The aim of this Sub-project is to develop tools for the reduction of the seismic vulnerability of buildings and infrastructures. These tools are of different nature, depending on the level of knowledge available for one given technique or method and depending also on the type of structure considered. Examples of the questions to which answers when trying to assess and reduce the seismic vulnerability of buildings and infrastructures are:

• What is the best adapted existing retrofitting technique in order not to interrupt service of buildings that are in use? How much would be the relative cost of one retrofitting technique compared to another one?

• How can one deal with potential hammering or support loss at expansion joints or at joints between buildings?

• How can the reinforcement of a bridge pier be made using FRP (Fibre Reinforced Polymer) and considering the contribution of existing rebars? What is the most effective technique to apply FRP?

• How can a structure retrofitted with FRP be modelled? What about the durability of the reinforcement?

• Can base isolation apply to historical buildings and how? Can base isolation using semi active devices be modelled and how?

• How can one design dissipative connections for precast concrete structures? How should a truss bracing steel structure using dissipative connections be designed?

• How should one implement retrofitting measures considering a complete building stock?

• Is there a need and a means for improving the seismic capacity of underground train stations in soft soils?

The answers to these pertinent and contemporary questions will be found by means of an extensive programme theoretical, numerical and experimental research activities, together with the application to a series of case studies. The Sub-project is coordinated by ULIEGE, featuring also the participation of CIMNE, IST, ITU, METU, NECSO and UBRIS.

Disseminated material related to this Sub-Project can be found by clicking each of the links below:

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A summary of each of the tasks involved in this Sub-Project is provided below:

Task 2.2b.1: Selection of buildings case studies

A set of 5 reinforced concrete buildings will be selected. They have to provide elements with realistic dimensions and reinforcement detailing, which will serve as a common basis for all applications of vulnerability reduction techniques. The reference structures are:

• 2 medium rise RC buildings and one medium high rise buildings (12 storeys) made with cast in situ concrete; all are weak design with respect to the seismicity of their zone of construction; amongst the 2 medium rise building, one will be selected for the case study on masonry infills.

• one precast concrete building.

• one structure with steel truss bracings.

Task 2.2b.2: Selection of infrastructure case studies

For the case studies on seismic risk reduction in infrastructures, real case structures of the following nature will be selected, serving as the basis for comparison between as-built and post-retrofitting behaviour, considering a gamut of seismic retrofitting solutions.

• One real case situation of underground station in soft soils, with all geometrical data and plans will be selected, together with data on soils and materials of the structure.

• One real case bridge with low ductility pier

• One real case pipeline

Task 2.2b.3: Optimised upgrading of structures by existing techniques.

Adding new structural members to upgrade a structure offers a classical and sound engineering solution for the reduction of seismic risk. Commonly adopted solutions like adding infill walls to reinforced concrete frames, jacketing reinforced concrete columns, and so on, have usually been applied to damaged buildings evacuated after an earthquake. Risk reduction implies interventions before an expected earthquake. In this case, a major difficulty arises with the implementation of retrofitting in buildings that are in service. Research on optimised upgrading of structures will be tackled through activities on four sub-tasks, described in what follows:

Sub-task 2.2b.3.1: Developing global upgrading methods that cause minimum disturbance to the building usage and occupation 

This calls for the definition of the less intrusive techniques and for the determination of minimum acceptable performance levels expected after a probable earthquake; this a new concept and the procedure to determine the least added capacity to achieve the performance targets will be established.

Practical design recommendations with new added members satisfying the specified minimum performance objectives and field implementation techniques offering acceptable solutions for buildings in use will be developed. Relative costs of different intervention techniques will be established, based on recent experience in Turkey.

Sub-task 2.2b.3.2: Pounding between adjacent buildings 

A special study about the problem of existing joints between buildings or expansion joints; these can contribute to critical situations in case of an earthquake, due to pounding between 2 buildings causing degradations in columns and to loss of support of beams over expansion joints. Parameters of the problems and technical solutions like smart reconnection of blocks and reduction of impacts through the introduction of damping product that fills the expansion joints will be identified and developed.

Sub-task 2.2b.3.3: Structural retrofitting at urban scale

In urban areas where the number of deficient structures is significant, urban rehabilitation/renewal methods may have broader acceptability and efficiency compared to individual building retrofit. Urban rehabilitation is not only engineering and requires a coordinated effort of engineers with city planners, sociologists, economists and even non governmental organisations since community participation is essential for the acceptance of the proposed solutions.

The work will consist in considering selected pilot urban regions in Europe (see Sub-Project 11) where urban renewal methods can produce acceptable solutions and implementing for those cases the practical outcome of a large scale urban seismic rehabilitation study which will be completed in Istanbul in 2003.

Sub-task 2.2b.3.4: Application to case studies

Examples of application of the various forms of optimised upgrading will be developed in case studies.

Task 2.2b.4. Upgrading of reinforced concrete and masonry structures by fibre-reinforced polymers (FRP's) 

Retrofitting of existing structural members using FRP jacketing is a relatively new technique. Its efficiency is qualitatively demonstrated, but many aspects are not well assessed and thus difficult to quantify in practical applications. This issue will be addressed through activities on five sub-tasks, described in what follows:

Sub-task 2.2b.4.1. Modelling of external FRP jacketing

Firstly, the combined confinement effect of existing transverse reinforcement and external FRP jacketing, which determine the behaviour of retrofitted elements, will be examined experimentally and analytically on both damaged and undamaged RC elements.

