Recorded and Numerical Strong Motion Response of a Base-Isolated Bridge

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1 Recorded and Numerical Strong Motion Response of a Base-Isolated Bridge B. Bessason, a) M.EERI, and E. Haflidason b) Since 1983, 12 Icelandic bridges have been base isolated for seismic protection. Lead-rubber bearings have been used in all the cases. The Thjorsa River Bridge, built in 1950 and retrofitted with base isolation in 1991, is instrumented by strong-motion accelerometers. The bridge has one 83-m-long main span and two 12-m-long approach spans. Only the main span, a steel arch truss with concrete deck, is base isolated. In June 2000, two major earthquakes of magnitude 6.6 and 6.5 occurred in South Iceland; the epicenter was close to the Thjorsa River Bridge. In the first earthquake, a peak ground acceleration of 0.53 g was recorded at the bridge site, and in the second earthquake, a peak ground acceleration of 0.84 g was recorded. The Thjorsa River Bridge survived the earthquakes without any serious damage and was open for traffic immediately after the earthquakes. [DOI: / ] INTRODUCTION Seismic base isolation is a relatively new method in earthquake-resistant design. Scientists and engineers in New Zealand, the United States, Japan, and Italy have been active in developing the concept and using it in buildings and bridges (Kelly 1986, Buckle and Mayes 1990, Skinner et al. 1993, Naeim and Kelly 1999). The method has also been applied in other countries as well. Since 1983, base isolation has been used for seismic protection of 12 Icelandic bridges. Seven of the bridges are located in South Iceland and five in North Iceland. Two of the base-isolated bridges in South Iceland are instrumented with strong-motion accelerometers. In June 2000, two major earthquakes, of magnitudes 6.6 (M w ) and 6.5 (M w ), struck South Iceland; the epicenter was close to one of the instrumented bridges, i.e., the Thjorsa River Bridge (see Figure 1). The Thjorsa River Bridge was built in 1950 and retrofitted in 1991, using lead-rubber bearings (LRB). In both these earthquakes, ground motion at the bridge site, as well as the structural response of the bridge, was recorded. High peak ground acceleration (PGA) was recorded at the bridge site in both earthquakes. The bridge survived the earthquakes without any serious damage. The objective of this paper is threefold: first, to describe briefly the application of seismic base isolation in Iceland; second, to describe the seismic retrofit program of the base-isolated Thjorsa River Bridge; third and mainly, to report the recorded strongmotion data from Thjorsa River Bridge, as well as to analyze the earthquake response of the bridge during the South Iceland earthquakes of June a) University of Iceland, Hjardarhagi 2-6, 107 Reykjavik, Iceland b) Public Roads Administration, Borgartún 7, 105 Reykjavik, Iceland 309 Earthquake Spectra, Volume 20, No. 2, pages , May 2004; 2004, Earthquake Engineering Research Institute

2 310 B. BESSASON AND E. HAFLIDASON Figure 1. The Thjorsa River Bridge. One 83-m-long main span and two 12-m-long approach spans. OVERVIEW APPLICATION OF BASE ISOLATION IN ICELAND From 1983 to 2003, twelve Icelandic bridges have been seismically base isolated (see Table 1). Ten of these are in-situ cast concrete-beam bridges designed with respect to seismic base isolation; one is the steel-arch bridge, which was seismically upgraded, Table 1. An overview of Icelandic base-isolated bridges Bridge Construction year Type of structure Length of spans (m) Number of bearings Sog at Thrastarlundur 1983 Concrete beams Stora Laxa 1985 Concrete beams Eyjafjardara 1986 Concrete beams Ölfusaros 1988 Concrete beams Thjorsa (old) 1950/91 Steel arch Bæjarhals 1994 Concrete beams VesturósHéraZsvatna 1994 Concrete beams Thvera 2002 Concrete beams Lonsos 2002 Concrete beams Svarfadardalsa 2002 Concrete beams 30 4 Skidadalsa 2002 Concrete beams Thjorsa (new) 2003 Composite steelconcrete arch arch: 78 total length:

3 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 311 Figure 2. Cross section of lead-rubber bearing used in the Thjorsa River Bridge. and one is a composite steel-concrete arch bridge. The bridges are located in or close to either of the two major seismic zones in Iceland, i.e., the South Iceland Seismic Zone and the Tjörnes Fracture Zone. In both these zones, earthquakes with magnitudes of up to seven have been detected (Einarsson 1991). Many isolation systems have been proposed through the years (Naeim and Kelly 1999), but systems based on elastomeric bearings are most common in modern seismic base isolation. To fulfill flexibility requirements for the bearings, a sufficient thickness of rubber is selected. Several solutions for obtaining the damping and rigidity properties for low load levels are available. This can be achieved from the rubber elastomeric compound properties alone, by using a lead core in the middle, by combining the elastomeric bearing with sliding friction, and by using secondary mechanical devices based on plastic deformations of metal or by using viscous dampers. The system with the lead core, i.e., the LRB, is most common. LRB have been used for all the Icelandic bearings (Figure 2). LABORATORY TEST OF LEAD-RUBBER BEARING The common way to install the lead core in an LRB is to manufacture a bearing with a hole and then press a precast, oversized, lead plug into the hole. For the Icelandic bridges, a nontraditional method has been used. A hole is drilled into a conventional elastomeric bearing, and then the lead plug is cast directly into the hole. Two identical bearings made by this method were dynamically tested at the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway, in The dimensions and material properties of these bearings are shown in Table 2. Table 2. Dimensions and material properties for the lead-rubber bearings used in the Thjorsa River Bridge Length 450 mm Installation height 141 mm Width 350 mm Hardness of rubber 60 Shore A Number of rubber layers 7 Shear module of rubber 1 MPa Thickness of one rubber layer 11 mm Diameter of lead cylinder 124 mm Thickness of steel plates 4 mm Height of lead cylinder 141 mm Total thickness of rubber 77 mm

