Strong Column-weak Beam Research Articles

Design recommendations for achieving “strong column-weak beam” in RC frames

This paper deals with the realization of ductile failure of RC frames with cast in-situ slabs. The cyclic loading experiments of two spatial reinforced concrete (RC) frames, a control specimen and a RC frame with cast in-situ slabs, are carried out. The failure pattern and the role of slabs are experimentally studied. Tests results indicate that slabs can change the failure pattern of RC frames from a typical “strong-column-weak-beam” failure to the “strong-beam-weak-column” failure. The slab reinforcement enhances the resistance moment of the beams, which is the main reason to form “strong-beam-weak-column”. The parameters such as axial compression ratio, thickness of slab, concrete strength, reinforcement ratio of slab and stiffness of transverse beam are investigated by Finite Element Analysis (FEA). Strut and tie model of slab is proposed to calculate the effective slab width. Simple equations are derived by a 95% guarantee rate based on 200 models. The required ratio of column-to-beam strength is studied based on the tests and FEA data. A reasonable required ratio is proposed to avoid the brittle failure of the RC frames for seismic design.

Dynamic response of a full-scale reinforced concrete building frame retrofitted with FRP column jackets

Many existing reinforced concrete buildings designed in accordance with pre-1971 codes are generally dominated by weak column-strong beam behavior under seismic loading due to inadequate reinforcement detailing. This behavior can lead to premature failure under seismic loads from damage concentrated in the first story of the structure. This paper presents the results of an experimental investigation into the seismic response of a full-scale, two-story non-ductile reinforced concrete frame. The frame was retrofitted with a fiber-reinforced polymer jacketing system on the first story columns to mitigate seismic vulnerability. Shake weight testing was performed to investigate the dynamic performance of the retrofitted building structure in terms of the modal response, inter-story drift, and effectiveness of the fiber-reinforced polymer jacketing system. The results demonstrate that the retrofit scheme helped develop a more uniform story drift distribution, working to counter the soft-story mechanism commonly found in reinforced concrete frames designed during this period.

Retrofitting of a seismically deficient building

Most available seismic retrofitting methods have certain degree of difficulty to be implemented on site and are thus more expensive. A relatively new retrofitting technique coupling the conventional braces with the recently developed viscoelastic dampers is proposed to improve the seismic performance of deficient structures in more effective and efficient way. The relatively new damper is tubular in shape as opposed to the flat shape seen in the conventional dampers. The effectiveness of the proposed retrofitting technique is validated through the three-dimensional finite element analyses performed on real structures, in which three different brace configurations (diagonal, X and K) and small to strong earthquakes are considered. The vibration periods, interstorey drifts and formation sequences of plastic hinges are compared to each other between the original and the retrofitted structures. The study results show that the energy dissipation resulting from the proposed retrofitting scheme is significant, especially for X-type bracing systems. The proposed retrofitting method also alters the overall framing behaviour from strong beam–weak column to the more desirable strong column–weak beam. The retrofitted structure is shown to satisfy the modern seismic design requirements. The proposed retrofitting technique is also shown to be easier to be installed and cost-effective.

Study on seismic performance of connection joint between prefabricated prestressed concrete beams and high strength reinforcement-confined concrete columns

As the common cast-in-place construction works fails to meet the enormous construction demand under rapid economic growth, the development of prefabricated structure instead becomes increasingly promising in China. For the prefabricated structure, its load carrying connection joint play a key role in maintaining the structural integrity. Therefore, a novel end plate bolt connecting joint between fully prefabricated pre-stressed concrete beam and highstrength reinforcement-confined concrete column was proposed. Under action of low cycle repeated horizontal loadings, comparative tests are conducted on 6 prefabricated pre-stressed intermediate joint specimens and 1 cast-inplace joint specimen to obtain the specimen failure modes, hysteresis curves, skeleton curves, ductility factor, stiffness degradation and energy dissipation capacity and other seismic indicators, and the seismic characteristics of the new-type prefabricated beam-column connecting joint are determined. The test results show that all the specimens for end plate bolt connecting joint between fully prefabricated pre-stressed concrete beam and highstrength reinforcement-confined concrete column have realized the design objectives of strong column weak beam. The hysteretic curves for specimens are good, indicating desirable ductility and energy dissipation capacity and seismic performances, and the research results provide theoretical basis and technical support for the promotion and application of prefabricated assembly frames in the earthquake zone.

