A SYNERGIC DISSIPATION APPROACH TO RETROFIT FRAMED STRUCTURES WITH A SOFT FIRST STOREY

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1 9 th World Seminar on Seismic Isolation, Energy Dissipation and Active Vibration Control of Structures, Kobe, Japan, June 13-16, 2005 A SYNERGIC DISSIPATION APPROACH TO RETROFIT FRAMED STRUCTURES WITH A SOFT FIRST STOREY Alberto Parducci, Fabrizio Comodini, Matteo Lucarelli Department of Civil and Environmental Engineering University of Perugia (Italy) ABSTRACT This paper deals with retrofitting problems arising when the structural configuration of the building is inadequate to oppose strong earthquakes. One of the most frequent dangerous configurations is that of the "soft storey" which was widely used in Italy at the first level of framed reinforced concrete buildings. Instead of strengthening the structural elements, an alternative consists in the inserting of appropriate dissipating systems. The paper illustrates the functioning of a synergic strategy designed to retrofit a group of residential buildings built before the site was classified as a seismic zone. The idea is based on the performance of a dissipating compound system, made of mechanical dampers, inserted into the open frames of the "pilotis" floor, running together with the inelastic dissipation of the columns plastic hinges. To reach this target, the inelastic capacity of the critical ends of the columns is improved by a confinement achieved by rehoplastic concrete and FRP wrapping. 1. INTRODUCTION The selection of the appropriate configuration of the structural system, on which the actual seismic performance of the buildings mainly depends, is one of most important subjects in seismic design. Nevertheless, little attention is generally paid to this aspect in seismic design (Arnold and Reitherman, 1982, Parducci 1999, 2000, 2001, Mezzi et Al. 2004). In spite of its real importance, only few of the recent design codes underline the problem (Eurocode 8, Ordinanza 3274). Hence, the problem has not been taken into sufficient consideration in the current professional practice, starting from the first steps of the architectural design, when the morphology of the building is defined. This anomalous situation can have dramatic consequences in the retrofitting design of the existing buildings which were built before the area where they are was recognized to be seismic. As a matter of fact, many architectural shapes which are usually designed in non seismic zones give rise also to structural configurations which are gainst stronger earthquake. The configuration of the buildings having a "soft first storey" (the "pilotis" buildings) is one of the most recurrent. Since this composition can provide attractive and useful solutions from the architectural point of view, it was encouraged in Italy for the architectural design of typical multi-storey reinforced concrete buildings, and it was largely applied, both in seismic and non seismic zones.

2 On the other hand, thanks to updated seismological analyses, remarkable enlargements of the seismic zones were introduced by the Italian code in the last few years, the latest in 2003 (Ordinanza 3274). So, many residential, commercial and public buildings, located in zones that have recently been recognized to be under seismic risk, were designed not only with poor lateral resistance but also, which is even worse, built with the unsafe configuration of the "pilotis" building. Since this configuration has been acknowledged to be one of the main causes of the most dangerous collapses provoked by main earthquakes (Figure 1), defining the best design criteria to eliminate this potential dangerous effects is important. The energy approach seems to be one of the most attractive ways to achieve this target. Figure 1 - Typical examples of the well-known dangerous effects of the "soft storey" configuration. The typical situation of the "pilotis" effect is emphasized in a residential plant of the Modica town, in Southern Italy, due to the peculiar configuration which is illustrated in this paper. A special unusual dissipating arrangement conceived to retrofit these buildings, is based on the synergic dissipating behaviour of mechanical damping elements and on the hysteretic response of plastic hinges improved at the ends of the columns by means of a special local workmanship. Paradoxically, the possibility to apply a retrofitting system like this derives from the configuration of the "soft first storey", because appropriate works can transform this structural shape in a suitable configuration to withstand the deformations which are necessary to put in action the damping system. Tthe project has been approved by the Ministry of Public Work which has acknowledged the interest of this demonstrative project and approved the allocation of special funds to carry out the required experimental research. 1. LAYOUT OF THE BUILDING PLANT AND RETROFITTING OPTIONS 1.1. Layout of the Buildings Plant Figure 2 shows the structural arrangement of the residential plant of Modica. A single 14-storey (building A) and two contiguous 6-storey (building B) buildings emerge over a large pedestrian platform giving rise to a "pilotis" effect. A partial presence of the "pilotis" scheme is extended also to some frames of the first storeys above the platform.

