M. Petkovski

Energy absorbing elements in regular and composite steel frame structures

Intensive investigations in the field of the control of structural behaviour have been conducted at the Institute of Earthquake Engineering and Engineering Seismology in Skopje. Part of them, relating to energy absorption elements are presented in this paper. In order to provide a more sophisticated presentation of the characteristics and capabilities of these energy absorption elements, a short review of the currently available and most frequently applied passive control systems is included. To prove the efficiency of the energy absorption elements quasistatic tests on single-span models CR10 and CR20 of the characteristic frame have been performed. The results proved that the CR20 models are of considerably higher strength, even six times higher at ultimate state, ductility and energy dissipation capacity. A mathematical model has been formulated for the nonlinear behaviour of the main elements of the system, whereby a trilinear P-Δ diagram has been used, as developed at the Institute. The analysis of the hypothetical prototype, a five-storey building, with energy absorption elements incorporated in six spans, subjected to the seismic excitations of two PGA levels, 0.12 g and 0.15 g, has shown very good results. Further investigations in this field resulted in the development of new energy absorbing elements (through friction), SBC, which when placed at the joints of the frame introduce hysteretic damping to the structure in the case of low level excitations, when the rest of the system is in the linear range.

Experimental Detection of Damage Evolution in Concrete under Multiaxial Compression

The nonlinear inelastic stress-strain relationship of concrete is a result of changes to the fabric of the material caused by several damage mechanisms. Existing experimental research on concrete provides very little information on the evolution of these mechanisms. Instead, most experimental studies are limited to measuring the external loads and deformations of concrete specimens. The aim of the research presented here is to provide a direct link between macroscopically observed global stress-strain behavior and the development of damage mechanisms within the specimens. A novel experimental procedure is introduced to monitor the changes in the material during a conventional multiaxial compression test. This procedure, comprising small stress probes repeated at selected points of the stress-strain history, is designed to provide data on the tangent stiffness operator and the associated acoustic tensors. The acoustic tensor with the minimum value of its determinant provides information on both magnitude and orientation of the damage. The stress-probing methodology and some of the key practical aspects and limitations of the experimental procedure are illustrated by using five multiaxial compression tests as examples of different stress-strain histories. The results of the test methodology presented in this paper give an indication of the extent of damage and accurate information about its orientation. This paper therefore shows that the stress-probing procedure can be incorporated in multiaxial tests designed to monitor the relationship between damage evolution in the material and macroscopically observed stress-strain response of the specimen.

Self-adaptive approach for optimisation of passive control systems for seismic resistant buildings

The concept of passive control of the seismic response of structures was introduced to improve the performance of structures by increasing their energy dissipation and reduce or eliminate damage in the structural elements. The key task in the design of passive systems is to determine the forces in the control devices (yield/slip or post-tensioning) at each floor, that will result in best performance (e.g. minimum inter-storey drift). This can be achieved by large parametric studies in which both the maximum control force (e.g. at ground level) and the distribution of forces along the height of the structure are varied. Alternatively, optimum forces in the devices can be achieved by semi-active control, where the structure self-adapts to the earthquake. Both solutions are expensive: the first requires hundreds of non-linear response simulations in the design stage; the second needs a system of sensors, controllers and electromechanical devices. Presented here is a new Self Adaptive Optimisation Approach (SAOA) in which the self-optimisation of a semi-active system is used in the design stage and the resulting distribution of control forces is adopted as a passive system. The new approach was evaluated through comparing the simulated dynamic responses of two relatively simple benchmark structures (braced and post-tensioned) with three sets of control forces: (1) passive system with forces obtained in parametric study, (2) semi-active system with self-adapting control forces, and (3) passive system with SAOA-optimized forces. The results show good performance of the SAOA systems, indicating that SAOA offers a simple and effective solution that can replace the existing optimisation approaches for the design of passively controlled earthquake resistant structures. This study presents a novel idea of using the semi-active control as a tool for optimising a passive control system. The passive control systems can be further improved by a larger study in which the semi-active control algorithms are also optimised.

Performance-based optimisation of RC frames with friction wall dampers using a low-cost optimisation method

Friction-based dampers can be considered as one of the suitable passive control systems for seismic strengthening and rehabilitation of existing substandard structures due to their high adjustability and good energy dissipation capability. One of the main issues in the design of these systems is to obtain the magnitude of the maximum slip force and the distribution of slip forces along the height of the building. In this study, a practical performance-based optimisation methodology is developed for seismic design of RC frame buildings with friction energy dissipation devices, which allows for an accurate solution at low computational cost. The proposed method aims at distributing the slip loads of the friction dampers to achieve a uniform distribution of damage along the height of the building. The efficiency of the method is evaluated through the optimum design of five different low to high-rise RC frames equipped with friction wall dampers under six natural and six synthetic spectrum-compatible earthquakes. Sensitivity analyses are performed to assess the reliability of the method using different initial height-wise slip load distributions, convergence parameters and earthquake records. The results indicate that optimum frames exhibit less maximum inter-storey drift (up to 43%) and global damage index (up to 75%), compared to uniform slip load distribution. The method is then developed to obtain the optimum design solution for a set of earthquakes representing a design spectrum. It is shown that the proposed method can provide an efficient tool for optimum seismic design of RC structures with friction energy dissipation devices for practical purposes.

Shake Table Tests on Deficient RC Buildings Strengthened Using Post-Tensioned Metal Straps

The European research project BANDIT investigated the effectiveness of a novel Post-Tensioned Metal Strapping (PTMS) strengthening technique at improving the seismic performance of deficient RC buildings using shake table tests. A full-scale two-story structure was designed with inadequate reinforcement detailing of columns and beam-column joints so as to simulate typical deficient buildings in Mediterranean and developing countries. Initial shaking table tests were carried out until significant damage was observed in the beam-column joints of the bare frame. Subsequently, the damaged building was repaired and strengthened using PTMS and additional tests were performed. The results of this study show that the adopted strengthening strategy improved significantly the seismic performance of the substandard RC building under strong earthquake excitations.