Wing walls for enhancing the seismic performance of reinforced concrete frame structures

A building retrofitted with wing walls in the bottom story, which was damaged during the 2008 M8.0 Wenchuan earthquake in China, is introduced and a corresponding 1/4 scale wing wall-frame model was subjected to shake table motions to study the seismic behavior of this retrofitted structural system. The results show that wing walls can effectively protect columns from damage by moving areas that bear reciprocating tension and compression to the sections of the wing walls, thus achieving an extra measure of seismic fortification. A ‘strong column-weak beam’ mechanism was realized, the flexural rigidity of the vertical member was strengthened, and a more uniform distribution of deformation among all the stories was measured. In addition, the joint between the wing walls and the beams suffered severe damage during the tests, due to an area of local stress concentration. A longer area of intensive stirrup is suggested in the end of the beams.

Pseudo-Dynamic and Quasi-Static Testing of an Irregular Steel Concrete Composite Frame with Wing Walls

This paper presents a series of pseudo-dynamic tests (PDTs) and quasi-static tests (QSTs) on a dual wing-walled frame system, represented here by a 1/7-scaled composite moment frame with steel reinforced concrete (SRC) columns and reinforced concrete (RC) wing walls. Special characteristics of this scaled system are irregular story layout, strong-beam weak-column mechanism and large axial load. A series of scaled El-Centro (NS) waves were used as the input ground motion for the PDTs, the results of which showed that the seismic behavior was significantly improved by the RC wing walls. With the strong-beam weak-column connections, severe damages sustained by the longitudinal wing walls (LWW) prevented the potential collapse of column, and the transverse wing wall (TWW) efficiently avoided the fragile shear failure of short columns and panel zone of beam-column joints. The failure mechanisms were identified indicating that wing walls improved the ductility for the bare frame. This study provides a solid experimental support on the evaluation of seismic behavior of irregular SRC frames with RC wing walls, which could be applied in the main factory buildings of thermal power plants (TPP).

Seismic Behavior of RC Beam to Partially Steel Tubed Concrete (PSTC) Column Joints Under High Axial Load and Cyclic Loading

A new type of partially steel tubed concrete (PSTC) column is proposed that is suitable to be used in new high rise reinforced concrete (RC) buildings. Three exterior joint specimens consisting of RC beams and PSTC columns and two exterior RC joint specimens were designed and tested under high axial load and cyclic loading to investigate the joint behavior in terms of failure pattern, hysteresis response, deformation, energy dissipation capacity and degradation of strength and stiffness. Test results indicate that the PSTC column can benefit the performance of the joint in terms of strength, ductility and energy dissipation capacity and can partly compensate for the unfavorable effect induced by slab. The strong column–weak beam mechanism can also be ensured in RC beam to PSTC column joint.

Study on Strengthening of RC Beam Column Joint Using Hybrid FRP Composites

Beam-Column joints are critical zones in reinforced concrete structures which are most vulnerable to earthquake forces. Hence strengthening beam-column joint is vital to save the structure and its inhabitants in case of seismic forces. Numerous retrofitting works using fibre reinforced polymer (FRP) composites are being undertaken worldwide. This work aims to investigate the effectiveness of strengthening beam-column joints using natural and artificial fibres. In this study, basalt (natural fibres) as monolithic composite (BFRP) and as hybrid composite along with glass (artificial fibres) were used for strengthening of beam-column joints. Totally six specimens were prepared and tested under monotonic loading. Specimen details used were: two control specimen, two specimens for monolithic wrapping and remaining two specimens for hybrid wrapping. The test results were compared with control and rehabilitated specimens. The performance of the treated joints was studied using the following parameters: initial and ultimate cracking loads, energy absorption, deflection ductility and stiffness at ultimate. From the test results, it was found that the hybrid combination of Basalt and Glass FRPs were found to be more effective in the treatment of beam-column joints. The strong column weak beam concept was achieved by failure in beam portion which helped in preventing the catastrophic failure of the entire structure.

