MOMENT RESISTING WELDED CONNECTIONS: AN EXTENSIVE REVIEW OF DESIGN PRACTICE AND EXPERIMENTAL RESEARCH IN USA,JAPAN AND EUROPE

Welded connections, widely used in seismic moment resisting frames (MRFs) in USA, Japan and Europe, have been extensively investigated during the 1990s in order to improve their performance under severe earthquakes. In order to correctly evaluate the results of experimental research on welded connections, the differences among the American, Japanese and European current practices in designing the frame structural systems and in detailing the beam-to-column connections should be firstly appraised. In this paper, the major aspects characterising the USA, Japan and Europe design practice in moment resisting welded connections are reported, the differences are underlined and the main issues recently addressed in experimental research are reviewed. Among the several parameters which affect the connection performance, attention is focused on the effect of the beam cross-section size. Thus conclusions are drawn in terms of plastic rotation capacity as dependant on the beam size.     

Development of Neural Network Based Hysteretic Models for Steel Beam-Column Connections Through Self-Learning Simulation

Beam-column connections are zones of highly complex actions and deformations interaction that often lead to failure under the effect of earthquake ground motion. Modeling of the beam-column connections is important both in understanding the behavior and in design. In this article, a framework for developing a neural network (NN) based steel beam-column connection model through structural testing is proposed. Neural network based inelastic hysteretic model for beam-column connections is combined with a new component based model under self-learning simulation framework. Self-learning simulation has the unique advantage in that it can use structural response to extract material models. Self-learning simulation is based on auto-progressive algorithm that employs the principles of equilibrium and compatibility, and the self-organizing nature of artificial neural network material models. The component based model is an assemblage of rigid body elements and spring elements which represent smeared constitutive behaviors of components; either nonlinear elastic or nonlinear inelastic behavior of components. The component based model is verified by a 3-D finite element analysis. The proposed methodology is illustrated through a self-learning simulation for a welded steel beam-column connection. In addition to presenting the first application of self-learning simulation to steel beam-column connections, a framework is outlined for applying the proposed methodology to other types of connections.     

STRUCTURAL RESPONSE AND DAMAGE ASSESSMENT BY ENHANCED UNCOUPLED MODAL RESPONSE HISTORY ANALYSIS

The Uncoupled Modal Response History Analysis (UMRHA) method developed by Chopra et al. is modified in this paper to estimate damage to welded moment-resisting connections in a steel frame (MRSF) subjected to earthquake ground motions. The behaviour of these connections is modelled by a moment-rotation relationship that accounts for the cracking of the beam flange-to-column flange groove weld. The behaviour of the frame is approximated by a sequence of single-degree-of-freedom (SDOF) models for the first three modes to allow for the contribution of higher modes of vibration. The dynamic properties of these SDOF systems are determined by nonlinear static pushover analyses of the building frame. Because of the significant drop in connection strength caused by beam-to-column weld cracking, the pushover procedure uses a changing rather than invariant distribution of horizontal loads, while the structural responses are calculated from shapes that are based on the displaced shape of the frame after damage occurs. The accuracy of the method is demonstrated by a comparison with the results of a nonlinear time history analysis of the frame. This method can be used for rapid assessment of seismic damage or damage potential and to identify buildings requiring more detailed investigation.     

Seismic Performance Assessment of Steel Frames with Shape Memory Alloy Connections. Part I — Analysis and Seismic Demands

This article is the first of two companion articles that evaluate the seismic performance of steel moment-resisting frames with innovative beam-to-column connections that incorporate shape memory alloys (SMAs) to dissipate energy and provide recentering effectively during large earthquakes. Two types of SMA elements are considered: (1) superelastic SMA elements with recentering capability and (2) martensitic SMA elements with high energy dissipation capacity. This article describes the fundamental engineering characteristics of these SMA connections, their modeling in connections for nonlinear dynamic finite element analysis of building frames, and the validation of these connection models using data from full-scale experimental tests that were performed in previous research at Georgia Institute of Technology. Using three- and nine-story partially restrained (PR) moment frames selected as case studies from the SAC Phase II Project, nonlinear time history analyses of frames with and without SMA connections were conducted using suites of ground acceleration records. The beneficial effects of SMA connections on peak and residual deformation demands are quantified and discussed.     

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.     

Assessment of Perforated Steel Beam-to-Column Connections Subjected to Cyclic Loading

This article presents a study of fully fixed (welded) perforated beam-to-column connections, used as strengthening techniques to seismic-resistant design. The effect of using non-standard novel web opening configurations of variable depths and positions is investigated. The improvements on the structural behavior foreshadow the enhancements gained using these perforated members. It is concluded that using large isolated perforations is an effective way of improving the behavior of connections enhancing their ductility, rotational capacity and their energy dissipation capacity. Moreover, the connections with novel openings outperform the conventional ones; therefore, they can be suitably used in the aseismic design of steel frames.     

