Marwan T. Shedid

Alternative Strategies to Enhance the Seismic Performance of Reinforced Concrete-Block Shear Wall Systems

In this paper, seven reinforced concrete-block shear walls with aspect ratios of 1.5 and 2.2 (two- and three-storey high) were tested under displacement-controlled cyclic loading. The response of rectangular, flanged, and end-confined walls, designed to have the same lateral resistance when subjected to the same axial load, is discussed. In general, high levels of ductility accompanied by relatively small strength degradation were observed in all walls with a significant increase in ductility and displacement capabilities for the flanged and end-confined walls compared to the rectangular ones. For both aspect ratios evaluated, the drift levels at 20% strength degradation were 1.0, 1.5, and 2.2% corresponding to the rectangular, the flanged, and the end-confined walls, respectively. The ductility values of the proposed flanged and end-confined walls were, respectively, 1.5 and 2 times those of their rectangular wall counterparts (with the same overall length and aspect ratio). In addition to the enhanced d...

Seismic Performance Parameters for Reinforced Concrete-Block Shear Wall Construction

The focus of the current study is to analyze previously reported test results to evaluate equivalent plastic hinge lengths for concrete-block shear walls and to extract related seismic performance parameters. Inelastic curvatures at the base of the walls are the main source of plastic deformation for flexurally dominated walls. With adequate estimation of the plastic hinge length and more realistic values for inelastic curvatures at the base of the wall, top wall displacement can be predicted more accurately. For the walls analyzed, measured compressive strains close to the base of the wall at maximum load were significantly higher than the strains specified by North American codes. The strains, although may not alter the wall strength, significantly affect the displacement ductility at maximum load. The analysis showed that the equivalent plastic hinge length varied between approximately 30 and 60% of the wall length at a drift of 1%. The product of the seismic force modification factors (for ductility a...

Characteristics of Rectangular, Flanged, and End-Confined Reinforced Concrete Masonry Shear Walls for Seismic Design

This paper contains detailed analyses of an experimental study conducted to evaluate the ductility, stiffness degradation and energy dissipation characteristics of rectangular, flanged, and end-confined reinforced masonry (RM) shear walls failing in flexure. The test program consisted of seven two- and three-story RM shear walls, with aspect ratios of 1.5 and 2.2, tested under reversed cyclic lateral displacements simulating seismic loading effects. Documentation of the compressive strains at the wall toes, wall base curvatures, and ductility levels attained are presented. The paper focuses on determining the extent of plasticity over the wall height, evaluating the contribution of flexure and shear deformations to the overall wall lateral displacements, identifying the trend of stiffness degradation, and quantifying the amount of energy dissipation. The rectangular walls displacement predictions at ultimate loads using Canadian Standards Association (CSA) S304.1 were in better agreement with the experimental results compared to the Masonry Standards Joint Committee (MSJC) code predictions. However, both the MSJC code and the CSA S304.1 significantly overestimated the test results for the flanged and end-confined walls. Analysis of the measured displacements showed that the contribution of shear displacement to the overall wall displacement was, on average, 21 and 25% of the total displacement for the walls with aspect ratio of 2.2 and 1.5, respectively. The relationship between the energy dissipation and the ratio of the postyield to the yield displacements was found to be almost linear for the test walls. In addition, the wall stiffnesses degraded rapidly to about 60% of their gross stiffness at very low drift levels (0.1% drift). Measured compressive strain at the wall toes were almost double those specified in both North American codes. Extent of plasticity over the wall height was about 75% of the wall length. The data presented in this paper is expected to facilitate better understanding of RM wall behavior under in-plane load to researchers, practicing engineers, and code developers. This study aimed at presenting the flanged and end-confined categories as cost-effective alternatives to enhance the seismic performance of midrise RM construction in North America.

Plastic Hinge Model and Displacement-Based Seismic Design Parameter Quantifications for Reinforced Concrete Block Structural Walls

AbstractA practical alternative to the traditional rectangular cross sections of reinforced masonry structural wall systems is to alter the wall ends to allow for smaller compression zone depths, and thus higher curvatures to develop under increased seismic lateral loads. Despite the significantly enhanced seismic performance of flanged and end-confined masonry structural walls compared with their rectangular counterparts, seismic design parameters related to the former two types of walls have not been widely investigated. In addition, prescriptive design requirements for rectangular walls are under continuous development to meet the ongoing research findings in this area. The focus of the current study is to extract specific seismic design parameters of these three types of masonry walls having different end configurations for different aspect ratios when tested under reversed cyclic loads. The parameters investigated include the equivalent plastic hinge lengths, lp, the hysteretic damping levels and the...