3 building A building B reinforced concrete "pilotis" platform Figure 2 - General layout of the building plant. Figure 3 - The buildings above and below the pedestrian platform

4 The pedestrian platform is much larger than the horizontal plan of the buildings and extends as far as about 3000 square meters about. It is made by a set of reinforced concrete slabs that are separated by small expansion gaps of less than 2 centimetres. The whole platform is supported by reinforced concrete columns directly founded on the outcropping volcanic soil. The structures were designed before the seismic classification of the town. Under current conditions, the natural periods of the building A and B have been estimated to be 1.25 and 0.76 seconds respectively. The natural periods of the each section of the pedestrian platform are smaller and very different Three Retrofitting Options The present Italian classification put Modica town in seismic zone 2, where a PGA = 0.25g on firm rocky soil must be applied to define the elastic response spectrum having the probability of 10% to be exceeded in 50 years. The demands defined by the elastic design spectrum for the two buildings, directly founded on volcanic soil (soil B - V s = m/s), are 0.313g and 0.651g respectively. According to the seismic code, these values can be divided by a q-factor to check the resistance of the structure at the ultimate conditions. Due to the structural irregularities of the buildings and the lack of ductile details consistent with the capacity design criterion, the q-factor is equal to q = 3.03 for the building A (framed structure with small shear walls) and q = 3.28 for the building B (simple framed structure). These values have been confirmed by performing elasto-plastic pushover analyses with a preliminary structural model. The seismic demand corresponding to these coefficients was much higher than the capacity of the structures under current conditions. Three different design options can be compared in order to retrofit these buildings: (a) a traditional approach, consisting on the strengthening of the existent structures; (b) the reduction of the seismic input through a base isolation system; (c) the application of a special dissipating strategy. Option (a) requires very invasive works. By the new Italian code, the irregular internal configuration of the structural frames, which were not conceived for a seismic resistance, requires a substantial increasing of the horizontal design forces. The insertion of new shear walls, which also require important modifications of the foundation structures, would be necessary. Therefore, this option involves the temporary evacuation of the inhabitants during the retrofitting works and is the most expensive of the three. Option (b) is less expensive than (a), but only in terms of the cost of the structural works (cutting of the base of the structure, provisional supports and insertion of the isolating devices). Carrying out the works in safe conditions also requires the evacuation of the inhabitants. In addition, a sufficient enlargement of the expansion gaps of the pedestrian platform around the building is necessary to make the large lateral displacement of the isolated structures possible. The technical details of option (c) are described in the following point. This is the most interesting option from any point of view, including cost, and it can be carried out in safe conditions, avoiding the provisional evacuation of the inhabitants.

5 2. THE SYNERGIC ENERGY DISSIPATION SYSTEM 2.1. Layout of the Compound Dissipating System A preliminary design has been carried out using a simplified numerical model to check the feasibility of the dissipative system discussed in this paper. The objective is reaching the required seismic protection of the unusual configuration of these buildings, supported by a soft storey system, against main earthquakes, when damages are possible, but they must remain within the limits required by the seismic code. Some secondary works are foreseen to close the open frames at the first storeys above the platform, where the rooms will be used for commercial activities. The basic idea consists in achieving a high dissipating effect of the seismic energy by inserting under the platform of a compound dissipation system (Figure 4). This objective is reached by interconnecting all the sections of the concrete platforms and linking them to the buildings by means of rubber connectors which can allow for only small thermal deformations (the shrinkage of the concrete has been run out). closing of the open frames above the platform potential plastic hinges at both the ends of all the columns primary damping system below the buildings Figure 4 - The compound dissipating system to retrofit the buildings. pedestrian platform 1 2 confined elements 1 - elimination (hydrodemolition) of the peripheral concrete. 2 - elasto-plastic concrete and wrapping with confining FRP. M/f cbh 2 confined hinge element exisent element damper concrete wall concrete wall damper β u β u(conf) Figure 5 - General arrangement of the compound daping system.

6 The primary dissipating system includes a set of mechanical dampers located in the peripheral positions of the plans below the buildings, inside the columns of the open framed elements supporting directly the structures. Figure 5 shows an example of the arrangement of the dampers. Moreover, the critical zones at both ends of all the columns, including those of the entire continuous platform, will be modified to improve the plastic behaviour of the potential plastic hinges, that can be mobilized when the seismic demand exceed an assigned level of the damaged limit state. This structural modification has the objective of a double target. On the one hand, they make the large deformations that are required possible thus making the running of the dampers more effective against main earthquakes. On the other, in addition to the lateral hysteretic performance of the columns, they make it possible to take advantage directly of the large potential dissipating effect deriving from the plastic deformation of the large number of columns supporting the platform around the buildings The Structural Modification of the Columns Figure 5 shows the works that have to be carried out to modify the ends of all the columns located under the pedestrian concrete platform. Increasing the inelastic flexibility of the potential plastic hinges at both the critical zones of the columns, so that large displacements of the entire platform level can be allowed, is the target. This arrangement will be achieved by removing the concrete thickness around the reinforcement and by replacing it with a special rheoplastic concrete. Then, the concrete core and the rheoplastic thickness will be confined through a FRP wrap. In this way, the vertical bearing capacity is mainly assigned to the central core, while high ultimate plastic deformations of the entire confined section may occur. These works may favour the dissipative running of the main dampers, and give rise to a synergic dissipative effect deriving from the improved potential hysteretic behaviour of the plastic hinges when the elastic limits of the steel reinforcement and the concrete are overtaken. 3. RESULTS OF THE PRELIMINARY ANALYSIS A preliminary study has been carried out to check the feasibility and the effectiveness of the retrofitting works. Some pushover analyses have been developed using a simplified numerical model consisting of an equivalent multi-storey frame. This numerical model has been defined to simulate the equivalent mechanical behaviour of the higher building (building A). The ATC-40 analytical procedure (ATC-40) has been applied to the model to estimate both the elastic and the ultimate resistant displacements of the structure both under current conditions and after the execution of the retrofitting works. The pushover analyses have been performed using the energy approach of Parducci (2004), that modifies the classical method indicated by (Fajifar 2000) and (Chopra and Goel 1999). The results have been illustrated in the capacitive ADS representation ("Acceleration Displacement Spectrum") shown in Figure 6. In the picture the seismic demands are compared with the effective inelastic capacities of the structures. The graphs show:

