BEHAVIOUR OF CONFINED MASONRY WALLS WITH INTERMEDIATE REINFORCED CONCRETE BEAMS

BEHAVIOUR OF CONFINED MASONRY WALLSWITH INTERMEDIATE REINFORCED CONCRETEBEAMS SUBJECTED TO LATERAL LOADSR. Barragán, S. Sánchez & R. ArroyoUnidad Académica de Ingeniería, Universidad Autónoma de Guerrero, MéxicoF. J. SánchezCivil engineer by Universidad Autónoma de GuerreroSUMMARY:A confined masonry wall with an intermediate beam tested under lateral cyclic loading is presented. In this model, all reinforced concrete elements had identical cross section. In order to take into account typical longitudinal and transversal reinforcement commonly used in masonry constructions, the model was reinforced according to local practice in Mexico. Resistant mechanism was identified through the analysis of recorded and observed results. Structural capacity was assessed on strength, stiffness, deformation capacity and energy dissipation terms. The experimental response of two walls subjected to quasi-static loads was compared each other. Results indicate that, compared with a wall using an intermediate beam, a wall without such a structural element is seismically more suitable since it shows, at the same level of lateral distortion, better initial stiffness besides to an improved lateral load capacity and low structural damage levels.Keywords: confined masonry walls, solid clay bricks, lateral cycling loading.1. INTRODUCTIONThe Mexican Pacific Coast has the highest seismic risk in México; particularly the Guerrero Gap, located along the Guerrero State coast, is a subducting area where no major earthquakes have been produced for almost nine decades. Therefore, it has a high probability of occurrence for an earthquake magnitude greater than 8.Among civil infrastructure systems, low-cost housing is the most vulnerable by both its non-technical and auto-construction nature. Confined masonry is a traditional method for low-cost housing projects even in seismic regions. It consists of load-bearing walls surrounded by small cast-in-place reinforced concrete columns and beams. Actually, local constructors are increasing the use of intermediate beams. This structural element is widely used for masonry housing, principally in the southern region of México. Nevertheless, Mexican building codes do not recommend using such kind of element on confined masonry walls. Therefore, this paper aims to identify experimentally the seismic performance of this local structural modified system.The study is focalized on handmade solid clay brick walls, as it has been widely tested in national and local research (Institute of Engineering of the National University Autonomous of Mexico, II-UNAM; National Center for Disaster Prevention, CENAPRED; and Academic Unit of Engineering of Autonomous University of Guerrero, UAI-UAGro).2. EXPERIMENTAL PROGRAMThe experimental program was carried out at Laboratory of Structures of UAI-UAGro, aiming to evaluate the mechanical behaviour of masonry walls with intermediate beam under lateral loads. Masonry materials were also characterized (units, mortar and masonry as composite material).2.1. AntecedentsIn Mexico, the II-UNAM and CENAPRED are the leading institutions in experimental investigation of masonry elements and systems, which have been the basis of changes to Mexico City Code. Studied issues include: quality mortar and units, wall dimensions, type and amount of reinforcing steel in both masonry and confining elements; degree of coupling between walls, vertical load application, strength and stiffness degradations to alternating loads, energy dissipation capacity and ductility, and strengthening of openings.Building construction based on confined masonry walls is a method widely used in countries located in seismic zones in Latin America and to a lesser extent in some Asian and European countries. There is evidence of such structural systems primarily in Chile, Argentina, Peru, Colombia, Ecuador, Mexico, Costa Rica, Guatemala, Nicaragua, Honduras, El Salvador, Italy, Greece, Slovenia, Indonesia, China, among others (Moroni et al., 1994; Iiba et al., 1992; Tomaževič et al., 1996; Alcocer and Meli, 1995).In Peru, Pastorutti and San Bartolomé (1995) tested four confined masonry walls of 2.4 x 2.4 m subjected to cyclic loading; the objective was the effect of horizontal reinforcement in seismic behaviour. They built a control wall without horizontal reinforcement in masonry panel, MR1, while in MR2 steel was concentrated at middle height of wall in a reinforced concrete element. This last was considered for comparison with this work.Beginning in 2005 at UAI-UAGro, Muñoz (2007), Romero and Ortega (2007), Sánchez (2009), Delgado (2010) and Pastrana (2011), have developed experimental tests on masonry. In all cases they used mortar in proportion 1:3 by volume (cement-sand) and units from Atliaca, Guerrero.2.2. Description, construction and instrumentation of modelThere is no information in Mexico about tests of