Shurun-Urgun Highway Composite Steel Bridge Elastomeric Bearings Under Seismic Force

This study examines the parameters of Elastomeric Bearing (EB) for affecting the seismic design and behavior for Shurun-Urgun Composite-Steel Bridge is located in Paktia province in eastern part of Afghanistan. Laminated elastomeric bearings used in addition to provide connection between bridge superstructure and substructure, reduce the response of bridge during an earthquake by increasing fundamental period of vibration of bridge, and might reduce the acceleration of superstructure and inertia force passed to the substructure. As an analysis has been done using computer models for two type of piers (tall & short) for similar bridge with or without elastomeric bearing pads and a manually calculation for this numerical investigation to determine the magnitude of forces, displacements and deformations. USGS seismic hazard maps for Afghanistan, and referred to AASHTO, FHWA, and Caltrans manuals where needed.


Introduction
The mountainous heart of Asia-Afghanistan is shaped by tectonically active Alpine-Himalayan orogenic belt that in reflex to the collision between the Indian plate, Arabian plate, and Eurasian plate in Late Paleocene to current period ( fig. 1) (Ruleman, & others, 2007). In such geologically hazardous region with annually moderate to strong earthquakes cause significant damages and fatalities, earthquakes pose a real significant menace to the people and reconstruction of Afghanistan. Now by the efforts of the U.S. Geological Survey (USGS) a series of probabilistic seismic hazard maps to predict the frequency and strength of ground shaking quantity throughout the country (Boyd, & others, 2007;. Composite bridge as a perfect combination of steel girder with concrete slab is easy to erect and economic with lighter structural loads on the piers and foundations compared to the construction of concrete girder commonly used in short-to-medium span (Chen & Duan, 2000). Suran-Urgun Highway Composite-Steel Bridge is supported by laminated (steelreinforced) elastomeric bearings ( fig. 3). Seismic isolation bearings used may lower the response of bridge during an earthquake. Isolation bearings should be stiff under service conditions, and flexible under seismic loads. This is as isolators transmit service loads to the bridge's piers without developing harmful overstress with minimum amount of movement, but as a flexible layer between superstructure and substructure protect structure from damaging action of earthquake by partially decoupling with low stiffness and high damping characteristics. In addition, elastomeric bearings would permit lateral movement due to temperature change, traffic movements, wind, shrinkage, creep, foundation movement, seismic actions, dynamic forces and vibrations. Moreover, elastomeric bearings extend the life of bridge by reducing the wear and tear of the bridge materialsmuch like the shoes allow human beings walk long distances.
The superstructure essentially acts like a rigid body, thus reducing the inter-story movement. When the period is increased, pseudo-acceleration is decreased and hence the force in the structure gets reduced. However, the displacement of the system increases drastically. Flexibility in elastomeric isolation systems is provided by elastomeric bearings (laminated rubber bearings reinforced with steel plates). Energy-dissipation capacity is provided by inherent damping capacity of the rubber, as in high-damping elastomeric bearings (AASHTO, 2012;Caltrans, June 1994& FHWA, 2006. Figure 1. Location of study area in the greater Alpine Himalaya orogenic belt of Asia. Vectors show relative plate motions and velocities between the Indian, Eurasian, and Arabian plates (plate boundaries shown and labeled in yellow). Plate velocities from (Jade, 2004;Bird, 2003;Vernant et al., 2004;and Sella et al., 2002).

General Modeling Assumption
Dynamic analysis of the bridge exposed to earthquakes geometric configuration, mass and stiffness of bridge components and mechanical properties of elastomeric bearing are included in computer modeling. Link elements in a bridge model link elements present elastomeric bearing where their properties are defined as hard spring under compression and weak spring under shear. Elastomer flexibility gives bridge maximum longitudinally release and blocked transversely. Fundamental period, shifts and variations in internal force distribution in the bridge members are expected.

Example Bridge
The plotted dimensions belong to a three equally (30m) spans of real constructed Shurun-Urgun Highway Composite Steel Bridge ( fig. 5a, b & c), which is located in fourth earthquake zone in eastern part of Afghanistan, with 6.5m height piers. Elastomeric bearing pads are supporting deck on the substructure. Computer models using SAP2000 have been performed for analyses with a hand calculation and the numerical results are compared. Structural analysis is carried out using three-dimensional multi-mode model of bridge and simplified model (SDOF-system: using bridge's rigidity and mass) considering and ignored the elastomeric bearing pads. The processes repeated on the bridge with taller piers and then result from simple analysis (manually calculation) will be compared to find out the impact of elastomeric bearings on seismic response of bridge. Only half of superstructure weight is considered on pier.

c. Structural Model Without Elastomeric Bearing
Supposing the bridge without its elastomeric bearing resistance entirely at seismic loading (modeling as simple fixed or sliding hinges) analysis and design. Connection between superstructure and substructure is free to move at abutments and top of the pier. Abutments carry only vertical loads and only pier inertia resists longitudinal movement.

d. Structural Model with Elastomeric Bearing
In this case, we suppose elastomeric bearings as connection between superstructure and substructure of the bridge that represented as link elements in computer analysis in (CSI, SAP2000) to connect superstructure with substructure (piers and abutments). These adjustments in stiffness would influence the fundamental period of the bridge earthquake load as a result the bridge design.

f. Analysis (Manually Calculations)
All of the three bridge models are analyzed with SAP2000 software. Fundamental periods in the bridge longitudinal direction, internal forces, and displacements obtained from each model will be given for comparison purposes in (table 4). In an alternative analysis (manually analysis for the bridge with short piers), the period of bridge will be found using structure's rigidity, pier stiffness, total stiffness of elastomer and mass (table 3) on the pier of the bridge (Caltrans(Metric), October 1995):

Result Collation
The following observations can be made from two bridges analyses with the same superstructure but one is taller and less stiff to show the effect of substructure's rigidity on seismic response. Results achieved from each bridge analysis are summarized in table 4. In the table T refers to period of bridge, M is moment at pier bottom, V is shear force at one pier, VK is lateral force at one abutment, dsub is displacement of substructure, dels is displacement of elastomeric bearing.

Conclusion
For investigating, the effects of elastomeric bearing on the example Shurun-Urgun Composite Steel Bridge both multi-mode and simplified (SDOF system) analyses have been considered in computer models in cases of considering and ignoring the elastomeric bearings properties. Fundamental period shifts, displacements and changes in internal forces distribution in bridge members have been examined, also to precisely come to show effect of substructure rigidity on seismic response, same bridge with taller but flexible piers is analyzed.
Positive reasons from this investigation show that elastomeric bearings have a positive response against the seismic forces applies on the bridge that immensely relies on geometry of bridge, particularly the pier rigidities. Elastomeric bearings for bridge with short piers by absorbing applied forces and appropriately lateral loads between piers and abutments reduce base shear of bridge about 58%, moment about 86%, and 52% in displacement. On the other hand, comparing results with bridge with tall piers in which reduction of base shear is about 8%, moment about 10% and displacement about 38%. In addition, elastomeric bearings increase fundamental period, of the time that bridge undergoes one complete cycle of oscillation, of bridge with short piers about 14% more than bridge with taller piers. Consequently, elastomeric bearings in generally for all bridges and specifically bridges with short piers are effective. Elastomers by absorbing applied forces cause extra displacement of substructure increase displacement of substructure. It becomes more significant if the height of piers increases. Despite, the numerical investigations reveal the positive affect of elastomeric bearing in seismic response, it is highly dependent on geology of bridge, and pier rigidity.