PRESERVATION OF EXISTING SOFT-FIRST-STORY CONFIGURATIONS BY IMPROVING THE SEISMIC PERFORMANCE

M Mezzi, University of Perugia, Italy A Parducci, University of Perugia, Italy

Abstract

A very widespread structural configuration in existing buildings is characterised by the absence of claddings at the ground floor, while they are present at the elevation stories. It is the so called "pilotis" configuration characterised by a soft first story. This configuration allows for a good use and distribution of the space at the ground floor but it is very dangerous from a seismic point of view, because the lateral response of these buildings is characterised by a large rotation ductility demand concentrated at the extreme sections of the columns of the first story, while the superstructure behaves like a quasi-rigid body. The paper illustrates a solution proposed for the preservation of a particular architectonic double soft-story configuration. The case study consists of the preliminary project carried out to retrofit two residential buildings which structures, r/c frames, were designed thirty years ago as non seismic structures, while at present the site has been included in the Italian medium intensity seismic zone. The two buildings are fourteen and six stories high, starting with a "pilotis" story from a large r/c pedestrian platform supported by a number of reinforced concrete columns forming another "pilotis" system. The special retrofit arrangement is based on the synergic dissipating behaviour of damping elements and improved hysteretic plastic hinges at the ends of the columns.

Keywords: soft story, pilotis configuration, energy dissipation system, ductility, retrofitting

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 the most important subjects in seismic design. Nevertheless, little attention is generally paid to this aspect in seismic design [1, 2, 3, 4, 5] and, in spite of its real importance, only few of the recent design codes underline the problem [6, 7]. 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 implies that often the structural system is not designed as an effective earthquake resistant system integrated in the behaviour of the whole building, but only "calculated" ignoring the actual behaviour it can have, under severe seismic attacks, due to the building shape and configuration. This anomalous situation can have dramatic consequences in the retrofitting design of the existing buildings built in non seismic zones and therefore having architectural shapes acknowledged as inefficient or even dangerous from a seismic point of view. They require a seismic rehabilitation when their site is recognised as a seismic one and their configurations must be reviewed, but respected, with the aim of resisting without collapse undergoing the same damage expected for ordinary buildings.

2. The soft-first-story configuration A very widespread structural configuration in existing buildings is characterised by the absence of claddings at the ground floor while they are present at the elevation stories. It is the so called "pilotis" configuration characterised by a soft first story. This configuration allows for a good use and distribution of the space at the ground floor but it is very dangerous from a seismic point of view, because the lateral response of these buildings is characterised by large rotation ductility requests concentrated at the extreme sections of the columns of the first story, while the superstructure behaves like a quasi-rigid body. The configuration of the buildings having a "soft first story" (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-story reinforced concrete buildings, and it was largely applied in Italy, as in other countries, both in seismic and non seismic zones in the last fifty years. 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 [7]. 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 optimum design criteria to eliminate these potential dangerous effects is important. The energy approach seems to be one of the most attractive ways to achieve this goal.

Figure 1 - Typical examples of the well-known dangerous effects of the "soft storey" configuration.

3. Energy dissipating retrofitting systems Whit the aim of defining an effective retrofitting strategy for the seismic protection of the soft-first-story configuration, a design simulation has been carried out to check the feasibility of the use of a special dissipative arrangement allowing, without important secondary works, for the effective seismic retrofitting of the building. The basic idea consists of the reduction of the seismic input by inserting an energy dissipation devices in the first story and transforming it in a compound energy dissipation system. The primary system is made by dissipating devices located among the columns of the framed elements supporting the building. The secondary system use, as dissipating units, the extreme critical zones of all the columns of the first story which will be modified for increasing their ductility. The modification consists of confining the concrete with FRP in order to increase the maximum plastic deformability of the compressed concrete, therefore increasing the potential horizontal deformability. The special dissipating arrangement conceived to retrofit these buildings, is based on the synergic dissipating behaviour of mechanical damping elements and on the hysteretic response of the plastic hinges at the ends of the columns, improved by means of a special local work. Paradoxically, the possibility to apply a retrofitting system like this one, derives from the configuration of "soft-first-story", because appropriate works can transform this structural shape in a configuration suitable to withstand the deformations which are necessary to put in action the damping system. The soft-first-story configuration is equivalent to a base isolation scheme where the isolating devices are the first story columns (Figure 2a) which limit the building base shear, V(0)

max, to the value corresponding to their flexural resistance, My, that is:

