 100x100 bared frame; (b) bared first storey; (c) bared Base shear (N) 2.5E+05 200x200 second storey; (d) bared third storey; (e) bared fourth 300x300 2.0E+05 400x400 storey and (f) completely infilled frame 1.5E+05 Fig.8 and Fig.9 show that infill has always 1.0E+05 a benefit effect of the lateral seismic behavior of the 5.0E+04 portal frame as the obtained capacities are always 0.0E+00 higher independently of where the infill has been 0 0.2 0.4 0.6 0.8 placed. As this can be seen from Fig.8b, if the first Roof drift (m) storey is not infilled, then there is no need to seek(8c) enhancing the seismic behavior of the building, by inserting infills in the upper stories. Also, as seen 3.5E+05 from Fig. 9a, if the infill quantity is not enough, only 100x100 insignificant changes will be observed on the 3.0E+05 200x200 300x300 capacities independently from where the weak Base shear (N) 2.5E+05 400x400 storey exists. The infill will affect in this case only 2.0E+05 the initial stiffness and insignificant variations 1.5E+05 appear in the lateral capacity. 1.0E+05 1.0E+05 5.0E+04 8.0E+04 0.0E+00 Base shear (N) 0 0.2 0.4 0.6 0.8 6.0E+04 Roof drift (m) Bared frame Bared first storey(8d) 4.0E+04 Bared second storey Bared third storey Bared fourth storey 2.0E+04 Complete filled frame 3.5E+05 3.0E+05 0.0E+00 Base shear (N) 2.5E+05 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Roof drift (m) 100x100 2.0E+05 200x200 (9a) 1.5E+05 300x300 1.6E+05 400x400 1.0E+05 5.0E+04 1.2E+05 Base shear (N) 0.0E+00 0 0.2 0.4 0.6 0.8 8.0E+04 Roof drift (m) Bared frame Bared first storey(8e) 4.0E+04 Bared second storey Bared third storey Bared fourth storey Complete filled frame 0.0E+00 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Roof drift (m) (9b) 1182 | P a g e
  6. 6. Sabri Attajkani, Abdellatif Khamlichi, Abdellah Jabbouri / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 1, January -February 2013, pp.1178-1183 2.5E+05 stories should be first infilled and the infill quantity Bared frame Bared first storey should be significant. Bared second storey 2.0E+05 Bared third storey The obtained results have shown also that Bared fourth storey some infill configurations with bared stories areBase shear (N) Complete filled frame 1.5E+05 more advantageous than the complete infilled frame 1.0E+05 in terms of ductility, while the highest stiffness is always achieved by the configuration where all the 5.0E+04 stories are infilled. 0.0E+00 REFERENCES 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Rood drift (m) [1] H. Holmes. Steel frames with brickwork (9c) and concrete infilling. Proceedings of the 3.0E+05 Institute of Civil Engineers 1961; 19:473- 478. 2.5E+05 [2] B. Stafford Smith. Behaviour of the square 2.0E+05 infilled frames. Journal of Structural Div.Base shear (N) ASCE 1966; 92:381-403. 1.5E+05 [3] R.E. Klingner, V.V. Bertero. Earthquake 1.0E+05 resistance of infilled frames. Journal of 5.0E+04 Bared frame Bared first storey Structural Engineering, ASCE 1978; Bared second storey Bared fourth storey Bared third storey Complete fillled frame 104:973-989. 0.0E+00 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 [4] T.N. Valiasis, K.C. Stylianidis, G.G. Roof drift (m) Penelis. Hysteresis Model for weak brick (9d) masonry infills in R/C frames under lateral Figure 9: Pushover curves as function of the infilled reversals. European Earthquake configuration for different infill sections; (a) Engineering 1993; 1:1–9. 100 100 ; (b) 200 200 ; (c) 300  300 ; (d) [5] T.B. Panagiotakos, M.N. Fardis. Seismic 400 400 ; (e) bared fourth storey and (f) response of infilled RC frames structures. completely infilled frame Proceedings of the 11th World Conference Some frame configurations with partially on Earthquake Engineering, Acapulco, infilled stories are more advantageous than the Mexico. Paper no. 225. Oxford: Pergamon, complete infilled frame in terms of ductility as this 1996. can be seen from Fig.8e, Fig.8f , Fig.9c and Fig.9d. [6] Applied Technology Council. NEHRP The bared fourth storey will have quantitatively Guidelines for the seismic rehabilitation of higher ductility than the complete infilled frame buildings. FEMA 273, prepared by ATC even if the initial stiffness shows the reverse (project 33) for the Building Seismic Safety behavior. This behavior can be beneficial if Council. Washington (DC): Federal confirmed by experimental tests in order to increase Emergency Management Agency; 1997. seismic performance of buildings. It can be assessed [7] Applied Technology Council. Evaluation of also through a dynamic modelling of the building, as earthquake damaged concrete and masonry irregularity from bared stories can have a drastic wall buildings - basic procedures manual. effect on the results that could not be assessed FEMA 306, prepared by ATC (project 43) through only nonlinear static analysis. for the Partnership for Response and Recovery. Washington (DC): The Federal 6. CONCLUSION Emergency Management Agency; 1998. The effect of infills on seismic performance [8] ASCE. FEMA 356 Prestandard and of reinforced concrete building was analyzed. This commentary for the seismic rehabilitation was achieved through using the concept of of buildings. Reston (VA): American equivalent compression diagonal strut that enables to Society of Civil Engineers; 2000. model the infill mechanical behavior. Considering [9] R.J. Mainstone. On the stiffnesses and regular buildings for which the seismic response can strengths of infilled frames. Proc Inst Civil be sought by means of the equivalent portal frame Eng 1971(Suppl. iv):57-90 [7360 S]. subjected to lateral static equivalent loads to seismic [10] B. Stafford Smith. Behavior of square action, pushover curves were derived by using infilled frames. Journal of Structural ZeusNL software package. Engineering, ASCE 1966:381-403. The obtained results have shown that infill [11] A.S. Elnashai, V.K. Papanikolaou, D.H. enhances always seismic performance. This Lee, 2008. Zeus NL A system for inelastic enhancement is however largely affected by the analysis: User Manual, Version 1.8.7, distribution of infill through the levels of the University of Illinois at Urbana Champaign, building stories. For infill to be beneficial, the lower Mid- America Earthquake Center. 1183 | P a g e