The information deduced will be used to develop structural models for the study of retrofitted reinforced concrete structures repaired by external bonding/wrapping of fibre-reinforced polymer composites. A formulation for the basic substances of composite materials using the mixing theory and a non-linear formulation of the homogenisation theory, as well as a general formulation for the treatment of the anisotropy for composite materials, will be proposed.

Sophisticated constitutive models will combine effects of stiffness degradation with plasticity and viscosity, allowing interpreting the results in terms of damage and/or fracture. The behaviour of anchorages and unions between the new material and the concrete, which often restricts the use of FRP will be studied. These models will allow performing of numerical studies on the reliability of FRP retrofitted structures.

Sub-task 2.2b.4.2. Modelling of FRP reinforcement of infill panels

For masonry infills, simplified analytical models can produce probabilistic estimates of their vulnerability. Models to account for FRP reinforcement of infill panels will be calibrated by experimental studies.

Sub-task 2.2b.4.3. Application of fibre-reinforced polymers

There are uncertainties about the best way to apply fibre-reinforced polymer on concrete. This will be studied, considering the following parameters: lay up, impregnation, cure problems on site, selection of temperature and use of vacuum, value of applied pressure, fibre prestressing. Different process of reinforcement will be defined depending on the environmental conditions. Guidelines for design and application of the proposed solutions, defining standard methodologies/ technologies and a design tool as a decision support system to assist construction managers in evaluating the reinforcement solutions, will be prepared.

Sub-task 2.2b.4.4. Durability and fatigue resistance of FRP retrofitting

Tests will be made on joints and structural elements to study the durability and fatigue resistance of FRP retrofitting. Dynamics tests in linear regime and pseudodynamic tests simulating seismic loading up to failure will be performed.

Sub-task 2.2b.4.5. Application to case studies 

Examples of application of the FRP use and performance will be developed in case studies of buildings, bridge and pipelines, considering both the elements behaviour and the global response of the structures.

Task 2.2b.5. Base isolation 

Seismic base isolation systems use a flexible system implemented between the foundation and the superstructure in order to increase the natural period of the structure and the damping capacity. The superstructure experiences small deformations while large deformations may occur in the isolation systems. Research on base isolation will be tackled through activities on two sub-tasks, described in what follows:

Sub-task 2.2b.5.1. Displacement-based design of base isolated historical structures

The first objective of the research concerns the application of passive base isolation to historical structures. Analytical studies will be made in order to adapt the methodology of displacement-based design to base isolated buildings, with special consideration to low ductility structures. Vulnerability functions for base isolated structures will then be developed, based on an integrated solution considering both the structure and the isolation system. Simple design rules and guidelines will be established.

Sub-task 2.2b.5.2. Real-time stiffness adjustment of base isolation devices

The other research steps deal with the control method of base isolation devices using a new technology. In order to avoid the drawback of large isolators with viscous dampers, which reduce base isolator displacements but can increase the displacements and accelerations in the superstructure, a new technology has been developed, in which the stiffness and damping properties of the isolator can be adjusted with almost no external power, in order to protect the structure from the penalising effects mentioned above. A nonlinear control method for this new type of isolator is needed.

Causal easy-to-implement control algorithms will be developed and compared to optimal controller derived from the solution to the Euler-Lagrange equations for case study buildings and bridges structures submitted to different earthquakes. It will then be checked how well the control method developed fits for semi active devices using controllable fluids. These are substances able to reversibly change in milliseconds from a free-flowing fluid to a semisolid with controllable yield strength when exposed to an electric or a magnetic field. To exploit the capabilities of these smart materials a true understanding of their nonlinear material behaviour is indeed required. The performance for controllable fluids of the easy-to-implement control algorithms developed will be compared to optimal controllers in case examples of structures submitted to earthquake excitations.

Task 2.2b.6. Energy dissipation devices

Energy dissipation devices or connections absorb the energy from the dynamic loads and reduce the action on the main structural members. Contrarily to base isolation schemes, the energy dissipation devices are generally present at many places in all the structures. Connections between industrialised elements could advantageously be designed to be dissipating energy, because they are anyway a place for stress concentration. Classical seismic design of frames with overstrength design of connections generates costly structures.

Research activity on dissipative devices will bear on two types of building structures, precast concrete frames and steel truss braced structures, and will aim to define adequate design approach of dissipative connections and structures using them. For both types of structures, the dissipation devices will be characterised numerically and the developed model will be implemented within a nonlinear software for seismic analysis of structures. The effectiveness of the energy dissipation devices in the design of new structures will then be evaluated in case studies of new and rehabilitated structures in seismic zones.

Whilst for the case of precast RC buildings, the research will focus on single storey and multi storey portal frames and vulnerability/fragility curves will be established using Monte Carlo simulations, for steel braces, different typologies of braces (X, V, inverted V's) will be considered and the research will focus on the design of the structure, in terms of the optimum distribution of strength of dissipative connections throughout the structure. Based on the analysis results, guidance documents for conceptual design of precast concrete and steel truss braced frames with dissipative connections will then be produced.

Task 2.2b.7. Seismic upgrade of underground structures

The majority of existing underground structures have withstood well earthquake action, because soil conditions or depth were such that the earthquakes imposed almost a rigid body movements with very small forces. In soft soil however, collapse may take place (e.g. the Dakai tube station during the Kobe earthquake).

Within the scope of the present task, a set of design, assessment and re-design methodologies applicable to large reinforced concrete underground structures in soft soils will be developed. This will allow the establishment of conception criteria for the design of new structures, the identification of weak points in existing structures and the most adequate intervention strategies for the vulnerability reduction of existing underground structures not designed to withstand earthquake effects.