International Concrete Abstracts Portal

Thirty-nine large-scale reinforced concrete beams were testedunder monotonic three-point bending to investigate the use of stirrups with mechanical anchors (heads) or hooks and Grade 80 (550) reinforcing steel. Grade 60 and 80 (420 and 550) No. 3, No. 4, and No. 6 (0.375, 0.5, and 0.75 in. [10, 13, and 19 mm]) bars wereused as stirrups, which were spaced at one-quarter to one-half ofthe member effective depth. Other variables included beam depth(12 to 48 in. [310 to 1220 mm]), beam width (24 and 42 in. [620and 1070 mm]), longitudinal reinforcement strain correspondingto the nominal beam shear strength (nominally 0.0011, 0.0017, or0.018), and concrete compressive strength (4000 and 10,000 psi[28 and 69 MPa]). Headed stirrups that: a) engage (are in contactwith) the longitudinal bars; or b) have a side cover of at least sixheaded bar diameters and at least one longitudinal bar within theside cover, produce equivalent shear strengths as hooked stirrups,and both details allow stirrups to yield. The results affirm thatbeams designed for the same Vn with either Grade 60 or 80 (420 or550) stirrups exhibit equivalent shear strengths. A nominal shearstrength based on a concrete contribution equal to 2 √ fc bwd may beunconservative when ρtfytm < 85 psi (0.59 MPa) in members witha/d = 3, h ≥ 36 in. (910 mm), ρ < 1.5%, and no skin reinforcement.

Recent tests showed that anchorage failure could be the primary mechanism that limits the strength and deformation capacity of column-footing connections. An experimental program consisting of the reversed cyclic load testing of 16 approximately full-scale column-footing subassemblages was thus conducted to investigate the effect of various reinforcement details on connection strength, drift capacity, and failure mode. The main parameters evaluated were type of anchorage for the column longitudinal bars (either hooks or heads), extension of column transverse reinforcement into the footing, and longitudinal and transverse reinforcement ratios in the footing. Test results indicate that even when column longitudinal reinforcement extends into the joint with a development length in accordance with ACI 318-19, a cone-shaped concrete breakout failure may occur, limiting connection strength and deformation capacity. The use of transverse reinforcement in the connection over a region extending up to one footing effective depth away from each column face proved effective in preventing a concrete breakout failure. However, for the specimens with column headed bars, extensive concrete crushing adjacent to the bearing side of the heads and spalling beyond the back side of the heads led to significant bar slip and “pinching” in the load versus drift hysteresis loops at drift ratios greater than 3%. The use of U-shaped bars in the joint between the column and the footing or slab, as recommended in ACI 352R-02, led to improved behavior in terms of strength and deformation capacity, although it did not prevent the propagation of a cone-shaped failure surface outside the joint region. Based on the test results, the basic concrete breakout strength, Nb, corresponding to a 50% fractile, in combination with a cracking factor ψc,N = 1.25, is recommended when using Section 17.6.2. of ACI 318-19 for calculation of concrete breakout strength in connections similar to those tested in this investigation.

Lap-splicing of longitudinal reinforcing bars in shear walls is often encountered in practice, and the transfer of forces in lap-spliced reinforcing bars to the surrounding concrete depends on the bond strength. Buildings with shear walls during an earthquake develop plastic hinges in the shear walls, particularly where the reinforcing bars are lap-spliced. Brittle failure is commonly observed in lap-spliced reinforced shear walls, which needs to be minimized by choosing the appropriate percentage of lap-spliced reinforcing bars. Therefore, it is essential to address the detailing of the lap-spliced regions of reinforced concrete (RC) shear walls. Several seismic design codes provide guidelines on lap-spliced detailing in shear walls related to its location, length of lap-splice, confinement reinforcement, and percentage of reinforcing bars to be lap-spliced. In this study, the percentage of reinforcing bars to be lap-spliced at a section is examined with staggered lap-splicing of 100, 50, and 33% of longitudinal reinforcing bars, in addition to a control RC shear wall without lap-splicing. This study tested four half-scale RC shear walls with boundary element (BE), designed as per IS 13920 and ACI 318, under quasi-static reversed cyclic loading. From the experimental study, it is observed that the staggered lap splicing of reinforcing bars nominally reduces the performance of shear walls under cyclic load in terms of the reduced flexural strength, deformation capacity, energy dissipation, and ductility of the shear walls compared to the control shear wall without lap splicing. It is also observed that the unspliced reinforcing bars do not sustain the cyclic loading in staggered lap-splice after the postpeak. Current provisions of ACI 318, Eurocode 2, and IS 13920 recommend staggered lap-splice detailing in shear walls. However, from the current study, shear walls with different percentages of staggered lap-splices show that the staggered lap-splice detailing in shear walls does not improve its seismic performance.

In this study, a shear strengthening method for lightly reinforced concrete columns with partial height masonry infills was proposed. Perforated steel jackets are attached to one face or both faces of the column without removing the cover concrete and mortar finish. The steel jackets were designed to provide additional shear resistance to the column through the interlocking of the ribs at both ends. To investigate the seismic strengthening effects, six column specimens with partial masonry infills were tested under cyclic loading. The tests showed that the specimens with double-face jacketing exhibited an improved seismic performance, whereas there was little or no strengthening effect for the specimens with single-face jacketing. For further investigation on the short column effects due to partial height infills, modeling parameters to define the stiffness and force-deformation relation of the column and masonry walls were proposed, and the modeling results were compared with the test results. Based on the investigation results, the detailing requirements of steel jacketing and the nonlinear modeling methods of the columns with partial masonry infills were discussed.

Slender reinforced concrete (RC) shear walls have become an integral part of the lateral load-resisting mechanism of modern high-rise structures, especially in earthquake-prone regions. Since the seismic behavior of the wall is governed by its design and detailing, a number of past studies are available regarding the same. However, limited studies have been carried out regarding the influence of the confinement effect in the web and the boundary elements on the wall behavior. To address this concern, an experimental study of an isolated slender shear wall with enlarged Boundary Element (BE) designed in conformance to IS 13920:2016 is undertaken. The specimen was subjected to lateral slow cyclic displacement-controlled loading. The relation between the global and local response of the wall, by means of strain profiles, is documented in the present study. The evolution of the strain profiles clearly depicts the participation of the transverse reinforcement, indicating the mobilization of the confinement effect.