Rotational Ductility Research Articles

Damage assessment of square RC bridge columns subjected to torsion combined with axial compression, flexure, and shear

Reinforced Concrete (RC) bridge columns can be subjected to combined flexure, shear, axial and torsional loadings under multidirectional earthquake motions and significant vertical motions, specifically in skewed and curved bridges, bridges with specific structural restraints, and bridges with unequal spans or column heights. This paper presents experimental and analytical studies to assess the damage progression of square RC bridge columns under combined loadings including torsion. The main variable in this study is the ratio of Torsional-to-bending Moments (T/M). The effects of combined loadings on the torsional and flexural hysteretic responses, strength, plastic hinge distribution, and progression of damage states are highlighted in this paper. A unified equivalent damage index model is proposed to couple the flexural and torsional actions for combined loadings. Quantified damage indices value under combined loadings can be correlated to the categorized damage states from a performance-based point of view. Based on the experimental and analytical results, the progression of damage was found to be amplified by the torsion effect, and the rotational ductility was decreased due to the effect of combined loadings.

A proposal for plastic hinges modification factors for damaged RC columns

This paper proposes experimental based formulations for modifying the moment–rotation plastic hinges of RC columns that have entered the plastic range, introducing suitable expressions of modification factors for stiffness, strength and displacement capacity as a function of the rotational ductility demand. Those expressions may be used in order to assess the performance loss of a building that has been damaged by an earthquake. Considering typical RC members of buildings in Mediterranean regions, with design characteristics non-conforming to present-day seismic provisions, a database of 23 cyclic tests results on columns reinforced with deformed bars and of 13 tests for columns with smooth bars was assembled and studied. The trends resulting from experimental data show that member’s stiffness decreases quite steeply with ductility demand, while an almost parabolic increase with ductility of the ratio of residual drifts versus yielding drift ratio can be observed. The obtained modification factors can be used to simulate the damage state in RC members that have experienced a given ductility demand after an earthquake, either retrieved via analytical simulations (e.g. pushover analyses) either inferred by observation of real damage on the structural elements.

Influence of incident angles of earthquakes on inelastic responses of asymmetric-plan structures

This paper presents the influence of incident angles of earthquakes on inelastic dynamic responses of asymmetry single story buildings under seismic ground motions. The dynamic responses such as internal forces and rotational ductility factor are used to evaluate the importance of the incident angles of ground motions in the inelastic range of structural behavior. The base shear and torque (BST) response histories of the resisting elements and of the building are used to prove that the shape of the BST surface of the building can be a practical tool to represent those of all resisting elements. This paper also shows that the different global forces which produce the maximum demands in the resisting elements tend to converge toward a single distribution in a definable intensity range, and this single distribution is related to the resistance distribution of the building.

Effect of Strength Eccentricity on Torsional Behaviour of RC Frame Buildings

This study attempts to understand the inelastic seismic behaviour of asymmetric multistory RC buildings using non-linear dynamic time history analysis. Both inherently asymmetric and artificially generated eccentric building models were considered. Two categories of artificially generated models were considered. In the first category, groups of mass symmetric systems (MSS) having strength, stiffness and both strength and stiffness eccentricities were considered. In the second category of models, mass eccentricity was introduced in otherwise symmetric models. Building systems were modelled as 3D space frame with lumped plasticity. The response is evaluated using the peak rotational ductility demand of beams of the different frames as measures of their inelastic response. Investigations of first category of artificially generated MSS showed that the response can be better co-related with the strength eccentricity. The results from the second category of systems not designed for torsion indicated that there is significant variation in the beam ductility demands of different frames, whereas systems designed for torsion indicated a shift in the center of strength towards the center of mass and exhibited almost uniform ductility demand in beams of various frames for smaller eccentricities.

Lightly Damped Moment-Resisting Steel Frames: A Design-Based Approach

The current U.S. seismic design provisions for steel moment-resisting frames generally result in structures for which stiffness is the controlling factor in the design. The design for stiffness often provides considerable overstrength, which reduces rotational ductility demand on the plastic hinges in the structure. Even though the reduction in ductility demand may be considerable, the design provisions do not allow the detailing rules to be waived, resulting in designs that may not be economically optimized. This paper presents the results of a study in which a variety of steel frames were designed for strength and that used added energy dissipation in the form of linear viscous dampers to control the drift. The goal of the study was to provide only enough damping to control the drift, and to this end, it was found that total system damping of 10% critical was sufficient. As shown in the paper, the added damping provided the required drift control and had the added advantage of minimizing the dispersion, which typically occurs in response history analyses carried out under several appropriately scaled ground motions. Such dispersion control is illustrated through incremental dynamic analysis of damped nine-story buildings in Seattle, Washington.

