Dipti Ranjan Sahoo

Effect of Steel Fiber Content on Behavior of Concrete Beams with and without Stirrups

An experimental study is conducted on a series of 12 reinforced concrete (RC) and steel fiber-reinforced concrete (SFRC) beam specimens to study their shear and flexural strengths, failure mechanisms, and ductility response under monotonic loadings. The main parameters varied in this study are the concrete compressive strength, percentage of longitudinal reinforcement, fiber content, shear span-depth ratio, and amount of transverse stirrups. End-hooked steel fibers of volume fraction ranging from 0.5 to 1.5% are used in the specimens. Test results showed that the addition of steel fibers enhanced the flexural and shear strengths and the ductility of the flexural members. The addition of a minimum of 0.5% fiber content in the beams with shear stirrups changed the mode of failure from brittle to ductile, whereas a minimum fiber content of 1.0% is required to achieve the ductile response of the beams without shear stirrups. Using a curve-fitting method on the available test data, simple expressions are also derived to predict the shear strengths of medium- to large-scale flexural members with varying fiber contents.

Postyield Cyclic Buckling Criteria for Aluminum Shear Panels

Aluminum shear panels can dissipate significant amount of energy through hysteresis provided strength deterioration due to buckling is avoided. A detailed experimental study of the inelastic behavior of the full-scale models of shear panels of 6063-O and 1100-O alloys of aluminum is conducted under slow cyclic loading of increasing displacement levels. The geometric parameters that determine buckling of the shear panels, such as web depth-to-thickness ratio, aspect ratio of panels, and number of panels, were varied among the specimens. Test results were used to predict the onset of buckling with proportionality factor f in Gerard’s formulation of inelastic buckling. Moreover, a logarithmic relationship between buckling stress and slenderness ratio of the panel was observed to predict experimental data closely. These relations can be further used to determine the geometry of shear panels, which will limit the inelastic web buckling at design shear strains. DOI: 10.1115/1.2793135

Seismic Behavior of Steel Buildings with Hybrid Braced Frames

AbstractThis paper presents the seismic performance of buildings with a hybrid bracing system in which buckling-restrained braces (BRBs) are used at the lower stories with the higher level of ductility demands, and conventional buckling-type braces consisting of steel hollow structural sections (HSS) are used in the upper stories with relatively smaller ductility demands to minimize the probability of their fracture under reversed cyclic displacements. This type of hybrid braced frame (HBF) could prove economical, especially for retrofitted buildings in seismically active regions. A series of nonlinear time-history analyses was conducted to investigate the seismic performance of 3- and 6-story buildings with the hybrid bracing system. The main parameters studied are the seismic design parameters, maximum interstory drift ratios, fracture response of the HSS, and the optimal deployment of the BRBs. The seismic performance of the HBFs was compared with conventional concentrically braced frames and buckling-...

A Novel Technique of Seismic Strengthening of Nonductile RC Frame using Steel Caging and Aluminum Shear Yielding Damper

A novel strengthening scheme for seismically-weak RC frames is proposed which utilizes external steel caging to improve flexural/shear strength of columns and aluminum shear-yielding damper (Al-SYD) to further enhance lateral strength, stiffness and overall energy dissipation capacity of the frame. This paper describes the effectiveness of this scheme as evidenced in an experimental study on a reduced scale (1:2.5) single-story, single-bay, gravity-only designed reinforced concrete (RC) frame. The strengthened frame was simultaneously subjected to gravity loads and reversed cyclic lateral displacements as per ACI-374 loading protocol. An innovative connection scheme was designed to transfer a portion of frame lateral load to the energy dissipation device (Al-SYD). Besides the significant increase in lateral strength and stiffness of the strengthened frame, RC frame members did not suffer any major damage during the entire test protocol. This indicates significant reduction in force demand on existing RC members because of enhanced energy dissipation through hysteretic shear yielding of aluminum panels. Moreover, the simple connection scheme proposed in this study proved very efficient in transferring the frame lateral load to strengthening elements.

Cyclic Response of Non-ductile RC Frame with Steel Fibers at Beam-Column Joints and Plastic Hinge Regions

An experimental study has been conducted on a reduced-scale gravity-load designed test frame to investigate its overall performance due to the addition of steel fiber-reinforced concrete (SFRC) at the critical regions. Two geometrically similar specimens, namely, reinforced concrete (RC) and SFRC, are tested under slow-cyclic lateral loading. End-hooked steel fibers (aspect ratio = 80) of 1.0% volume fraction were used in the SFRC mix for a distance of one-and-half times the member size near the joint regions. The addition of steel fibers improved the damage tolerance, lateral load resisting capacity, lateral stiffness, ductility, and energy dissipation of the frame.

