SEISMIC BEHAVIOR OF CONFINED MASONRY WALLS REINFORCED WITH WELDED STEEL AND DUCTILE STEEL

Transcription

1 15 th International Brick and Block Masonry Conference Florianópolis Brazil 2012 SEISMIC BEHAVIOR OF CONFINED MASONRY WALLS REINFORCED WITH WELDED STEEL AND DUCTILE STEEL San Bartolomé Angel 1 ; Quiun, Daniel 2 1 Professor, Pontifical Catholic University of Peru, Civil Engineering Department, asanbar@pucp.edu..pe 2 Professor, Pontifical Catholic University of Peru, Civil Engineering Department, dquiun@pucp.edu.pe An experimental research was performed to investigate the possibility of using industrially prepared welded steel bar assemblies in the columns of confined masonry buildings, instead of the conventional ductile deformed bars. The tests were performed at the Laboratory of Structures of the Catholic University of Peru. Six full scale confined masonry walls were built varying the type of steel assemblage in the confining columns. In three of these walls, ductile deformed bars were used, with 9.5mm, 12.7mm and 16.0 mm diameter. In the other three walls, welded steel bar assemblies were used, which are equivalent to the ductile ones. All the walls were tested under cyclic lateral load for comparison of the seismic behaviour. Among the conclusions obtained, concerning the failure type, a flexural failure was observed in two of the walls with less amount of vertical reinforcement in the columns; the other four walls had a shear failure predominantly. In all cases, the walls with equivalent steel assemblies had similar behaviour. In overall, it was found that it is possible the use of welded bar assemblies instead of the traditional bars, provided a similar tension yield force in the columns, which let us save around 16% in steel. Keywords: Confinements, reinforcement, columns, welded, assemblies Theme: Research and testing INTRODUCTION The Peruvian Masonry Code (Sencico 2006) specifies the use of ductile steel, with a minimum elongation of 9% as structural reinforcement in masonry constructions. In the case of welded steel bars this specification is not satisfied. However, an experimental research was conducted to study the effects of welded steel bars assemblies over the strength, stiffness and ductility of the confined masonry walls subjected to seismic loads. Such bar assemblies can be prefabricated and could increase the speed of construction of masonry buildings. The tests were performed at the Laboratory of Structures of the Catholic University of Peru. For this research, six full scale confined masonry walls were built with variations in the type of steel bars assemblies in the columns. For walls W1, W3 and W5, four ductile bars of 9.5mm (3/8 ), 12mm (1/2 ) and 16mm (5/8 ) were respectively used, as they are usual reinforcement bars in masonry buildings. For the other walls, W2, W4, and W6, welded steel bar assemblies were used with areas equivalent to the ductile ones. All six walls were subjected to cyclic lateral loads in order to compare the seismic behaviour of both types of reinforcement used in the confining columns. NOMINAL CHARACTERICTICS OF MATERIALS In this project good quality materials were used, with the objective that no other parameter will influence in the one under study. The bricks were clay solid units, of class IV according

2 to the Masonry Code. The mortar was prepared with a 1:4 cement: sand volumetric proportion. Concrete for confinement elements was specified to reach a resistance of f c=17.5 MPa with a maximum aggregate size of 12mm (1/2 ). The ductile steel bars were grade 60 with yield stress of 420 MPa, while the welded bar assemblies were brought from the factory and have a yield stress of 500 MPa. REINFORCING STEEL BAR ASSEMBLIES The confining columns in all cases were rectangular of 130x200 mm, and contain the bar assemblies. The 4 reinforcement bars in walls W1, W3 and W5 were ductile with fy=420 MPa, with 6mm (1/4 ) tie bars. The 4 reinforcement bars in walls W2, W4, and W6 were welded with fy=500 MPa, with 5.5mm tie bars. By using the same yield tension force, the equivalent area for the last 3 walls was obtained, and summarized in Table 1. A reduction of about 16% in steel area was attained in the welded bar assemblies. Table 1: Reinforcement bars for Columns W1 W2 W3 W4 W5 W6 Longitudinal bars 4-9.5mm 4-8.7mm 4-12mm 6-9.5mm 4-16mm 6-9.5mm mm Ties 6 mm 5.5 mm 6 mm 5.5 mm 6 mm 5.5 mm Type Ductile Welded Ductile Welded Ductile Welded W1 and W2 equivalent W3 and W4 equivalent W5 and W6 equivalent The 6mm ties for the ductile steel were spaced 1 at 50 mm, 4 at 100mm and rest at 200mm. These ties had 1 ¾ loop so that they let the concrete pouring easily (Figure 1-upper left), and small diameter wire #16 was used to fix them. The 5.5 mm ties for the welded steel bars have a fixed spacing of 200 mm, so at the column ends extra 5.5 mm ties at 200 mm were added to satisfy the Code specification of 100 mm maximum spacing between ties (Figure 1-upper right). Some defects in welding points and the ties were observed in the assemblies for walls W4 and W6, which were corrected prior to construction. Welding Wire #16 Hook 135 º W1, W2, W3, W5 W4 W6 Figure 1: Reinforcement bar assemblies