Prediction of available rotation capacity and ductility of wide-flange beams: Part 1: DUCTROT-M computer program

This paper is a development of the [1], presenting the new improvements in determining the rotation capacity of wide-flange beams. It deals with the available rotation capacity of steel beams, using the local plastic mechanism methodology. For these purposes, a more advanced software was elaborated at the “Politehnica” University of Timisoara, namely DUCTROT-M, substituting the old DUCTROT-96 computer program presented in [1]. The new version considers two different forms the in-plane and out-of-plane plastic mechanisms, as well as the application of gradient or quasi-constant moments. A CD-ROM containing the free DUCTROT-M computer program can be finding in Appendix of [2] or free on the site [3]. Thus, the present paper is focused on the phenomenological aspects of the utilized plastic collapse mechanisms, while the companion paper [41] is devoted to applications in design practice exploiting the new capabilities of the aforementioned current software version.

Prediction of available rotation capacity and ductility of wide-flange beams: Part 2: Applications

Exploiting the capabilities of the DUCTROT-M computer program, a comparison between cross-section ductility classes, recommended in codes, and member ductility classes, proposed in technical literature, is performed, showing very important differences. A numerical analysis was carried out examining the main influencing factors such as collapse modes, fabrication details (welding, hot-rolling, connection details), material properties, geometrical beam dimensions and type of action. Because the out-of-plane mechanism produces very high ductility degradation during a cycling earthquake, the first purpose of the study is to determine the measures to eliminate this mechanism mode. For the welded beams, geometrical proportion of the flange and web thickness is the key, while for hot-rolled beams, the constructional detailing of the joint. The contribution of the junction between flange and web that increases the rotation capacity was revealed. Consequently, rolled and welded sections possess different available ductility, but this effect does not reflected to the current Eurocodes

A New Laboratory Evaluation Method for the Adhesive Performance of Crack Sealants

Abstract Crack sealing is a practice used for routine and preventive maintenance as part of a pavement’s preservation strategy. However, crack sealant failures are common in Texas, particularly within the first 3 years of application (or service life). The major causes of sealant failures can be classified under two categories: Adhesion failure and cohesion failure. Although both failures can lead to a significant reduction in the service life of a pavement structure, adhesive failure is in most cases the dominant failure type. Several laboratory tests such as rotational viscosity, penetration, softening point, ductility, and bond tests are currently used to evaluate the properties of crack sealants. Among these tests, the bond test is specifically used to evaluate the adhesive failure of crack sealants and the procedure of the bond test is well documented in ASTM D5329-07. However, the bond test often takes several days to complete, and the pass/fail criterion is determined through visual observation, which is a very subjective process. Furthermore, the correlation between the bond test and adhesive failure performance of crack sealants in the field was found to be either weak or non-existent. In an attempt to minimize these problems, a simple, fast, and adhesive performance-related laboratory test method is developed in this paper to ensure the proper selection of a sealant for a given project.

Effect of retrofit strategies on mitigating progressive collapse of steel frame structures

In this study, the effect of three retrofit strategies on enhancing the response of existing steel moment resisting frames designed for gravity loads is investigated using Alternate Path Methods (APM) recommended in the General Services Administration (GSA) and the Department of Defense (DoD) guidelines for resisting progressive collapse. The response is evaluated using 3-D nonlinear dynamic analysis. The studied models represent 6-bay by 3-bay 18-storey steel frames that are damaged by being subjected to six scenarios of sudden removal of one column in the ground floor. Four buildings with bay spans of 5.0 m, 6.0 m, 7.5 m, and 9.0 m were studied. The response of the damaged frames is evaluated when retrofitted using three approaches, namely, increasing the strength of the beams, increasing the stiffness of the beams, and increasing both strength and stiffness of the beams. The objective of this paper is to assess effectiveness of the studied retrofit strategies by evaluating the enhancement in three performance indicators which are chord rotation, tie forces, and displacement ductility demand for the beams of the studied building after being retrofitted.

