Strong Column-weak Beam Research Articles

Lessons learned from the “5.12” Wenchuan Earthquake: evaluation of earthquake performance objectives and the importance of seismic conceptual design principles

Many different types of buildings were severely damaged or collapsed during the May 12, 2008 Great Wenchuan Earthquake. Based on survey data collected in regions that were subjected to moderate to severe earthquake intensities, a comparison between the observed building damage, and the three earthquake performance objectives and seismic conceptual design principles specifi ed by the national “Code for Seismic Design of Buildings GB50011-2001,” was carried out. Actual damage and predicted damage for a given earthquake level for different types of structures is compared. Discussions on seismic conceptual design principles, with respect to multiple defense lines, strong column-weak beam, link beam of shear walls, ductility detailing of masonry structures, exits and staircases, and nonstructural elements, etc. are carried out. Suggestions for improving the seismic design of structures are also proposed. It is concluded that the seismic performance objectives for three earthquake levels, i.e., “no failure under minor earthquake level,” “repairable damage under moderate earthquake level” and “no collapse under major earthquake level” can be achieved if seismic design principles are carried out by strictly following the code requirements and ensuring construction quality.

Building Damage Subject to The 27th May 2006 Yogyakarta Earthquake: As Consequence of Unclear Code, Desemination, or Construction?

Nowadays, there are many problems existing in both design and construction of reinforced concrete struc-tures. An example of those problems was structural damage caused by the 27th May 2006 Yogyakarta Earthquake. In order to reduce similar structural damage in the future, it is necessary to investigate the buildings in many aspects including current and previous codes dissemination as well as material and con-struction qualities in the reinforced concrete structures. Field investigation on the structural damage cov-ering several aspects has been conducted. The investigation included several engineered reinforced con-crete structures not only for heavy and moderate damaged structures but also light damaged buildings as well. Inadequacy of the structural design and its building constructions has been identified to meet the cur-rent codes, i.e. PBI 1971, SK SNI 1991 and RSNI 2002. The result shows that serious damages in the rein-forced concrete structures were mainly caused by the several basic defects in the design and construction, such as inappropriate seismic-design philosophy, lack of structural strength, building configuration prob-lem, building sway mechanism problem, short element effect, shear reinforcement problem, and poorly code dissemination process. Based on this investigation, valuable lesson learned for future structural dam-ages in the reinforced concrete can be withdrawn. Keywords: strong column weak beam, joint failure, shear failure, short column, structural pounding

STRUCTURAL PERFORMANCE OF A PLANE FRAME FOR COMPOSITE CES STRUCTURAL SYSTEMS

A static loading test was carried out on a two-bay two-story concrete encased steel (CES) frame in which the beams and columns consisted of only steel and fiber reinforced concrete (FRC). In this test, the structural behavior of the frame such as the failure modes, hysteresis characteristics and deformation capacity was examined. Although the frame designed to be a weak beam-strong column system with the flexural yielding at the both ends of beams and the bottom of the first story columns, the shear yielding in the joint panel of the interior beam-column joints also occurred. The results showed that the use of FRC in the CES frame controlled the damage of the concrete. In addition, the frame had stable hysteresis loops with almost no degradation in load carrying capacity until the average story drift angle of 0.05 rad. Static inelastic analysis with reversal loading was also conducted to simulate the story shear versus story drift response of the frame. It is shown that the analytical results agree well with the test results in this paper.

Seismic Design of Steel Joist Girder Structures

Current seismic design provisions emphasize detailing that results in large system ductility and energy dissipation. This may not be the best approach for flexible structures that may be able to withstand large earthquakes in the elastic range or with very limited inelasticity. Among these types of structures are tall one-story frames often constructed utilizing joist girder systems. The main objective of this study is to provide a safe and efficient seismic design methodology for steel joist girder structures; typically one-story industrial or commercial structures intended primarily to carry light roof loads. A new design procedure based on the weak column—strong beam concept is developed and experimental and analytical verification studies are described. A full-scale cyclic test and pushover analyses show that the intended mechanism can be achieved if the required design strength of the joist girder members and connection to column are based on a column flexural strength of 1.2RyMpc. This design requirement is needed in order to avoid a buckling failure of the compression angle members of a joist girder and failure of connections at the end of the joist girder.

