Search Results (36)

This study investigates two novel prefabricated frame joints: prestressed steel sleeve-connected prefabricated reinforced concrete joints (PSFRC) and non-prestressed steel sleeve-connected prefabricated reinforced concrete joints (SSFRC). A total of three PSFRC specimens, four SSFRC specimens, and one cast-in-place joint were designed and fabricated. Seismic performance tests were conducted using different end-plate thicknesses, grout strengths, stiffener configurations, and prestressing tendon configurations. The experimental results showed that all specimens experienced beam end failures, and three failure modes occurred: (1) failure of the end plate of the beam sleeve, (2) failure of the variable cross-section of the prefabricated beam, and (3) failure of prefabricated beams at the connection with the steel sleeves. The load-bearing capacity and initial stiffness of the structure are increased by 35.41% and 32.64%, respectively, by increasing the thickness of the end plate. Specimens utilizing C80 grout exhibited a 39.05% higher load capacity than those with lower-grade materials. Adding stiffening ribs improved the initial stiffness substantially. Specimen XF2 had 219.08% higher initial stiffness than XF1, confirming the efficacy of stiffeners in enhancing joint rigidity. The configuration of the prestressed tendons significantly influenced the load-bearing capacity. Specimen YL2 with symmetrical double tendon bundles demonstrated a 27.27% higher ultimate load capacity than specimen YL1 with single centrally placed tendon bundles. An analytical model to calculate the moment–rotation relationship was established following the evaluation criteria specified in Eurocode 3. The results demonstrated a good agreement, providing empirical references for practical engineering applications. Full article

In order to enhance the durability of concrete frame beams, a U-shaped engineered cementitious composites (ECC) protective layer is applied at the end of the frame beams. The bond between the ECC protective layer and the concrete is reinforced by incorporating notches and grooves in the occupancy plate. The development and resistance to cracking of reinforced concrete (RC) frame beams and frame beams with an ECC protective layer were investigated using monotonic loading tests. The test results show that the average value of crack spacing in the negative moment zone of the RC frame beam specimen is in close agreement with the crack spacing calculated according to the GB50010 Code for Design of Concrete Structures. While the dispersion of crack width in the negative moment zone of the RC frame beam specimens is considerable, the distribution pattern of crack width undergoes a gradual change with increasing load. When the maximum crack width calculation method of GB50010 is employed in the negative moment zone of RC frame beams, the crack width should be increased by approximately 1.25 times. Furthermore, the crack spacing and crack width of the ECC protective layer are markedly smaller than those of RC frame beams. Full article

Precise determination of structural elastic–plastic displacement and component states under rare earthquakes is crucial for structural design. This article proposes a quasi-elastic–plastic optimization method for reinforced concrete structures. First, an approximate formula for calculating the yield bending moment of shear walls is provided through analysis of 64 shear walls. Second, a quasi-elastic–plastic analysis method is proposed. Using the elastic response spectrum analysis, strain energy for each component is calculated, and stiffness reduction factors for walls, beams, and columns are derived based on the energy equivalence principle. Finally, combining the elastic response spectrum analysis and the quasi-elastic–plastic analysis, various constraint indicators at the elastic and elastic–plastic design stages are calculated, and structural size optimization is completed using the particle swarm optimization method. The feasibility of this method is validated with examples of a 15-story reinforced concrete frame structure and a 15-story frame–shear wall structure. The quasi-elastic–plastic optimization with the particle swarm optimization efficiently completes elastic–plastic optimization for reinforced concrete structures, determining section sizes that meet performance standards while reducing material usage. Full article

In this article, the response of reinforced concrete frames concurrently subjected to both horizontal and vertical seismic motions is assessed. The article is not limited to the variation in response quantities but aims to identify which specific design parameters are affected and how, as well as which specific code provisions could be violated due to the omission of vertical oscillations during the design process. Furthermore, the consequences that a design against vertical ground motion would cause in both technical and economic terms were investigated. For this purpose, six eight-storey 2D frames were designed, neglecting the vertical seismic component in compliance with code provisions. Subsequently, the seismic response of the frames to the concurrent action of horizontal and vertical ground motion was evaluated by applying both modal response spectrum and inelastic dynamic analyses. It was found out that several code violations occurred, mainly due to the fluctuation of the columns’ normalized axial forces and the amplification of up to two times or more of the beam bending moments. Thereafter, the frames were redesigned without neglecting the vertical seismic component, and the changes in the members’ cross-sectional dimensions and reinforcement were determined. Finally, it was estimated that the economic impact of redesigning did not exceed 4% of the initial total construction cost of the frames. Full article