3 WALL CHARACTERISTICS AND CONSTRUCTION In Figure 2, the wall characteristics are depicted. All had the same geometry and materials, with exception of the confinement column assemblies. The walls are a representation of the first story walls in a 3-story or more building. In that sense, according to the Masonry Code, continuous horizontal reinforcement was included (one 6mm bar every two layers) to give a minimum ratio of Figure 2: Common characteristic of the walls (dimensions in cm) For the six walls the same hand labour was used and the same construction sequence (Figures 3, 4 and 5). In the foundation the column bar assemblies were anchored. The bricks were watered during a half hour the afternoon prior to construction to reduce their natural suction. The masonry walls were completed in two days. The mortar used was 1:4 cement : sand, of 15 mm joints and a vertical connection between the columns and the wall. The horizontal reinforcement was placed in the bed joints at this time. Figure 3: Bending 90 the welded column bars (left), and horizontal reinforcement in the layer (right)

4 Figure 4: Vertical connection column-wall (left), collar beam for W6 (centre) and three of the constructed walls (right) One day after the masonry was completed, the column forms were installed and concrete was poured and vibrated. Lastly, the collar upper beam was built. All the RC elements were cured for over 3 days, using a hose. No voids were noticed in the columns. WALL TESTING A set of LVDT displacement instruments were placed to record the wall response. The cyclic lateral load was applied by controlling the topmost displacement D1, in several steps of increasing value (Table 2). Each step consisted in several cycles until the hysteretic loop is stabilized. No vertical load was applied. Table 2: Test steps Step D mm Cycles The Peruvian Seismic Code (Sencico 2003) establishes that for a wall to be repairable, its maximum drift should be limited to (1/200), which corresponds to step 7 of the cyclic test (D1=12mm). However, in order to take into account the possibility of the effects of vertical load that reduces the wall ductility and the damage generated by intensive seismic vibrations, the test was extended up to step 10, D1=20 mm. WALL BEHAVIOR Wall W1 which had the lowest amount of steel, exhibited a combined failure of flexure and shear friction in the lower end of the columns (Figure 5). Equivalent wall W2 had also a flexure failure that ended in shear friction at the base (Figure 6). Both walls reached up to step 9 with the buckling of the vertical reinforcement of the columns, as inspected after the test, so it can be concluded that they had similar behaviour. Wall W3 had a combined failure by a diagonal shear cracks and shear friction over the first layer (Figure 7). Equivalent wall W4 had also diagonal shear cracks and shear friction over the base (Figure 8). Both W3 and W4 reached step 10 of the test with severe damage to the columns end but no buckling of the vertical reinforcement. Therefore it can also be concluded that these two walls had similar seismic behaviour. Wall W5 had a diagonal shear failure crack, and at the end of the test (step 10) only the upper joints had crushing, and the bottom ends were in good shape (Figure 9). The equivalent wall W6 behaviour was very similar to that of wall W5 (Figure 10).

5 Figure 5: Wall W1 after step 9 and further inspection Figure 6: Wall W2 after step 9 and further inspection Figure 7: Wall W3 during step 10

6 Figure 8: Wall W4 during step 10 Figure 9: Wall W5 during step 10, see upper joint Superior Joint in M6 Figure 10: Wall W6 during step 10, see upper joint The hysteretic loops of the lateral load V and horizontal displacement D1 for the six walls are given in Figure 11. It can be noted that regardless the steel bar assembly in the confinement columns, the loops are closed and go toward the origin. This behavior is typical of systems that have low energy dissipation and that have degrading strength and stiffness. Similar wall behavior can be seen in older projects by San Bartolome (2007).

