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.     

STRENGTH REDUCTION FACTORS FOR DUCTILE STRUCTURES WITH PASSIVE ENERGY DISSIPATING DEVICES

In recent years, current seismic codes started contemplating the design of structures with passive energy dissipating devices. One important issue for the rational seismic design of these devices and the structure that contains them is the formulation of numerical methods to estimate their design seismic forces. From the study of the dynamic response of single-degree-of-freedom systems subjected to accelerograms recorded in Mexico during the last two decades, expressions to estimate the strength reduction factor that should be used to reduce the elastic design strength spectra for 5 percent damping, to establish the design seismic forces for structures having different combinations of plastic and viscous energy dissipating capacities, are formulated.     

DESIRABLE STRENGTH DISTRIBUTION FOR ASYMMETRIC STRUCTURES WITH STRENGTH-STIFFNESS DEPENDENT ELEMENTS

Recent studies have shown that for many reinforced concrete lateral force-resisting elements (LFRE) stiffness is dependent on strength, and as a result strength assign-ment to these elements would affect both the strength and stiffness distributions in a structure. As a consequence, stiffness distribution cannot be considered known prior to strength assignment. This implies that in assigning strength to LFRE, the designer has the ability not only to prescribe the strength distribution, but also indirectly control the stiffness distribution in the structure. In this paper, a study is made on the seis-mic performance of a number of single-story structures to reconfirm that the “balanced CV-CR location” criterion, previously suggested by the writers, constitutes a desirable strength/stiffness distribution for minimising torsional response of asymmetric reinforced concrete structures.     

Seismic Response of a Four-Story Miniature Building with Replaceable Plastic Hinges

To emphasize on linear and nonlinear seismic behavior of building systems in education, a four-story miniature moment-resisting frame steel building was designed, built, and tested in a shaking table at the Structures Laboratory of the Department of Civil Engineering at the University of Canterbury, New Zealand. A prominent feature of the building is the incorporation of elements designed to form plastic hinges that can be easily replaced after a test with minimum effort and at a very low cost. This model is mainly aimed at education in undergraduate and graduate structural dynamics/earthquake engineering courses and it has also been used to support research. This article describes in detail the main features of the building, its design, and discusses the response of the building to two input ground motions. Because the use of pushover analyses is becoming an industry standard, the some relevant results will be compared with those predicted by such kind of analyses. This article is written in very simple terms and is aimed at the undergraduate and graduate student, at educators in structural design and at structural engineers involved in seismic design of building structures. This article covers many aspects that are seldom highlighted in building behavior to earthquake excitation and that are not always covered in design codes or guidelines.     

Fragility-Based Performance Evaluation of Asymmetric Single-Story Buildings in Near Field and Far Field Earthquakes

Previous studies have shown that both distribution and intensity of response parameters in asymmetric buildings are dependent on their stiffnesses and strength distributions. The locations of centers of strength and rigidity relative to the center of mass provide suitable metrics for strength and stiffness distributions. In general, the proper locations of these centers are a function of earthquake ground motion characteristics and the level of building's nonlinear responses. In this article, using nonlinear response evaluation of single-story building models with a wide range of uncoupled torsional to lateral frequencies subjected to near field and far field earthquake excitations the proper location of building centers is studied . Diaphragm rotation, interstory drift, hinge plastic rotation, and ductility demand are selected as damage measure parameters. To compare the performance of models in each limit state, fragility representation of responses is used. It is concluded that proper configuration of building centers in a torsionally stiff building fundamentally depends on the chosen demand parameter. The proper configuration of centers in a torsionally stiff building for a specific demand parameter can converge the probability and distribution of related damages to those in the symmetric building counterpart. When the critical demand parameter for a case is identified, its corresponding arrangements of centers for a suitable seismic behavior may also be recognized. By rearranging the configuration of centers based on the attained configuration, the adverse effects of asymmetry can be avoided.     

Out-of-Plane Seismic Response of Unreinforced Masonry Walls: Conceptual Discussion,Research Needs,and Modeling Issues

ABSTRACT

Modeling unreinforced masonry walls, subjected to seismic loads applied normal to their plane, has received much attention in the past. Yet, there is a general lack of conformance with regard to what aspects of seismic response a computational model should reflect. Boundary conditions are certainly an important aspect, as the response can involve two-way bending or just one-way bending and, in the second case, along vertical or horizontal directions. In this respect, flexural restraint of wall intersections can be significant in addition to size and placement of openings. Moreover, in-plane damage can modify the boundary conditions and the overall out-of-plane performance. Proper modeling of actions is also relevant, as they can be a result of distortions imposed upon wall elements and/or inertial forces along the span of a wall. Axial forces can markedly affect the out-of-plane response of the wall, particularly vertical compressive forces, which can enhance out-of-plane strength. The outcome of static verifications can be more conservative than that of dynamic analyses, but the latter are much more complex to carry out. These topics are discussed with reference to previous research, observations in the field and in the laboratory, as well as numerical analyses on three-dimensional models.     

对城市社会文化要素经营的探讨

在经济全球化与经济文化一体化的趋势中,社会文化要素对经济活动的影响日渐深刻。本文分析了社会文化要素对城市竞争力和城市经营的影响,认为城市经营必须重视对社会文化要素的经营。在分析了城市社会文化要素经营内容的基础上,提出了当前中国城市社会文化要素经营的着力点。     

Influence of Frequency Content and Peak Intensities in the Rocking Seismic Response of Rigid Bodies

The rocking response of a family of bodies given by its width and height due to synthetic and recorded strong ground motions is presented; this allow us to identify the main parameters that govern their response: peak acceleration and velocity, dominant frequency, and acceleration time-history. Recorded strong ground motions were scaled to the same peak acceleration and also to the same peak velocity to analyze the effect of both parameters in the response of rigid bodies. These three parameters were used to build an equation to obtain the height/width overturning plots. The proposed equation was tested for many well-known strong ground motions and the results were compared to other methods shown to be more accurate. This parametric equation does not need iterations or numerical approximations to be solved and can provide engineers, in a very practical way, minimum design requirements or particular specifications to protect non structural elements.     

World Trade and Workers' Rights: In Search of an Internationalist Position

Trade unions and NGOs have been divided sharply over the issue of a workers' rights clause in WTO trade agreements, and have failed to reach a consensus despite heated debate. This appears to be due to elements of protectionism and nationalism in positions on both sides. Arguments against a workers' rights clause can be classified into those opposing (1) globalisation, (2) the WTO, (3) any linkage between workers' rights and trade, and (4) the proposed mechanisms for enforcement. The first three types of objections can be traced to nationalistic considerations, which subordinate the interests of workers to a "national interest" that represents various business groups. The fourth type, however, includes valid criticisms and reveals elements of protectionism in the current proposals for a workers' rights clause. If the proposal is revised to eliminate these elements, it should be possible to arrive at a consensus among progressive labour unions and NGOs. This would be an important step towards an internationalist strategy to fight for minimum labour rights worldwide.     

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.     

Fiber-Based Modeling of Circular Reinforced Concrete Bridge Columns

This article presents the application of fiber-based analysis to predict the nonlinear response of reinforced concrete bridge columns. Specifically considered are predictions of overall force-deformation hysteretic response and strain gradients in plastic hinge regions. This article discusses the relative merits of force-based and displacement-based fiber elements, and proposes a technique for prediction of nonlinear strain distribution based on the modified compression field theory. The models are compared with static and dynamic test data and recommendations are made for fiber-based modeling of RC bridge columns.     

SEISMIC TORSIONAL EFFECTS ON DUCTILE STRUCTURAL WALL SYSTEMS

With a distinct design rather than analysis-oriented approach, some torsional phenomena, arising during the elasto-plastic seismic response of building structures, are addressed. It is postulated that existing code recommendations, expressed only in terms of the properties of elastic structures, are largely irrelevant when the design needs to be based on ductile system behaviour. Torsional restraint provided by elastic elements of a system, is claimed to be the desirable property controlling the amplification of inelastic translatory deformations of critical elements by storey twist. With the identification of the sources of inelastic element deformations, a simple behaviour-based design strategy is proposed. This should ensure that the displacement ductility demand, expected to be imposed on the system, does not result in deformations that, as a consequence of storey twist, may exceed the displacement ductility capacity of critical elements.     

Optimum Design Approaches for Improving the Seismic Performance of 3D RC Buildings

In this article, a number of design approaches for 3D reinforced concrete (RC) buildings are formulated in the framework of structural optimization problems and are assessed in terms of their performance under earthquake loading. In particular, three design approaches for RC buildings are considered in this study. In the first, the initial construction cost is considered as the objective function to be minimized. The second one is formulated as a minimization problem of the torsional response, while a combined formulation is also examined as the third design approach. The third approach is considered with two distinctive formulations. According to the first approach, the torsional behavior is minimized by minimizing the eccentricity between the mass and rigidity centers, while the second one is achieved by minimizing the eccentricity between the mass and strength centers. It is shown that the optimized designs obtained according to the minimum eccentricity of the rigidity center behave better in frequent (50/50 hazard level) and occasional (10/50 hazard level) earthquakes, while the designs obtained according to the minimum eccentricity of the strength center formulation was found better in rare (2/50 hazard level) events. Designs obtained through a combined formulation seem to behave equally well in the three hazard levels examined.     

BEHAVIOUR OF REHABILITATED STRUCTURAL WALLS

An experimental program for identifying the causes of failures in structural walls under earthquake loading and investigating potential rehabilitation schemes was undertaken. Large-scale models of the plastic hinge region of the walls were tested. An innovative test setup that provides the possibility of controlling the ratio of the shear force to both bending moment and axial load was constructed. A control wall was tested and failed prematurely in shear reproducing the failure observed in the field. Two different rehabilitation schemes to improve the behaviour of the wall using biaxial fibre reinforced polymer (FRP) sheets were designed to prevent the shear failure. To improve the ductility, the end column elements of the walls were confined using anchored FRP. The two schemes were tested and proved to be effective in increasing shear strength, ductility, and energy dissipation capacity of the walls.     

POSTPOSITIVISM AND THE LOGIC OF THE AVANT‐GARDE

The purpose of this article is to explore the conditions under which the postpositivist interest in rewriting or reinterpreting history could operate legitimately from a historical point of view. The first part of the article outlines and explains some of the key thematic elements of historical postpositivism. The second proceeds to investigate how these elements can be configured and related to each other within Arthur Danto's influential account of the development of contemporary art, and especially the avant‐garde. The intention is to acquire a sense of the working dynamics of postpositivist thought, so as to better understand its possible implications for the writing of history. In the concluding section an argument is proposed to the effect that, although the postpositivist interest in the rewriting of history can in principle be admitted as entirely legitimate, its legitimacy depends on introducing some substantive constraints on content, in addition to the formal considerations that postpositivist discourse generally tends to favor. It is further suggested that this constraint should take the form of a requirement on historical literacy whose meaning is, finally, elucidated by drawing a contrast with historical common sense.     

B/ordering nature and biophysical geopolitics: A response to Hirsch

This response to Hirsch's examination of Mekong geopolitics raises challenges to the geo-economics’ capture of common property resources and fragmentation of the Mekong commons by stressing flows, migrations and socio-ecological relations that should be central biophysical elements within deliberative geopolitics in the region.     

Rotten to the core: Against a core curriculum for geography in UK higher education

Abstract

This paper examines some of the main reasons why a core curriculum for UK higher education is seen as being desirable, and challenges these arguments with particular reference to geography. It suggests that a core curriculum would be damaging for six main reasons: that problems exist over the identification of central elements which could provide the basis of a core; that the higher education experience should be enlightening rather than dehumanising; that the strength of geography as a very broad discipline would be damaged by the imposition of a core; that much of the most exciting geographical ‘knowledge’ is created at the research frontier rather than in any potential core; that there are problems over the choice of people who might determine any core; and that there are serious questions over precisely in whose interests a core might be created.     

Simulation Of Shake Table Tests on Out-of-Plane Masonry Buildings. Part (II): Combined Finite-Discrete Elements

ABSTRACT

Out-of-plane response of unreinforced masonry elements is frequently the most critical aspect of the seismic performance of existing masonry buildings. The response of such elements is usually governed by equilibrium rather than strength. Hence, it is customary to resort to rigid-body models, accounting for possible rotations, and/or sliding. However, the results of such analyses depend on the initial choice of the mechanism. In this article, the shaking-table experiments on a brick-masonry specimen, and on a stone-masonry specimen have been modeled by resorting to a combined finite-discrete element strategy. Despite the coarse discretization of both discrete and finite elements, the three-dimensional models are able to capture the experimentally observed multi-degree-of-freedom mechanisms, without any a priori assumption on the mechanism. A sensitivity analysis is carried out, addressing eight different parameters. The identification of the mechanism is sufficiently robust, but the assessment of its activation and failure is best done by combining the finite-discrete element model with a simplified model of the recognised mechanism.     

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.