4 312 B. BESSASON AND E. HAFLIDASON The bearings were tested for different load conditions, i.e., various combinations of horizontal displacement amplitude (shear strain amplitude), frequency of the horizontal displacement strokes, vertical force amplitude (static load simulating dead load of superstructure), and environmental temperature. For the low displacement amplitudes, corresponding to 40% shear strain (i.e., the ratio of horizontal displacement amplitude and total rubber thickness), the frequency was 0.5 Hz, and for the high amplitude corresponding to 120% shear strain, the frequency was 0.15 Hz. Details of the tests and the test results are given in Bessason (1992). A bilinear hysteresis model was fitted to the test results. Based on this, the following parameter set was introduced: K d K r A rg r T r (1) K u 11.6 K r (2) Q d yp A p (3) where K d is the post-stiffness, K u the initial- and unloading-stiffness, and the parameters A r, G r, and T r are plane area, shear modulus, and total thickness of the rubber in the bearing, respectively. It should be noted here that the parameter 11.6 in Equation 2 is higher than normally found in the literature. Furthermore, Q d is the characteristic yield strength at zero shear strain in the bilinear hysteresis model, A p is the cross-sectional area of the lead plug, and yp 8.0 MPa is the estimated effective yield shear stress of the lead plug. This yield stress is about 20% lower than commonly found in other documented results where the lead plug is oversized and pressed into the hole (Skinner et al. 1993). The reason for this low effective yield stress can be explained primarily by the method used for insertion of the lead core and by larger thickness of each rubber layer than is standard practice. Both effectively reduce the confining pressure on the lead core. The main conclusion from the laboratory test was that the bearings behaved similarly to other tested bearings reported in the literature, and the bearings were capable of undergoing a few cycles with shear strains in the range 100% to 120% without any significant damage. THE SEISMIC RETROFITTING PROGRAM FOR THE THJORSA RIVER BRIDGE EARTHQUAKE ANALYSIS BEFORE UPGRADING The Thjorsa River Bridge on Highway 1 is located in the middle of the South Iceland Seismic Zone. The bridge is an important link between the eastern and western part of the South Iceland Lowland, which is the main agricultural region in Iceland. If the bridge is seriously damaged or collapses during an earthquake, the traffic will have to be diverted to an 80-km bypass on a secondary road. The Thjorsa River Bridge, built in 1950, is a three-span bridge with one 83-m-long main span and two 12-m-long approach spans (see Figure 1). The structural system of the main span (superstructure) is a steel-

5 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 313 Figure 3. Schematic drawing of the Thjorsa River Bridge showing main parts and the soil profile. The black dots ( ) show the location of accelerometers. arch truss with a concrete deck supported by two rigid, approximately 1.5-m-high concrete piers. The mass of the superstructure is 415,000 kg. The approach spans are simply supported steel beams with concrete decks, connecting the main span to abutments. The bridge has four 50-mm-wide expansion joints, located between the abutments and the approach spans and between the approach spans and the main span, as shown in Figure 3. The western pier and abutment are founded on lava rock, overlaying alluvial deposits, and the eastern pier and abutment are founded on rock (dolerite) (Figure 3). In 1984, a linear elastic earthquake response analysis of the bridge was carried out using a Finite Element (FE) model combined with the normal mode method and the response spectral analysis approach (Bessason and Sigbjörnsson 1984). The analysis was based on two recorded events from California, i.e., the well-known Imperial Valley earthquake of 18 May 1940, M 6.3, recorded in El Centro with a PGA of 0.33 g, and the Kern County earthquake of 21 July 1952, M 7.7, recorded in Taft with a PGA of 0.22 g. The main conclusion, based on the earthquake response analysis, was that the ability of the bridge to withstand the seismic loads caused by the two above-mentioned earthquakes was not satisfactory. The weakest link was found in the bearing system of the bridge. The analysis indicated that earthquakes with a PGA exceeding 0.2 g would probably cause significant damage to the bearing system that could even lead to a total collapse of the bridge. Furthermore, the superstructure itself would probably not be able to withstand an earthquake with a PGA exceeding 0.3 g without plastic deformation and serious and unpredictable damage. As described later in this paper, it should be noted that the above PGA is not a unique or absolute measure of the earthquake intensity, but is derived from the two California earthquakes. The supporting piers are short and massive and not likely to be critical for the survival of the bridge during an earthquake. Based on this analysis, it was decided to seismically retrofit the bridge. MEASUREMENTS OF THE THJORSA RIVER BRIDGE S DYNAMIC PROPERTIES Before the upgrading process took place, measurements of the dynamic properties of the bridge were carried out in One transversely, one longitudinally, and four ver-