New factor to characterize mechanism of “strong column-weak beam” of RC frame structures

Most reinforced concrete(RC)frame structures did not achieve the “strong column-weak beam” failure mode in recent big earthquakes, resulting in a large number of casualties and significant property loss. To deal with this serious problem, a new column-beam relative factor was proposed to characterize the relative yield situation of column ends and beam ends. By limiting the column-beam relative factor, RC frame structures could achieve the “strong column-weak beam” failure mode under the excitation of strong ground motions. The limit values of column- beam relative factor were calculated, analyzed and verified by using structural simulation models for corner columns in the bottom story of structures, which are destroyed most seriously in earthquakes. The results show that the limit values should be analyzed under bi-directional ground motion and with different axial compression ratios of columns. The peak ground acceleration(PGA)of ground motions has no significant effect on the limit values, while the type of strong ground motions has a significant effect on the limit values.

SFRHPC interior beam-column-slab joints under reverse cyclic loading

Beam-column joints are highly vulnerable locations which are to be designed for high ductility in order to take care of unexpected lateral forces such as wind and earthquake. Previous investigations reveal that the addition of steel fibres to concrete improves its ductility significantly. Also, due to presence of slab the strength and ductility of the beam increases considerably and ignoring the effect of slab can lead to underestimation of beam capacity and defiance of strong column weak beam concept. The influence of addition of steel fibres on the strength and behaviour of steel fibre reinforced high performance concrete (SFRHPC) interior beam-column-slab joints was investigated experimentally. The specimens were subjected to reverse cyclic loading. The variable considered was the volume fraction of crimped steel fibres i.e., 0%, 0.5% and 1.0%. The results show that the addition of steel fibres improves the first crack load, strength, ductility, energy absorption capacity and initial stiffness of the beam.

Lessons learned from recent destructive Van, Turkey earthquakes

A destructive earthquake, the magnitude of this earthquake was 7.2, hit Van, Turkey on October 23, 2011. After this devastating earthquake, a moderate earthquake which had 5.7 magnitude on November 9, 2011 occurred in Edremit, Van. These earthquakes caused heavy damages and collapses in many reinforced concrete buildings with loss of lives. In this paper, characteristics of ground motions of these earthquakes were studied and, deficiencies in structural elements and engineering faults such as poor workmanship and quality of construction, soft and weak stories, strong beam-weak column, short column, large overhang, hammering and unconfined gable wall were investigated. According to the observations, it was seen that, low quality of structural materials, lack of engineering services, inappropriate design and construction with insufficient detailing of the structural elements were the main reasons of heavy damages.

Realization of the global yield mechanism of RC frame structures by redesigning the columns using column tree method

Global failure mechanism, i.e., the strong-column weak-beam mechanism, can provide higher total energy dissipation capacity with less ductility demand on components than other failure modes, and results in a more uniform story drift distribution and higher resistance to earthquake loads at the system level. However, the current code-based elastic design method cannot guarantee the global failure mechanism of frame structures under severe earthquakes. In this paper, a simple, but practical design procedure is proposed to ensure the global failure mechanism of reinforced concrete (RC) frame structures by redesigning the columns using the column tree method (CTM). CTM considers the yield limit state of all beams and column bases. The code-based design is firstly carried out to determine the section information of all beams and base columns. Then, the internal force demands applied on the column tree can be derived. Lastly, the column moments, shear forces and axial forces are determined according to the free-body diagram of CTM to finish the column redesign. Two RC frame structures with 6 and 12 stories are illustrated to verify the design procedure. The analytical results demonstrate the proposed approach can realize the global failure mechanism.

INFLUENCE OF STRONG COLUMN & WEAK BEAM CONCEPT, SOIL TYPE AND SEISMIC ZONE ON SEISMIC PERFORMANCE OF R C FRAMES FROM PUSHOVER ANALYSIS

Earthquakes are most devastating natural hazards among all the forces that structures are likely to be subjected to. Generally, structures are designed for expected seismic forces and designers rely on the reserve capacity and stiffness aspects of the structure to provide the necessary strength and ductility for resisting unexpected earthquakes. Hence it is very important to design the structure to resist moderate and severe earthquake. According to seismic design philosophy, the capacity design method which is currently used in practice indicates that the structure should have strong columns and not so strong beams to possess good ductility and preferable collapse mechanism. There are many instances of failure of structures during earthquakes due the poor construction practice of providing weaker columns and stronger beams. Pushover analysis is a nonlinear static approach for the seismic analysis of structures subjected to permanent vertical load and gradually increasing lateral load at very large strains up to failure. The present work focuses on emphasizing the need for “strong column-weak beam” concept to enhance the seismic performance of structures using Pushover analysis. The ratio of stiffness of column to that of beam is termed as stiffness ratio, which is varied by changing the cross sections of the columns and keeping the beam cross section the same. For this purpose, a 2D reinforced concrete frame is modeled in ETABS, a finite element software and analyzed is using pushover analysis. The effects of seismic zones and types of soil as per IS 1893 (Part 1): 2002 have been studied. Base shear carried, roof displacements experienced, status of performance point, ductility characteristics and vulnerability index are the parameters used to quantify the performance of RC frame. It is inferred that structures with weaker columns have low seismic capacity, and are most vulnerable to seismic excitations in severe zones and for loose soil condition.

