Strong Column-weak Beam Research Articles

DISASTER RESILIENCE IN BUILDING CONSTRUCTION THROUGH CONCRETE CASTING METHODS: IMPLICATIONS FOR THE USE OF STRONG COLUMNS AND WEAK BEAMS

<div><p class="Els-history-head">The difficulty in obtaining the construction of a strong-beam-weak column building will be encountered during the concrete casting work because in the implementation the beam may be cast using a higher quality concrete than that used for column casting. This has the potential to result in the construction of strong weak-beam columns which should be avoided in earthquake-resistant structures. This study aims to find the most effective concrete casting method that results in the construction of strong-weak beam-columns as intended above. Observations were made on the construction work of a four-story concrete construction building. Analysis by comparing all foundry works whose implementation uses ready mixed concrete made from one vendor, with those using different vendors for each casting of different building structural components. The data used are in the form of compressive strength test results for column casting samples and beam casting samples. From these data, a graph of the compressive strength sample of beam casting is made, then it is compared with the graph of the compressive strength sample of column casting. The results of the study concluded that using different vendors for casting different components of the building has a greater chance of producing a weak beam-strong column structure.</p></div>

Retrofit of non-code conforming moment resisting beam-to-column joints: A case study

The non-linear monotonic and hysteretic behaviour of as-built and retrofitted beam-to-column joints of an existing non-code conforming steel multi-storey building was investigated by means of finite element simulations. The building case study was originally designed to resist only gravity and wind loads without any capacity design principle. The as-built joints have continuous beam and interrupted columns that are bolted with end-plate splice to the tapered flange of the beams in the nodal zone. These joints were analysed to assess their non-linear response. The finite element analyses showed adequate resistance against gravity loads, but poor performance under seismic and column loss scenarios. Two retrofit strategies were designed in order to enhance the local and global performance in order to guarantee the weak beam-strong column condition, namely: (i) strengthening the column and its connection; (ii) weakening of the beam. Different types of interventions were designed per examined strategy and numerically analysed to assess the global and local response of both external and internal joints. A TOPSIS multicriteria analysis was also performed to identify the most suitable type of intervention.

A probabilistic model for the evaluation of strong column-weak beam provision for flexible-base buildings subjected to pulse-like ground motions

A probabilistic model for the evaluation of strong column-weak beam provision for flexible-base buildings subjected to pulse-like ground motions

PERILAKU SAMBUNGAN BALOK I DAN KOLOM TABUNG BAJA AKIBAT BEBAN SIKLIK

Using Concrete-Filled Steel Tube (CSFT) as a column component provides many advantages over steel columns and reinforced concrete columns. Steel tube serve as reinforcement as well as formwork for concrete filler material. Besides, restrain effect of steel tube prevent crack in the concrete. The concrete-filler on steel tube prevent local buckling and increases the strength and stiffness of the column. The lack of experience and complexity of the beam-column connection system becomes an obstacle to use the CFST as a column. In addition, strength and stiffness of the connection system on CFST should be sufficient to endure the earthquake loads. This research purpose to find alternate connection of CFST column beams and also to know hysterestis behavior, strength, stiffness and pattern of connection collapse due to cyclic loading. There are two specimens in this research, continuous joint of beam-column steel tube with concrete filler (BKM-T) and continuous joint of beam-column steel tube without concrete filler (BKM-K). BKM-K as a comparator to show an increasing strength and stiffness due to the concrete core. Load cycle of specimens refer to ACI 374.1-05. The test results showed the average maximum load (P peak ) of 32.59 kN on BKM-K increased to 34.08 kN on BKM-T. Elastic stiffness (K e ) at BKM-K of 0.78 kN/mm increased to 1.08 kN/mm on BKM-T. BKM-T also has a larger hysteretic loop than BKM-K. In addition, BKM-K and BKM-T adequate the strong column weak beam criteria with partial ductility.