6 314 B. BESSASON AND E. HAFLIDASON Table 3. Measured and computed modes of the Thjorsa River Bridge before it was base isolated Mode no. FIELD MEASUREMENTS (1989) FINITE ELEMENT ANALYSIS (1984) Natural Period Mode Natural Period (s) Type of mode no. (s) Type of mode Transverse (& torsion) Transverse Torsion Torsion Vertical (& longitudinal) Transverse Vertical Vertical Transverse (& torsion) Vertical Vertical & longitudinal Torsion Transverse tically directed accelerometers were used. More details are given in Bessason and Sigbjörnsson (1990) and in Bessason (1992). The main results from the measurements, based on Fourier analysis, are shown in Table 3. For comparison, the FE analysis results from the earthquake response analysis from 1984 are also shown in the table. The natural period in the FE model is longer in all cases than the period obtained by the field measurements. The stiffness of the concrete bridge deck was not included in the FE model in 1984, and this affected the result, especially for the first mode where the FE model period was 55% higher than the measured one. The dynamic measurement was also used to determine the damping in the steel grid. For the low amplitude/strain oscillations generated during the tests, it was found that the damping ratio was, on average, 0.5% for the first 12 modes, and in all cases less than 1%. BASE ISOLATION OF THE BRIDGE Following the consideration of several strengthening alternatives, it was decided to focus on seismic base isolation of the main span using LRB. This was considered a relatively simple solution to implement. By using hydraulic jacks, the basic idea was to lift the superstructure of the bridge, remove the original bearings, and then replace them with four suitable LRB. The design criterion for the bearing was that the maximum displacement should be less than u critical 50 mm. Otherwise, the superstructure would hit the land spans and cause unpredictable damage. Based on recommendations from New Zealand codes, it was decided to allow 70% shear strain to develop in the bearings for maximum displacement in the bearings. These criteria, along with earthquake response analysis of the bridge, resulted in four lead-rubber bearings, two in each end. Dimensions and material properties of the bearings are the same as those used in the dynamic laboratory tests at NTNU (see Table 2). In addition to the bearings, a secondary system was constructed. This system consists of tension bars to prevent large, transversal horizontal displacements and to avoid tipping of the bridge in major earthquakes. The bars were designed to be inactive for small horizontal displacements where the main load was supposed to be handled by the bearings. In larger displacements, the tension bars should become gradually more active.

7 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 315 Figure 4. Schematic figure showing lead-rubber bearings and tension bars on single pier. The force-displacement curves for both systems are shown on the right. A schematic figure of the tension bars and the bearings at one end, along with a forcedisplacement curve for both systems, are shown in Figure 4. Although the primary aims of the tension bars are to secure the superstructure for transversal displacements, the system is also active in the longitudinal direction with the same characteristic (forcedisplacement curves) as for the transverse direction. However, the transverse beam (Figure 4) connecting the tension bars to the superstructure is softer in bending and twisting (longitudinal action) than in axial deformations (transverse action). Earthquake hazard analysis of the site (Bessason and Sigbjörnsson 1990, Bessason 1992) showed that the bridge could expect PGA up to an order of 1 g. In order to control the displacements for such high accelerations, it was considered necessary to use lead plugs with a total yield shear strength corresponding to 10% of the weight of the bridge. Based on u critical 50 mm and the parameters in Table 1, the effective period of the system was estimated as T eff 1.03 s. In Bessason and Sigbjörnsson (1990), a bilinear single-degree-of-freedom (SDOF) hysteresis model was used to analyze the baseisolated bridge. The model assumes rigid body motion of the superstructure. The secondary system, i.e., the tension bars (Figure 4), was not included in the model. Simulated time series were used as earthquake excitation. The simulation was based on an evolutionary stochastic model for the earthquake motion, using an amplitude-modulated time series derived from the Kanai-Tajimi power spectra and the Jennings-Housner envelope function. The earthquake response was computed for different PGA (see Table 4). Table 4 shows that when PGA is 0.35 g (linear interpolation), the mean value for the maximum displacement is u max 0.05 m, meaning that there is approximately 50% probability that the superstructure will pound into the approach span. In Bessason (1992) and Bessason and Sigbjörnsson (1998), models that are more sophisticated are used to analyze the bridge in order to consider the effect of the flexible superstructure. A time-domain method and a simplified random-vibration approach were used for the special case of a single-span bridge with base isolation. For both methods, the bridge was idealized as a simply supported, symmetric beam with LRB at each end. The superstructure was modeled with six linear, elastic beam elements and the supporting bearings with either a bilinear hysteresis model or the nonlinear Bouc-Wen hyster-

8 316 B. BESSASON AND E. HAFLIDASON Table 4. Earthquake response of the base-isolated bridge modeled by bilinear SDOF hysteresis model as a function of peak ground acceleration. The table shows the estimated mean value and standard deviation for maximum displacement and maximum shear force (Bessason and Sigbjörnsson 1990). Peak ground acceleration (g) Max displacement u max (m) u,max (m) Max shear force in bearings F max (MN) F,max (MN) esis model (Bouc 1967, Wen 1976, Wen 1980). Comparison of an SDOF model shows that the bilinear and the Bouc-Wen hysteresis model give the same results if the parameters set for these models are adjusted properly (Bessason 1992). A comparative analysis of the time-domain method and the simplified randomvibration approach showed that results from both models agreed reasonably well. Hence it was considered justifiable to use the random-vibration approach to study the response of a base-isolated, simply supported beam for different dynamic properties of the structural system, as well as for different types of earthquake excitation. An important result from the random-vibration approach was that a flexible superstructure reduces the transverse response, compared with SDOF analysis where the superstructure is assumed rigid. This means that the response values in Table 4 are overestimated. More detailed results from this study are published in Bessason (1992), and Bessason and Sigbjörnsson (1998). The spatial variation of earthquake ground motion may be an important factor for the response of structures with spatially extended foundations. It is well known that the correlation of the ground motion at two separated points decays with increasing distance and increasing frequency. Low-frequency motion is better correlated (see, for instance, Harichandran and Vanmarcke 1986). In general, the earthquake response of seismically isolated structures is dominated by low-frequency motion. Therefore, it was considered reasonable to assume full correlation of the excitation at both supports for the mathematical models above. Furthermore, at the time the analysis was done, the difference between the soil profile on the east and the west sides (Figure 3) was not known. INSTRUMENTATION OF THE BRIDGE The western side and mid-span of the bridge were first instrumented in November In May 2000, the system was expanded to cover both ends of the bridge. The system is based on components from Kinemetrics Inc. In both piers, there is a triaxial,