A mid-story pin system to avoid moment-resisting frame column failure during seismic action was proposed recently. The solution consists of a reinforced concrete (RC) pier protruding from the foundations, the steel column connected with the superstructure, and plates and the anchor bolt working as a pinned connection in between. This paper utilizes shell finite element analysis (FEA) models to examine the demanded column-to-beam strength ratio to keep the column elastic and maximize the story drift at the moment of beam buckling of the frame. The method of calculating post-seismic residual strength based on maximal buckling deformation of the beam is also proposed. Full article

This paper aims to examine the seismic response of prestressed self-centering moment-resisting frames (PSC-MRFs) based on concrete-filled double steel tubular (CFDST) columns and RC beams. The beam of this novel connection is divided into two parts, connected by bolts and tendons, and the beam includes a gap opening feature, which could be regarded as a normal single beam in the field. Cyclic loading analysis was performed on one-story frames with different initial parameters arranged in adjacent bays. Nonlinear dynamic analysis was conducted on a six-story frame under two seismic hazard levels. The cyclic loading analysis showed favorable self-centering performance of the frame even when the hysteretic energy dissipation ratio reached 0.808. Seismic analysis results showed that compared with the in situ reinforced concrete frame, PSC-MRFs generally had similar maximum inter-story drifts under fortification earthquakes, but the residual inter-story drifts were reduced by 33%; under rare earthquakes, the maximum inter-story drifts and residual inter-story drifts of PSC-MRFs were reduced by 22% and more than 90%, respectively. In the adjacent bays on the same story of PSC-MRFs, connections with smaller imminent moments of gap opening opened earlier under earthquake, and the maximum opening angle was larger. The general seismic performance and self-centering of PSC-MRFs was significantly more advantageous than that of in situ reinforced concrete frames. Full article

Based on the design concept of a strong column and weak beam, a new type of reinforced concrete frame structure beam–column joint is proposed. Considering different column end amplification factors (beam–column bending moment ratio), the finite element method (FEA) is used to analyze the parameters that affect the seismic performance of RC frame structure beam–column joints. The reliability verification error is within 4.8% to 11.7%, meeting the requirements of engineering accuracy. Then, through parameter analysis, the effects of different concrete strengths, stirrup diameters, and axial pressures on the seismic performance of the joint are studied. The study results show that enhancing concrete strength has a significant effect on the seismic performance of the structure, especially when the amplification factor is 2.0. Compared with the C20 specimen, the bearing capacity of the C40 specimen increased by 26.88%. However, the increase in stirrup diameter did not significantly improve the performance of the specimen. In addition, a high axial pressure ratio may affect the bearing capacity of the structure. This study provides a new type of beam–column joint that conforms to the design concept of a strong column and weak beam and provides a theoretical basis for its application in engineering. Full article

This paper discusses the iteration of a seismic retrofit solution for shear-deficient end regions of 19 reinforced concrete (RC) moment-resisting frame (MRF) T-beams located in a 12-story RC MRF building in downtown Los Angeles, California. Local strengthening with externally bonded (EB) fiber-reinforced polymer (FRP) fabric was chosen as the preferred retrofit strategy due to its cost-effectiveness and proven performance. The FRP-shear-strengthening scheme for the deficient end-hinging regions of the MRF beams was designed and evaluated through large-scale cyclic testing of three replica specimens. The specimens were constructed at 4/5 scale and cantilever T-beam configurations with lengths of 3.40 m or 3.17 m. The cross-sectional geometry was 0.98 × 0.61 m with a top slab of 1.59 m in width and 0.12 m in thickness. Applied to these specimens were three different retrofit configurations, tested sequentially, namely: (a) unanchored continuous U-wrap; (b) anchored continuous U-wrap with conventional FRP-embedded anchors at the ends; and (c) fully closed external FRP hoops made of discrete FRP U-wrap strips and FRP through-anchors that penetrate the top slab and connect both ends of the FRP strips, combined with intermediate crack-control joints. The strengthening concept with FRP hoops precluded the premature debonding and anchor pullout issues of the two more conventional retrofit solutions and, despite a more challenging and labor-intensive installation, was selected for the in-situ implementation. The proposed hooplike EB-FRP shear-strengthening scheme enabled the deficient MRF beams to overcome a 30% shear overstress at the end-yielding region and to develop high-end rotations (e.g., 0.034 rad [3.4% drift] at peak and 0.038 rad [3.8% drift]) at strength loss for a beam that, otherwise, would have prematurely failed in shear. These values are about 30% larger than the ASCE 41 prescriptive value for the Life Safety (LS) performance objective. Energy dissipation achieved with the fully closed scheme was 108% higher than that of the unanchored FRP U-wrap and 45% higher than that of the FRP U-wrap with traditional embedded anchors. The intermediate saw-cut grooves successfully attracted crack formation between the strips and away from the FRP reinforcement, which contributed to not having any discernable debonding of the strips up to 3% drift. This paper presents the experimental evaluation of the three large-scale laboratory specimens that were used as the design basis for the final retrofit solution. Full article