Relocating plastic hinges in reinforced concrete beam-column joints by mechanically anchored diagonal bars

The implementation of strong column-weak beam concept in reinforced concrete (RC) moment-resisting frames results in the formation of plastic hinges at the beam ends, which could lead to the strain penetration of beam longitudinal reinforcement into beam-column joint (BCJ) cores. This causes high shear forces inside joint cores, bonding deterioration of longitudinal reinforcement and consequently jeopardizes the seismic behaviour of BCJs. Plastic hinge relocation by mechanically anchored diagonal bars has been proposed to mitigate such potential damage to BCJ cores. This paper investigates the effect of relocating distance of plastic hinge on the seismic performance of BCJs with mechanically anchored diagonal bars. Five interior BCJ specimens including one control specimen reinforced with conventional stirrups and four others reinforced with mechanically anchored diagonal bars were tested under cyclic load. Test results indicate that the plastic hinges can be generally relocated by different distances to the anchorage ends of diagonal bars. Increasing the relocating distance can enhance the loading capacity and stiffness of BCJs, but also increase the potential of joint shear failure. The ductility, energy dissipation and joint shear deformation of BCJs are first improved then reduced as the relocating distance increases. Although the strain penetration of beam longitudinal reinforcement into the joint cores can be gradually alleviated as the plastic hinges are moved farther away from the joints, the induced bending moments at the beam ends are amplified and subsequently increase the joint shear forces. Moreover, increasing the length of anchorage segments increases the forces sustained by the mechanically anchored diagonal bars. Therefore, a lower bound of the relocating distance of plastic hinges is recommended to take advantage of the benefits from reduced strain penetration, while an upper limit needs to be specified to avoid joint shear failure. In addition, an analytical model is developed to predict the failure modes and loading capacities of BCJs with the plastic hinges relocated by different distances. A satisfactory agreement between the prediction and the experimental results is obtained.

Behavior of T-shaped CFST column and U-shaped steel-concrete composite beam joints

This paper intends to study the seismic and shear behavior of joints for T-shaped concrete-filled steel tubular (CFST) column and U-shaped steel-concrete composite beam (USCB). In the joint details, the C-shaped slots cut in the T-shaped steel tube allowed the double-C channels used to connect the thin-walled U-shaped steel (USS) passing through the panel zone directly. A total of six full-scale specimens constructed according to the concept of strong column-weak beam were tested: two specimens were tested under the monotonic loading to obtain the load-carrying capacity and the other four specimens under cyclic loading to evaluate the seismic performance . The variables included the details of USCB, the axial load level, and the beam-column linear stiffness ratio . Test results demonstrated that the proposed joint details exhibited good seismic and shear performance. Three typical failure modes, i.e. welding fracture between the double-C channels and the steel tube, local buckling of the steel tube, and fracture of the double-C channels were observed. Some indexes of seismic performances, such as load-carrying capacity, deformability , degradation of load-carrying capacity, stiffness degradation , and energy dissipation capacity were evaluated. Finally, the joint shear resistance of the specimens was discussed according to the ASCE design guidelines. • An innovative joint details for T-shaped concrete-filled steel tubular column and U-shaped steel-concrete composite beam was proposed. • Some indexes of seismic performances for the joints, such as load-carrying capacity, deformability, degradation of load-carrying capacity, stiffness degradation, and energy dissipation capacity were evaluated. • The joint shear capacity was evaluated according the methods given by the ASCE Task Committee and by the previous findings of the authors.

Improved Substitute-Frame (ISF) model for seismic response of steel-MRF with vertical irregularities

This paper attempts to evaluate and improve the Substitute Frame (SF) model recently proposed by the authors to be used in simplifying moment resisting frames with various vertical irregularities besides the non-fulfilled Strong-Column/Weak-Beam (SCWB) concept. Accordingly, two improvements related to the setback irregularities and SCWB issues have been proposed. For those cases where the current SF model makes inadequate estimations of the original frame, the proposed improvements make it possible to overcome the existing problems and get more accurate results. For this purpose, a combination of 24 full-frame 2D steel frames has been designed and modeled in OpenSEES software. Then, the analysis results were compared to those of the corresponding SF models in terms of modal, nonlinear static, and dynamic analyses. In this regard, three sets of ground motions were selected by the Conditional Spectrum method including weak far-fault, strong far-fault, and severe near-fault ground motion records. To gain further insight and achieve reliable outputs, more than 4000 runs of dynamic time history analyses have been carried out. The results show that the SF model can predict periods and modal shapes of the original frame with excellent accuracy. For performing nonlinear analyses, by applying the proposed improvements for the required cases, the Improved Substitute Frame (ISF) model can estimate the global behavior and maximum interstory drifts of the original frame with high accuracy.