7 (a) the elastic design spectrum (R = 475 years return period) defined by the code (PGA = 0.25g on firm rock); (b.1) the performance curve of the structure under current condition, pushced to its ultimate state; (b.2) the acceleration spectrum reduced taking into account the dissipation capacity reached by the structure (b.1) at its ultimate state; (c.1) the performance curve of the retrofitted structure, pushed to its ultimate state; (c.2) the acceleration spectrum reduced taking into account the dissipation capacity reached by the retrofitted structure (c.1) at its ultimate state. The dissipating effect of the main dampers has been considered by assuming an elasto-plastic behaviour of the devices. This effect has been improved by taking into consideration the parallel contribution deriving from the hysteretic cyclic behaviour of the plastic hinges of the columns. The allowable displacements at their ultimate states have been estimated by using the Kent and Park criterion (Park and Paulay 1975) and taking into account the confining effect of the FRP wrap around the concrete. The confining effects due to the FRP that wraps the concrete have been estimated by using the criterion of (Arduini et Al. 1999). The "pi-delta" effect has been considered in the numerical calculations (a) elastic design spectrum (R = 475 years, ξ = 5%) (b.2) reduced design spectrum of the existing structure (ξ eq 25%) (c.2) reduced design spectrum of the retrofitted structure (ξ eq 34%) 0.4 S e (g) (c.1) performance curve of the retrofitted structure (b.1) performance curve S d (mm) of the existing structure Figure 6 - Capacitive spectrum representation. Through the retrofitting system the limit displacement at the platform level, corresponding to the collapse of the first column, can reach 82 mm instead of 32 mm of the existing structure. The consequence is clearly shown by the graphs of the picture: under current conditions a performance point cannot be reached, while a suitable performance point can be achieved if the retrofit of the structure is carried out by using the dissipating system illustrated in this paper.

8 ACKNOWLEDGEMENTS The structural preliminary design has been performed by A. Parducci, consultant of the Federcasa Agency (owner of the buildings), for the public administration of Modica town (RG). The "Studio Ing. Agosta" of Modica is responsible for the technical and administrative activities of the general design. REFERENCES Arduini M., Di Tommaso A., Manfroni O., Ferrari S., Romagnolo M. (1999), Il Confinamento Passivo di Elementi Compressi in Calcestruzzo con Fogli in Materiale Composito (in Italian), L'Industria Italiana del Cemento - Roma, 1999/11. Arnold C. and Reitherman R. (1982) Building Configuration and Seismic Design, John Wiley & Sons, ATC 40 (1996) Seismic Evaluation and Retrofit of Concrete Buildings, Applied Technology Council, Report No. ATC 40. Chopra A. K., Goel R. K. (1999) Capacity Demand-Diagram Methods for Estimating Seismic Deformation of Inelastic Structures, Report PEER-1999/02, Berkeley, April Clough R.W. and Penzien J. (1993) Dynamics of Structures, 2nd edn, McGraw Hill, New York. Eurocode 8 (2001) Design of Structures for Earthquake Resistance, prdraft No.3, May Fajifar P. (2000), Structural Analysis in Earthquake Engineering - A Breakthrough of Simplified Non Linear Methods, 12 th European Conference on Earthquake Engineering, Elsevier Science Ltd, Mezzi M., Parducci A. Verducci P. (2004), Architectural and Structural Configurations of Buildings with Innovative Aseismic Systems, 13th World Conference on Earthquake Engineering (Paper No. 1318), Vancouver, B.C., Canada, August, Ordinanza PCM 3274/03 Primi Elementi in Materia di Criteri Generali per la Classificazione Sismica del Territorio Nazionale e di Normative Tecniche per le Costruzioni in Zona Sismica (in Italian), May Parducci A. (1999), Seismic Isolation: Why, Where, When - Design options for ordinary buildings: the Italian experience, International Post-Smirt Conference Seminar Isolation, Energy Dissipation and Control of Vibration of Structures Cheju (Korea), August Parducci A. (2000), Seismic Isolation and Structural Configurations, Technical Meeting held in San Francisco - Forell/Elsesser Engineers, Inc. August, Parducci A.(2001) Seismic Isolation and Architectural Configuration, Special Conference on the Conceptual Design of Structures, Singapore, August Parducci A., Comodini F., Mezzi M. (2004) Approccio Energetico per Analisi Pushover (in Italian), XI National Congress "Seismic Engineering in Italy", Genova (Italy), January Park R., Paulay T. (1975), Reinforced Concrete Structures, Wiley & Sons, New York, 1975.