RETROFIT OF EXISTING BUILDINGS FOR EARTHQUAKE RESISTANCE

Buildings sited on soft soils are sometimes subjected to tremors due to earthquakes occurring some 400 to 700 kilometers away as a result of the amplifying effect of soft soils on low-frequency, long-distance waves. This study focuses on the seismic vulnerability of existing reinforced concrete (RC) buildings in Singapore that are designed primarily for gravity loads, and examines the use of externally bonded glass fiber-reinforced polymer (FRP) systems in retrofitting these buildings to resist lateral forces due to seismic action. Two case studies were considered: (1) a four-story frame building, representing typical low-rise buildings; and (2) a 25-story shear wall-frame building, representing typical high-rise buildings. Pushover tests were carried out correspondingly on 1/2-scale sub-frames and 1/5-scale shear walls. The one-and-a-half bay, two-storey frame specimens represent the critical part of the low-rise building while the I-shaped wall specimens represent the lower critical 2.6 stories of the high-rise building. Test results revealed a strong column–weak beam failure mechanism for both the un-retrofitted and retrofitted frames. The retrofitted frame was 30 percent higher in ultimate strength but 12 percent lesser in ultimate drift ratio compared to the un-retrofitted frame. For the wall specimens, sudden failure of the un-retrofitted shear wall was observed at the base of the side walls due to shear. Failure of the retrofitted wall was however more ductile with FRP debonding, followed by concrete crushing and FRP rupture at the compressive base of the side wall. The ultimate load capacity and lateral displacement of the retrofitted wall increased respectively by 45 and 66 percent.

The impact of the 26 December 2004 earthquake and tsunami on structures and infrastructure

A field investigation of the 26 December 2004 south east Asia earthquake- and tsunami-affected areas in Thailand and Indonesia was conducted. The objective of the study was to evaluate the effects of the event on buildings, bridges and infrastructure. The effect of the tsunami on structures and infrastructure produced surprising behaviour with several new lessons to learn. Failure of critical infrastructure such as bridges, harbour docks, hospitals and communication systems delayed search and rescue operations and relief efforts, which increased the suffering of the survivors. The causes of structural failures due to the earthquake included soft story, strong beam–weak column designs, short columns, deficient beam–column joints, non-ductile detailing, unreinforced masonry and inadequate construction quality. It is concluded that it is necessary to design critical structures and important infrastructure to survive tsunami effects. Well-designed and constructed structures with attention to details and no significant additional cost survived the earthquake and the resulting tsunami with minimum damage.

Direct Inelastic Earthquake Design Using Secant Stiffness

A new earthquake design method performing iterative calculations for secant stiffness was developed. Since basically the proposed design method uses linear analysis, it is convenient and stable in numerical analysis. Moreover, the proposed design method can accurately estimate the inelastic strength and ductility demands of structural members through iterative calculations. In the present study, the procedure of the proposed design method was established, and a computer program incorporating the proposed design procedure was developed. Design examples were presented to verify the advantages of the proposed method. The proposed method, an integrated analysis and design method, can address the earthquake design strategy intended by the engineer, such as limited ductility of the member and the concept of strong column-weak beam. The strength and ductility demands satisfying the given design strategy can be directly determined for all the members. As a result, structural safety and economical design can be achieved.

Shaking table tests of reinforced concrete wide beam–column connections

AbstractReinforced concrete wide beam–column connections have been used in low‐to‐moderate seismicity regions despite little information being available on their seismic performance. This research was conducted to clarify experimentally the hysteretic behaviour and ultimate energy dissipation capacity (UEDC) of this type of existing connection under lateral dynamic earthquake loadings. For this purpose, ⅔ scale models were constructed and tested on a shaking table until they collapsed. The exterior connection behaved as a strong column–weak beam mechanism, and the interior connection as a weak column–strong beam mechanism. The averaged UEDC of the connections in each domain of loading, normalized with respect to the product of the yield strength and yield displacement, were about 6 and 5 for the exterior and interior connections, respectively. Copyright © 2005 John Wiley & Sons, Ltd.

