Benefits and Advantages of the Application of Anti-seismic Devices on Bridges and Structures

Agamoni Das

Asst Manager-Projects, mageba India adas@mageba.in, Kolkata, India

Santanu Majumdar CEO, mageba India

smajumdar@mageba.in , Kolkata, India 1. Introduction The design of critical structures to withstand the effects of earthquakes continues to gain importance all over the world. The main objective of any seismic protection system would always be the people’s safety. However, the integrity of the structures and their serviceability immediately after an earthquake play an important role in the speed of the emergency response, particularly bridges, hospitals and schools. Additionally, the cost associated with repair or reconstruction of damaged structures is likely to be small compared to the economic impact caused by disruption of serviceability after an earthquake and during the long reconstruction phase. In order to assure functionality of buildings and bridges, they must be designed to safely withstand the devastating forces of seismic ground movements. Past earthquakes have served as full-scale tests and the often tragic results have forced engineers to reconsider design principles and philosophies. The need for safer structures has stimulated the adoption of a common earthquake protection strategy which has seen conventional bearings being replaced by seismic isolation devices. An isolation system placed between the building structure and its foundations is generally capable of increasing both flexibility and energy dissipation. Flexibility in the horizontal plane will lower the frequency of the bridge, decreasing earthquake-induced acceleration, while the energy-dissipating capacity of the seismic isolators will considerably reduce the damaging energy exerted to other structural members. Moreover, when isolation bearings are installed in the structure, the lateral force from the foundations during a seismic event can be distributed among all structural elements, avoiding the concentration of lateral forces at specific locations. Seismic isolation systems provide an alternative to conventional earthquake resistance design such as strengthening of structural elements (columns or beams), and have the potential for significantly reducing seismic risk without compromising safety, reliability, and economy of structures. Together with the adoption of new performance-based design criteria, this has resulted in seismic isolation technologies already becoming the preferred option for many structural engineers. As an alternative to seismic isolation, energy dissipation becomes essential in terms of seismic protection. The use of effective devices able to dissipate high amounts of energy ensures that other structural elements do not undergo excessive demands that could cause significant damage. An effective seismic isolation system shall provide the following four functions:

a) Performance under all service loads, vertical and horizontal, shall be as effective as conventional structural bearings

b) Provide enough horizontal flexibility in order to reach the target natural period for the isolated structure

c) Re-centering capabilities even after a severe earthquake so that no residual displacements could disrupt the serviceability of the structure.

d) Provide an adequate level of energy dissipation: in order to control the displacements that otherwise could damage other structural elements.

Figure 1. Reduction of accelerations by period shifting Figure 2. Reduction of accelerations by added damping

a) Lead Rubber Bearing

b) High Damping Rubber Bearing

c) Single Pendulum Isolator

d) Double Pendulum Isolator

Figure 3. Most accepted seismic isolation technologies worldwide.

2. Available seismic isolation technologies The main objective of a seismic isolation system is to increase the natural period of a structure. However, instead of elongating the period to high values, an adequate seismic design emphasizes increased energy dissipation capability and distribution of lateral forces to as many foundations or substructures as possible. Buildings are ideal candidates for the adoption of seismic isolation technology due to the effectiveness and simplicity of these devices in ensuring the safety and comfort of the people during an earthquake. Among the different seismic isolation systems, elastomeric isolators such as lead rubber bearings and high-damping rubber bearings have found wide application in all type of structures, as have curved surface sliders, also known as pendulum isolators. This is due to their simplicity and the combined isolation and energy dissipation functions in a single compact unit. Both types of isolators provide a high level of damping. In terms of seismic protection, this is a crucial aspect

