DUCTILITY AND LOAD CARRYING CAPACITY PREDICTION OF STEEL BEAM-TO-COLUMN CONNECTIONS UNDER CYCLIC REVERSAL LOADING

Abstract

The economy and reliability of steel-framed buildings in seismic areas depend basically on the hysteretic behaviour of its individual components, such as members and joints. With reference to the latter, despite the recent semi-continuous frame approach (which appears generally very convenient for the design of low- and medium-rise steel buildings), the present state of knowledge does not allow for a complete understanding of the behaviour and the low-cycle fatigue life of beam-to-column connections under dynamic loads.

This paper presents a criterion for the definition of the low-cycle fatigue strength of steel connections, and proposes two approaches for the design of steel frames in seismic zones via the assessment of the fatigue damage, which is evaluated alternatively on the basis of either the ductility or of the load carrying capacity.     

Constant and Variable Amplitude Cyclic Behavior of Welded Steel Beam-to-Column Connections

This article presents a study on welded beam-to-column joints of moment-resisting steel frames. The main features of the joint specimens are summarised, in order to identify key parameters influencing the joint response as well as their low-cycle fatigue endurance. The cyclic behavior and the low-cycle fatigue strength of the connections were initially assessed by cyclic quasi-static testing, carried out at the Technical University of Milan. Analysis of the results has been carried out in order to verify the validity of a linear damage accumulation model combined with a low-cycle fatigue approach based on S-N lines concept. Moreover, a criterion to predict the type of failure and a procedure of appraising the fatigue endurance are presented and their validity proved by the results of variable amplitude tests.     

Improving Low-Cycle Fatigue Performance of High-Performance Buckling-Restrained Braces by Toe-Finished Method

The purpose of this research is to improve the low-cycle fatigue life of the buckling-restrained brace (BRB) for the development of the high-performance buckling-restrained brace (HPBRB) used in bridge engineering. The HPBRB is expected to suffer three strong earthquakes without severe damage during the lifecycle of a bridge. This article employs a low-cycle fatigue experiment, including two series of BRB specimens, all of which had been tested to failure. In the second batch, the weld toes of the rib had been ground to improve the weld's low-cycle fatigue life. According to test results under constant and variable strain amplitudes, the specimens have a great low-cycle fatigue performance. The weld of the rib clearly affects the performance of the as-welded specimens tested with the relatively small strain amplitude. The toe-finished method effectively improves the BRB's low-cycle fatigue performance. Besides, the in-plane gap width of the BRB slightly affects the low-cycle fatigue life. Finally, the low-cycle fatigue curves are compared between the BRB and the materials and the low-cycle fatigue damage evaluation formulae of the as-welded and toe-finished BRBs are recommended for the strain-based evaluation of the low-cycle fatigue damage.     

FATIGUE ANALYSIS OF MOMENT RESISTING STEEL FRAMES

A procedure for the fatigue assessment of steel building structures subjected to earthquakes is presented. The procedure constitutes an extension of the present, high-cycle, fatigue assessment to cases of low-cycle fatigue. It may serve as a basis for the introduction of a fatigue limit state in the earthquake design of steel structures. It may be also used for the damage assessment of existing steel buildings subjected to past earthquakes. By means of parametric studies, the effects of various parameters on the fatigue susceptibility of several moment resisting steel frames are studied. The influence of a number of parameters such as the type of ground motion, type of structural typology, local fatigue behaviour, overall frame design and semi-rigidity of joints on the susceptibility to damage are investigated.     

Nonlinear Performance of Explicit Pseudodynamic Algorithms

A previously published explicit method has been proved to have the same numerical properties as those of constant average acceleration method for linear elastic systems. Although its application to the pseudodynamic testing of nonlinear systems with high frequency modes has been conducted and thus unconditional stability for nonlinear systems was indicated, there is still lack of an analytical proof. In order to explore the nonlinear performance of this explicit pseudodynamic algorithm, a new parameter of instantaneous degree of nonlinearity is introduced to monitor the stiffness change between the stiffness at the end of a time step and the initial stiffness. This parameter enables basic analysis and error propagation analysis for a nonlinear system. In addition, it can also be applied to construct the rough guidelines to select an appropriate time step to conduct a pseudodynamic test although it is almost impossible to achieve this goal using the currently available techniques for a nonlinear system. Analytical results reveal that this algorithm can have unconditional stability for instantaneous stiffness softening and linear elastic systems while it has conditional stability for instantaneous stiffness hardening systems. The proposed rough guidelines for selecting a time step to yield a reliable pseudodynamic test are confirmed with numerical examples.     

