List of publications
Publication: The behavior of infill walls in RC frames under combined bidirectional loading (2016)
Journal of Earthquake Engineering
Preparation and upload by:
Filip Anic, Faculty of Civil Engineering and Architecture Osijek, Josip Juraj Strossmayer University of Osijek
List of test setups
Publication abstract (click to enlarge):
Publication abstract (click to shrink):
Large-scale reinforced concrete frames with five different infill conditions, namely three single-leaf, two double-leaf walls with and without Z-ties, and a bare frame were tested. Under progressively increased in-plane actions, as the main aspect the behavior of walls under simultaneously acting monotonic out-of-plane loads was investigated. All infill walls have doubled the initial stiffness
Large-scale reinforced concrete frames with five different infill conditions, namely three single-leaf, two double-leaf walls with and without Z-ties, and a bare frame were tested. Under progressively increased in-plane actions, as the main aspect the behavior of walls under simultaneously acting monotonic out-of-plane loads was investigated. All infill walls have doubled the initial stiffness and energy absorption capacity of the frames, and the ultimate strength has increased about 50%. Z-ties have helped the double-leaf wall to maintain its integrity up to higher inter-story drift levels, and improve the out-of-plane stability of the wall.
Tests were done in order to assess a potential technique to improve the out-of-plane behavior of single- and double-layer infills under combined in-plane and out-of-plane loadings.
AIM AND SCOPE
The main objective of this study is to determine the behavior of the five different infill wall types under combined biaxial loading. More specifically, under regularly increasing cyclic in-plane drifts, out-of-plane stiffness, lower-bound out-of-plane strength of walls corresponding to specific in-plane drifts, and the out-of-plane stability issues are investigated. The other objective of the study is to observe in-plane interaction between reinforced concrete frame and commonly used infill wall types in Turkey.
DESIGN AND CONSTRUCTION OF THE TEST SPECIMENS:
Geometry and reinforcement details shown in Fig. 3 have identical columns with a height of 240 cm, and a cross-section of 250 x 400 mm2. The clear span of the beam was 320 cm and beam has a cross-section of 250 x 400 mm2. Longitudinal reinforcement ratios of column and beam sections were 0.012 and 0.006, respectively. Slab also contained 6F12 longitudinal reinforcements. The grade of the reinforcement and the target compressive strength of concrete were S420 and 25 MPa, respectively. In order to obtain confined beam-column joints, and to guarantee that the joints are not the first parts to be damaged, the joints were constructed so that they possess transverse beams at the beam columns joint locations. Test frames were constructed in Dokuz Eylul University, Structural Engineering Laboratory, and ready-mixed concrete was used. The frames were assumed identical regarding geometrical and mechanical aspects, and infill walls were constructed. after the reinforced concrete frames were cast. Plaster on both sides of infill with a thickness varying between 15–20 mm is a common value for outer walls used in Turkey.
Frame With Pumice Blocks – PWF. The wall had ten rows of pumice blocks in vertical direction with mortared bed joints, and locked head joints. Pumice blocks were manufactured under TS EN 771-3 standard [TSE, 2012b] with dimensions 390 x 190 x 190 mm3, and each block unit included eight parts of vertical hollows.
Frame with Hollow-Fired Clay Heat Insulation Bricks – IWF. The wall had eight rows of hollow-fired clay insulation bricks in vertical direction with mortared bed joints and locked head joints. Brick units were manufactured under TS EN 771-1 standard [TSE, 2012a] with dimensions 240 x 240 x 235 mm3, and each block unit included 30 parts of vertical hollows as shown in Fig. 4b.
Frame with Autoclaved Aerated Concrete Blocks – AWF. The wall had ten rows of autoclaved aerated concrete blocks in vertical direction with mortared bed and head joints using a specific mortar used for this particular type of blocks. Autoclaved aerated concrete blocks were manufactured under TS EN 771-4 standard [TSE, 2012c] with dimensions 600 x 200 x 250 mm3.
Frame with Double-Leaf InfillWalls without Z Ties – SWF. Double-leaf wall without Z ties (SW) is composed of two layers of wall with different thicknesses, and formed with bricks of dimensions 190 x 190 x 135 mm3, and 190 x 190 x 85 mm3 as shown in Fig. 4d and Fig. 4e, respectively. Brick units were manufactured under TS EN 771-1 standard [TSE,2012a]. The wall has ten rows of bricks in the vertical direction with mortared bed and head joints. 30 mm gap between the leafs where a styrofoam layer with a density of 16 was filling the gap. Between the layers, no mechanical links (i.e., no ties) was used as is the case in real life applications. The thin wall leaf was constructed on the back of the thick wall leaf where the out-of-plane loads were applied.