to minimize the seismic energy flow to the superstructure and to limit the horizontal displacements of the isolators. In lead rubber bearings, the lead plug deforms plastically and thus dissipates energy through hysteretic damping. In the curved surface sliders, the energy is dissipated by friction between the sliding material and a curved stainless steel sliding surface. 2.1. Lead Rubber Bearings (LRB) Lead Rubber Bearings consist of alternate layers of rubber and vulcanized reinforcement steel plates of limited thickness and a central lead core. They can dissipate energy up to 30% damping due to the high damping capacity of the lead core, as shown in Fig. 3.1-a. LRBs work on the principle of base isolation and limit the energy transferred from the ground to the structure in order to protect it. The rubber/steel laminated bearing is designed to carry the weight of the structure and make the post-yield elasticity available. The rubber provides the isolation and the re-centering. The lead core deforms plastically under shear deformations, while dissipating energy through heat. 2.2. High Damping Rubber Bearings (HDRB) High Damping Rubber Bearings consist as well of alternate layers of rubber and vulcanized reinforcement steel plates of limited thickness. HDRB are essentially elastomeric bearings manufactured with chemically improved rubber. They allow isolation of the structure dissipate energy up to 16% damping (Fig. 3.1-b). 2.3. Curved Surface Sliders (Pendulum Isolators, PI) Pendulum isolators are based on the working principle of a pendulum. The isolators allow the horizontal displacement of the structure, while providing the necessary natural period shifting required by the seismic isolation system. Once activated by an earthquake, the isolators will allow the virtual decoupling of the supported structure from the ground. Subsequently, the restoring force due to gravity will bring it back toward the center position. The performance of the device mainly depends on the radius of curvature and the coefficient of friction. Pendulum isolators can be design with single or double curved surfaces. Fig. 4 shows an example of the single pendulum isolator. Fig. 5 shows the general construction of a double pendulum isolator. The working principle of the pendulum isolators is shown in Figure 6.

Figure 4. Pendulum isolator with one curved surface Figure 5. Pendulum isolator with double curved surfaces

Figure 6. Functioning principle of pendulum isolators

2.4. Benefits of Elastomeric Isolators

a) Transmit the vertical loads due to permanent and accidental effects in seismic condition; it is possible to cover a wide range of loads about up to 50,000 kN

b) Very wide range of plan diameters

c) Support the horizontal loads due to service load conditions with low displacements

d) Damping ratio from 10% to up to 30% equivalent viscous damping value to reduce the horizontal displacement of the isolated structure respect to the ground

e) Guarantee the stability at the maximum horizontal displacement due to seismic excitation; it is possible to guarantee a wide range of displacements function of the applied vertical load and horizontal stiffness

f) Significant dissipation of energy during a large scale earthquake, leading to a reduced structure size and total bridge cost

g) Effective solution for several types of bridges and buildings. It is highly suitable to be applied in crucial buildings as hospitals, schools or research facilities

h) Effective solution for the retrofitting or upgrade of existing structures

i) Among the different isolation system, the effectiveness of LRBs has been proven by numerous articles as well as during numerous earthquakes

j) The isolation of structures by LRBs is the most used seismic isolation technology in the world

k) Very well-known technology with 20-30 years of experience in many applications for buildings and for bridges all around the world

l) Very simple maintenance mainly limited to a periodic visual inspection for all the design life

m) High capacity of reducing the seismic forces on the structure; this implies simplification of the structure

design and reduction of the structure construction costs

n) Very easy simulation of the device response due to a simple bi-linear modeling 2.5. Benefits of Curved Surface Sliders (Pendulum Isolators)

a) Reduction of the dynamic impact on structural elements allowing slender as well as economic structures

b) Significant increase of the seismic safety of the structure and its users

c) High load bearing capabilities with compact geometry

d) Re-centering capabilities allowing the structure to return to the initial position after excessive displacements

e) Simplicity in design and adaptability to any type of structure

f) Applicable for new structures as well as for retrofitting of existing ones

g) Longer life of the devices due to highest quality standards for all components

h) Virtually maintenance free due to high durability of the corrosion protection and high performance sliding material

i) Proven seismic protection technology throughout the years in structures all over the world