ANALYSIS,TESTING AND DESIGN OF STEEL ROOF DECK DIAPHRAGMS FOR DUCTILE EARTHQUAKE RESISTANCE

An experimental program was conducted to study the inelastic response of steel roof deck diaphragms for low-rise steel buildings subjected to seismic loading. Tests were performed on 3.6 m×6.1 m diaphragm specimens made of corrugated steel deck panels. The parameters examined were the thickness and configuration of the sheet steel panels, the type and spacing of the fasteners, the applied loading history and the influence of end lap joints. Diaphragms built with screwed side lap fasteners and nailed deck-to-frame connectors exhibited a pinched hysteretic behaviour, but could sustain large inelastic deformation cycles with limited strength degradation. This type of diaphragm construction could be designed to resist earthquake effects in the inelastic range. Higher shear resistance and less pinching was observed for systems that included welded with washer connections. However, their strength decreased rapidly after the peak load was reached, and hence, these systems should be designed for limited inelastic response. Deck systems with button punched side laps and frame welds without washers showed a brittle response and should be designed to remain elastic under severe earthquake motions. The inelastic demand was found to increase when the spacing of the fasteners was reduced. Specimens constructed with an internal overlap joint exhibited extensive warping of the cross section mainly due to the shorter panel length.     

Reinforced Concrete Exterior Waffle Flat Plate-Column Connections Subjected to Lateral Earthquake Loading

An experimental investigation of an exterior waffle flat plate-column connection subjected to combined gravity and cyclic lateral loading is presented. The test model was designed according to construction practices used over ten years ago in the Mediterranean area. Test results are discussed in conjunction with previous studies on flat-slab connections and ACI 318–05 code idealizations. The specimen exhibited a “strong column-weak plate” mechanism, whose behavior was controlled by the torsion cracks. First yielding and failure occurred at 1% and 5.5% drift-ratios, respectively. The results show that ACI 318–05 code significantly underestimates strength; as an alternative, a simple model is proposed.     

Seismic Performance Assessment of Steel Frames with Shape Memory Alloy Connections,Part II – Probabilistic Seismic Demand Assessment

This article is the second of two companion articles that evaluate the seismic performance of steel moment-resisting frames with innovative beam-to-column connections that incorporate shape memory alloy (SMA) elements to enhance the energy dissipation characteristics of such frames. Building upon the finite element models of the three- and nine-story frames that were developed in the first article, the seismic demands on partially restrained frames with and without SMA elements are evaluated within a probabilistic framework. The results of this evaluation, expressed in the form of demand hazard curves, depict the effectiveness of the SMA connections in enhancing building performance over a range of demand levels. Martensitic SMA connections are most effective in controlling deformation demands on the frame from high levels of seismic intensity. In contrast, the recentering capability of superelastic SMA connections make them most suitable for reducing residual deformations in the structure, a reduction that is achieved at the expense of increased deformation demands during strong excitation. However, neither connection is uniformly beneficial at all hazard levels, suggesting that SMA systems must be tailored to the specific performance objectives for the building structural system.     

STIFFNESS AND FRACTURE CHARACTERISTICS OF THE NORTHRIDGE STEEL MOMENT RESISTING CONNECTIONS

The Northridge fractures demonstrated the inability of the welded beam-to-column connection, typically used in California prior to the 1994 earthquake, to fully develop the beam's plastic moment and, in some cases, to even reach yield. The unanticipated character of these failures implies that loads at which fracture occurs under Northridge-type conditions as well as the effects the latter has on a building's stiffness need to be quantified. This paper addresses these issues in a quantitive manner. To simplify matters, the behaviour of a simple single degree-of-freedom (SDOF) frame, with connection characteristics modelled in a way that reflects the Northridge conditions, is examined by taking into account the random nature of the connection's resistance to fracture. The fracture event is found to result in a modest reduction in the frame's lateral stiffness, with the post-fracture behaviour being governed by the behaviour of the remaining connecting elements.     

FRAGILITY ASSESSMENT OF SLAB-COLUMN CONNECTIONS IN EXISTING NON-DUCTILE REINFORCED CONCRETE BUILDINGS

Fragility functions that estimate the probability of exceeding different levels of damage in slab-column connections of existing non-ductile reinforced concrete buildings subjected to earthquakes are presented. The proposed fragility functions are based on experimental data from 16 investigations conducted in the last 36 years that include a total of 82 specimens. Fragility functions corresponding to four damage states are presented as functions of the level of peak interstory drift imposed on the connection. For damage states involving punching shear failure and loss of vertical carrying capacity, the fragility functions are also a function of the vertical shear in the connection produced by gravity loads normalised by the nominal vertical shear strength in the absence of unbalanced moments. Two sources of uncertainty in the estimation of damage as a function of lateral deformation are studied and discussed. The first is the specimen-to-specimen variability of the drifts associated with a damage state, and the second the epistemic uncertainty arising from using small samples of experimental data and from interpreting the experimental results. For a given peak interstorey drift ratio, the proposed fragility curves permit the estimation of the probability of experiencing different levels of damage in slab-column connections.     