masonry walls with intermediate beam, so this work developed at Laboratory of Structures of UAI-UAGro, intend to lay the groundwork for comprehensive studies of masonry houses with intermediate beam (reinforced concrete element with the same characteristics than confining elements).Due to load testing equipment available at UAI-UAGro, to the experimental tests carried out in CENAPRED and II-UNAM, to studies developed at local level and to physical characteristics of the masonry walls of affordable housing representing in Guerrero State, in this paper was proposed a wall of 250 x 190 cm for length and height, made on solid clay bricks and dimensions intermediate beam of 12 x 18 cm. It’s foundation consisted of a reinforced concrete structural element of 0.5 x 0.7 x 2.9 m in height, width and length, respectively, which has three holes from the bottom to the top of 25 x 25 cm, distributed along the element , which have the function of allowing the attachment of vertical confining elements.Construction process was carried out according to common practice, as might be expected, the inclusion of a reinforced concrete element at middle height do not permit continue immediately with the upper wall (Fig. 1).The concrete elements have four longitudinal bars 9.53 mm diameter yield stress fy=46 MPa. Besides, stirrups 6 mm diameter and yield stress fy=25 MPa were placed every 20 cm. Eight and five stirrups were placed at ends of columns, down and up, respectively.Four linear variable differential transformers, LVDT, displacement capacity equal to 30 mm measured the wall displacements. Two LVDT measured the shortening-extension of columns and two LVDT measured the lateral displacement on top of the wall. Pressure transducers connected to the actuators hoses measured indirectly the load (Fig. 2).Figure 1. Construction process of confined masonry wall with intermediate beamFigure 2. Foundation slab, load frame and instrumentation of wall3. TEST RESULTS3.1. Hysteretic curveThe hysteretic curve in terms of lateral load and lateral distortion are shown in Fig. 3. It is also drawn on it, the strength prediction using the Mexico City Code requirements, MCC, (Gobierno, 2004), and envelope curve. Experimental resistance developed was 35% higher with respect to the theoretical.Cycles within elastic limit experienced some hysteresis attributed to wall flexural cracking at initial stages. As it is common in confined masonry structures, specimens attained their strength to higher loads than those that are associated to the first inclined cracking.3.2. Stiffness and rotationTo assess stiffness degradation, peak-to-peak stiffness, K, it was calculated. Fig. 4 shows the initial stiffness value, K 0, and its variation, K/K 0. Stiffness decay was observed at low lateral distortion, even before first inclined cracking became visible. This phenomenon is attributed to incipient wall flexural cracking, and perhaps, to some micro-cracking in masonry materials, local loss of mortar bond and adjustment of brick position. After first inclined cracking, but before reaching the strength, the decay increased with the lateral distortion. Fig. 5 shows the rotation of the wall, which was determined from the displacement transducers located on the axis of the columns.-90-60-3003060-0.9-0.6-0.30.00.30.60.9L a t e r a l l o a d [k N ]Lateral distortion [%]Figure 3. Hysteretic behaviour and envelope curveFigure 4. Stiffness degradation4. COMPARING RESULTSComparisons between a masonry wall with intermediate beam and the works done by Pastorutti and San Bartolomé (1995) and Sánchez (2009) are presented. As each specimen was subjected to a particular experimental program, to facilitate comparison among these specimens, three limit states were defined: elastic, E, (first diagonal cracking in the masonry wall); maximum or strength, M, (maximum base shear was resisted); and ultimate, U, (highest lateral distortion).For identification purposes, establishes the following classifications: M1, confined masonry wall without intermediate beam, made of solid clay bricks (Sánchez, 2009); M1DI, confined masonry wall with intermediate beam (identical geometry and materials to M1); MR2, confined masonry wall with horizontal reinforcement concentrated at middle height, made of industrial bricks (Pastorutti and San Bartolomé, 1995).MCC MCCFigure 5. Rotation of the wall4.1. Failure modeFig. 