THE γ-FUNCTION PSEUDODYNAMIC ALGORITHM

Practical applications of the γ-function dissipative explicit method to pseudodynamic tests are thoroughly investigated herein. Detailed implementation of this pseudodynamic algorithm is schematically sketched. Numerical experiments and verification tests strongly indicate that the γ-function dissipative explicit method can effectively filter out the spurious participation of high frequency responses while the lower mode responses can be obtained very accurately. In addition, error propagation analysis also shows that the γ-function dissipative explicit method possesses much better error propagation characteristics when compared to the Newmark explicit method. Thus, this pseudodynamic algorithm is very suitable for the test system where the responses are dominated by the low frequency modes and the high frequency responses are of no interest.     

Shock Response from Pseudodynamic Test Using Momentum Equations of Motion

The feasibility of using pseudodynamic techniques to yield shock responses from impulses is studied herein. A direct integration method is often used to solve the force equation of motion in performing a conventional pseudodynamic test. However, this technique might not be applicable to obtain an accurate shock response from an impulse as a load discontinuity occurs at the end of the impulse. This is because this discontinuity will lead to an extra amplitude distortion and the amount of this amplitude distortion is increased with the increase of time step. Hence, a small time step is needed to reduce the extra amplitude distortion and thus the displacement increment for each time step might be smaller than or as small as the resolution of the displacement transducer. As a result, the displacement increment cannot be accurately imposed upon the specimen and the responses will be contaminated by the noise. In addition, the test duration is drastically increased. Alternatively, this difficulty might be overcome if the momentum equation of motion instead of the force equation of motion is solved pseudodynamically. Hence, an external momentum is used in the solution of the momentum equation of motion. Since the external momentum is a resultant of the time integration of the external force the discontinuity problem will automatically disappear. Consequently, reliable shock responses can be obtained from pseudodynamic tests.     

A SUMMARY OF RESULTS OF SIMULATED SEISMIC LOAD TESTS ON REINFORCED CONCRETE BEAM-COLUMN JOINTS,BEAMS AND COLUMNS WITH SUBSTANDARD REINFORCING DETAILS

The results of some simulated seismic load tests on reinforced concrete one-way interior and exterior beam-column joints with substandard reinforcing details typical of buildings constructed in New Zealand before the 1970s are described. The tests were conducted using both deformed and plain round longitudinal reinforcement. The interior beam-column joint cores lacked transverse reinforcement and the longitudinal bars passing through the joint core were poorly anchored. The exterior beam-column joint units contained very little transverse reinforcement in the members and in the joint core. In one exterior beam-column joint unit the beam bar hooks were not bent into the joint core. That is, the hooks at the ends of the top bars were bent up and the hooks at the ends of the bottom bars were bent down. This anchorage detail was common in many older buildings constructed before the 1970s. In the other exterior beam-column joint unit the hooks at the ends of the bars were bent into the joint core as in current practice. The improvement in performance of the joint with beam bars anchored according to current practice is demonstrated. In addition, tests were conducted on interior joints with lap splices in the beam longitudinal reinforcement bars near the column face. The tests were conducted using both deformed and plain round longitudinal reinforcement. Tests were also conducted on columns with plain round bar longitudinal reinforcement and inadequate transverse reinforcement.

The reinforcing details were close to identical to those in an existing seven storey reinforced concrete building that was designed and built in New Zealand in the late 1950s.

The test results give an indication of the performance of beam-column joints and members with the above now out-of-date reinforcing details.

The test results reported are a summary of results reported in a number of publications written since 1994.     

Predicting Ductile Crack Initiation of Steel Bridge Structures Due to Extremely Low-Cycle Fatigue Using Local and Non-Local Models

The strain-based prediction model combining the Miner's rule and Manson-Coffin's relationship provides a local parameter for evaluating the ductile crack initiation of steel structures, and some modified models based on it were proposed to evaluate extremely low-cycle fatigue (ELCF) behaviors of steel structures. Previous research has confirmed these local models to be an accurate index for ductile crack initiation in steel bridge piers, however it is found to quite depend on the mesh size of the numerical model used. In this study, a non local damage parameter is presented and successfully applied to ductile crack initiation life assessment of steel bridge piers subjected to earthquake-type cyclic loading. The non local damage parameter is based on averaging the strains over the effective plane using a weight function in the exponential form, and introduces the non local damage parameter to replace the local state variable. Finite element analysis with three different mesh sizes is employed. Comparisons of the local and non local solutions with those of experiments indicate that the non local prediction model can predict the ductile crack initiation of steel bridge piers with good accuracy regardless of the specimen geometries and loading histories, meanwhile the mesh independent nature of the non local model is also demonstrated.     