The Effects of Yield Mechanism Selection on the Performance based Plastic Design of Steel Moment Frame

This study concentrates on the effect of selecting a desirable yield mechanism that has a significant effect on seismic design and response of structures designed by Performance Based Plastic Design (PBPD) method. In PBPD method, the design base shear is obtained on the basis of energy-work balance equation implementing pre-selected target drift and yield mechanism. With considering the importance of selection of yield mechanism, a parametric study has been done considering the different types of yield mechanisms to design a model of steel moment frame. The results obtained by nonlinear time history analyses shown that the complete sway mechanism (strong columns-weak beams) is the most efficient in terms of required design base shear for a given target story drift. It also should be noted that it is often impossible to estimate the column moment demand during the event of sever ground motions because they undergo large moments not only from those delivered from beams but also from their own deformation. Therefore, assuming only the criteria of strong column-weak beam mechanism in PBPD method can’t prevent the yielding in columns. In this paper to improvement of PBPD method, some solutions are represented to precisely obtain the required moment of columns to prevent their yielding. Because the yielding of columns leads to form undesirable mechanisms before the structure reaches the pre-selected target drift. For example, a 10 story frame has been designed based on PBPD method using strong column-weak beam mechanism. The frame has been redesigned considering all represented equations of this study (modified PBPD frame). The results obtained by nonlinear static and dynamic analyses show that the modified PBPD frame perform according to expectations in terms of yield mechanism and target drift levels whereas the PBPD frame suffer large story drifts due to flexural yielding of the columns.

Experimental research on seismic behavior of a composite RCS frame

To promote greater acceptance and use of composite RCS systems, a two-bay two-story frame specimen with improved composite RCS joint details was tested in the laboratory under reversed cyclic loading. The test revealed superior seismic performance with stable load versus story drift response and excellent deformation capacity for an inter-story drift ratio up to 1/25. It was found that the failure process of the frame meets the strong-column weak-beam criterion. Furthermore, cracking inter-story drift ratio and ultimate inter-story drift ratio both satisfy the limitation prescribed by the design code. Additionally, inter-story drift ratios at yielding and peak load stage provide reference data for Performance-Based Seismic Design (PBSD) approaches for composite RCS frames. An advantage over conventional reinforced concrete and steel moment frame systems is that the displacement ductility coefficient of the RCS frame system is much larger. To conclude, the test results prove that composite RCS frame systems perform satisfactorily under simulated earthquake action, which further validates the reliability of this innovative system. Based on the test result, some suggestions are presented for the design of composite RCS frame systems.

Nonlinear seismic responses and lateral force transfer mechanisms of RC frames with different infill configurations

Due to the architectural efficiency of masonry-infilled reinforced concrete frames, the frames are highly common structural forms for buildings. However, the infills can significantly modify the structural behaviour of these frames, which can be detrimental to the seismic performance of buildings. This study investigates the seismic response and failure mechanisms of infilled RC-frame structures with five different infill configurations: (1) full infills, (2) 2/3-storey-height infills, (3) a soft first storey, (4) infills with window openings and (5) infills with door openings. The nonlinear response history behaviour of the masonry-infilled RC frames under four realistic earthquakes, namely, the 1979 El Centro, 1987 Superstition Hills, 1995 Kobe and 1999 Chi-Chi earthquakes, were simulated using discrete-finite element analysis with damage-based constitutive relations. The analysis indicated that the degrees of continuity and regularity of the infill panels crucially affect the seismic performance of structures. As long as out-of-plane collapse of infills does not occur, full-height and continuous-infill panels can enhance the overall stability and energy dissipation of frame structures. By contrast, discontinuous infills can inflict serious damage localised at the points of discontinuity in the frame members. Furthermore, the analysis revealed that the design concept of “strong column–weak beam” may not be always applicable to infilled frames.