3. Available energy dissipation technologies While seismic isolation is a proven strategy to mitigate seismic damage, the complex dynamic response of the structure often requires additional devices in order to control the horizontal displacements. Another alternative to ensure a safe structure is by combining seismic isolation and energy dissipation. This provides the structure with a higher damping, and therefore a better dynamic response during a seismic event. In structures where seismic isolation is not a recommendable solution (e.g. soft soils), damping systems with high dissipation capabilities become the best seismic protection alternative. 3.1. Shock Absorbers (Viscous Dampers, SA) Shock absorbers are velocity dependent devices, consisting primarily of a piston, a piston rod and a cylinder pipe. They allow free movements of a structure during service conditions, but provide displacement control and dissipate energy during sudden movements caused by events such as earthquakes, exceptional traffic or high wind forces. The resistance force depends on the flow of a viscous fluid that passes from one chamber of the cylinder pipe into the other. The fluid is squeezed through small holes in the piston whose size determine the damping characteristics of the shock absorber (Figure 7 and 8). Shock absorbers dissipate energy from sudden, exceptional loading, and thus reduce the impact on the structure. This allows the design of the structure to be optimized, avoiding conventional strengthening which might be rarely or never needed during the lifetime of the structure. Shock absorbers can provide over 30 %

Figure 7. Model of a shock absorber (viscous damper) Figure 8. Installed shock absorber

of additional damping, significantly reducing design loads acting on the adjacent structural members. The devices are ideally combined with seismic isolators such as lead rubber bearings, high damping rubber bearings or pendulum isolators to further reduce forces and control the movements of the structure. 3.2. Shock Transmission Units (STU) Shock transmission units consist of a piston, a piston rod and a cylinder pipe. They are temporary (dynamic) connecting devices allowing free movements during service conditions while locking up during a shock loading from an earthquake or due to traffic/train breaking. In such cases, they transmit the forces to the connected elements (Figure 9 and 10). Shock transmission units – also known as lock-up devices – work on the principle that the rapid passage of a viscous fluid through a narrow gap, orifice or port generates high resistance, while slow passage at low velocity generates only minor resistance. Consequently, RESTON®STU devices lock-up during a quick motion, while delivering a very small reaction force caused by friction during slow displacements such as from thermal expansion or contraction. STUs do not dissipate energy and consequently there is no reduction of the load impacts. However, by locking up in certain extraordinary events, the STUs temporarily change the static system of a structure, e.g. from simply supported to continuously supported elements. The main purpose is to control the load distribution and to share the forces with several structural elements. In addition, STU devices avoid large movements of the structural elements such as bridge decks in case of sudden load impacts.

Figure 9. Shock transmission unit Figure 10. STU installed on a bridge

3.3. Preloaded Spring Damper (PSD) Preloaded spring dampers are on top of conventional viscous dampers, especially designed to perform the following functions:

1. For general loads due to traffic, creep, shrinkage and thermal variations, the PSD devices act as fixed points of the structure and do not allow any movements.

2. In a seismic event, the PSD devices allow the structure to move. The units dissipate seismic energy and control displacements simultaneously.

3. After a seismic event, the PSD devices automatically re-center them-selves back to their initial position.

The proper definition of the preloaded value F0 is very important, as the units will prevent any displacement before reaching this threshold. It has to be further taken into account that F0 varies in relation to the temperature. Preloaded spring dampers can dissipate over 30 % of the introduced energy due to a dynamic event. This allows the structures to be protected at a lower cost as compared to conventional strengthening methods. The re-centering capability is given by the internal compression. The return force has to be defined in advance and is an important design parameter of the device. In any case, return force and friction force must be higher than the friction force of the structure’s sliding bearing, which ensures its return to initial position. These devices can be produced in the following options:

1. Compression in one direction

2. Compression in two directions

3. Traction only

4. Traction and compression. 3.4. Benefits of energy dissipation devices The viscous fluid used for seismic devices is protected against aging by special additives, while the fluid itself protects the device from inner corrosion. In the case of temperature variations, viscosity remains nearly constant. This characteristic causes the mechanical system to be thermally compensated. The sealing is the most critical element of the hydraulic system and requires highest quality standards. Consequently, manufacturers employs a high grade sealing that demonstrates a quasi-zero natural wear and an absolute physical chemic compatibility with the adopted viscous fluid. All these considerations allow these devices to provide the following benefits:

1. Significant increase in the safety of the structure and its users

2. Longer lifespan of the devices due to finest quality standards for all components

3. Devices tailored to the needs of the client

4. Applicable for new structures as well as for retrofitting of existing ones

5. Re-centering of the structure after the event of an exceptional load (earthquake)

4. Recommendation for the design of seismic protection systems In order to properly reach and optimized seismic isolation strategy for each structure, it is recommended to follow with the following steps in order to reach the most suitable seismic isolation system:

a) Structural engineer evaluates the dynamic characteristic of the proposed isolator together with the designer of the device

b) The bilinear model of the isolator’s hysteresis is applied to the seismic analysis of the building, based on the information provided by the manufacturer

c) Required stiffness and horizontal displacements are evaluated and defined.

d) Structural engineer provides the manufacturer with final parameters, such as stiffness, displacements, loads and dimension limitations.

e) The manufacturer shall optimize the design of the units once target horizontal stiffness and displacements are fully defined

f) Structural engineer shall review and approve the final design of the devices For the above process, a nonlinear seismic analysis of structures is recommendable in order to find the most effective seismic protection system. This advanced optimization process provides safe and economical structures. Such analysis can be performed in accordance with any seismic norm (EN, AASHTO, JSCE, IBC, BS, JPN, etc.). A nonlinear seismic analysis allows the precise calculation of forces and displacements that could potentially occur during a determined earthquake. 5. Performance and CE certification of seismic devices Seismic devices are designed and manufactured in accordance with European Standard EN 15129, EN 1337 and with European Technical Approval ETA-08/0115. Bearings are marked with the CE mark of conformity, which confirms that they satisfy all requirements of these Standards and Approval, without exception. The seismic devices can also be designed, manufactured and tested in accordance with international specifications, such as the “AASHTO Guide Specification for Seismic Isolation Design”, Japanese Specifications, National Norms, etc. 6. Conclusions The use of a seismic protection strategies such as seismic isolation and energy dissipation by added damping has proven to be a sensible approach to the challenges presented by the need to make important and key structures seismically safe in accordance with current seismic design standards. By providing an alternative to conventional earthquake resistance design measures, it saves the major strengthening works which would otherwise be required, if the benefits of energy dissipation and damping were not incorporated in the design. The application of seismic protection technologies thus demonstrate the potential such an approach, and such devices, have to significantly reduce seismic risk without compromising the safety, reliability, and economy of structures. References

1. CEN (European Committee for Standardization). 2009. EN 15129 - Anti-seismic devices. Brussels, Belgium.

2. Mendez Galindo, C, Spuler, T, Moor, G, & Stirnimann, F. 2012. Design, full-scale testing and CE-certification of anti-seismic devices according to the new European norm EN 15129: Elastomeric Isolators. 15th World Conference on Earthquake Engineering. Lisbon, Portugal.

3. Mendez Galindo, C, Spuler, T, Moor, G, & Stirnimann, F. 2012. Design, full-scale testing and CE-certification of anti-seismic devices according to the new European norm EN 15129: Curved Surface Sliders. 15th World Conference on Earthquake Engineering. Lisbon, Portugal.

4. Moor, G, Mendez, C, and O’Suilleabhain, C. 2011. Measuring Up, Bridge Design & Engineering, Issue 63. London, England.