Experimental Study on Low-Cycle Fatigue Performance of Weld-Free Buckling-Restrained Braces

A type of weld-free buckling-restrained brace is proposed to eliminate the influence of welding on the low-cycle fatigue performance. The core member is manufactured with no weld existing along the overall length of the member. Three welded and three weld-free specimens under different strain amplitudes were tested, and the hysteretic behavior and low-cycle fatigue performance of the specimens were analyzed. The test results indicate that the ductility and the cumulative plastic deformation of the weld-free specimens are much higher than that of the welded ones, which are much closer to the performance of the material capacity.     

PREDICTION OF DAMAGE IN R/C SHEAR PANELS SUBJECTED TO REVERSED CYCLIC LOADING

In this paper, the damage prediction of shear-dominated reinforced concrete (RC) elements subjected to reversed cyclic shear is presented using an existing damage model. The damage model is primarily based on the monotonic energy dissipating capacity of structural elements before and after the application of reversed cyclic loading. Therefore, it could be universally applicable to different types of structural members, includeing shear-dominated RC members. The applicability of the damage model to shear-dominated RC members is assessed using the results from reversed cyclic shear load tests conducted earlier on eleven RC panels. First, the monotonic energy dissipating capacities of the panels before and after the application of reversed cyclic loading are estimated and employed in the damage model. Next, a detailed comparison between the analytically predicted damage and the observed damage from the experimental tests of the panels is performed throughout the loading history. Subsequently, the effects of two important parameters, the orientation and the percentage of reinforcement, on the damage of such shear-dominated panels are studied. The research results demonstrated that the analytically predicted damage is in reasonably good agreement with the observed damage throughout the entire loading history. Furthermore, the orientation and percentage of reinforcement is found to have considerable effect on the extent of damage.     

Seismic Performance Assessment of Non-Compliant SMRF-Reinforced Concrete Frame: Shake-Table Test Study

Seismic performance assessment is carried out for reinforced concrete structure built in low-strength concrete lacking confining ties in beam-column joint. Shake-table tests were performed on 1/3rd scaled two-story frame using design-spectrum-compatible accelerogram, scaled to various target levels. The frame is observed with beam longitudinal bar slip and pullout. Joints with no confining ties experienced extensive damage, observed with cover/core concrete spalling. The frame could resist 70% of the design ground motion to remain within the code-specified drift limit. The code requirement for minimum column depth will not avoid joint damageability in case of low-strength concrete and joints lacking confining ties.     

NONLINEAR ANALYSIS OF REINFORCED CONCRETE SHEAR WALL UNDER EARTHQUAKE LOADING

A constitutive model for predicting the cyclic response of reinforced concrete structures is proposed. The model adopts the concept of a smeared crack approach with orthogonal fixed cracks and assumes a plane stress condition. Predictions of the model are compared firstly with existing experimental data on shear walls which were tested under monotonic and cyclic loading. The same model is then used in the finite element analysis of a complete shear wall structure which was tested under a large number of cyclic load reversals due to earthquake loading at NUPEC's Tadotsu Engineering Laboratory. Two different finite element approaches were used, namely a two-dimensional and a three-dimensional representation of the test specimen. The ability of the concrete model to -reproduce the most important characteristics of the dynamic behaviour of this type of structural element was evaluated by comparison with available experimental data. The numerical results showed good correlation between the predicted and the actual response, global as well as local response being reasonable close to the experimental one.     

A DAMAGE MODEL FOR THE ANALYSIS OF THE SEISMIC RESPONSE OF MONUMENTAL BUILDINGS

In this paper the Author proposes a damage model for the analysis of masonry plates and shells, which is based on an improvement of a previous constitutive model. The modifications introduced, connected to the head joint damage, allow us to study the influence of masonry texture on the damage modes once the mechanical characteristics of the elements constituting the masonry and the results of tests on simple assemblages are known. Having a nonlinear constitutive model is certainly one of the basic elements for understanding the damage mechanisms in masonry buildings. If, in fact, an elastic-linear constitutive model may be used under normal loading conditions, in critical situations it is necessary to model the damage and the dissipation mechanisms that occur between the elements, stone (brick) and mortar, in correlation with their characteristics and kind of masonry. To validate the model a comparison is made between the numerical and experimental results, in the case of tests available in the literature in masonry panels subjected to out-of plane loading and in a real structure through the observation of the damage in Umbria (Italy) surveyed after the 1997 earthquake.     

Liquefaction Resistance of Silty Sands and Dynamic Properties of Cohesive Soils from Düzce,Turkey

This article presents results from a laboratory investigation into the cyclic and dynamic properties of soils from Düzce, Turkey, conducted after the destructive November 12, 1999 earthquake. The investigation was mainly conducted by means of monotonic and cyclic triaxial, as well as resonant-column tests. The triaxial tests allowed the determination of the liquefaction resistance of silty sands, as well as their critical state behavior, whereas the resonant-column tests allowed the determination of shear modulus and damping ratio of cohesive soils. The results are presented and then discussed together with their pertinence to soil behavior when subjected to earthquake loading.     