9 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 317 FBA-23 strong-motion accelerometer (see Figure 3). As the piers are short and stiff (rigid), these sensors can be assumed to measure the ground motion. In addition, three single-axes, FBA-11 accelerometers are located in the superstructure. Two of them are installed just above the southern LRB on the western pier, measuring horizontal motion in the longitudinal and transverse direction, and one is located in the middle of the bridge, measuring the transverse motion (see Figure 3). The sensors are connected to an Altus K2 data acquisition system, which is in turn connected by modem to the Earthquake Engineering Research Center at the University of Iceland in Selfoss. The objective of the instrumentation is threefold: first, to obtain information about the earthquake response of the base-isolated bridge; second, to collect data about the ground motion at the site during major earthquakes; and third, to study how the ground motion is correlated at the eastern and the western piers. All this was important when defining design criteria for the new bridge over the Thjorsa River, which was constructed in It should be noted that the longitudinal direction is, as mentioned earlier, approximately in the north-south direction, and the transverse direction runs east-west. THE SOUTH ICELAND EARTHQUAKES OF JUNE 2000 TECTONIC In June 2000, two major earthquakes struck the South Iceland Seismic Zone. The first earthquake, with an estimated magnitude of M w 6.6, struck on 17 June 2000 at 15:41 (GMT). The epicenter, with an approximate focal depth of 6.3 km, was 16 km east-northeast from the Thjorsa River Bridge. The earthquake was a right-lateral strikeslip earthquake, with fault striking in the north-south direction. Observations showed an approximately 20-km-long surface fault rupture. The closest point from the bridge site to the surface fault rupture was 13 km. The second earthquake, with an estimated magnitude of M w 6.5, struck on 21 June 2000 at 00:52 (GMT). The epicenter, with an approximate focal depth of 5.3 km, was 5 km west-northwest from the Thjorsa River Bridge. This earthquake was also a right-lateral strike-slip earthquake, with fault striking in the north-south direction. Observations showed an approximately 23-km-long surface fault rupture. The closest point from the bridge site to the surface fault rupture was 2 to 3 km. INSPECTED DAMAGE OF THE BRIDGE The bridge was subjected to large earthquake forces during the 17 June and 21 June earthquakes. However, there is no evidence of significant damage to the bridge. The western and the eastern piers moved approximately 40 mm toward each other, leaving the bearings with permanent inclinations corresponding to approximately 25% shear strain. Site evidence also shows that the main span has pounded lightly into the two approach spans and shifted them towards the abutments. This means that the longitudinal relative displacements of the main span have been greater than the 50-mm-wide expansion joints. The lead starts yielding at a displacement of 5 mm (Figure 3), meaning that the bearings have deformed plastically and dissipated energy. Furthermore, the connecting bolt for one of the tension bars broke. Assuming that the forces on the tension bars

10 318 B. BESSASON AND E. HAFLIDASON Figure 5. Recorded acceleration time histories at the western and the eastern piers for longitudinal, transverse, and vertical directions during the South Iceland earthquake on 17 June are equally distributed, the connecting bolts should start breaking at a displacement of 70 mm if statically loaded. The missing bolt could not be found. Finally, one wing wall suffered minor damage when rotating about the end attached to the abutment. MEASURED GROUND MOTION AT THE BRIDGE SITE Time Histories In Figure 5 the measured acceleration time histories at the western and eastern piers for the 17 June earthquake are compared for the longitudinal, transverse, and vertical directions. The acceleration is in all cases lower at the eastern pier than at the western, despite the fact that the epicenter is on the eastern side of the bridge. As both concrete piers are short and stiff, it can be concluded that the measured PGA has a ground motion characteristic. In Figure 6, the same comparison is shown for the 21 June earthquake. Again, the acceleration is lower at the eastern pier than the western pier. In Figures 7 and 8, the displacement time histories (based on double integration of the acceleration) are compared for the east-end pier and the west-end pier for the 17 June and 21 June earthquakes, respectively. The correlation between the measured displacements on both piers is good, and one would have to look carefully to see the small differences in the figures. Response Spectrum In Figure 9 the elastic response spectra (5% damping ratio) for both the horizontal components on each side of the Thjorsa River Canyon are shown for the South Iceland earthquake of 17 June. For comparison, the El Centro response spectrum (comp. S00E) is also shown. In the earthquake analysis of the bridge from 1984, it was concluded that

11 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 319 Figure 6. Recorded acceleration time histories at the western and the eastern piers for longitudinal, transverse, and vertical directions during the South Iceland earthquake on 21 June the El Centro spectrum was close to the limits of what the superstructure could withstand. For the natural periods above 0.5 s, which are of main interest for the bridge, the El Centro spectrum is far more critical for the superstructure than the 17 June earthquake. Figure 7. Comparison of recorded displacement time histories, at the west pier (solid line) and at the east pier (dashed line), during the South Iceland earthquake on 17 June The comparison is shown for the longitudinal, transverse, and vertical components.