Simplified Procedure for Seismic Evaluation of Piles with Partial-Moment Connection to the Deck in Marine Oil Terminals

This paper presents development of a simplified procedure for seismic evaluation of piles with partial-moment connection typically used in marine oil terminals. The current seismic evaluation procedure of the piles in marine oil terminals includes monitoring material strains specified in the Marine Oil Terminal Engineering and Maintenance Standard (MOTEMS) during the nonlinear static pushover analysis to estimate the displacement capacity of piles. This investigation developed closed-form formulas for estimating the displacement capacity of piles by using a simple pile-deck connection system. The displacement capacity estimated from these formulas ensures that the material stain limits specified in the MOTEMS is not exceeded. These formulas are demonstrated to be accurate by comparing results from these formulas against those from the nonlinear finite-element analysis. The formulas developed in this investigation utilize the curvature ductility capacity of the pile section and rotation ductility capacity of the connection at the selected seismic design level, along with the parameter β which depends on the relative stiffness of the pile and the connection and the parameter η which depends on the relative strength of the connection and the pile.

Reply to discussion by S. Guravich and S. Skarborn on “Double-angle shear connections with short outstanding legs”Appears in the Canadian Journal of Civil Engineering, 35(8): 786–795.

The authors thank S. Guravich and S. Skarborn for their thoughtful discussion. First, the authors would like to emphasize that while the bending of the outstanding legs is a significant contributor, it is not the only source for the rotational ductility of double-angle shear connections. Shortening of the outstanding legs does reduce the rotational capacity of the connections. However, for the tested connections in the study, they still possessed a rotational capacity at the level of 0.03 radians. As pointed out by S. Guravich and S. Skarborn, column face widths less than 203 mm may be affected. The paper gave 152 mm as a column face width that would definitely create the problem rather than may create the problem. For face width 203 mm, the fillet welds can often run onto the corner radius of the hollow structural section column to eliminate the need for shortening the outstanding legs. The initial thought for group C specimens to adopt filled flush flare bevel groove welds (FBGW) was to reflect such a fact that ‘‘in general, there is a tendency for flare groove welds to be oversized and filled flush’’ (quote from Packer and Frater 2005). This tendency was likely due to the following two reasons: (1) the CISC Handbook (CISC 2006) only listed filled flush for flare bevel and flare V-welds in its Welding Practice in Part 6; and (2) the concept of a flare bevel fillet weld was introduced for the first time only recently in CSA W59 (Packer, and Frater 2005) and in Supplement No.1 (January 2005) to CSA-S16-01 (CISC 2006). Some extra work on the FBGWs was reported by the first author in a subsequent paper (Gong 2008). For the tested connections, the average corner radius was 22 mm, which was considerably less than the maximum value 36 mm given by the CISC Handbook. The subsequent study found that the weld face width was a better characteristic dimension than corner radius to predict the effective throat of the FBGWs. The authors believe the groups A and B specimens, which used fillet welds, also provided useful information for the design of FBGW using unaltered outstanding legs. For example, corresponding to 6 mm fillet weld, the required effective throat for FBGW is 4.3 mm. To avoid further stress concentration, a weld return can be made at the top of the angles and then ground. In addition, the designer has the option to increase the effective throat of the FBGW. The unique merit of the unaltered outstanding leg is that it is able to provide extra space for stronger FBGW, if necessary. The authors agree that the paper would have been enhanced if additional connections using smaller size flare bevel fillet welds had been tested. The authors will consider doing additional testing subject to financial support being available. As for other preferred solutions, such alternative connection types were beyond the scope of the paper.

Hysteretic behaviour of flush end plate joints to concrete-filled steel tubular columns

The results of an experimental research study involving the testing of four bolted moment-resisting connections under simulated seismic loading conditions are presented. Each test specimen modeled the interior joint of a moment-resisting frame consisting of H-shaped steel beams and circular or square concrete-filled steel tube (CFST) columns using high-strength blind bolts. In order to investigate the seismic behaviour of the blind bolted flush end plate joints to CFST columns, the hysteretic performance, failure modes, stiffness degradation and energy dissipation of the connection type are evaluated in detail. The test parameters varied included the column section type and the thickness of the end plate. The experimental results indicate that both the blind bolted connections with circular and square sections exhibited excellent hysteretic behaviour in terms of their moment–rotation response, strain distributions and energy dissipation. Under cyclic loading, all tested specimens displayed large rotation ductility capacities, and the failure modes were similar to those under monotonic loads. The effects of cyclic loading on the behaviour of the composite joint were obvious, especially on load bearing and stiffness of the connections. The joint type exhibited excellent seismic performance, so that it can be effectively utilized in moment-resisting composite frame structures.