SIMULATION OF THE CYCLIC BEHAVIOUR OF R/C RECTANGULAR HOLLOW SECTION BRIDGE PIERS VIA A DETAILED NUMERICAL MODEL

In the search for robust constitutive models suitable for reproducing the performance of bridge piers during a seismic event, this paper details the simulation of the cyclic responses of four rectangular hollow section R/C bridge piers. These four R/C bridge piers were built at scale 1/2.5 and tested experimentally. Both tall and short piers are considered, covering situations where bending or shear are of relevance. Furthermore, the four piers were reinforced according to rather different design strategies: (I) the first is a 30-year-old bridge designed without allowance to the seismic action, and (ii) the second is a bridge fulfilling the EC8 provisions. The detailed constitutive model that provides the numerical predictions includes two submodels: one with two scalar damage variables, reproducing the tensile and compressive degradations of concrete, and the other is based on the Giuffre-Menegotto-Pinto formulation, simulating the cyclic behaviour of the re-inforcement. The Damage Mechanics submodel is implemented at the Gauss points of the finite elements that discretize the concrete, whereas the steel submodel is implemented on the 2-noded truss elements adopted for the rebars. A comparison between the numerical and the experimental results is discussed in detail in this paper.     

Cumulative Ductility Spectra for Seismic Design of Ductile Structures Subjected to Long Duration Motions: Concept and Theoretical Background

A seismic design procedure that does not take into account the maximum and cumulative plastic deformation demands that a structure will likely undergo during severe ground motion could lead to unreliable performance. Damage models that quantify the severity of repeated plastic cycling through plastic energy are simple tools that can be used for practical seismic design. The concept of constant cumulative ductility strength spectra, developed from one such model, is a useful tool for performance-based seismic design. Particularly, constant cumulative ductility strength spectra can be used to identify cases in which low-cycle fatigue may become a design issue, and provides quantitative means to estimate the design lateral strength that should be provided to a structure to adequately control its cumulative plastic deformation demands during seismic response. Design expressions can be offered to estimate the strength reduction factors associated to the practical use of constant cumulative ductility strength spectra.     

SEISMIC DESIGN AND RESPONSE OF BARE AND MASONRY-INFILLED REINFORCED CONCRETE BUILDINGS. PART I: BARE STRUCTURES

Abstract

Eurocode 8 is applied for the complete design of 26 multi-storey reinforced concrete buildings to study its operationally and compare the implications of trading strength for ductility through designing the same structure for a different Ductility Class. The difference between the conventional full Capacity Design of columns in bending and the relaxed one allowed by Eurocode 8 is quantified, and the implications on the column capacities are examined. About half of the designed buildings, representative of the class of regular frames, are subjected to nonlinear dynamic response analyses to spectrum-compatible motions with intensities up to twice that of the design motion. Nonlinear modeling is very simple, but gives satisfactory agreement with available quasistatic or pseudodynamic test results on full scale structures. Results show that the three Ductility Classes of Eurocode 8 are essentially equivalent in terms of material quantities and seismic performance. Within the limitations of the nonlinear modelling, the response results suggest very satisfactory performance of structures designed to Eurocode 8, even under twice the design motion intensity. Softening of the structure due to concrete cracking and steel yielding significantly reduces the seismic force demands and contributes to the satisfactory performance, despite the increased P — 6 effects. Another important contributor to the good performance is the significant overstrength of the members considered in the analyses with their average as-built properties. Beam overstrength due to the contribution of the slab to flexural capacity is large enough to overcome the effects of the application of the relaxed Capacity Design rule to columns in bending. However, the resulting column plastic hinging does not lead to drift concentrations suggesting formation of storey-sway mechanisms.     

不同方法加固古建筑榫卯构架抗震试验

为保护古建筑,采用试验方法,研究了不同方法加固榫卯节点后木构架的抗震性能。基于故宫太和殿某开间梁柱节点的具体尺寸,制作了1:8比例的4梁4柱木结构空间框架模型,其中梁和柱考虑为燕尾榫形式连接。采用人工加载方式,进行了低周反复加载试验,研究了马口铁、CFRP布和钢构件加固榫卯节点后对木构架抗震性能的影响。基于试验结果,获得了加固前后构架的力-侧移滞回曲线和骨架曲线,分析了构架耗能能力、刚度退化规律及变形能力,对不同方法加固木构架的抗震效果进行了对比,对其工程应用提出了建议。