A possible way to improve the structural safety and robustness of precast building structures is to develop effective precast frame systems with layered beams, which combine prefabricated parts with cast-in situ ordinary concrete, high-performance concrete, fiber concrete, or FRP. The paper provides a new type of precast reinforced concrete frame system with layered beams for rapidly erected multi-story buildings resistant to accidental actions. Using a combination of the variational method and two-level design schemes, a simplified analytical model has been developed for structural analysis of the precast reinforced concrete frame system, both for serviceable and ultimate limit states as well as for accidental actions. The proposed model allows for determining shear deformations and the formation and opening of longitudinal cracks in the intermediate contact zone between precast and monolithic parts of reinforced concrete structural elements of the frame, as well as the formation and opening of normal cracks because of the action of axial tensile force or bending moment in these elements. The design model was validated by comparing the calculated and experimental data obtained from testing scaled models of the precast reinforced concrete frame system with layered beams. The paper investigates and thoroughly analyzes the factors affecting the stiffness and bearing capacity of the intermediate contact zone, discusses the criteria for the formation of shear cracks along the contact zone of precast and monolithic concrete, and examines the change in the stiffness and dissipative properties of layered elements at different stages of their static–dynamic loading. The robustness of the experimental models of the structural system was not ensured under the specified load, section dimensions, and reinforcement scheme. Following an accidental action, longitudinal cracks were observed in the contact joint between the monolithic and prefabricated parts in the layered beams. This occurred almost simultaneously with the opening of normal cracks in adjacent sections. A comprehensive analysis of the results indicated a satisfactory degree of agreement between the proposed semi-analytical model and the test data. Full article

Steel–concrete composite structures have advantages in terms of strong bearing capacity and full utilisation of performance, and thus, composite frame beams are widely used in building construction. However, in the design and use of existing composite frame beams, the composite effect of a slab and steel beam cannot be completely taken into account. In this study, the effective flange width method is utilised to calculate the contribution of the slab reinforcement to the section moment of inertia to check the beam-end crack width via simulations using the general finite-element software MSC.MARC 2020. A parameter sensitivity analysis of the reinforcement tensile stress is conducted to determine critical influential geometric parameters for the side-column and centre-column hogging moment regions. Finally, design formulae for calculating the effective flange widths of the side- and centre-column hogging moment regions are proposed. In the formula for the side-column hogging moment region, the half column width ($R$) and steel-beam height ($h_{\mathrm{s}}$) are critical variables, whereas, in the formula for the centre-column hogging moment region, the steel-beam height ($h_{\mathrm{s}}$), slab width ($b_{\mathrm{c}}$), and half clear-span length ($l$) are critical variables. Both formulas are verified via a multiparameter simulation, which enables more accurate crack-checking calculations for the hogging moment region in the serviceability limit state. This study provides an important reference for fine finite-element simulations of serviceability limit states and shows the factors affecting the effective flange width that differ from those in the ultimate limit state. Full article

The use of friction-based beam-to-column connections (BCCs) for earthquake-resistant moment-resistant frames (MRFs), aimed at eliminating damage to beam end sections due to the development of plastic hinges, has been prevalent since the early 1980s. Different technical solutions have been proposed for steel structures, and some have been designed for timber structures, while a few recent studies concern friction joints employed in reinforced concrete structures. Research aimed at characterizing the behavior of joints has focused on the evaluation of the tribological properties of the friction materials, coefficient of friction, shape and stability of the hysteresis cycles, influence of the temperature, speed of load application, effects of the application method, stability of preload, the influence of seismic excitation characteristics on the structural response, statistical characterization of amplitude, and frequency of the slip excursion during seismic excitation. Studies aimed at identifying the design parameters capable of optimizing performance have focused attention mainly on the slip threshold, device stiffness, and deformation capacity. This review compiles the main and most recent solutions developed for MRFs. Furthermore, the pros and cons for each solution are highlighted, focusing on the dissipative capacity, shape, and stability of hysteresis loops. In addition, the common issues affecting all friction connections, namely the characteristics of friction shims and the role of bolt preload, are discussed. Based on the above considerations, guidelines can be outlined that can be used to help to choose the most appropriate solutions for BCCs for MRFs. Full article