Carbon Fiber-Reinforced Polymer for Continuity in Existing Reinforced Concrete Buildings Vulnerable to Collapse

Carbon fiber-reinforced polymer (CFRP) can provide continuity in reinforced concrete beams and thereby reduce the likelihood of progressive collapse if a supporting column were lost due to extreme loading (blast or impact). Seven half-scale specimens representing two spans of a reinforced concrete (RC) frame with a center supporting column removed were tested. The capacity of vulnerable RC building beams with discontinuous reinforcing steel were evaluated and compared with beams using CFRP to provide continuity, and with beams designed with continuous reinforcing steel. It was found that CFRP is capable of providing sufficient continuity to withstand the loss of a supporting column through either catenary (or cable) action, which reduces material usage, or flexural action, which reduces deflections. Furthermore, beams with continuous reinforcement may not be able to withstand the loss of a center-supporting column due to limited rotational ductility of the beam.

Tying capacity of web cleat connections in fire, Part 1: Test and finite element simulation

The tying capacity of connections between beams and columns is very important in maintaining structural integrity when beam deflections are high due to accidental loads such as fire, but has not so far been thoroughly studied. The project which is the subject of this paper has investigated the robustness of common types of steel connection when subjected to fire. The results reported here concern the performance of web cleat connections in fire, and are drawn largely from experimental investigations. During the testing programme, short cantilever stub beams were subjected to different combinations of shear and tying force. The rotational capacities and resistance to tying forces of their connections at high temperatures were investigated in the presence of other concurrent actions. Test results show that web cleat connections have excellent rotational ductility, and that their resistance reduces rapidly with increase of temperature. Web cleat connections can fail in a number of modes, the selection of which is highly dependent on the connection temperature. Finite element simulations of the test results have been shown to be able to reproduce the behaviour accurately up to the stage at which material failure happens. However, as the ultimate behaviour of connections is often controlled by material fracture, finite element analysis is limited in predicting the ultimate resistance of connections. Investigation of the behaviour of the connection, with some proposed modifications to the general finite element model, showed that finite element analysis can help to interpret the test results and expand the test observations to other similar applications.

Experimental verification of bridge seismic damage states quantified by calibrating analytical models with empirical field data

Bridges are one of the most vulnerable components of a highway transportation network system subjected to earthquake ground motions. Prediction of resilience and sustainability of bridge performance in a probabilistic manner provides valuable information for pre-event system upgrading and post-event functional recovery of the network. The current study integrates bridge seismic damageability information obtained through empirical, analytical and experimental procedures and quantifies threshold limits of bridge damage states consistent with the physical damage description given in HAZUS. Experimental data from a large-scale shaking table test are utilized for this purpose. This experiment was conducted at the University of Nevada, Reno, where a research team from the University of California, Irvine, participated. Observed experimental damage data are processed to identify and quantify bridge damage states in terms of rotational ductility at bridge column ends. In parallel, a mechanistic model for fragility curves is developed in such a way that the model can be calibrated against empirical fragility curves that have been constructed from damage data obtained during the 1994 Northridge earthquake. This calibration quantifies threshold values of bridge damage states and makes the analytical study consistent with damage data observed in past earthquakes. The mechanistic model is transportable and applicable to most types and sizes of bridges. Finally, calibrated damage state definitions are compared with that obtained using experimental findings. Comparison shows excellent consistency among results from analytical, empirical and experimental observations.

Seismic Behaviour of Semi-Rigid Joints to Concrete Filled Steel Tubular Columns

This paper discussed the results of experiments on bolted moment connection joints of square or circular concrete filled steel tubular (CFST) columns and H-shaped steel beam using high-strength blind bolts under cyclic loading. The objective of this work was to study the seismic performance of the blind bolted flush endplate connections to CFST columns. The test parameters varied were the column section type and the thickness of the endplate. The feasibility of the proposed beam-column connection is successfully verified by the experiments. The test results showed that under cyclic loading the tested specimens displayed large rotation ductility capacities and could satisfy the request of the structural seismic design. When subjected to cyclic loading, most of failure modes of the tested joints are similar to those under monotonic loading. Moreover, the energy dissipation of the type joints is influenced by the column section type and the thickness of the endplate.