12 320 B. BESSASON AND E. HAFLIDASON Figure 8. Comparison of recorded displacement time histories, at the west pier (solid line) and at the east pier (dashed line), during the South Iceland earthquake on 21 June The comparison is shown for the longitudinal, transverse, and vertical components. In Figure 10, the same comparison is done for the 21 June earthquake. For the transverse component, the El Centro spectrum is quite similar to the 21 June earthquake spectra for natural periods above 0.6 s. For the longitudinal components, the 21 June spectra are higher than the El Centro spectrum. Furthermore, it should be noted that for both the June 2000 earthquakes, the response spectra from both sides of the Thjorsa River Bridge site are quite similar for periods above 1.0 s, but long natural periods are of Figure 9. Acceleration response spectra (5% damping ratio) for the western pier (solid thin line) and the eastern pier (dashed line) based on recordings during the South Iceland 17 June 2000 earthquake. Both the longitudinal and the transverse components are shown. Furthermore, the El Centro response spectrum (S00E component) from 1940 is shown for comparison in both graphs.

13 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 321 Figure 10. Acceleration response spectra (5% damping ratio) for the western pier (solid thin line) and the eastern pier (dashed line) based on recordings during the South Iceland 21 June 2000 earthquake. Both the longitudinal and the transverse components are shown. Furthermore, the El Centro response spectrum (S00E component) from 1940 is shown for comparison in both graphs. main interest for base-isolated structures. This is the displacement-controlled part of the response spectra, and it agrees well with the displacement time histories, which are quite similar on both sides of the river canyon (figures 7 and 8). Peak Ground Acceleration and Effective Peak Ground Acceleration In Table 5 recorded peak ground acceleration (PGA) and calculated effective peak ground acceleration (EPGA) for the horizontal components on the piers are shown for the South Iceland earthquakes of June The EPGA is defined as the average spectral acceleration over the period range from 0.1 s to 0.5 s, divided by 2.5 (elastic 5% acceleration response spectrum). Furthermore, for comparison, the PGA and EPGA for the El Centro (comp. S00E) time histories from 1940 are shown in the bottom line. Table 5. Measured PGA and EPGA at the Thjorsa River Bridge site during the June 2000 earthquakes LOCATION OF ACCELEROMETERS Earthquake of 17 June 2000 Earthquake of 21 June 2000 PGA (g) EPGA (g) PGA (g) EPGA (g) WESTERN PIER: Longitudinal Transverse Vertical EASTERN PIER: Longitudinal Transverse Vertical EL CENTRO (1940) S00E component

14 322 B. BESSASON AND E. HAFLIDASON It should first be noted that both the PGA and the EPGA are significantly larger at the western pier (lava rock overlying alluvial deposits) than at the eastern pier (rock site) in both earthquakes as expected from the time histories plots and the response spectra (figures 5 and 6, and figures 9 and 10). It has been shown by processing recorded data and by analytical studies that the sediments underlying the lava rock significantly amplify the ground motion (Bessason et al. 2002, Bessason and Kaynia 2002). Second, although very high PGA is measured at the piers, the EPGA is lower and is probably more representative of the seismic load than the PGA. For instance, comparing the eastern response spectra (rock site) for the 21 June earthquake with the El Centro spectrum shows that these spectra are quite similar for a wide period range. This is reflected in similar EPGA, while the difference in PGA is far higher. Focusing on the natural periods in the range from 0.6 to 1.0 s, which are important with respect to the earthquake response of the Thjorsa River Bridge, the El Centro response spectrum (PGA 0.33 g) and the recorded Thjorsa River Bridge response spectrum (PGA 0.84 g) are quite similar. This example shows that it is not a unique or absolute measure to use the PGA as a representative ground-motion parameter when describing earthquake excitation. It has also been shown that PGA is very instrument dependent. EARTHQUAKE RESPONSE OF THE BRIDGE General Remarks Examining how the bridge responded during the June 2000 earthquakes, based on the recorded time histories and numerical analysis, is interesting and informative: first, to find out if the bearings isolated the bridge as expected; second, to verify the use of mathematical models to predict the response of the bridge; and third, to compare the earthquake response of the bridge for fixed-base versus base-isolated design. These questions will be addressed below. Recorded Effect of the Lead-Rubber Bearings In order to examine the effect of the LRB on the earthquake response of the superstructure, one can compare the measurements in the pier and at the top of the LRB on the west side (Figure 3). A fixed-base superstructure is excited by the pier motion, as represented by the pier records. On the other hand, a superstructure supported by the LRB is excited by the motion at the top of the LRB, represented by the recordings there. For these comparisons, it is illustrative to use response spectra based on the recorded accelerograms from these locations. Figure 11 shows the results for the 17 June earthquake and Figure 12, the 21 June earthquake. Results for both the longitudinal and the transverse components are shown. For the longitudinal direction, it can be seen from these figures that there is a significant difference between the response spectra in the pier and at top of LRB for the short natural periods. This is valid for both the 17 June and 21 June earthquakes. Furthermore, for the 21 June earthquake, there is an amplification for natural periods above 0.8 s. Taking in to consideration that the fundamental natural period of the superstructure is close to 0.8 s, it can be stated the LRB are dissipating energy and reducing earthquake action on the superstructure for this direction.