The Effect of Stiffened Floor and Roof Diaphragms on the Experimental Seismic Response of a Full-Scale Unreinforced Stone Masonry Building

An extensive experimental program was carried out at EUCENTRE, within a research project on the evaluation and reduction of the seismic vulnerability of stone masonry structures. The main part of the experimental program has been devoted to the shaking table tests on three full-scale, two-story, single-room prototype buildings made of undressed double-leaf stone masonry. The first building tested was representative of existing unreinforced stone masonry structures with flexible wooden diaphragms, without any specific anti-seismic design nor detailing. In the second and third buildings, strengthening interventions were simulated on structures theoretically identical to the first one, improving wall-to-floor and wall-to-roof connections and increasing diaphragm stiffness. In particular, in the third specimen, steel and r.c. ring beams were used to improve the diaphragm connection to the walls and collaborating r.c. slab and multi-layer plywood panels were used to stiffen floor and roof diaphragms, respectively. This article describes the strengthening interventions applied to the third building prototype and presents the experimental results obtained during the shaking table tests. The results obtained permitted the calibration of a macroelement model representative of the nonlinear behavior of the structure.     

Interconnected Epistyles of Marble Monuments Under Axial Loads

The restoration of destroyed connections of epistyles joined together by means of metallic connectors is among the most difficult problems encountered by scientists working for the conservation/restoration of monuments made of marble. The complexity of the geometry and the interaction of three completely different materials (metal-filling material-marble) constituting the connection render the study (either experimental or analytic) extremely complicated. In this direction a numerical analysis is presented here in order to deeply understand the response of a typical ancient connection subjected to axial loads as well as to quantify the influence of some geometric parameters on its response. The parameters studied include the length of the connector, the width of its flanges, the depth of the groove (mortise), the existence or not of relieving space and the filling material’s volume. The numerical model was calibrated and validated according to the results of an earlier experimental study realized on the worksite of the Parthenon Temple. It was concluded that the interventions required to relieve the stress field by changing the size of the connector are disproportional to the respective positive effect on the response of the connection. Moreover it was proved that the most beneficial parameter is the relieving space.     

Development and Seismic Behavior of Precast Concrete Beam-to-Column Connections

A new precast concrete beam-to-column connection for moment-resisting frames was developed in this study. Both longitudinal bar anchoring and lap splicing were used to achieve beam reinforcement continuity. Three full-scale beam-to-column connections, including a reference monolithic specimen, were investigated under reversal cyclic loading. The difference between the two precast specimens was the consideration of additional lap-splicing bars in the calculation of moment-resisting strength. Seismic performance was evaluated based on hysteretic behavior, strength, ductility, stiffness, and energy dissipation. The plastic hinge length of the specimens is also discussed. The results show that the proposed precast system performs satisfactorily under reversal cyclic loading compared with the monolithic specimen, and the additional lap-splicing bars can be included in the strength calculation using the plane cross-section assumption. Furthermore, the plastic hinge length of the proposed precast beam-to-column connection can be estimated using the models for monolithic specimens.     

OUT-OF-PLANE SEISMIC RESISTANCE OF MASONRY WALLS

Seismic vulnerability of unreinforced masonry buildings is studied by means of simplified out-of-plane collapse mechanisms that take into account connections with transversal walls. According to experimental evidence, the analysis assumes that failure is reached with a rigid body motion of a part of the facade that falls down. Two classes of mechanism are examined: the overturning of the facade due either to a vertical crack at the connection or a diagonal crack on the transversal wall, both defined resorting to a simple model of masonry fabric, viewed as a regular assembly of rigid blocks and elastic plastic joints with friction but no cohesion. The use of simplified mechanisms give rise to an explicit evaluation of the seismic resistance to changes in the geometry and in the masonry fabrics, that could be used by practising engineers. This formulation is developed for both static horizontal actions and ground velocity peak, in the belief that the latter probably gives a better approximation of seismic action, while also providing, by comparison with the results of static forces, an estimate of the behaviour factor for unreinforced masonry. Eventually, the analytical forecasts are compared with numerical results obtained by means of the distinct element method.     