Seismic rehabilitation of damaged reinforced concrete frames using combined metallic yielding passive devices

Abstract This study is focused on the effectiveness of a strengthening technique using the combined metallic yielding devices (CMDs) as passive energy dissipation systems to enhance the lateral load resistance and the deformation capacity of the damaged reinforced concrete (RC) frames. An experimental investigation has been conducted on two geometrically-identical damaged RC frames under the lateral cyclic loading condition. In addition to the CMDs, the beam and columns of the damaged frame have been fully/partially strengthened using the additional steel elements. This study highlights the importance of the positioning of CMDs, the selection of local strengthening strategy, and the proper installation of CMDs on the lateral load resistance and energy dissipation potential of the damaged RC frames. The strengthened frames exhibited the enhanced lateral strength, lateral stiffness, lateral deformation capacity, energy dissipation and damping potential up to a storey drift of 6%. Test results are further used to validate a design methodology proposed for the proportioning of CMDs required to achieve the desired lateral resistance of the damaged RC frames.

Steel Shear Panels as Retrofitting System of Existing Multi-story RC Buildings: Case Studies

The seismic retrofitting of existing reinforced concrete buildings is currently a major economic and social activities for the redevelopment of congested urban areas. Innovative reversible technologies by metal devices are capable of protecting buildings from damage, providing high levels of structural safety. The current paper is framed within this context, where steel plate shear walls (SPSWs) as retrofitting system of multi-story residential RC buildings, designed for vertical loads and located in the historic center area of Torre del Greco (Naples, Italy), have been used. The vulnerability analysis performed on the inspected buildings have shown their inability to withstand seismic actions. Therefore, the retrofitting design of such buildings through partial bay and full bay SPSWs has been done. The retrofitted buildings behaviour has been assessed through numerical analysis carried out in the non-linear field. The achieved results have shown a significant improvement of the structural performance of the tested improved buildings in terms of strength and stiffness. As a further research development, the available information on the design of SPSWs for retrofitting existing RC buildings will allow to design useful design charts capable of providing the optimal plate geometric dimensions as a function of predetermined performance targets.

Influence of Steel and Polypropylene Fibers on Flexural Behavior of RC Beams

Seven full-scale concrete beam specimens are tested under gradually increasing monotonic loading to investigate their overall flexural response due to the addition of both metallic and nonmetallic fibers to the concrete. Steel and polypropylene fibers of 0.5% and 1.0% volume fraction are used in the fiber-reinforced concrete (FRC) beam specimens. No improvement is noticed in the compressive and splitting tensile strengths of concrete due to the addition of polypropylene fibers only. However, an improvement of 25-100% in the concrete splitting tensile strength is noticed when either steel or combined fibers are added to the concrete. Although an increase in fiber content in the combined FRC improves various mechanical properties, its influence on the peak load-resisting capacity of the full-scale beam specimens is rather limited. An increase of both steel and polypropylene fibers in excess of 0.5% volume fraction does not improve the ultimate flexural resistance of beams due to the uneven distribution of similar sizes of fibers in the presence of reinforcing steel bars. However, displacement ductility of the beam specimens is improved by 120% as compared with the RC specimen, when only polypropylene fibers of 1% volume fraction are added to the concrete. A better postpeak residual strength response is noticed in case of all FRC beam specimens due to multiple cracking associated with the fiber-bridging action.

Stiffness-based design for mitigation of residual displacements of buckling-restrained braced frames

AbstractBuckling-restrained braced frames (BRBFs) are often used as primary seismic force resisting systems to achieve the desired seismic performance of building frames. However, significant residual displacements of these systems may cause concerns on their postearthquake performance, especially under long duration earthquakes and strong aftershocks. Although backup moment frames (MFs) along with the BRBFs, referred to as dual systems, can be used to mitigate their residual interstory drifts, there is no design criterion on how to determine the section sizes in the MF for this purpose. In the research reported in this paper, a simple mitigation technique was developed, which relies on a stiffness-based design in which the residual interstory drifts can be effectively controlled by increasing the elastic story stiffness using column sections with a higher moment of inertia. Nonlinear time-history analyses were carried out on low-to-high rise BRBFs for a set of 20 design basis earthquake (DBE) level groun...

Optimum range of slenderness ratio of hollow steel square braces for special concentrically braced frames

Past studies have shown that the slenderness ratio and the width-to-thickness ratio of braces are primarily responsible for achieving the desired cyclic response of special concentrically braced frames. An increase in brace slenderness ratio results in a reduction in its energy dissipation capacity along with a simultaneous increase in the ductility nearing its fracture. Since both energy dissipation capacity and ductility of braces are essential parameters in quantifying the seismic performance of special concentric braced frames, there is a need of establishing the optimum range of brace slenderness ratio and width-to-thickness ratio. Hence, an extensive finite element parametric study has been conducted on a wide range of values of these parameters on braces of hollow steel square section using a commercial software ABAQUS. The finite element models account for the inelastic hysteretic characteristics and the fracture behavior of braces. The results of simulation models matched very well with the past experimental results with respect to the performance points, namely, global buckling, local buckling, fracture initiation, complete fracture, and ductility. Finally, a relationship between the lower limit of slenderness ratio and the width-to-thickness of square braces has been established based on the simulation results.