Frame with Double-Leaf Infill Wall with Z Ties – SWZF. SWZ was identical to SW in terms of wall type, workmanship and material properties. The difference was that the wall included Z-shaped metal ties, connecting the wall leafs to each other with a certain reinforcement type, reinforcement ratio, and spacing which were recommended for doublewythe load-bearing masonry walls (cavity walls) by Masonry Standards Joint Committee [MSJC, 2011]. These ties were installed on the wall leafs horizontally during the construction of the wall in a vertical orientation to the wall plane by punching through the styrofoam layer while the leafs were constructed to the same height, Z ties were made of cold-drawn plain steel wire with a diameter of 4 mm. Before the mortar was laid, Z ties were placed with a horizontal spacing of 60 cm (1 tie per 3 rows), and with a vertical spacing of 40 cm (1 tie per 2 rows) resulting in approximately 1 metal tie per 0.25 m2 wall area, and were staggered as proposed in the code.
i. The infill walls increased the strength, stiffness, and the energy dissipation capacity of the bare frame of similar levels up to 1.75 drift ratio. The infill walls have doubled the initial stiffness, and the energy dissipation capacity of the frames at 1.75% drift ratio, and also increased the ultimate strength up to 50%.
ii. Z ties were effective on maintaining the out-of-plane stability of double-leaf wall after 1.0% drift ratio, therefore infill wall continued to contribute to the in-plane strength, and the energy dissipation capability of the frames.
iii. Although the strength envelopes of infilled specimens in in-plane direction are similar, the damage mechanisms, and the drift ratios corresponding to certain performance limits of infills were different. Two different failure modes of infills were observed, namely corner crushing in vertical hollowed blocks in PWF and IWF specimens, and a mixed mode of failure with diagonal tension, and shear sliding in AWF, SWF, and SWZF specimens. The second group demonstrates the shear sliding behavior at a smaller ultimate strength, and a more stable post-peak behavior with respect to the first group.
iv. Failure modes of infills also affected the IO performance level of frame members, the top drifts of PWF and IWF specimens corresponding to the IO limit state occurred at a lower drift level with respect to the other specimens. The IO performance level of the specimens excluding PWF is controlled by the behavior of infills.
v. In PWF, IWF, and SWF specimens the drift ratios corresponding to the LS performance limit state of walls are smaller than the ones for the frame members. Thus, for these specimens the LS performance level is controlled by the behavior of infills. In contrast to these specimens, the LS performance level is achieved by the damage level of frame members for AWF and SWZF specimens, and the infills of these specimens continued to preserve their stability within the deformation limits of the LS performance level which is the design goal for residential buildings. Moreover, 0.6% drift ratio limit given in ASCE/SEI-41/06 for the LS performance level is found to be very conservative for the walls considered in the test program
a. For infill walls with horizontal hollow clay bricks (SWZ), and autoclaved aerated concrete blocks (AW), a failure mode not fitting the classical arching action was observed under biaxial loading. The damage leading the infill wall to collapse developed not on a horizontal line, but with the diagonal failure of the wall.
b. Applied maximum load along the out-of-plane direction was set to be 30% of the load resisted by the infill wall for each specimen in the in-plane direction. All of the wall types, except SW, resisted the applied loads without any out-of-plane instability up to the targeted drift ratios, and satisfied and passed the lower-bound strength determined based on their in-plane strength.
c. At the end of 1.0% drift ratio, the outer leaf of SW wall (without Z ties) moved out of the wall plane in the order of 30% of its thickness (10 cm with plaster), and thus provided a stable out-of-plane behavior up to this drift level.
d. SWF and SWZF showed a very similar in-plane and out-of-plane behavior up to 0.75% drift ratio. However, the out-of-plane stiffness of SWZ wall is reduced to 13% of its initial value at 2.0% drift ratio, the wall clearly showed a more stable out-of-plane behavior by maintaining its integrity up to 2.75% drift ratio compared to double-leaf wall without Z ties.
RECOMMENDATIONS FOR FUTURE RESEARCH
Recommendations and Limitations:
i. The contact forces on the wall sides during in-plane action are resisted by the weak axis of vertical hollowed clay bricks of IWF which causes corner crushing of the wall in early drifts. To overcome this problem, the insulation bricks may be placed horizontally without using mortar during construction of this type of wall. This change of use may reduce the lateral stiffness but also reduces the strength degradation of wall in the post-peak behavior. It is recommended that the behavior of this type of wall under combined biaxial loadings needs to be verified.
ii. In FEMA-356  and EC8 , some equations are provided to determine the out-of-plane strength of slender walls, some limits are specified for infill wall damages corresponding to different earthquake performance levels for the structure, and the out-of-plane collapse of infills in the LS performance level for the structure is prohibited. Since the upper limit of inter-story drift ratio is given as 2.0% in Turkish Earthquake Code , the walls are prone to be damaged under this specified drift ratio. For this purpose, it will be crucial to improve the earthquake design code in Turkey by enforcing the determination of minimum out-of-plane strengths, and the limits on wall damage.
iii. The double-leaf wall is generally constructed by moving the outer leaf 3–4 cm out (unsupported offset of 3–4 cm) of its plane at outer facades of buildings in Turkey. This application, which is very frequent, aims to maintain the same level of plaster with the outer face of the wall after the insulation layers placed on beam and column members. This application comes with an undesirable effect of increasing the slenderness; increased slenderness reduces the arching action of the outer leaf of the wall. Although in the test setups this aforementioned shift of the outer leafs from the frame-plane was not introduced, this effect was indirectly taken into account by exerting the out-of-plane load on the thicker side therefore leaving the thinner leaf on the opposite side (i.e., the out-of-plane force was pushing the thinner leaf out).
iv. The test results concerning the above-mentioned infills are valid under the mechanical properties of the materials used for the tests, dimensions of the wall and RC frame, workmanship, boundary conditions of the wall, and limited number of specimens tested. The factors mentioned above should carefully be taken into account while evaluating the test results.
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