6 shows the final state of damage in the three walls. It is observed that the failure mode was governed by shear deformations of masonry walls, regardless of presence of intermediate beam. It is appreciated more damage in M1 and MR2 than M1DI because were subjected to more cycles, inducing in them more distortions and lateral loads.MR2 M1 M1DIFigure 6. Final state of damage in wallsGenerally develop cracks initiated at approximately in central zone of walls, which are propagated towards corners with increasing lateral load application, once beam-column joints are cracked, it is reached to the maximum resistance of wall, then have relatively large displacements without significant increase in lateral load.In both cases where there are intermediate beam, when cracks extend from central area of wall to ends, they found the reinforced concrete beam, so that cracks propagate horizontally to intercept columns, at that moment, there are a masonry wall with two large blocks (one above and one below to intermediate beam), then to increase lateral load shows more damage in lower (M1DI).K 0 = 629.80 kN/cmK 0 = 99.94 kN/cmK 0 = 1537.23 kN/cm M1M1DI MR24.2. Response envelope and stiffness degradationFig. 7 shows the response envelope for each model in terms of the lateral force against distortion of wall, also values of initial lateral stiffness of each wall. Table 1 presents the above limit states for the models, which are expressed: maximum and minimum values for lateral load and distortion, and lateral stiffness degradation expressed as the coefficient of stiffness in the limit state and initial stiffness of wall.MR2 exhibited an initial stiffness and lateral force capability very high compared with M1, which in turn has greater values than M1DI. M1 and M1DI are completely identical in geometry and materials (pieces of bricks, sand, gravel and steel reinforcement), the only difference between them was manpower and industrialized masonry units which were used in MR2.-300-200-1000100200300-0.6-0.30.00.30.6L l a t e r a l L o a d [k N ]Lateral distortion [%]Figure 7. Response envelopesThe final state of damage is consistent with stiffness degradation shown in Table 1, for example, at final test M1DI was 32% of its initial stiffness, while M1 and MR2 were 27 and 18%, respectively. Table 2 provides a summary for comparison and justification of the seismic behavior of each wall, the geometry and properties of materials used in the construction of each wall.Table 1. Limit states of the wallsModelLimit state Lateral load [kN] Lateral distortion [%] K/K 0 Minimum Maximum Minimum Maximum M1Elastic -52.09 70.53 -0.04 0.07 0.67 Strength -87.28 100.94 -0.29 0.35 0.27 M1DIElastic -52.09 45.62 -0.15 0.19 0.51 Strength -66.79 69.36 -0.29 0.45 0.32 MR2 Elastic161.87 0.05 0.81 Strength 250.35 0.43 0.18CONCLUSIONSBecause there is no prior information about behaviour of confined masonry walls with intermediate beam subjected to lateral loads, and differ with the closest comparison (a confined masonry wall with horizontal reinforcement placed at middle height on a concrete element), then results must be taken with reservation.Table 2. Geometrical and mechanical properties of materials of the wallsConcept M1, M1DI MR2 Dimensions of wall, length x height, cm 240 x 190 240 x 240Test type Static: lateral cyclic loads Manufacturing of units Handmade IndustrialMortar proportion: cement-lime-sand 1:0:3 1:1:5Concrete proportion: cement-sand-gravel 1:2:3 Compressive strength of masonry units, MPa 9.4 (30 units, CV=17%) 10.15 (5 units) Compressive strength of mortar, MPa 28.0 (31 cubes, CV=11%) 12.0 (41 cubes) Compressive strength of concrete, MPa 22.0 (15 cylinders) Compressive strength of masonry, MPa 4.5 (12 prisms, CV=20%) 8.13 (3 prisms, CV=8.6%) Diagonal compressive strength of masonry, MPa 0.7 (12 walls) 0.88 (3 walls, CV= 45) Comparing M1 and DIM1 (identical in material and geometry, except for manpower and the time of year they were built), is concluded that walls without intermediate beam are more appropriate, since for the same level lateral displacement, M1 showed the higher values in stiffness and lateral load capacity, exhibiting thus less damage.Direct comparison between M1DI and MR2 is not suitable since, even though both are confined masonry walls, M1 has an intermediate beam with longitudinal and transverse reinforcement identical to confining elements, on the other hand, MR2 has a concentrated amount of horizontal reinforcement of 0.16% in a concrete element, located at middle height of wall. Both specimens differ in quality of the pieces, mortar and concrete used.The author MR2, built and tested three more models, where the study variable was the effect of horizontal reinforcement (a confined wall with two columns and two beams, another confined and reinforced wall with 2 bars 6.35 mm diameter every 3 rows, and one more confined and reinforced wall with bars of the same size every 6 rows). They concluded that is not desirable to concentrate all the horizontal reinforcement in intermediate zone of wall, it generates concentration of stress in anchorage zone with columns.REFERENCESAlcocer, S. M. and Meli, R. (1995). Test program on the seismic behavior of confined masonry structures, The Masonry Society Journal, Vol 13, Num 2, 68-76.Delgado, D. (2010). Comportamiento de muros de mampostería confinada de tabique rojo recocido ante cargas laterales, Universidad Autónoma de Guerrero, Mexico.Gobierno del Distrito Federal (2004). 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