Nowadays, fiber-reinforced polymer (FRP) has become a widely accepted alternative reinforcement to steel bars in concrete members due to its many sustainability traits, as represented by its high strength-to-weight ratio, corrosion resistance, non-conductive properties, and electromagnet neutrality. However, FRP bar exposure for an extended period of time to harsh environmental conditions and chemicals can have an adverse effect on their mechanical properties. In this investigation, glass FRP bars were exposed to indoor controlled temperature, outdoor direct sunlight, outdoor shade, seawater, and alkaline solution for six months prior to using them as reinforcement in concrete flexural members. This research involves the fabrication and testing of five pairs of 3 m-long concrete beams with 200 mm by 300 mm cross-sections embedded in the tension zone with the exposed GFRP bars. The 10 beams were instrumented with strain gauges and tested following a four-point loading scheme using a hydraulic jack attached to a rigid steel frame. Crack width records from the tests showed the inferior serviceability of the beams that contained rebars stored in an outdoor environment relative to the control beams. GFRP bar exposure to an alkaline solution or outdoor direct sunlight slightly affected the cracking and ultimate moment capacities, reducing them by 5% and 3% in terms of the same parameters as the controlled indoor exposure, respectively. The influence of GFRP bar exposure to open-air shade or sunlight decreased the pre-cracking stiffness by 25% and flexural ductility by 10–20% when compared with the control specimens. The predicted ultimate flexural strength using the ACI 440 provisions gave comparable results to the experimentally obtained values. A simple mathematical equation that envelops the moment–deflection relationship for GFRP over-reinforced concrete beams and only requires information about initial cracking and ultimate flexural conditions is proposed. Full article

Beam–column connection zones are high regions of interest in reinforced concrete (RC) structures, which are expected to respond elastically to seismic loads. Using carbon fiber-reinforced polymers (CFRP) to improve these connections, performance is critical in retrofitting deficient RC frames because existing slabs may pose numerous limitations in the design and wrapping of CFRP sheets in joints. The main aim of this research is to develop a new design for flange-bonded CFRP retrofit of frames, including slabs, for the relocation of plastic hinges of the connection area toward the beam and to develop beam–column joint capacity and building stability in cases of subjection to dynamic loads. The performance of these proposed retrofittings was explored both experimentally and numerically. Two full-scale fabricated interior RC joints of a real moment-resisting frame with moderate ductility were subjected to monotonic loads before and after retrofitting, and the results were used to detail the numerical progress and verify of the beam–column connection. Moreover, a parametric study was conducted on CFRP sheets’ optimal thickness to examine its influence on plastic hinge relocation in the connection region. Results show that the retrofitting method can efficiently relocate the plastic hinge to the mid-span of the beam, which, in turn, leads to improved capacity and achievement of the RC frame and guarantees better structural safety a lower cost. Full article

Precast concrete (PC) structures have many advantages, but their use in the construction of middle- to high-rise buildings is limited. The construction of PC structures requires skills in various operations such as transportation, assembly, lifting, and structural soundness. In particular, regarding the seismic design of PC structures, it is necessary to clearly evaluate whether they have the same structural performance and usability as integral RC (cast-in-place) structures. In this paper, an experimental study was conducted to investigate whether PC members can achieve a seismic performance equivalent to that of RC members in beam–column joints, which are representative moment-resisting frames. The main variables are the two types of structural systems (intermediate and special moment-resisting frames) and the design flexural strength ratio of the columns and beams. The experimental and analytical results showed that the seismic performance of the PC specimens was equivalent to that of the RC specimens in terms of strength, stiffness, energy dissipation, and strain distribution, except for the specimen with splice sleeve bond failure of the column reinforcement (poor filling of the internal mortar). In addition, the I series satisfied the present emulation evaluation criteria for special moment-resisting frames of PC structures, confirming the possibility of applying intermediate moment-resisting frames. Full article

The effectiveness of a recently proposed methodology for the identification of damage in planar, multistory, reinforced concrete (RC) moment frames, which develop a plastic yield mechanism on their beams, is showcased here via the examining of a group of such existing multistory frames with three or more unequal spans. According to the methodology, the diagram of the instantaneous eigenfrequencies of the frame in the nonlinear regime is drawn as a function of the inelastic seismic roof displacement by the performance of a sequence of pushover and instantaneous modal analyses with gradually increasing target displacement. Using this key diagram, the locations of severe seismic damage in an existing moment frame can be evaluated if the instantaneous fundamental eigenfrequency of the damaged frame, at an analysis step within the nonlinear area, is known in advance by “the monitoring and the identification of frequencies” using a local network of uniaxial accelerometers. This is a hybrid technique because both procedures, the instrumental monitoring of the structure and the pushover analysis on the frame (M and P technique), are combined. A ductile, five-story, planar RC moment frame with three unequal spans is evaluated in this paper by the M and P technique. The results show that the seismic roof displacement, the lateral stiffness matrix, and, finally, the damage image of this existing frame, are fully compatible with the eigenfrequencies identified by the monitoring and are calculated with high accuracy. Full article