7 W1 W2 W3 W4 W5 W6 Figure 11: Hysteretic loopsv-d1 for walls W1 to W6 The comparison on the seismic behavior between similar walls, W1-W2, W3-W4, and W5- W6 is given in the following. In Figure 12 the hysteretic envelope is displayed regarding the stable cycle of each step and with the more critical branch. In this case it was the positive branch because it had the larger strength degradation. In Figure 12 up, it is clear that behavior of walls W1 and W2 do not differ. Therefore, in such cases of flexural failures that end as shear friction, it would be the same to use ductile or welded assemblies for the columns. Respect to walls W3 and W4, Figure 12 center, shows that wall W3 with ductile steel has 20% larger capacity, from the early elastic range in which the reinforcement has insignificant effects. It can be said that the difference could be due to the masonry dispersion rather than the steel assembly. Therefore, in such cases of diagonal shear failure of masonry, the use of either steel assembly would provide similar behavior; notice that both walls could reach the same level of inelastic deformation.

8 The comparison between walls W5 and W6 (figure 12 bottom) indicates a similar behavior up to step 8 (D1=15 mm). A horizontal crack appeared in W6 at step 8 that produced a decrease in strength, but the same problem happened to wall W5 in step 9, so both walls reached the same level o resistance in step 10. The failure in both walls was in the masonry, so the reinforcement bar assemblies (ductile or welded) behaved similarly. W1 W2 W3 W4 W5 W6 Figure 12: Envelope curves for similar walls MATERIAL PROPERTIES The bricks used were solid with holes near to 30% in the bed area. Their nominal dimensions are 240x130x90 mm and the compressive strength is f b=17 MPa. They classified as IV according to the Peruvian Masonry Code. Three masonry prisms were built for test under axial compression, leading a strength of f m=11 MPa. Also three wallets of 600 mm side were built giving the shear strength value of 1.05 MPa. The concrete of the confining columns was found that the compressive strength was f c=17.6 MPa, very similar to the nominal design value.

9 The welded steel bars were subjected to tension tests, with a 400mm length bar welded at the mid length to a 5.5 mm short bar to simulate the tie bars (Figure 13). Two samples were tested of each diameter and the average results are shown in Table 3, and the loaddeformation curves of Figure 14. Figure 13: Tested steel bars welded (left) and ductile (right) Table 3: Steel bar tension test results Diameter Steel Type Yield stress Maximum Elongation % MPa stress MPa ¼ or 6 mm Ductile /8 or 9.5mm Ductile ½ or 12 mm Ductile /8 or 16 mm Ductile mm Welded None mm Welded None Axial stress MPa Displacement mm Figure 14: Tested steel bars

10 CONCLUSIONS The welded bar assemblies used were of good quality, if some placement or welding defects are found, these must be corrected before the construction. The welded bars used in this research showed 6% elongation, which is close to the required 9% for ductile bars of the Masonry Code. Regarding walls W1 and W2, they had a flexural failure which is quite unusual in masonry walls of real buildings. Anyhow, their behavior was similar all the test. The walls with larger reinforcement, W3, W4, W5, and W6, had diagonal shear failures, which is the usual way of confined masonry. The corresponding pairs of walls (W3-W4 and W5-W6) had similar behavior regardless of their internal reinforcement assemblies in the columns. All four walls showed a ductile behavior, reaching drifts larger than the repair limit of Finally, under the limitations of the tests performed, it can be concluded that it is possible to use welded bar assemblies in the construction of the columns of confined masonry walls, in replacement of the traditional ductile bar assemblies. The area of the welded bars could be obtained using the maximum tension stress, to provide the same tension force as the ductile bars area with their corresponding yield stress. ACKNOWLEDGEMENTS The authors wish to thank PRODAC factory who provided the financial support for this work. REFERENCES San Bartolome, A. Masonry research Blog (Blog de Investigaciones en Albañileria in Spanish) (in Spanish) SENCICO, Peruvian Seismic Code (Norma E.030 Diseño Sismorresistente in Spanish), National Building Code, SENCICO, Peruvian Masonry Code (Norma E.070 Albañileria in Spanish), National Building Code, edition of 2007.