Seismic Analysis of Multi Storey Building Using Strong Column Weak Beam Ratio (SCWB) in Different Zones of India

An earthquake structures are mandatory to avoid significant damages (i.e., collapse) and aims that structure withstand a major earthquake without collapse. The design approach adopted is to ensure that the columns of the structure more capable to resist moments than beam; to avoid progressive collapse of structure due to failure of columns in lower level; it is necessary the columns have stronger than beams (strong column weak beam). The concept of SCWB is to ensure that plastic hinge formed in the beam not in the column; this help in dissipating the more energy along with providing ductility to the structure. If the plastic hinge is formed on the both ends of column then, the column is not able to spread the plasticity and collapse which are leads to global failure. The failure modes in all past earthquake is exactly opposite i.e, strong beam weak column; and comes to sway mechanism and fails to collapse. For this it is foreseen that the values of ratio of Mc/Mb (ratio of sum of ultimate moment of resistance of columns to sum of ultimate moment of resistance of beam) in the beam-column joint are stated by many design codes and the values are different ranging from 1.2,1.3,1.4,1.5 to 2, etc. Another effect of ratio Ic/Ib (ratio of moment of inertia of column to moment of inertia of beam) have been studied but the exact meeting of SCWB behaviour in the structures at the time of collapse not stated clearly. The Mc/Mb and Ic/Ib ratio are very important to prevent damage in the structure under seismic action. In the present work, attempts are made to achieve exact ratio of exact strong column weak beam. In this study, the combined effect of two ratio (Mc/Mb, Ic/Ib) simultaneously investigated in different zones of India to find out exact SCWB ratio’s value for to meet the SCWB behaviour. Different numerical examples are presented of combine ratios (Mc/Mb,Ic/Ib) and pushover analysis is performed on each ratio’s. The result of the investigation highlighted on the objective that is to find exact SCWB ratio value considering the parameter like target displacement, ductility ratio, hinge response etc.

Numerical Study on the bending behavior of Reinforced Concrete Beams with kinked rebar

During the past earthquakes, the formation of plastic hinges in columns has given rise to global structural damage in moment resisting frame structures. If the plastic hinges are formed in the beams, the most suitable energy dissipating mechanism in structure will happen. The kinked rebar has locally curved regions (usually near the inflection points in beams) which can be gradually straightened under tension. Due to lower initial yielding flexural capacity compared with that of a cross section reinforced with traditional straight bars, the beam section reinforced with kinked rebars will yield first when the RC frame is subjected to seismic loading, and thus, the strong column-weak beam hierarchy can be realized. In this study, first the load-deflection behavior of a reinforced concrete beam was numerically simulated by ABAQUS software, and the reliability of the finite element model was verified by comparing with the experimental results of other researchers. Then, the load-deflection response of the RC beams with kinked bars which has two-step behavior were investigated and described in steps by using finite element modeling results. Finally, the control beam was used for further analyses to investigate the effect of important parameters including: concrete compressive strength, reinforcement ratio, and the ratio of beam span to depth on load-deflection curve of beams. Results showed that the ultimate bearing capacity of RC beam increases as the reinforcement ratio increases, however, it doesn’t have an important effect on the first yielding point. The increase of concrete compressive strength also contributes to greater initial yielding. In addition, the increase of beam span can have negative effects on the flexural behavior of RC beams with kinked bars, and reduce initial strength and stiffness of beams.Finally, the behavior of an internal connection including a beam with kinked rebar to the column was evaluated.