Effects of strain hardening of steel reinforcement on flexural strength and ductility of concrete beams

In the design of reinforced concrete beams, it is a standard practice to use the yield stress of the steel reinforcement for the evaluation of the flexural strength. However, because of strain hardening, the tensile strength of the steel reinforcement is often substantially higher than the yield stress. Thus, it is a common belief that the actual flexural strength should be higher than the theoretical flexural strength evaluated with strain hardening ignored. The possible increase in flexural strength due to strain hardening is a two-edge sword. In some cases, it may be treated as strength reserve contributing to extra safety. In other cases, it could lead to greater shear demand causing brittle shear failure of the beam or unexpected greater capacity of the beam causing violation of the strong column-weak beam design philosophy. Strain hardening may also have certain effect on the flexural ductility. In this paper, the effects of strain hardening on the post-peak flexural behaviour, particularly the flexural strength and ductility, of reinforced normal- and high-strength concrete beams are studied. The results reveal that the effects of strain hardening could be quite significant when the tension steel ratio is relatively small.

철골 연성 모멘트 골조에 대한 다자유도 시스템의 연성계수 평가

연성 계수는 반응수정계수(R)의 핵심구성요소로서 내진설계에서 중요한 역할을 하는 계수이다. 본 연구에서의 연성계수()는 단자유도 구조물의 연성계수()에 다자유도 보정계수()를 곱하여 평가하였다. 단자유도 시스템의 연성계수는 지진하중을 받는 단자유도 구조물의 변위 연성요구도와 주기에 따른 비선형 동적해석으로부터 산정하였다. 다자유도 시스템의 영향을 고려하기 위한 다자유도 보정계수()는 기존의 연구에 근거하여 제시하였다. 철골 연성 모멘트 골조의 연성계수를 평가하기 위하여, 구조물의 층수, 골조시스템(외곽골조, 분배골조), 붕괴 메카니즘(강-기둥 약-보, 약-기둥 강-보), 토질조건 및 지진구역을 변수로 하여 총 108개의 예제 구조물을 설계하였다. 구조물의 층수, 붕괴 메카니즘 및 토질조건은 연성계수에 큰 영향을 미치는 반면, 골조 시스템 및 지진구역은 연성계수에 영향을 미치지 않는 것으로 나타났다. Ductiluty factor has played an important role in seismic design as it is key component of response modification factor(R). In this stuty, ductility factors() are calculated by multiplying ductility factor for SDOF systems() and MDOF modification factors(). Ductility factors() for SDOF systems are computed from nonlinear dynamic analysis undergoing different level of displacement ductiluty demands and period when subjected to a large number of recorded earthquake ground motions. The MDOF modification factors() are proposed to account for the MDOF systems, based on previous studies. A total of 108 prototype steel frames are designed to investigate the ductility factors considering the number of stories(4, 8 and 16-stories), framing system(Perimeter Frames, PF and Distributed Frames, DF), failure mechanism(Strong-Column Weak-Beam, SCWB and Weak-Column Strong-Beam, WCSB), soil profiles(SA, SC and SE in UBC 1997) and seismic zone factors(Z=0.075, 0.2 and 0.4 in UBC 1997). It is shown that the number of stories, failure mechanisms (SCWB, WCSB), and soil profiles have great influence on the ductility factors, however, the structural system(Perimeter frames, Distributed frames), and seismic zones have no influence on the ductility factors.

Hysteretic energy response of steel moment‐resisting frames with vertical mass irregularities

AbstractDynamic analyses were carried out to study the seismic response of high‐rise steel moment‐resisting frames in 16‐storey buildings. The frames are intentionally designed using three different design procedures: strength‐controlled design, strong column–weak beam controlled design, and drift‐controlled design. The seismic performances of the so‐designed frames with vertical mass irregularities were discussed in view of drift ratio, plastic hinge rotation, hysteretic energy input and stress demand. A demand curve of hysteretic energy inputs was also presented with two earthquake levels in peak ground accelerations for a future design application. Copyright © 2004 John Wiley & Sons, Ltd.

