Strengthening of RC framed structure with masonry infill

Transcription

1 Strengthening of RC framed structure with masonry infill ADRIANA SCURT, CORNELIU BOB, SORIN MĂRGINEAN, DAN DIACONU Civil Engineering Department Politehnica University of Timisoara Timisoara, Traian Lalescu Street, no 2 ROMANIA adriana.scurt@yahoo.ro, cbob@mail.dnttm.ro, sorin.marginean@gmail.com, dan.diaconu@ct.upt.ro Abstract: The paper presents the experimental tests on strengthened RC framed structure as well as on framed structures with masonry infill. The behavior of the framed structure, without and with masonry infills, strengthened with carbon fibers products is analyzed. The strengthening will affect with positive implication the damaged elements: the horizontal load, the stiffness and the total ductility are the same for the initial structure and for the strengthened damaged structures at service limit states. Key-Words: RC framed structures, masonry infill, experimental program, strengthening of damaged structures, carbon fibers products, structure stiffness, structure ductility, shear resistance. 1 Introduction Reinforced concrete structures and/or composite structures are to be repaired/strengthening in cases when the general damage is limited and demolished when structural safety is greatly affected and the rehabilitation cost is very high [1]. Strengthening of reinforced concrete/composite structures takes into account the increase of strength, stiffness and ductility. In the case of reinforced concrete framed structures the increase in stiffness and ductility is to be achieved by jacketing of beams, columns and joints. The jacketing is performed by reinforced concrete, steel profiles, carbon fiber reinforced polymer (CFRP), etc. CFRP may be used for increasing ductility and slightly increasing the stiffness. For composite and masonry structures the increase of bearing capacity is obtained by: erection of RC cores at appropriate distances, combined (if necessary) with straps at each level, masonry lining with reinforced concrete, masonry lining with reinforced concrete, masonry confinement with steel profile, utilization of CFRP strips at appropriate distances etc. [2], [3]. In the case of reinforced concrete frame wall structures the increase of bearing capacity is obtained by coating the core, the flange and the coupling beam. Experimental program reported in present paper is dedicated to the behavior of the RC framed structure, without and with masonry infill, strengthened with carbon fibers products. The reference RC frame was with one span and one level without masonry infill and the composite structure was with coupled masonry infill (solid bricks and bricks with vertical hollows). The structures were loaded with a constant vertical forces and horizontal actions which were applied in two directions of the frame plan. Experimental horizontal action and the drift were measured. Similar determinations concerning behavior of RC frame with masonry infill walls were reported in some papers [4], [5], [6], [7]. On the other hand the strengthening of RC frame buildings with masonry infill walls was theoretically treated in many works but tests of such structures are only few. 2 Experimental program The tests were performed on the same structures as it was reported in the paper The Influence of Masonry Infill on RC Framed Structure Behavior [6]. The geometrical dimensions, reinforcement of structural elements, loaded models and arrangement of instruments are given in citated work of the authors. The photos with the RC framed structure, framed structure with masonry infill, strengthened framed structure with CFRP materials and frame with masonry infills strengthened by using CFRP wrap layers are presented in Figures 1, 2, 3 and 4. ISBN:

2 Fig.1 Reference RC framed structure Fig.3 Strengthened framed structure with CFRP materials Fig.2 Framed structure with masonry infill The strengthening solution adopted was based on carbon fiber polymer composites (CFRP) as it is illustrated in Figures 3 and 4. For RC frame the columns were strengthened by longitudinal Sika Carbodur S 512 strips, on each side of 50 mm width and 1.2 mm thickness. As shear strengthening a single layer of SikaWrap -230C/45 closed jacked was used on 0.40 m height at the ends of the column; the sheets had 600 mm width and 0.12 mm thickness (Fig.3). In the case of composite structures, the walls were strengthened on both faces, by a single layer of SikaWrap -230C/45; three layers of Sika Wrap of 600 mm width and 0.12 mm thickness were used on each face (Fig.4). CFRP material characteristics used for strengthening are E f = 231 KN/mm 2 and Ԑ fu = The bond of Sika wrap materials to the existing materials (concrete and brick) was ensured by specific adhesives (Sikadur 30 for CFRP longitudinal strips and Sikadur 330 for CFRP sheets). The longitudinal layers of Sika wrap bonded on masonry were anchored by bond fixing on the beams of the RC frame (100 mm). Fig.4 Framed structure with masonry infill strengthened by using CFRP wrap layers 3 Test results The experimental program was done on two types of structures: reference frame without masonry infill, before and after strengthening; the frame with coupled masonry infill, before and after strengthening. The masonry was made of bricks with vertical hollows. The structures were loaded with vertical forces and horizontal actions in the two directions of the frame plan; the horizontal actions were applied in many steps till the maximum experimental value. The shear resistance and the drift were measured. In the Figures 5 and 6 the correlation between horizontal action and deflection at the top of the frame in both horizontal direction for the four tests is presented: reference frame and strengthened frame (Fig.5); reference frame, frame with coupled masonry and frame with coupled masonry after strengthening (Fig.6). ISBN:

