SUSTAINABLE AND LOW COST ROOM SEISMIC ISOLATION FOR ESSENTIAL CARE UNITS OF HOSPITALS IN DEVELOPING COUNTRIES

Enrique Morales1, Andre Filiatrault2,3, Amjad Aref2

ABSTRACT

Throughout history, people in Central and South America have suffered devastating economic, social, and environmental consequences from natural disasters particularly earthquakes. Undoubtedly, one of the worst impacts resulting from earthquake hazard is the effect on healthcare infrastructure, principally hospitals. For example, Ecuador suffered major earthquakes in 1987 and 1998 that severely damaged hospitals such as the José María Velasco Ibarra and Miguel H. Alcívar hospitals, as shown in Fig. 1. These hospitals were not adequately built for such events. Since the facilities were not operational after the earthquakes, the number of deaths resulting from the earthquakes was probably higher than it would have been had they been designed or retrofitted to provide seismic resiliency. After the 1998 earthquake, the Ecuador Ministry of Public Health embarked on a program to retrofit hospitals by means of jacketed columns and concrete shear walls, with the intent of strengthening these structures. Noting that non-structural elements in the hospitals suffered major damage, the Ministry also found it important to improve performance of these elements against seismic events so that the hospitals could continue to provide services during times of crisis.

Figure 1. Structural and non-structural damage of Karina, Los Corales and Miguel H. Alcívar hospitals during the1998 earthquake in Ecuador (Courtesy of Aguiar R.).

Using Ecuador as an example, developing countries need to recognize the importance of considering structural control technology such as passive systems to reduce the dynamic response of structures, such as housing, hospitals, schools and public infrastructure. Passive systems such as base isolation have been developed and provide a good means of controlling the

1 PhD Candidate, State University of New York at Buffalo, USA2 Professor, State University of New York at Buffalo, USA3 Professor, Institute for Advanced Study IUSS Pavia, Italy

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demand imposed by an earthquake event. Past events have abundantly demonstrated the effectiveness of techniques such as base isolation by reducing response level in the building from that of a non-isolated structure. A key idea in base isolation is that de-coupling will be accomplished using an isolation scheme that makes the effective fundamental period of the isolated structure much longer than that of the structure above the isolation system.

Base isolation systems have become quite common in developed countries like Japan, primarily due to their rapid development after the M7.2 Kobe earthquake in 1995. However, there are far fewer applications in developing countries due to economic considerations, type of construction and other factors. There is a global need to develop safety standards for an economically-viable protective system that can be implemented within new or existing structures to reduce the dynamic response and increase the performance of structures and their contents so that the overall earthquake risk is markedly lowered.

The slow adoption of protective systems in developing countries has provided the motivation for an innovative design in this project that uses a cost-effective, recycled material, i.e. recycled automobile tires, for isolating designated floors or rooms in health care facilities. The proposed system consists in inserting between the structural slab and a top floor, recycled tires cut in half through their diameter, as illustrated in Fig. 2. The arrangement of this Recycled Tire Bearing (RTB) system is symmetric in plane and each tire is compressed under the gravity load of the top floor and of its equipment and content. Unlike conventional seismic isolation system, the RTB system is able to deform both in the horizontal and vertical directions during an earthquake, thereby providing seismic isolation under horizontal and vertical floor motions. Also, potential vibrations caused by the RTB system during the normal operation of the isolated room can be eliminated by the use of lock bolts between the bottom slab and top floor that could be designed to fail at a predetermined floor acceleration in order to activate the RTB system.

Figure 2. Illustration of the application of the Recycled Tire Bearing (RTB) system in the operation theatre of a hospital.

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A tire is a very complex structure, formed by layers made of different materials that includes rubber, polyester, nylon and steel. It is difficult to define a rubber tire analytically because of its complex geometry, the material nonlinearities of its various elements, and the large deformations impinged on it by the mechanical loads. In this project, tire analyses were performed using the Finite Element Method (FEM) implemented in the LS-DYNA platform. RTB specimens were also tested under vertical and horizontal loading to obtain their mechanical characteristics, as shown in Fig. 3. The maximum displacement, max, effective (secant) stiffness, keff, energy dissipated per cycle, EDC, and the effective damping ratio, , were calculated for each test based on three loading cycles at an applied maximum force Fmax. The experimental results were then compared to the numerical analysis predictions. The predictions of the numerical analyses and results of the RTB tests are consistent, as clearly shown in Table 1 for RTB specimens made of 175/70 R13 tires. Note that under horizontal load, the equivalent viscous damping provided by an RTB specimen is of the order of 15% of critical, which is significantly higher than conventional isolation bearings.