Experimental and numerical research on the multi-stage seismic failure modes of steel frame with end-plate connection

To study the multi-stage seismic failure mode of semi-rigid plane steel frame with end-plate connection, one 1/2-scale specimen of the quasi-static cyclic test was conducted. According to the failure criteria, the mechanical characteristics and the failure process of the specimen at three performance stages (the elastic stage, the elastoplastic stage, and the ultimate stage) were studied. The bearing capacity, stiffness degradation, and the plastic energy dissipation of the test specimen were also analyzed. Moreover, a nonlinear finite element model of the specimen was implemented in the ABAQUS platform to compare and verify the test results, and a parametric analysis was conducted to investigate the influence of the thickness ratio (δ = te/tf) of the end-plate (te) to the column flange (tf) on frame performance. The specimen has reliable energy dissipation capacity and ductility, and the lateral displacement of the first story accounted for about 40% of the second story displacement, while this ratio increased to 50% in the ultimate stage. The increase of δ can significantly improve the lateral stiffness and bearing capacity of the specimens, but the ductility decreases gradually. The failure order of the specimen was the yield of the end-plate, the yield of the beam end, the buckling of the column base, and the failure of the whole frame, which indicated that the specimen satisfied the design criteria of “strong column weak beam”.

On the FE modelling of RC beams with a fibre-reinforced polymer (FRP)-strengthened web opening

In order to realize the strong column-weak beam hierarchy in existing reinforced concrete (RC) frames, a novel seismic retrofit technique (i.e., beam opening technique or BO technique) was proposed. The BO technique involves the cutting of a web opening in the RC T-section beam to weaken its flexural capacity, in combination with the installation of a fibre-reinforced polymer (FRP) strengthening system around the opening to offset the loss in shear capacity caused by the opening. While extensive experimental studies have verified the effectiveness of the BO technique, this paper presents a finite element (FE) study on the structural behaviour of RC beams with an FRP-strengthened web opening, for a more comprehensive and in-depth understanding of the performance of such beams. Two constitutive models of concrete were investigated, namely, the concrete damaged plasticity (DP) model and the brittle cracking (BC) model. Through detailed comparisons, it was found that for specimens failing in a flexural mode, the DP model is more appropriate; while for specimens failing in a shear mode, the BC model with the secant modulus of concrete adopted provides better predictions. A simple method for the selection of a proper FE approach is finally proposed.

Experimental and numerical study on seismic performance of steel reinforced concrete spatial frame with irregular section columns

In order to research the seismic performance of steel reinforced concrete (SRC) spatial frame with irregular section columns, a 1/4 scaled model of two-bay and five-story spatial frame was constructed and tested under low cyclic reversed loading. The failure pattern, hysteresis curves, ductility, drift angle, stiffness degradation, strength degradation and energy dissipation capacity were analyzed. The results showed that the plastic hinges firstly appeared at the end of beams, which obeyed the design principle of “strong-column weak-beam”. The hysteresis curves were plump, indicating that the spatial frame had excellent energy dissipation capacity, but pinched at the later loading stage. The strength degradation coefficients and displacement ductility coefficients were larger than 0.8 and 4.0, respectively, and the ultimate drift angle reached 1/19, showing that the spatial frame had stable bearing capacity, good ductility and strong ability to collapse resistance. Based on the test, finite element analysis software OpenSees was employed to evaluate the seismic performance of the spatial frame and the influence of loading direction, axial compression ratio and length-width ratio of column limb was analyzed. The research results can provide reference for the application of SRC frame with irregular section columns in earthquake fortification zone.

Strong-column–weak-beam criterion for reinforced concrete frames subjected to biaxial seismic excitation

In several earthquakes, numerous reinforced concrete (RC) frames subjected to seismic excitation exhibit a collapse pattern characterised by column hinges, although these frames are designed according to the strong-column–weak-beam (S-C–W-B) criterion. The effect of biaxial seismic excitation on the disparity between the design and the actual performance of RC frames is investigated in this study. First, a modified load contour method was proposed to derive a closed-form equation of the biaxial bending moment strength, which was verified based on numerical and experimental results. Subsequently, a group of time-history analyses of a simple frame modelled using fibre beam-column elements subjected to biaxial seismic excitation were conducted to verify that the current S-C–W-B criterion is inadequate for preventing the occurrence of column hinges. A biaxial overstrength factor was developed based on the proposed equation, and the reinforcement of columns was appropriately amplified using this factor to prevent the occurrence of column hinges under biaxial excitation; this approach was proved to be effective through another group of time-history analyses. Thus, the findings of this study can serve as a basis for improving the seismic design of RC frames, which will enable the construction of safer engineering structures.