Shear Strength of Fiber-Reinforced Polymer Reinforced Concrete Beams Subject to Unsymmetric Loading

This study is concerned with the determination of the effects of shear span-to-depth ratio (a/d) and beam depth, or size, on the concrete contribution to the shear resistance of beams longitudinally reinforced with carbon fiber-reinforced polymer (CFRP) bars. One of the distinguishing features of the study is the unsymmetrical nature of the applied load, which creates two distinct a/d ratios in the same beam and allows the effect of the a/d ratio on shear strength to be clearly seen. Six simply supported large size CFRP reinforced concrete beams without shear reinforcement were tested, each under a single concentrated load. The test variables were the a/d ratio, varying from 1.0–11.5, and the beam depth varying from 200–500 mm. All the beams failed in shear, but the failure load and location for some of these beams could not be predicted by the shear design recommendations of American Concrete Institute (ACI) Committee 440. The reason is that these recommendations do not account for the effects of a/d and beam size on shear strength. Suggestions are made for the inclusion of these parameters in the shear design equations.

Seismic Response Modification Factors for Reinforced Masonry Structural Walls

The current North American design standards provide seismic force modification factors for the rectangular masonry structural walls category only; no similar provisions for flanged and end-confined masonry structural walls exist. This study demonstrates that seismic force reduction factor (R) values calculated for rectangular walls was close to 5.0, which is consistent with the value stipulated by the ASCE 7, and was 36 and 90% higher for the corresponding flanged and end-confined walls. The deflection amplification factor (Cd) values calculated for rectangular walls were higher than specified in the ASCE 7 for the special reinforced masonry wall category. Values of the ductility-related force modification factor (Rd) for flanged and end-confined walls were, respectively, at least 30 and 100% higher than those of rectangular walls specified in the National Building Code of Canada (NBCC). Quantification of the seismic response parameters within this study is expected to facilitate adoption of the flanged a...

Seismic Response Analysis of a Reinforced Concrete Block Shear Wall Asymmetric Building

AbstractThis paper presents detailed analyses of experimental results pertaining to the cyclic behavior of a reduced-scale reinforced masonry (RM) asymmetric building with walls aligned in two orthogonal directions. The current study focuses on analyzing the influence of twist, as a system-level effect, on the displacement and strength demands of the building’s individual seismic force resisting system (SFRS) wall components. The study evaluates the individual wall contributions to the overall building response characteristics within both the elastic and the inelastic response phases. Documentation of the compressive strains at the wall toes is also presented and correlated to the damage levels anticipated for each wall. In addition, the building center of rotation and center of strength are determined and the corresponding twist angles and moments and building torsional stiffness values are evaluated throughout the loading history. Moreover, the trends of stiffness degradation for the walls and for the b...

Behavior of Beams Strengthened with Novel Self-Anchored Near-Surface-Mounted CFRP Bars

A commonly observed failure mode in laboratory tests involving surface bonded fiber-reinforced polymer (FRP) laminates or near-surface-mounted (NSM) bars is premature delamination, that is, the separation of the FRP from the substrate well before the FRP reaches its ultimate strain capacity. To delay the onset of delamination and to ensure that the NSM FRP reinforcement continues to contribute to member strength after partial delamination, a new self-anchored carbon fiber-reinforced polymer (CFRP) bar was developed and tested for this investigation. This bar is made with a series of monolithic spikes that can be anchored deep inside the concrete. In addition to cutting grooves into the concrete cover for the placement of the primary reinforcing bar, holes are drilled deep into the concrete to insert the spikes. To test the performance of this bar, six large, simply supported, reinforced, concrete beams were retrofitted with NSM bars and tested in four-point bending. Two beams were strengthened with NSM ba...

Seismic Response Evaluation of Ductile Reinforced Concrete Block Structural Walls. I: Experimental Results and Force-Based Design Parameters

AbstractThe reported experimental study documents the performance of six fully grouted reinforced concrete block structural walls tested under quasistatic cyclic loading. The walls fall under the ductile shear walls and the special reinforced masonry walls seismic force resisting system (SFRS) classification of the Canadian and U.S. standards, respectively. The test matrix consisted of one rectangular, one flanged, and two slab-coupled walls, all with an overall aspect ratio of 1.4. In addition, two rectangular walls, representing the individual components of the slab-coupled wall systems, were tested to quantify the wall slab coupling effects. In addition to discussing the experimental results, the study also presents key force-based seismic design (FBSD) parameters, such as the wall lateral load capacity, plastic hinge length, wall failure modes, and displacement ductility capacities. Moreover, the effects of wall cross-sectional configuration and slab coupling on the cyclic response and deformation cap...

System-Level Seismic Performance Assessment of an Asymmetrical Reinforced Concrete Block Shear Wall Building

AbstractIn this study, a two-story reinforced concrete block scaled building was tested to failure under fully reversed quasi-static displacement-controlled loading. The building’s seismic force-resisting system (SFRS) consisted of eight structural walls in total, with four walls, aligned along the loading direction, placed asymmetrically to result in a center of rigidity eccentricity from the floor center of mass of approximately 20% of the building width, evaluated on the basis of elastic analysis. The other four orthogonal walls were placed symmetrically around the building floor center of mass to provide torsional restraints to the building. As such, the focus of the paper is on evaluating the influence of twist as a system-level aspect on the ductility capacity of the building and the ductility and strength demands of its wall components. This paper presents the details of the building SFRS and wall configurations and characteristics and the main test observations and results. This is followed by ana...