Figure 3: Vertical Testing of RTB specimen.

Table 1.Comparison of experimental results and numerical analysis predictions using LS-DYNA software for 175/70 R13 RTB specimens.

Vertical TestsCycle Fmax

(kN)max (cm) Keff (kN/cm) EDC (kN-cm) eff (%)

Test

LS-DYNA Test LS-DYNA Test LS-DYNA Test LS-DYNA

1 0.52 3.01 3.49 0.17 0.15 0.38 0.21 7.77 3.712 1.00 5.20 5.75 0.19 0.17 1.14 0.96 6.97 5.323 1.50 6.96 7.19 0.21 0.21 1.92 2.00 5.88 5.83

Horizontal Tests with 1.5 kN Gravity LoadingCycle Fmax

(kN)max (cm) Keff (kN/cm) EDC (kN-cm) eff (%)

Test

LS-DYNA Test LS-DYNA Test LS-DYNA Test LS-DYNA

1 0.22 1.76 2.10 0.13 0.10 0.17 0.19 13.7 14.22 0.44 5.80 4.17 0.08 0.11 1.27 0.92 15.8 14.5

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3 0.61 10.8 7.00 0.06 0.09 3.22 2.23 15.5 14.2Based on the test results shown in Table 1, a tributary vertical load of 1.5 kN was considered adequate for each RTB bearing. The vertical deflection under this gravity loading (approximately 7 mm in Table 1) was considered reasonable and allow sufficient controlled vertical movement before contact with the structural slab (bottoming out of the bearings) or separation (uplift) take place. Considering that the uplift can introduce tensile stresses, the boundary condition or support of the tire is designed to allow uplift under tensile load. For this level of tributary gravity load, the vertical effective isolated period Th-eff = 0.53s and horizontal effective isolated period Tv-

eff = 1.03s. The ratio of the vertical stiffness to the horizontal stiffness is 3.68. The horizontal effective isolated is below the traditional target of 2 s for base isolation system. However, the higher damping ration provided by the RTB system, would still provide significant reduction in floor response of compared to rigid floors.

The proposed design and application of the RTB system is intended for high priority rooms such as hospital operating rooms and intensive care units in developing countries where the seismic risk is high. The retrofitted Miguel H. Alcívar Hospital located in the city of Bahia de Caráquez in Ecuador was selected as a case study in order to implement the proposed RTB system in operating rooms and intensive care units. It is noteworthy that the hospital under study suffered considerable damage to its structure and non-structural elements during the 1998 earthquake, making it inoperable for a period of four years. It was later reinforced and now basically has a much stronger and more rigid structural system in which its seismic response under design level earthquake ground motions should be in the elastic range.

The case study hospital was analyzed using SAP 2000, as illustrated in Figure 4. The structural system was assumed to remain elastic and it performance was evaluated by performing an Incremental Dynamic Analysis (IDA) under design level ground motions. The fundamental period of the building is approximately 0.4 s. For each analysis, the floor spectrum acceleration and displacement at the first floor where the Emergency and Recovery rooms are located was obtained in each direction. Fragility curves for various performance objectives were generated. Without floor isolation, the accelerations levels would cause serious damage to the medical equipment such as diagnostic equipment and treatments will be seriously affected. The median floor spectrum acceleration in the horizontal directions shows that using the effective horizontal isolation period of 1.03 s and the associated equivalent viscous damping ramping ratio of 14%, the acceleration demands at the top of the isolation plane remained under 0.15 g. The associated median spectral displacement remains under 10 cm, which could be accommodated by leaving proper gaps between the top isolation plane and the structural elements of the building.

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Figure 4. Numerical model of case study hospital.

As the final stage of this project, seismic testing on a complete prototype system consisting of four RTB bearings is in progress on a six degrees-of-freedom at the University at Buffalo. The experimental procedure is divided into three phases, namely: (i) System identification by hammer and impulse tests, (ii) Seismic testing of the isolated structure with RTB and (iii) Resonance (destructive) test.

Figure 5. Seismic testing of RTB system on a shake table.

The proposed RTB base isolation system developed in this project is a novel and envisioned to be cost-effective strategy that is intended to improve the seismic response of essential hospital facilities in developing countries. The analytical and numerical analysis, developed in this study, show that the RTB system has relatively high damping and low horizontal effective stiffness, which significantly reduces the transmitted acceleration to the critical care unit intended for seismic protection. The case of study Miguel H. Alcívar Hospital in Ecuador was selected in order to verify the implementation of the system in essential care units. This study focuses on static, dynamic and shake table test to evaluate the proposed isolation system.

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