2/H

MV y)0(

max∑

= (1)

Moreover, the experienced behaviour of these buildings under strong earthquakes shows that the portion of the building above the "isolating" story is undamaged, as it happens for base isolated buildings. The problem consists of the large displacement requested to the soft story which results in a flexural ductility demand of the columns larger than their capacity. The rotation, θ (0)

max, of the plastic hinges at the column ends, disregarding the elastic component of the deformation, results

H/s )0(max

)0(max =θ (2)

The lateral behaviour of the building can be represented by its capacity curve in terms of base shear versus lateral displacement reported schematically in Figure 2b.

s

My

V

s(0)max

V(0)max

V

s sy

(a) building deformation (b) lateral capacity curve

Figure 2 - Lateral behaviour of the ordinary soft-first-story building. The natural behaviour of the "soft-first-story" configuration can be improved by the compound dissipating system previously described consisting of the insertion of a dissipating device and the enhancement of the flexural ductility of columnsso obtaining an effective protection system (Figure 3). In this case the maximum base shear increases to the value

)0(maxVd

y)1(max VfF

2/H

MV ⋅=+=

∑ 1fV > (3)

where Fd is the threshold force of the dissipating device. Moreover, the enhancement of the flexural ductility, corresponding to a rotation capacity )0(

max)1(

max θθ >> , allows for a larger value of the maximum lateral displacement

)0(maxs

)1(max

)1(max sfHs ⋅=⋅= θ 1f s > (4)

Comparing the new capacity curve of Figure 3b with that one of the existing building (Figure 2b), the increase in strength and ductility expressed by (3) and (4) is evident. But the very important result consists of the increase in the energy dissipation which is proportional to the product of the strength and ductility increase

( ) )0(sV E1ff4E −⋅=∆ (5)

s

My

V

Fd

s(1)max

V(1)max

sy

V

s

E(0)

E(1)

(a) building deformation (b) lateral capacity curve

Figure 3 - Lateral behaviour of the enhanced soft-first-story building. It is evident that the increase of energy dissipation associated with the insertion of the only dissipating device cannot be so large because it is approximately equal to the product Fd⋅smax and the value of Fd

cannot be large because it implies the increase of the base shear and consequently larger forces in the superstructure elements. Therefore, a significant increase of the energy dissipating capacity can be obtained only if a significant increase of the maximum displacement can be reached thanks to the ductility increase of the first story columns. The consequence is that only the described compound dissipating system allows for an effective enhancement of the seismic performance of the building. Figure 4 shows different schemes for the insertion of dissipating devices within the frames of the first story: figure reports typical oil dampers, but also plastic or friction metal devices or viscous-elastic devices can be used. The system dissipates energy in conjunction with the interstory drift amplitude that requests high flexural deformations to the extreme portions of the columns. Figure 5 shows the works that have to be carried out to modify the ends of all the columns of the first story with the aim of increasing the inelastic flexibility of the potential plastic hinges at both the critical zones of the columns, so that large displacements of the entire level can be allowed. This arrangement will be achieved by reconfiguring the end parts of the columns through the following operations: • removing the thickness of the concrete cover around the reinforcement, this operation must be

carried out avoiding the cracking of the concrete core, therefore mechanical tools have to be avoided, while hydro-demolition is the better technique to be used;

• replacing the cover layer with special rheoplastic concrete, that is with concrete characterised by special mixing design, including the use of special admixtures, allowing for a large compressive deformation;

• confining the reconstructed element, consisting of the old concrete core and the newly casted rheoplastic cover, by means of FRP strips wrapped around the sections.

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 synergetic dissipative effect deriving from the improved potential hysteretic behaviour of the plastic hinges when the elastic limits of the steel reinforcement and concrete are overtaken.

Figure 4 - Layout of the primary damping system.

Figure 5 - Works and behaviour of the secondary damping system.

confinement operations 1 - Removal of the concrete cover

(hydrodemolition). 2 - Rheoplastic concrete recasting.

3 - Wrapping with confining FRP strips.

2 1

first story floor

M/fcbh2

βu(conf) βu

confined hinge element existing element

confined portions

Moment-curvature curve

3

concrete wall

damper

concrete wall

damper

damper

steel frame

4. Case study The typical situation of the "pilotis" effect is emphasized in a residential plant of the Modica town (Sicily, Italy), due to the peculiar configuration that will be described in the following together with the solution proposed for the preservation of this particular double-soft-story architectonic configuration. The case study consists of the preliminary project carried out to retrofit two residential buildings of the mentioned complex which plant and view are reported in Figure 6. The structures consist of r/c frames designed 30 years ago without any considerations on earthquake resistant design. At that time the site was not considered as a seismic area, while, at present, it has been included in the Italian medium intensity seismic zone characterised by a design PGA equal to 0.25 g. The building height is 14 stories for Building A and 6 stories for Building B. The specified number of the stories starts, with a "pilotis" story, from the level of a large pedestrian platform made of many r/c slabs sections separated each other by small expansion joints having gaps about 2 cm large, not large enough for avoiding seismic hammerings. The whole platform is supported by a number of reinforced concrete columns, forming another "pilotis" system, directly founded on the outcropping volcanic soil. Under current conditions, the natural periods of the building A and B have been estimated to be 1.25 and 0.76 seconds respectively. Figure 7 shows the elevation of the buildings above the platform and figures 8 shows the story below the platform that is used for parking.

Figure 6 - General plant and front view of the complex in Modica

Figure 7 - Pictures of the buildings used as case study

Figure 8 - Pictures of the basement "pilotis" level

5. Retrofitting options

Different retrofitting options can be hypothesized aimed at the improvement of the seismic performance of the structural system of the buildings: both conventional and innovative strategies can be taken into account. Option 1 - Traditional approach. It consists of the strengthening and stiffening of the existing structural element. Moreover, also the insertion of new elements, typically shear walls, must be provided. It requires very invasive works and important modifications extended even to the foundation structures. This rehabilitation option is very expensive and involves the evacuation of the inhabitants during the works. Option 2 - Base isolation. It provides for the provisional supports of the elevation, the cutting of the basement columns, the insertion of the isolating devices, the widening of the gaps among the platform sections around the building. It is less expensive than the previous one, but it requires the inhabitants' evacuation as well. Option 3 - Dissipating bracing. Energy dissipating bracings are provided to be inserted strategically within the r/c frame mesh along the building height. The solution is such expensive as the previous one and requires the partial evacuation of the inhabitants. The system effectiveness is limited by the need to limit the story drift to respect the ductility availability of the existing elements. Option 4 - Synergetic dissipating strategy. It provides for the combination of new dissipating bracing elements inserted at the basement level and the enhanced dissipation capacity of the existing r/c elements, according to the methodology previously presented. The constructive details will be described in the following. The solution appears as the optimum one from both the cost and safety point of view.

6. Proposed approach A preliminary design has been carried out using a simplified numerical model to check the feasibility of the special dissipative retrofitting system discussed in this paper in order to reach the required seismic protection against main earthquakes of the unusual configuration of the case study buildings. The objective can be obtained if damage is accepted for the elements of the soft story system. In fact, as it happens in ordinary seismic-resistant structures, damage is possible, but it has to be controlled, that is it must remain within the limits of their ductility performance according to the requirements of the seismic code. The design goal is reached by linking all the sections of the concrete platform and inserting a compound dissipation system under the platform achieving a high dissipating effect of the seismic energy. The works to be carried out on the structures (Figure 9) are: • connection of all the slab sections and the building platforms by means of rubber connectors that

allow the only small thermal deformations (the shrinkage of the concrete has been run out); • installation of the primary system, made of dissipating devices located among the columns of the

framed elements which support directly the building; • improvement of the critical zones of all the columns under the platform by reconstructing the

concrete cover using rheoplastic concrete and confining the concrete with FRP in order to increase the maximum plastic deformability of the compressed concrete: the modification allows for increasing the flexural ductility and therefore the potential horizontal deformability;

• 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.

Figure 9 - The compound dissipating system used to retrofit the buildings.

7. Solution evaluation and results The goal of the designed works is to favour the mechanisms for a controlled energy dissipation and to obtain a parallel dissipative effect deriving from the performance of the dissipating devices and the improved potential hysteretic behaviour of the plastic hinges The solution allows for preserving the open space at the first story (the basement level of the case study buildings), using the plastic deformations of some sacrificial structural elements with improved ductility for dissipating energy and reducing the global building response. Preliminary evaluations have been carried out to check the feasibility and the effectiveness of the retrofitting works. Some pushover analyses have been carried out using simplified numerical models consisting of equivalent multi-story frames. The numerical models have been defined to simulate the equivalent mechanical behaviour of the highest building (building A), representing the most critical situation. The analytical procedure provide by ATC-40 [8] has been applied to the model to estimate both the elastic and the ultimate resistant displacements of the structure under both the current and the retrofitted condition. The pushover analyses have been performed using the energy approach reported in [9], that modifies the classical method [10, 11]. The results are summarised in the capacitive ADRS representation ("Acceleration Displacement Response Spectrum") shown in Figure 10. In the figure the seismic demands are compared with the effective inelastic capacities of the structures. The graphs show: (A) the elastic design spectrum defined by the code (return period = 475 years; PGA = 0.25g on

firm rock); (B1) the performance curve of the structure under current condition, pushed to its ultimate state; (B2) the acceleration spectrum reduced taking into account the dissipation capacity reached by the

structure (B1) at its ultimate state; (C1) the performance curve of the retrofitted structure, pushed to its ultimate state; (C2) the acceleration spectrum reduced taking into account the dissipation capacity reached by the

retrofitted structure (C1) at its ultimate state. The dissipating effect of the main dampers has been considered by assuming an elastic-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 [12] and taking into account the confining effect of the FRP wrap around the concrete estimated by using the criterion of [13]. "P-delta" effect has been considered in the evaluations. Through the retrofitting system the limit displacement at the platform level, corresponding to the collapse of the first column, increases from 32 mm, for the existing structure, to 82 mm. The consequence is clearly shown by the graphs of Figure 10: under current conditions (curves B1 and B2) a performance point cannot be reached, while a suitable performance point can be achieved (curves C1 and C2) if the retrofit of the structure is carried out by using the dissipating system illustrated.

(4) closing the open frames above the platform

(2) primary damping system below the buildings (3) enhancing plastic hinges at both the ends of all the columns

(1) connection of the platform sections

Figure 10 - Demand and capacity ADRS representation.

8. Conclusions A compound energy dissipation system for the retrofitting of buildings with soft-first-story has been illustrated, based on the synergetic effect of dampers, located at the first story, with the energy dissipation offered by the plasticization of the critical zones of all the columns of the first story, suitably modified for enhancing their ductility. The solution, allowing for preserving the open space at the first level, has been applied in the design of the seismic retrofitting of two buildings presenting a particular double soft-story configuration.

References:

[1] Arnold C. and Reitherman R. (1982) Building Configuration and Seismic Design, John Wiley & Sons, 1982.

[2] 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 1999.

[3] Parducci A. (2000), Seismic Isolation and Structural Configurations, Technical Meeting held in San Francisco - Forell/Elsesser Engineers, Inc. August, 2000.

[4] Parducci A.(2001) Seismic Isolation and Architectural Configuration, Special Conference on the Conceptual Design of Structures, Singapore, August 2001.

[5] 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, 2004.

[6] Eurocode 8 (2001) Design of Structures for Earthquake Resistance, prDraft No.3, May 2001. [7] 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 2003.

[8] ATC 40 (1996) Seismic Evaluation and Retrofit of Concrete Buildings, Applied Technology Council, Report No. ATC 40.

[9] Parducci A., Comodini F., Mezzi M. (2004) Approccio Energetico per Analisi Pushover (in Italian), XI National Congress "Seismic Engineering in Italy", Genova (Italy), January 2004.

[10] Fajifar P. (2000), Structural Analysis in Earthquake Engineering - A Breakthrough of Simplified Non Linear Methods, 12th European Conference on Earthquake Engineering, Elsevier Science Ltd, 2000.

[11] Chopra A. K., Goel R. K. (1999) Capacity Demand-Diagram Methods for Estimating Seismic Deformation of Inelastic Structures, Report PEER-1999/02, Berkeley, April 1999.

[12] Park R., Paulay T. (1975), Reinforced Concrete Structures, Wiley & Sons, New York, 1975. [13] 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.

0.0

0.2

0.4

0.6

0.8

0 50 100 150 200

Sd (mm)

Sa(g) (A) elastic design spectrum (R = 475 years, ξeq = 5%) (A)

(C2) reduced design spectrum of the retrofitted structure (ξeq ˜ 34%)

(B1) performance curve of the existing structure

(C1) performance curve of the retrofitted structure

32 82

(B2) reduced design spectrum of the existing structure

(B1)

(B2)

(C1) (C2)