15 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 323 Figure 11. Recorded response spectra in pier at top of LRB on the western pier during the 17 June earthquake. For the transverse direction, it can be seen that the response spectra in the pier and at top of the LRB are quite similar in both the earthquakes, indicating that there was little or no effect of the LRB for the transverse direction. The relative transverse displacements between the pier and the top of the LRB are small in both earthquakes. In the 17 June earthquake, it is 4 mm, and in the 21 June earthquake, it is 16 mm (see Tables 6 and 7). As the lead core first starts yielding at around 5 mm (Figure 4), one could not expect great effect from the LRB in the 17 June earthquake. On the other hand, in the 21 June earthquake, the relative displacement was well above the yield limits (16 mm), and therefore, it would have seemed logical to obtain some effect from the LRB. This may be explained, perhaps, by the fact that the first fixed-base vibrational mode of the superstructure is a transverse mode with longer or similar natural period to the first natural period of the base-isolated superstructure, which also is a transverse mode. Figure 12. Recorded response spectra in pier at top of LRB on the western pier during the 21 June earthquake.

16 324 B. BESSASON AND E. HAFLIDASON Table 6. Recorded and computed displacements of the superstructure relative to the piers, and recorded and computed absolute accelerations at the same points for the 17 June earthquake Earthquake excitation Response Direction Recorded Computed WESTERN PIER RECORDS EASTERN PIER RECORDS Displacement above LR-bearing Longitudinal 16 mm 20 mm Transverse 4 mm 5.4 mm Displacement at middle of bridge Transverse 22 mm 23 mm Acceleration above LR-bearing Longitudinal 0.30 g 0.32 g Transverse 0.41 g 0.40 g Acceleration at middle of bridge Transverse 0.26 g 0.33 g Displacement above LR-bearing Longitudinal 16 mm 9.9 mm Transverse 4 mm 2.4 mm Displacement at middle of bridge Transverse 22 mm 16 mm Acceleration above LR-bearing Longitudinal 0.30 g 0.20 g Transverse 0.41 g 0.16 g Acceleration at middle of bridge Transverse 0.26 g 0.13 g Table 7. Recorded and computed displacements of the superstructure relative to the piers, and recorded and computed absolute accelerations at the same points for the 21 June earthquake Earthquake excitation Response Direction Recorded Computed WESTERN PIER RECORDS EASTERN PIER RECORDS Displacement above LR-bearing Longitudinal 65 mm 59 mm Transverse 17 mm 13 mm Displacement at middle of bridge Transverse 61 mm 66 mm Acceleration above LR-bearing Longitudinal 0.66 g (0.9 g) 1 Transverse 0.73 g 0.72 g Acceleration at middle of bridge Transverse 0.44 g 0.44 g Displacement above LR-bearing Longitudinal 65 mm 44 mm Transverse 17 mm 8 mm Displacement at middle of bridge Transverse 61 mm 47 mm Acceleration above LR-bearing Longitudinal 0.66 g (0.8 g) 1 Transverse 0.73 g 0.55 g Acceleration at middle of bridge Transverse 0.44 g 0.37 g 1 These acceleration values were put in parenthesis as the response analysis showed very high longitudinal acceleration response values with very high frequencies (spikes) that were undoubtedly due to numerical problems rather than being based on physical responses.

17 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 325 Analytical Models A nonlinear, SDOF model was first used to simulate the earthquake response of the bridge. The main conclusion was that, with this simple model, the measured and computed response data was not consistent enough. Therefore, it was considered necessary to use a more sophisticated model. A three-dimensional, member-by-member, FE model was created for a nonlinear time history analysis of the bridge, using the computer program RUAUMOKO (Carr 2001). The FE model covers the superstructure, i.e., the steel-arch truss and the concrete deck, as well as the supporting LRB and the tension bars (see figures 1, 3, and 4). The two approaching spans, the piers, and the abutments are not included in the model. The model consists of 179 nodal points and has 1050 DOF. The superstructure is modeled by 504 linear elastic beam elements with 33 different section-properties. The LRB-bearings are modeled with four identical bilinear hysteretic spring elements, where the model parameters are determined by equations 1, 2, and 3 and the dimensions and material properties given in Table 2. Finally, the tension bars are modeled by four nonlinear elastic spring elements defined by F k o (4) where k 0 is a stiffness parameter, is relative displacement, and is a power-factor (Carr 2001). By adjusting the stiffness parameter and the power-factor, it was possible to fit the nonlinear force-displacement curve shown in Figure 4 fairly well (k GN/m, 2.99 [2 pieces]). In addition to the dissipation of energy through the hysteretic effect of the LRB, a viscous damping mechanism was also assumed in the superstructure. The viscous damping was based on the Caughey damping model (see, for instance Chopra 1995, p. 421), using a constant damping ratio for all modes. It was considered valuable with respect to other analysis and other structures to use only a simple damping model and examine how accurate fits to recorded data were possible to obtain by using a simple model. A lumped mass model was used. The total mass of the superstructure was adjusted to obtain a total mass of kg, i.e., the self-weight of the bridge. Finally, a time step of T 0.01 s was shown to be sufficient for the step-by-step integration. Modal Analysis Prior to the nonlinear time-history analysis of the bridge, a modal analysis was carried out. Natural modes were computed using the initial stiffness of the nonlinear elements. The first mode had a natural period of T s, and the second and third mode had T s and T s, respectively. If the bearings and tension bars are removed in the FE model and the superstructure is assumed to be fixed on both piers (only translation degrees are fixed), then the FE model gives T s. On the other hand, if the superstructure is fixed on the western pier but on rollers (in the longitudinal direction) on the eastern pier, the FE model gives T s. In both cases, T 2 and T 3 remain unchanged. These values compare well to the field measurements from 1989 (see Table 3) before the upgrading process took place. It is interesting to notice in this context that the initial stiffness of the LRB is too high to change the first mode of the system. As in the