3 Fig.5 Reference frame and strengthened frame Fig.6 Reference frame, frame with coupled masonry and frame with coupled masonry after strengthening The maximum horizontal actions were established function of interstorey drift limitations (EN ) as follows: (1) d ra SLS = xh = x1725 = mm for buildings having ductile non-structural elements, and (2)d ra SLS = 0.01xh = 0.01x1725 = mm for buildings having non-structural elements fixed in a way so as not to interfere with structural deformations. The most important facts which emerge from experimental determinations for horizontal load applied in the first step from left to right are: a) Horizontal load for the tested structures are in function of the interstorey drift such as: 36 kn for reference frame, 42 kn for strengthened frame, 78 kn for frame with masonry infill, 81 kn for strengthened frame with masonry infill, all at d ra SLS 13 mm for reference frame and d ra SLS 3.55 mm for frame with masonry infills; the maximum horizontal load was 39 kn for reference frame, 54 kn for strengthened frame and approximately 80 kn for not strengthened and strengthened frame with masonry infill; b) The stiffness calculated as ratio between the lateral load and the limited interstorey drift (as the secant modulus of elasticity) are: 2.71 kn/mm for reference frame and 2.76 for strengthened frame, 20,21 kn/mm for unstrengthen and kn/mm for strengthened frame with masonry infill; c) The ductility of the structures, calculated as the surface inside of charge-discharge curves, for the horizontal action applied in the first step of the cycle are: 274 knmm for the reference frame, 222 knmm for strengthened frame and 179 knmm for not strengthened and 143 knmmm for strengthened frame with masonry infill. The smaller ductility of the strengthened frame was due to elastic behavior at discharge branch (Fig.5); d) The aspect of the experimental elements, before and after strengthening are relevant for studied structures: frame with a lot of cracks after loaded (Fig.7), frame with masonry infill after first loaded (Fig.8), strengthened frame after second loaded (Fig.9), strengthened frame with masonry infill after second loaded (Fig.10). ISBN:

4 Fig.7 Frame with a lot of cracks after loaded Fig.10 Strengthened frame with masonry infill after second loaded The results for the horizontal actions applied in both directions and the average are presented in Table 1. Fig.8 Frame with masonry infill after first loaded Table 1 Frame Load Main characteristcs F [kn] k [kn/mm] D [knmm] Reference L-H frame R-H Mean Reference L-H frame R-H strengthened Mean Frame with L-H coupled R-H masonry Mean Frame with L-H coupled R-H masonry after Mean strengthening Legend: L-H : load from left hand R-H : load from right hand F: horizontal load; K: structure stiffness; D: structure ductility Fig.9 Strengthened frame after second loaded ISBN:

5 4 Conclusions From the experimental tests some useful ideas can be underlined: i. The strengthening of the structures will affect with positive implications the damaged elements. The horizontal load of strengthened frame is 1.34 times higher as compared with reference frame and 1.19 times higher for strengthened frame with masonry infill as compared with the not strengthened frame. ii. The stiffness of strengthened frame is only 1.13 higher compared with reference frame but the structure stiffness of the frame with coupled masonry (strengthened and not strengthened) is much higher as compared with reference frame (4.79 time and respectively 6.8). iii. The mean of the total ductility for the four structures are not very different. A very high ductility was noticed for the second step of loading (load from right hand) due to of damaged structure caused by the first step of loading. References: [1] Paulay, T., A Re-definition of the Stiffness of Reinforced Concrete Elements and its Implications in Seismic Design, Structural Engineering International, Vol. 11, No. 1, 2001, p [2] Bob, C.,Progress in Structural Engineering and Materials, vol. 3, p , Published by John Wiley and Sons, [3] Bob, C., Progress in Structural Engineering and Materials, vol. 6, p , Published by John Wiley and Sons, [4] Abrams, D.P., Proceedings of the NCEER Workshop on Seismic Response of Masonry, National Center for Earthquake Engineering, 1994 [5] Griffith, M., Seismic Retrofit of RC Frame Buildings with Masonry Infill Walls: Literature Review and Preliminary Case Study, JRC Scientific and Technical Reports, 2008 [6] Mărginean, S., Bob, C, Scurt, A., Gruin, A., The Influence of Masonry Iinfills on RC Framed Structure Behavior, European Conference Of Civil Engineering(ECCIE 12), 2012 [7] Mehrabi, A.B., Benson Shing, P., Schuller, M.P., Noland, J.L., Performance of Masonrz Infilled RC Frames under In-Plane Lateral Loads, Structural Engineering and Structural Mechanics Research Series, University of Colorado at Boulder, 1994 ISBN: