关于抗震设计的英文翻译

英文翻译
英文原文
Comparative Application of Capacity Models for Seismic Vulnerability Evaluation of Existing RC Structures Abstract. Seismic vulnerability assessment of existing buildings is one of the most common tasks in which Structural Engineers are currently engaged. Since, its is often a preliminary step to approach the issue of how to retrofit non-seismic designed and detailed structures, it plays a key role in the successful choice of the most suitable strengthening technique. In this framework, the basic information for both seismic assessment and retrofitting is related to the formulation of capacity models for structural members. Plenty of proposals, often contradictory under the quantitative standpoint, are currently available within the technical and scientific literature for defining the structural capacity in terms of force and displacements, possibly with reference to different parameters representing the seismic response. The present paper shortly reviews some of the models for capacity of RC members and compare them with reference to two case studies assumed as representative of a wide class of existing buildings.
Keywords: Assessment, Vulnerability, Capacity, Existing Structures, Reinforced Concrete.
INTRODUCTION
Seismic assessment of existing RC structures is a cutting-edge issue for structural engineers. The increased levels of safety required to new structures by the last generation of codes of standards indirectly emphasizes the lack in seismic performance usually affecting the existing ones. Seismic vulnerability evaluation can be carried out according to various code provisions about capacity of members of existing structures under earthquake actions. Different damage measures can be adopted for quantifying seismic capacity even depending on the type of structure considered. Three different parameters, besides other less common quantities, have been proposed for measuring seismic performance of reinforced concrete structures.Indeed, plastic rotations are considered in the U.S. code [6] for quantifying seismic capacity of beams and columns, while the total chord rotation is assumed as capacity measures for RC members in both the European [2] [3] and Italian [4] [5] seismic codes. Other proposals can be found within the scientific literature and the technical practice. For instance, the interstorey-drift angle is a parameter commonly used for quantifying seismic capacity and demand on structures [1]; furthermore, on the basis of particular assumptions about the mechanism most likely to occur, interstorey drift values can be somehow converted into global displacements [7]. According to the general Performance-Level framework of the current codes, various
levels of damage can be tolerated for every relevant Performance Level of the structures. Consequently,they assume different threshold values, in terms of the various measures mentioned above, for stating whether a given structure attains or not the Limit States of interest in seismic design and assessment. The present paper, after a critical review of the various proposals for all the relevant Limit States, compares the various definitions of structural capacity of members for two existing RC structures, designed for gravitational loads, assumed as case-studies.
OUTLINE OF CAPACITY MODELS FOR RC MEMBERS Several parameters and indices can be considered for measuring structural performance under horizontal actions induced by earthquake shaking. As a matter of principle, those parameters should look after the cyclic nature of the seismic response. Park&Ang Damage Index, Low-Cycle Fatigue Index [8] and other similar measures follow the cyclic evolution of the structural response defining suitable threshold values of the corresponding indices for defining the achievement of the relevant Limit States.Although those parameters explicitly consider the actual evolution of the structural response, they are not so easy to evaluate at both local and global level. Consequently, capacity is more often defined in terms of displacement parameters whose maximum value attained during the seismic response is considered. Figure 1 shows the three
different displacement measures which are more commonly considered within the scientific and technical literature:
θ, which can be evaluated as ratio - the interstorey drift angle ID
between the intestorey relative displacement ijδ a the storey height h;
- the plastic rotation plθwhich is defined as the concentrated rotation which is equivalent to the plastic curvatures arising around the column end throughout a length pl l which is the length of the so-called plastic hinge;
- the total chord rotation θwhich is defined as the ratio between the relative displacement of the point of zero curvature and the distance between that point and the column end, namely the shear length V L.
FIGURE 1. Displacement parameters for quantifying seismic capacity of beams-columns.
Alternative definitions can be derived for the relevant seismic performance levels (PL) or Limit States (LS) utilizing the three measures listed and briefly defined above. Indeed, three possible PLs are usually considered within the structural codes:
- Damage Limitation (DL), if no structural repair is needed after the event;
- Life Safety (LS) or Severe Damage (SD), if huge structural damage occurs, but life safety is guaranteed;
- Near Collapse (NC), if in the aftermath of the event the structure is only able to carry the gravitational loads.
The following sections summarizes the basic assumption of various seismic codes and other proposals available within the scientific literature for defining threshold values of the above displacement measures for the three defined limit states.
Eurocode 8 and Italian Code provisions
Both Eurocode 8 [2] [3] and the new Italian Seismic Code [4] [5] define displacement capacity and demand of frames in terms of total chord rotation. Two basic reference values are considered for defining that rotation at yielding and ultimate. In particular, the Italian Code provide the following relationship:
while a slightly different one is reported within Eurocode 8:
as a function of almost the same parameters, such as shear length V L cross section depth h, longitudinal bar diameter b d and yielding stress y f, and concrete compressive strength c f. Eq. 2 also involves the section lever arm z and the boolean factor V a which possibly takes into account the effect of the bending moment shift due to the shear force. Besides the slight differences between the two Eqq. 1 and 2, a similar structure, based on the sum of three terms related to flexural and shear deformation of the member and the possible small fixed-end rotation due to bar slip, can be easily recognized. Chord rotation at ultimate uθis also defined and two possible formulations are proposed in both the European and the Italian Seismic codes:
- an "empirical formula"
- a "theoretical formula"
involving the cross section curvatures yφand uφ… at yielding and ultimate, respectively, and the plastic hinge length pl L.。