18 326 B. BESSASON AND E. HAFLIDASON fixed-base case, as one could expect, the first mode is a transverse mode where the superstructure is deforming instead of behaving like rigid body on the isolators. Comparison of Recorded and Computed Response Only one sensor measures the response for the longitudinal direction of the superstructure. This is the sensor at the top of one of the LRB at the western pier (Figure 3). The sensor measures the absolute acceleration directly. By integrating the measured acceleration time history twice, it is possible to obtain the absolute displacement time history. Subtracting the displacement time history at the western pier from the displacement time history at the top of the LRB gives the recorded displacement time history for the superstructure relative to the pier in the longitudinal direction. Two sensors measure the response for the transverse direction of the superstructure. One is located at the top of the LRB at the western pier, and the other sensor is located in the steel grid at the middle of the bridge at the same level as the concrete deck (Figure 3). By using the same processing method as for the longitudinal direction, it is possible to obtain the recorded displacement time history for the superstructure at the end and in the middle relative to the west pier (or east pier) in the transverse direction. The earthquake excitation during the two South Iceland earthquakes of June 2000 can be classified as intense. The field measurements of the bridge from 1991 indicated low damping ratios for the first modes, i.e., less than 1% (Bessason 1992). These ratios were based on low-strain vibrations of the bridge. It is natural to expect considerably higher damping for intense (high-strain) vibrations. Therefore, a constant damping ratio of 5% (commonly used value) was assumed for all modes in the Caughey damping model. The recorded acceleration time histories from the western pier and the eastern pier in the 17 June and 21 June 2000 earthquakes were used as input motions. The analysis assumes full correlation of the earthquake motion on both the supporting piers. As this is not the actual case (see figures 5 and 6, or Bessason and Kaynia 2002), the analysis was run for two load cases for each earthquake. That is, one case where the input motion was defined by records from the western pier, and other case where the excitation was defined by records from the eastern pier. In both cases, the longitudinal, transverse, and vertical acceleration time histories act simultaneously. Based on the above assumptions, the response was computed by the FE model for the three response sensor degrees in the bridge, i.e., longitudinal and transverse responses above the other LRB on the western pier and the transverse response at the middle of the bridge. For the 17 June earthquake, the results of the response analysis are shown in Table 6 and in Figure 13. From Table 6, it can be seen that there is a better agreement between the recorded and the computed response when the excitation is based on the western pier records rather than the eastern pier records. The same trend can be observed from the plotted time histories in Figure 13, where figures a, b, and c show better correlation than figures d, e, and f. For the longitudinal response at top of the LRB, the recorded and computed time histories in Figure 13a are quite comparable. The main characteristic of the response can also be seen in Figure 13d. On the other hand, observing the transverse response at the top of the LRB, one can see considerable difference between the re-

19 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 327 Figure 13. Comparison of recorded (dashed line) and computed (solid line) relative displacement response in the superstructure just above lead-rubber bearing in longitudinal and transverse direction and at middle of bridge in transverse direction. The earthquake excitation is based on recorded acceleration time histories from the western pier (left-side graphs: a, b, and c) and from the eastern pier (right-side graphs: d, e, and f) in the 17 June earthquake. corded and computed time histories in figures 13b and 13e. Lots of short period response can be seen in Figure 13b, which are not seen in Figure 13e. This may be explained by the measured response spectra in Figure 9, which show great difference between the western pier spectrum and the eastern pier spectrum for the short natural periods, meaning that, in reality, little high-frequency energy comes from the eastern pier. Finally, the correlation between recorded and computed transverse response at the middle of the bridge is quite good (see figures 13c and 13f). Here the response is dominated by the first mode, T s, and as seen from Figure 9 at natural periods above 0.8 s, the spectral ordinates are quite similar for both piers. For the 21 June earthquake, the results of the response analysis are shown in Table 7 and in Figure 14. One can observe more or less the same results here as for the 17 June earthquake. That is, the recorded and the computed longitudinal responses above the bearings, as well as the transverse response at the middle of the bridge, are quite similar when the excitation is based on the western pier records. The transverse response above the bearing in the western end does not show the same degree of agreement. It should also be emphasized that the FE model does not model the pounding, i.e., when the superstructure hits the approaching spans. This indicates that the pounding must have been light and have had little effect on the response. Here, it should be kept in mind that although the pounding starts at a displacement of approximately 50 mm, the total clear-

20 328 B. BESSASON AND E. HAFLIDASON Figure 14. Comparison of recorded (dashed line) and computed (solid line) relative displacement response in the superstructure just above lead-rubber bearing in longitudinal and transverse direction and at middle of bridge in transverse direction. The earthquake excitation is based on recorded acceleration time histories from the western pier (left-side graphs: a, b, and c) and from the eastern pier (right-side graphs: d, e, and f) in the 21 June earthquake. ance is 100 mm in each direction. This is because, in addition to the 50-mm expansion joints at the ends of the superstructure, there are other 50-mm expansion joints at the ends of the approaching spans (Figure 3). The main conclusion here is that, by using the western pier excitation (more intense motion) and a constant damping ratio (5%) for all modes in the Caughey damping model, one can predict the response of the superstructure fairly well, except for the transverse motion at the top of LRB the western end. Comparison of Fixed-Base Versus Base-Isolated Response Having a reliable FE model of the bridge, it is informative to compare the earthquake response of the bridge with its original bearings to the response of the bridge after the installation of the LRB. For the comparison, only the excitation of the western pier from the 21 June earthquake is used (Figure 6), i.e., the most intense excitation from the South Iceland earthquakes of June 2000 with PGA 0.84 g. Computed base shear, normalized with the weight of the structure, is shown in Table 8 for fixed-base and base-isolated superstructures. Furthermore, computed normal stresses are shown for the most critical elements found in the earthquake response analy-