수직질량 비정형이 존재하는 고층 강 모멘트-저항골조의 지진 거동

Dynamic analyses were carried out to study the seismic response of high-rise steel moment-resisting frames in sixteen story buildings. The frames are intentionally designed by three different design procedures; strength controlled design. strong column-weak beam controlled design. and drift controlled design. The seismic performances of the so-designed frames with vertical mass irregularities were discussed in view of drift ratio. plastic hinge rotation, hysteretic energy input and stress demand. A demand curve of hysteretic energy inputs was also presented with two earthquake levels in peak ground accelerations for a future design application.

Guidelines: Seismic Design of Composite Reinforced Concrete and Steel Buildings

Seismic design guidelines for composite reinforced concrete and steel ~RCS! buildings consisting of reinforced concrete columns and structural steel beams were proposed in Japan as a practical outcome of the U.S.-Japan Cooperative Earthquake Research Program on Composite and Hybrid Structures. A design method satisfying the most prevalent seismic provisions used in Japan, which are based on the working stress design method, is described. At the same time, an alternative method, which is suitable for use in the context of the capacity spectrum method, is also briefly introduced. This paper outlines the design guidelines, giving priority to an explanation of the design method for the working stress seismic provisions. In this method, the weak beam-strong column mechanism is used as the design basis for buildings under severe earthquake. Therefore, moment magnification factors for both columns and joint panels are provided to secure the mechanism. Design equations for the ultimate shear strength of joint panels and associated hysteresis models are also included in the guidelines, and can be applied to a total of twelve details of RCS beam-column joints, including both the through beam and the through column types.

Seismic Behavior of Reinforced Concrete Column-Steel Beam Subassemblies and Frame Systems

In this paper, results are presented from tests of four reinforced concrete (RC) column-steel beam (RCS) subassemblies under large displacement reversals and dynamic analyses of RCS systems under various ground motions. The test specimens were designed following a strong column-weak beam philosophy and a deformation-based capacity design method for the connections. The effect of joint deformations on overall behavior of RCS frame systems was studied through a series of inelastic dynamic analyses performed on a six-story RCS frame with various joint modeling and design parameters. Results from this research show that RCS frame systems perform satisfactorily under seismic excitations. The test specimens showed a good strength and stiffness retention capacity with excellent energy dissipation up to displacement levels of approximately 5.0% drift. The joint deformation-based capacity design procedure was effective in controlling damage in the connection region, and thus forcing most of the inelastic activity to concentrate in the beam ends. From the analytical studies, it was found that joint deformations have a significant influence on maximum interstory drift of RCS frames. For the ground motions considered, an increase of up to 35% in maximum story drift was obtained when connection flexibility was considered in the analyses.

Modified iterated simulated annealing algorithm for structural synthesis

A new hybrid simulated annealing method is presented for the optimization of structural systems subjected to dynamic loads. The optimization problem is formulated as a structural weight minimization, with time-varying constraints on floor displacements, velocities, accelerations, or floor drifts, and structural member combined stresses. In addition, time-invariant constraints on structural frequencies and member sizes that will satisfy the strong column–weak beam philosophy of the building codes can be imposed. The method uses elements of existing simulated annealing algorithms and introduces certain new procedures. Firstly, the search range is automatically reduced, by using the updated information of the current design, at each iteration. Secondly, the inner and outer iteration loops are implemented. Thirdly, sensitivity analysis of the time-varying global displacements is performed with respect to the design variables that are the structural member cross-sectional areas. The results of the sensitivity analysis identify which design variables must be modified to decrease the global displacements in the most effective manner. However, once the variables are identified from the sensitivity analysis, the new values of these variables are determined in a random manner. The possibility of attaining a global minimum is thus maintained. The method is suited for structural optimization problems with time-varying constraints because the annealing is a random search technique and can locate global rather than local minima.