Increasing Seismic Energy Dissipation of Steel Moment Frames Using Reduced Web Section (RWS) Connection

Connections of steel moment frames are vulnerable to brittle failure. Providing a perforation near the beam-ends is suggested as a potential method to improve seismic behavior of these structures. This article presents a numerical study on the energy dissipation of steel moment connections with perforated beam. Models with elongated circular openings of different dimensions and location are analyzed and compared based on the global and local damage indices, predicted failure time and dissipated energy. Results show that an RWS connection with a proper opening size can develop reasonable inelastic deformations and provide an acceptable seismic improvement to moment-resisting frames.     

Experimental and Numerical Investigation of IPE Reduced Beam Sections with Diagonal Web Stiffeners

A reduced beam section (RBS) is a new type of connection in steel moment resistant frames. In addition to the major benefits, RBS has its own weaknesses, such as web local buckling and lateral torsional buckling. The purpose of this paper is to improve the performance of European I-beam profile (IPE) with an arched cut in the flange, using a diagonal stiffener of the beam web. With the help of laboratory tests and numerical models, it was found that the use of a diagonal stiffener in the area of an arched cut increased the energy dissipation and plastic rotation capacity of RBS connection.     

Seismic Evaluation and Retrofit of Asymmetric Multi-Story Wood-Frame Building

The presented research focuses on large-scale seismic testing under multi-directional ground motion of a three-story high, wood-frame residential building representing late 1960's California construction. Earthquake lateral resistance is provided by plywood shear walls around the perimeter of the building with an open front in the first story for tuck-under parking. Accordingly, the as-built structural configuration is asymmetric in plan and discontinuous in elevation with tendency to twist about a vertical axis and to form a weak story mechanism. The test results confirm this tendency. They also reveal the sensitivity of the response to multi-direction ground motion. Asymmetric damage patterns are induced by the multi-component motions in the walls oriented perpendicular to the open front for the as-built test structure, with or without finish materials. However, the observed damage remained noncritical as far as structural integrity is concerned even for ground accelerations exceeding 120% of that recorded during Northridge earthquake. This is viewed as a consequence of the better construction of the test building compared to actual construction. Investigated retrofit includes adding a welded moment resisting steel frame around of the garage opening and strengthening the diaphragm to header beam connections. The study indicates that the retrofit significantly reduced the maximum story drift in the open front. Moreover, the finish material and the retrofit greatly reduce the maximum rotation of the building about the vertical axis.     

Dynamic One-Sided Out-Of-Plane Behavior of Unreinforced-Masonry Wall Restrained by Elasto-Plastic Tie-Rods

Past earthquakes have shown the high vulnerability of existing masonry buildings, particularly to out-of-plane local collapse mechanisms. Such mechanisms can be prevented if façades are restrained by tie rods improving the connections to perpendiculars walls. Whereas in the past only static models have been proposed, herein the nonlinear equation of motion of a monolithic wall restrained by a tie rod is presented. The façade, resting on a foundation and adjacent to transverse walls, rotates only around one base pivot and has one degree of freedom. Its thickness is explicitly accounted for and the tie rod is modeled as a linear elastic—perfectly plastic spring, with limited displacement capacity. The model is used to investigate the response to variations of wall geometry (height/thickness ratio, thickness), tie rod features (vertical position, length, prestress level), and material characteristics (elastic modulus, ultimate elongation, yield strength) typical of historical iron. The most relevant parameter is the steel strength, whereas other characteristics play minor roles allowing to recommend reduced values for pre-tensioning forces. The force-based procedure customary in Italy for tie design is reasonably safe and involves protection also against collapse, although probably not enough as desirable.     

Ecofeminism and First Nations Peoples in Canada: Linking culture,gender and nature

Many ecofeminists see women's subordination as a result of linking women with nature. Thus one of their tasks has been to unravel the underlying dualistic structure of the categories ‘women’ and ‘nature’ and to argue for a reconceptualization of these categories. However, there exist amongst ecofeminists epistemological differences pertaining to the ways in which the women–nature connection should be addressed. Spiritual ecofeminists argue that the connection between women and nature is worth reclaiming and celebrating. In contrast, social ecofeminists contend that the connection represents a patriarchal artifice that reinforces oppression. In support of both perspectives, ‘Western’ ecofeminists have invoked the cultural beliefs and histories of Aboriginal peoples. Such use of Aboriginal beliefs and experiences within much of Western ecofeminist discourses is partial and uninformed. In this article an alternative approach is offered—one that emphasizes the importance of listening to Aboriginal voices describing contemporary connections to nature. Aboriginal voices are presented in the context of in-depth interviews conducted with Anishinabek (Ojibway and Odawa peoples) living in one First Nations community and three cities in Ontario, Canada. The interviews highlight the importance of listening to Anishinabek describe their connections to Mother Earth (nature) as they reveal counter-narratives that offer the potential to reconcile spiritual and social ecofeminism and to reconceptualize nature (Mother Earth) as an active and dynamic agent. Such counter-narratives may improve current understandings of gender–nature connections within Western ecofeminisms.