Investigation and Preliminary Analysis of Rural and Town Building Damages of Yunnan Ludian Ms6.5 Earthquake

The "8.03" Ms6.5 Ludian earthquake in Yunnan province is the largest earthquake of the area in recorded history. The PGA of epicenter (Longtoushan town) reaches 948gal and the intensity of meizoseismal area arrives to IX degree. The earthquake has caused severe casualties and huge property losses. Based on the earthquake damage investigation, it is found that structure types of affected towns mainly include mud-wood, brick-wood, masonry, frame, bottom frame and composite structures. In the IX degree area, most of the mud-wood, brick-wood, unfortified masonry and composite structures turn into severely damaged or destroyed. Most bottom frame structures obtain inclined shear cracks on bottom walls. Part of the masonry structures built after 2008, which are set with ring beams, construction columns, cast-in-place floors and roofs, show well earthquake performances and suffer slight damage. What’s more, most filler walls and other non-structural components of frame structures in the meizoseismal area are severely damaged, part of their column ends or beam-column joints fall into failure, which doesn’t achieve "strong column-weak beam" yielding mechanism of the seismic design code expectation. The ending of this paper summarizes a number of recommendations for post-earthquake recovery and reconstruction work according to local conditions.

Low Reversed Cyclic Loading Test for New Precast Concrete Beam to Column Connection

On the basis of the domestic and overseas scholars’ research, it is innovative to put forward a new precast frame beam-to-column connection. One full-scale beam-to-column connection in a precast concrete（PC）frame and a beam-to-column connection in a cast-in-place concrete structures were tested under uni-directional cyclic loading that simulated earthquake-type motions. The new-type beam-to-column joint in a precast concrete frame consisted of cast-in-place columns and precast beams. Test results showed that（a）The property and action of precast connections are similar to the cast-in-place connection, it showed that both precast connections and cast-in-place connection have similar seismic performance; (b) The cracking load of precast connection are similar to the cast-in-place connection; (c) All the columns of all specimens did not destroy, it illustrate that it conform to the strong column weak beam. (d) In this experiment test, because the control factor use the bending capacity of beam, it also play the same plastic hinge mechanism as the traditional component. All the longitudinal reinforcement destruction ahead of stirrup in beams, which should be confirmed the guiding ideology of strong shear weak bending. On the basis of the test results, design considerations for the beam-to column connection were recommended.

Experimental study on the seismic behaviour of the connection between concrete-filled twin steel tubes column and steel beam

This study evaluates the feasibility of a new type of connection between the concrete-filled twin steel tubes (CFTST) column and steel beams. The formation and feature of the new connection were described firstly; it mainly consists of vertical stiffeners and embedded anchorage plates. The experiments were conducted on seven one-half scaled specimens including six CFTST column-beam joints and one square concrete-filled steel tube (CFST) column-beam joint. And, seismic behaviours including hysteresis curve, typical failure modes, load-carrying capacity, ductility, energy dissipation capacity, etc. were studied. Connections behaved as predicted that plastic hinges were achieved in steel beams. Connections with ribbed anchorage plates are of higher load-bearing and deformation capacity than connections with no ribs; the extension of the vertical stiffener can move the buckling zone away from the column, so the joint’s initial stiffness was improved. The hysteretic characteristic of connections was stable; the energy dissipation capacity, strength and ductility were sufficient. In the whole loading, the stiffness degradation was obvious and bearing capacity degradation was slow. Test results show that connections with good aseismatic behaviour can easily achieve the anti-seismic design principle, namely strong column-weak beam and strong joint-weak member.

Solve Cast-in-Place Floor Slab in Frame Structure Buildings in the Study of \" Strong Beam Weak Column\" Issue

With the development of construction technology and construction equipment, now almost all industrial and civil buildings are changing to frame structure. As a result of the differences in floor height, span and the thickness of the cast-in-place floor slab, so the frame column and beam stress is different. When the stress exceeds a certain limit, then it will result in the problem of “strong beam weak column”, and this affects the quality of the buildings, thus how to solve the strong beam weak column caused by the cast-in-place floor slab becomes very meaningful.