Numerical simulation of the influence of bond strength degradation on the behavior of reinforced concrete beam-column joints externally strengthened with FRP sheets

Limited information is known about the effects of bond strength degradation during installation on the bond's quality and performance between fiber-reinforced polymer (FRP) reinforcement and substrate material. This research study's primary focus is to investigate the efficiency of the external FRP composites in rescuing the structural performance and controlling the mode of failure of the reinforced concrete (RC) beam-column joint with different bond strength degradation percentages nonlinear finite element analysis (NLFEA). Firstly, the RC beam-column model was validated against the published experimental results and then was expanded to consider the effect of the degradation percentages in bond strength between concrete and FRP composite (0 % (Fully bond), 10 %, 20 %, 30 %, 40 %, 50 %, 60 %, 70 %, 80 %, 90 %, and 100 % (control or un-strengthened joint). The structural performance was evaluated in terms of failure mode, stress distribution, pulling and pushing ultimate load capacity and corresponding displacement, horizontal load-displacement hysteretic loops, horizontal load-displacement envelopes, displacement ductility, energy dissipation, stiffness degradation, and equivalent hysteretic damping factor. The NLFEA results showed that the FRP strengthening technique with bond strength degradation percentages less than 30 % enhanced the cyclic performance (higher load capacity, larger horizontal displacement, higher displacement ductility, higher energy dissipation, and slower secant stiffness degradation). Also, the utilized FRP method with bond strength degradation percentage less than 30 % performed well in eliminating any surface debonding or buckling in the FRP composite because of the proper lateral support provided for the strengthening sheets. Finally, the bond strength degradation percentage less than 30 % could significantly enhance the deficient joints' seismic performance under strong beam-weak column conditions by changing its behavior to a more ductile one, including the beam flexural hinging. Moreover, the relocation of a plastic hinge in the beam provided more lateral strength for the joint specimens.

Seismic design of self-centering viscous-hysteretic devices used for steel moment-resisting frames

A force-based design (FBD) approach is proposed for steel moment-resisting frames (MRFs) with self-centering viscous-hysteretic devices (SC-VHDs) and numerical analysis is conducted on steel MRFs with SC-VHDs designed by the proposed FBD approach. The results indicate that all of these frames satisfy the peak and residual drift criteria under the design basis earthquake (DBE) and the column-to-beam strength ratio should be equal to or more than 1.8 for frames below 8 stories, whilst for frames with 8 stories or more the column-to-beam strength ratio should be equal to or greater than 1.5 to achieve the strong column-weak beam mechanism. Considering the effects of the column-to-beam strength ratio, the seismic intensity, the structural geometric characteristics, and the parameters of the SC-VHD, inelastic displacement profiles of steel MRFs with SC-VHDs are developed. Based on the proposed displacement profiles, a direct displacement-based design (DDBD) approach is proposed for steel MRFs with SC-VHDs, and the effectiveness of both the proposed DDBD approach and the displacement profiles are verified by conducting numerical analysis on steel MRFs with SC-VHDs designed by the proposed DDBD approach.

Experimental investigation on rcc external beam-column joints retrofitted with basalt textile fabric under static loading

The role of a beam-column joint is very vital in RCC framed structures. Most of the structural collapses due to earthquakes have been initiated by the failure of the beam-column joints and therefore understanding the behavior of beam-column joints is important. Large numbers of retrofitting techniques are being adopted worldwide to strengthen the beam-column joint. This paper presents the results of the first phase of experimental investigation on retrofitting of an external beam-column joint by externally bonded basalt fabrics. Eleven specimens of external beam-column joints using strong column-weak beam concept were prepared and tested under static loading conditions. Three specimens were tested and considered as control specimens. Remaining eight specimens were subjected to an induced damage by preloading them up to 50% of ultimate load recorded in control specimens. The damaged specimens were then retrofitted with basalt fiber fabric adopting four different configurations. The retrofitted specimens were tested for their ultimate capacity and the outputs were compared with that of the control specimens. The test results show that basalt textile reinforcement enhances the performance of the retrofitted specimens.