Evaluation of the seismic level of protection afforded to steel moment resisting frame structures designed for different design philosophies

Steel moment resisting frames have been considered as excellent systems for resisting seismic loads. However, after recent earthquakes (e.g., Northridge, California, in 1994 and Kobe, Japan, in 1995) the confidence in this structural system was reduced as a result of various types of damage that moment resisting steel frames suffered. This paper presents the results of the evaluation of seismic level of protection afforded to steel moment resisting frame buildings designed in accordance with the National Building Code of Canada. Six- and 10-storey office buildings located in a region of intermediate seismic hazard are designed in accordance with the current Canadian code provisions. Three different design philosophies are considered, namely strong column - weak beam (SCWB), weak column - strong beam (WCSB), and strong column - weak panel zone (SCWP). The performance of these frames is evaluated dynamically by subjecting an inelastic model to an ensemble of 12 actual strong ground motion records. The model takes into account both connection flexibility and panel zone shear deformation. The results are presented in terms of response parameters determined from static pushover analyses, as well as statistical measures of the maximum response parameters determined from the inelastic dynamic analyses. The computed performance of the frames is evaluated in order to assess both the overall level of protection of the frames and the preferred design philosophy. It is concluded that a well-designed and well-detailed ductile moment resisting frame designed using either the SCWB or SCWP design philosophy can withstand ground motions of twice the design level with very little likelihood of collapse, whereas a frame designed using the WCSB approach is ill-conditioned and may develop a collapse mechanism at an excitation level well below twice the design level.Key words: seismic, ductile, steel, frame buildings, performance, design, ductility, damage, inelastic, dynamic.

FRC-Steel Joist Composite Frames for Seismic Resistance

The research reported in this paper is concerned with development of fiber-reinforced concrete-encased open web steel joist composite frames for seismic resistance. This new and innovative system completely eliminates the need for any shear connectors between steel joists and surrounding concrete as well as that for conventional longitudinal and transverse reinforcement, all of which are quite labor intensive. In the initial phase of the study tests were carried out on half-scale beam specimens subjected to monotonic and reversed cyclic loading. The results showed that the presence of steel fibers as well as the open web steel significantly enhances the shear capacity of the system thus making it very suitable for seismic resistance. In the final phase, a half-scale, one-story, one-bay frame was designed by strong column-weak beam concept. The results showed ductile and stable hysteretic behavior. The study indicates that this type of composite framing system has excellent potential in seismic regions.

Correlation of damage and analysis of R/C building: Experience from the 1995 Kobe earthquake

During the 1995 Hyogoken-Nanbu Earthquake, a reinforced concrete building, called Jeunesse Rokko, suffered intermediate damage by forming a beam-yielding (weak-beam strong-column) mechanism, which has been regarded as the most desirable earthquake resisting mechanism throughout the world. High cost to repair damage at many beam ends and poor appearance expected after the repair work made the owner decide to tear down the building. Nonlinear earthquake response analyses were conducted to simulate the behavior of the building during the earthquake. The influence of non-structural members was considered in the analysis. The calculated results were compared with the observed damage, especially the location of yield hinges and compression failure of spandrel beams, and the degree of cracking in columns and in column-girder connections.

Seismic Performance of Moment-Resisting Steel Frame Structures Designed for Different Levels of Seismic Hazard

A large number of low and medium-rise buildings have moment-resisting steel frames as the primary lateral load resisting system. This type of structural system has been considered to be one of the most effective means for resisting strong seismic ground motions. This paper presents the results of an investigation into the performance which can be expected when code designed ductile moment-resisting steel frame buildings having ductile connections, designed for different seismic hazard levels, are subjected to excitation by strong seismic ground motions. Design variations at different seismic hazard levels include three design philosophies, namely strong-column weak-beam, weak-column strong-beam and strong-column weak panel zone. The analytical model used in the study takes into account both connection flexibility and panel zone shear deformation. Results show that the performance arising from a specific design philosophy may be acceptable in one seismic hazard region, while being unacceptable in a region having a different seismic hazard level.

鉄筋コンクリート造内部柱梁接合部の剪断破壊機構に関する実験的検討

Reinforced concrete earthquake-resistant frames are generally designed on the basis of “strong-column weak-beam”concept. Besides, shear failure of beam-column joints should be avoided in order that the frames exhibit adequate ductile performance. Joint shear failure occurs when the shear stress level is enough large, but the shear failure mechanism is different between the conditions with and without adjacent beam yielding. The experimental study shows that the joint shear failure results from the compressive fracture of joint concrete in which the compressive strength deterioration occurs due to shear cracking. And joint reinforcement makes a large influence on the joint shear strength when adjacent beams yield, because the joint reinforcement becomes the principal confinement for the joint concrete.