Recent developments in composite connections

AbstractEurocode 4 [1] suggests design rules for the evaluation of the mechanical properties of steel‐concrete beam‐to‐column composite joints (rotational stiffness, resistance and ductility). These rules cover situations where the joints are subjected to moments and shear forces, but in hogging regions only (negative bending moments). Recently, researches have been conducted on the behaviour of composite joints (Fig. 1) subjected to positive bending moments (called “sagging moments” in the following) and combined bending moments and axial loads; for instance, these loading situations may appear in composite sway frames under conventional loading and exceptional actions (impact, explosion …) leading to the loss of a structural column [2].In the present paper, the Eurocode 4 design rules are extended to joints in sagging regions and to joints subjected to combined bending moments and axial loads and the proposed methods are validated through comparisons with experimental test results. An easy‐to‐use and freely available software for the design of composite joints according to the Eurocode 4 principles is also described.

지반특성을 고려한 FCM 교량의 지진취약도 분석

본 연구에서는 확률적 지진취약도 분석방법을 3경간 FCM 교량에 적용하여 지반특성에 따른 지진취약 교각의 위치와 교각 상 하부에서의 소성힌지 발생 상태를 수치예제 결과의 분석을 통해 평가하였다. 이를 위해, 취약도 곡선을 최대지반가속도(PGA)의 변수에 대한 대수정규분포 함수로 가정하고 대수정규분포 함수의 주요 계수인 중앙값과 대수표준편차는 최우도추정법(Maximum Likelihood Method)으로 구하였다. 또한 지점별 상이한 지반특성은 "도로교표준시방서"에 제시되어 있는 지반의 등가 스프링을 사용하여 해석모델에 반영하였으며, 지진취약도 분석에 필요한 구조물의 손상지수로 교각의 소성힌지에서의 회전연성도를 이용하였다. This study investigates the influence of various soil properties on the seismic performance of a three-span FCM bridge. Piers that are vulnerable to seismic vibration are identified through numerical study of plastic hinges possibly occurring at the top and bottom of the piers. The fragility curve is obtained as a lognormal distribution function with respect to peak ground acceleration(PGA). The median and logarithmic standard deviation, which are two parameters of a lognormal distribution function, are estimated using the maximum likelihood method. In order to consider the different soil properties of each support, an equivalent spring based on the Korean Standard Specifications for Highway Bridges(KSSHB) is adopted in this study. For seismic fragility analysis, the rotational ductility demands of bridge piers are used as a damage index of the structure.

Estimation of seismic inelastic deformation demands in plane steel MRF with vertical mass irregularities

An extensive parametric study on the inelastic seismic response of plane steel moment-resisting frames with vertical mass irregularity is presented. A family of 135 such frames, designed according to the European seismic and structural codes, are subjected to an ensemble of 30 ordinarily (i.e., without near-fault effects) earthquake ground motions scaled to different intensities in order to drive the structures to different limit states. The statistical analysis of the created response databank indicates that the number of storeys, beam-to-column strength ratio and the location (top, midheight and bottom) of the heavier mass influence the heightwise distribution and amplitude of inelastic deformation demands, while the response does not seem to be influenced by the mass ratio. Nonlinear regression analysis is employed in order to derive simple formulae which reflect the aforementioned influences and offer, for a given strength reduction (or behavior) factor, three important response quantities, i.e., the maximum roof displacement, the maximum interstorey drift ratio and the maximum rotation ductility along the height of the structure.

Concrete Component of the Rotational Ductility of Reinforced Concrete Flexural Members

Reinforced concrete flexural members inherently rely on member ductility to ensure a safe design by allowing for: redistribution of applied stress resultants; quantification of drift for determining magnified moments; and for the absorption of seismic, blast and impact energy. Structural engineers have recognised that much of the member rotation is concentrated in a small region referred to as the plastic hinge and because of the complexity of the problem this has been quantified mainly through testing. In this paper, a new plastic hinge approach that is based on well established shear-friction theory is postulated. The generic behaviour of this novel shear-friction hinge is shown to agree with that exhibited in tests. Furthermore, the shear-friction hinge explains the mechanics of the benefits of confinement, such as that due to FRP encasement or steel stirrups, on the rotational capacity of RC members.