Influence of the column-to-beam flexural strength ratio on the failure mode of beam-column connections in RC frames

The column-to-beam flexural strength ratio (CBFSR) has been used in many seismic codes to achieve the strong column-weak beam (SCWB) failure mode in reinforced concrete (RC) frames, in which plastic hinges appear earlier in beams than in columns. However, seismic investigations show that the required limit of CBFSR in seismic codes usually cannot achieve the SCWB failure mode under strong earthquakes. This study investigates the failure modes of RC frames with different CBFSRs. Nine typical three-story RC frame models with different CBFSRs are designed in accordance with Chinese seismic codes. The seismic responses and failure modes of the frames are investigated through time-history analyses using 100 ground motion records. The results show that the required limit of the CBFSR that guarantees the SCWB failure mode depends on the beam-column connection type and the seismic intensity, and different types of beam-column connections exhibit different failure modes even though they are designed with the same CBFSR. Recommended CBFSRs are proposed for achieving the designed SCWB failure mode for different types of connections in RC frames under different seismic intensities. These results may provide some reference for further revisions of the SCWB design criterion in Chinese seismic codes.

Analysis of Seismic Performance of Reinforced Concrete Framed Structure

This paper deals with the study of behavior of RC frames design for gravity (GF) and earthquake load (EQF). Nonlinear push over analysis was done using SAP on the models (GF & EQF). The functioning of RC frame is estimated in terms of displacement, shear, and performance point and inters story drift. The results from push over analysis for GF and EQF are compared with each other. An approach of the performance level of a 2D frame (PBF) has been done by using the Deformation based earthquake design with the displacement required to be achieved for the performance level as specified in FEMA 356. Here the yield strength is calculated for the required target displacement and the PBF frame is redesign by strength based. Hinges are developed in the ground floor vertical member while carried out the seismic design and only in beams in EQF this represent the strong beam column weak beam concept is applied and inter-story drift is more in top floor for GF and almost similar in all the floors for EQF. Yield shear strength obtained is almost equal to that of the analysis for Performance based frame.

Cyclic loading test of abnormal joints in SRC frame-bentmain building structure

Due to functional requirements, SRC column-RC beam abnormal joints with characteristics of strong beam weak column, variable column section, unequal beam height and staggered height exist in the Steel reinforced concrete (SRC) frame-bent main building structure of thermal power plant (TPP). This paper presents the experimental results of these abnormal joints through cyclic loading tests on five specimens with scaling factor of 1/5. The staggered height and whether adding H-shaped steel in beam or not were changing parameters of specimens. The failure patterns, bearing capacity, energy dissipation and ductile performance were analyzed. In addition, the stress mechanism of the abnormal joint was discussed based on the diagonal strut model. The research results showed that the abnormal exterior joints occurred shear failure and column end hinge flexural failure; reducing beam height through adding H-shaped steel in the beam of abnormal exterior joint could improve the crack resistance and ductility; the abnormal interior joints with different staggered heights occurred column ends flexural failure; the joint with larger staggered height had the higher bearing capacity and stiffness, but lower ductility. The concrete compression strut mechanism is still applicable to the abnormal joints in TPP, but it is affected by the abnormal characteristics.

Comparative analysis of two seismic energy dissipation solutions in the case of a steel structure

The present paper aims at the comparative analysis of two seismic energy dissipation solutions for a steel structure in frames. We opted for non-braced frames and centrally braced frames. In the case of unbroken frames, the resistance structure consists of steel beams and columns. Potential plastic areas are formed mainly at the ends of the rulers and, to a lesser extent, in columns. Horizontal actions are mainly taken by bending the structural elements. To ensure the most efficient observance of the rigidity requirements for lateral actions, the solution of the rigid beam-column joint is required. The use of the concept of weak beam-strong column leads to the appearance of plastic joints in the beams, close to their joints with the columns. Centric braced frames can take over the horizontal actions, which are manifested in their plane, through axial forces. Energy dissipation occurs mainly by plasticizing the extended diagonals. The comparative analysis consists of highlighting the results obtained from the structural calculation with the help of a commercial automatic calculation program for the two solutions: non-braced frames and centrally braced frames. An office building with 2 levels was analyzed, located in Constanta, where the two types of the staff were provided in turn. The obtained results will be compared from the point of view of the amount of steel used, so from the economic point of view. Another comparison refers to the displacements to SLS and SLU and to the seismic energy dissipation capacity. Finally, another possible comparison refers to the over-resistance of the structural system.