21 RECORDED AND NUMERICAL STRONG MOTION RESPONSE OF A BASE-ISOLATED BRIDGE 329 Table 8. Comparison of computed base shear, and element normal stresses of fixed-base and base-isolated superstructure of the Thjorsa River Bridge during the 21 June earthquake Fixed-Base superstructure Base-Isolated superstructure BASE SHEAR Force/Weight Force/Weight Longitudinal Transverse ELEMENT STRESSES (MPa) (MPa) Foot-elements at western end Foot-elements at middle sis from Bessason and Sigbjörnsson in Those are the foot-elements in the truss supporting the bridge deck, i.e., the bottom elements in Figure 3. It should be emphasized that, in original bearing system, longitudinal earthquake action was only carried out by the two fixed bearings at the western pier. On the eastern pier, the bridge was supported by roller bearings. Therefore, the total base shear of 0.93W, where W is the weight of the superstructure, would only be resisted by the two bearings. This force is far above what they could have withstood. Therefore, the conclusion is that, in its original form, the 21 June earthquake would have broken the original bearing system, and the superstructure would have received unpredictable damage and even collapse. For the upgraded system, the reaction forces are divided among four bearings, two on the western pier and two on the eastern pier. Furthermore, in the baseisolated case, the tension bars take a large part of the reaction forces for large relative displacements and reduce the forces on the LRB. The situation is, therefore, far more satisfactory in the upgraded system. In the fixed-base case, the stresses in the foot-elements at the western ends are higher than the characteristic stress value for the steel used in the bridge ( y 240 MPa). It should be noted here that stresses due to bending of members and due to gravity are excluded. The gravity loads contributes little to the stresses in the foot-elements at the ends but around 90 MPa to the stresses in the foot-elements at the middle. Based on these high normal stresses and some contribution from bending as well as gravity, it is most likely that plastic deformations, as well as local buckling, would have occurred in the steel truss in the fixed-base case. This is given that the original bearing system would have been strong enough to transmit the earthquake action to the superstructure. In the base-isolated case, the stresses are far lower and less critical. CONCLUSIONS AND FINAL REMARKS In June 2000, two major earthquakes of magnitude 6.5 and 6.6 struck the South Iceland Seismic Zone. Both the earthquakes had epicenters close (5 km and 16 km) to the base-isolated Thjorsa River Bridge. The closest point to the surface fault rupture was 13 km and 2.5 km for the 17 June and 21 June earthquakes, respectively. The bridge is instrumented with nine accelerometers, monitoring both ground motion at the site and

22 330 B. BESSASON AND E. HAFLIDASON structural response. Valuable high-resolution strong-motion data (time histories) were recorded in both the earthquakes. In the first earthquake, a PGA of 0.53 g was recorded, and in the second one, a PGA of 0.84 g was recorded. The bridge was exposed to severe earthquake loads but survived both the earthquakes without any serious damage and was open for traffic immediately after each earthquake. The recorded data shows the earthquake excitation on each side of the river is significantly different for the short natural periods. For the long periods, which are most important for the response of the base-isolated bridge, the difference is less. The geology at the site and geotechnical borings show that the soil profiles are quite different on each side, and this explains the observed difference. Earthquake analysis before upgrading of the bridge showed that the original bearings were the bridge s weakest link. The recorded earthquake action, along with numerical analysis, show considerably higher reaction force than the bearings were expected to be able to resist before upgrading. The loads were also higher than the superstructure was expected to be able to resist. This means that the bridge would probably have been severely damaged in the South Iceland earthquakes in June 2000 if it had not been base isolated. For the 17 June earthquake, the recorded data and the numerical response analysis indicate that the LRB were effective in the longitudinal direction and dissipated energy. In the transverse direction, the measurements and the response analysis indicate that the LRB did not isolate the superstructure, because they did not deform plastically due to small displacements. For the 21 June earthquake, recorded data and the numerical analysis show that the isolating bearings sustained major displacements in the longitudinal direction and were quite effective in this direction. For the transverse direction, the recordings and the numerical analysis are more inconsistent. The recorded displacements indicate that the LRB should have dissipated energy. However, evaluated response spectra based on the recorded time histories in the western pier and at top of LRB indicate little effect of the bearings for the transverse direction. This may be explained, partly, by the fact that the first fixed-base vibrational mode of the superstructure is a transverse mode with longer or similar natural period to the first natural period of the base-isolated superstructure, which also is a transverse mode. By using three-dimensional member-by-member nonlinear FE model and by applying the western pier excitation (the more intense excitation), one can predict the response of the superstructure fairly well. In all cases, close agreement was obtained for maximum response values. There were some problems in simulating the earthquakeresponse time histories at top of the LRB in the transverse direction though. To improve the results, a next step might be to use multiple input ground-displacement histories to account for the uncorrelated ground motion on each pier. Finally, the recorded data from the Thjorsa River Bridge has been important in three senses; first, in showing and quantifying the soil amplification in the lava cap on the west