AN EXPLORATORY EXPERIMENTAL STUDY OF NEAR-FAULT GROUND MOTION EFFECTS ON REINFORCED CONCRETE BRIDGE COLUMNS ABSTRACT

Transcription

1 AN EXPLORATORY EXPERIMENTAL STUDY OF NEAR-FAULT GROUND MOTION EFFECTS ON REINFORCED CONCRETE BRIDGE COLUMNS V. Phan 1, M. Saiidi 2, J. Anderson 3, and H. Ghasemi 4 ABSTRACT Strong earthquakes of the past decade have revealed particularly destructive effects of motions in the vicinity of faults. Seismologist have identified the forward directivity and fling effects as the primary characteristics of near-fault ground motions. These characteristics make near-fault earthquakes unique compared to far field ground motions, which nearly all seismic design criteria are based on. The effect of near-fault motions on two, one-third scale bridge columns was studied through shake table tests and analytical studies at the University of Nevada, Reno. One column was designed based on the provisions of the California Department of Transportation and the other based on the Association of State Highway and Transportation Officials code. The Rinaldi record obtained during the 1994 Northridge earthquake was simulated in shake table tests. The measured results revealed the important role of ground motion characteristics on the column hysteretic behavior, ductility, and energy dissipation. The most unique aspect of the measured response was the high residual displacements even under moderate earthquakes. Other response parameters such as the strain rate and the plastic hinge length were found to be comparable to those for far-field motions. Introduction The unique high amplitude and short duration pulse that is a common characteristic of near fault ground motions has garnered attention due to the severity of damage these motions have caused in densely populated urban environments. Because a number of near fault ground motions have been recorded only in recent years, the effects of these motions on structures are not yet understood. Past research has dealt with impulsive loading on reinforced concrete bridge columns, but quasi-static or pseudo-dynamic loading protocols are typically used on the test specimens. Since the nature of an impulsive load is heavily dependent on time and velocity, a more accurate simulation of a near fault ground motion is warranted. This paper focuses on the 1 Graduate Student, Dept. of Civil & Environmental Engineering, Univ. of Nevada, Reno, Reno, NV, Professor, Dept. of Civil & Environmental Engineering, Univ. of Nevada, Reno, Reno, NV, Professor, Dept. of Geological Sciences and Engineering, Univ. of Nevada, Reno, Reno, NV, Federal Highway Administration, Office of Infrastructure R&D, McLean, VA 22101

2 dynamic shake table testing that was performed on two reinforced concrete specimens using an actual recorded near fault ground motion. The unique responses measured from the test specimens are also analyzed. In addition to shake table testing, a dynamic analysis of the columns was performed and a framework for the design of reinforced concrete bridge columns that incorporate critical near fault issues was introduced. These aspects of the study can be found in a report by Phan et al. (Phan et al. 2005). Forward Directivity Effect When an earthquake occurs, the velocity at which the fault ruptures is approximately the same as the velocity at which shear waves emulate (Somerville 2002). The accumulated energy is concentrated into the form of a short duration, high amplitude pulse perpendicular to the fault because shear waves are of transverse wave type. The same effect is not observed in the rear of fault rupture direction. As a result the energy is spread over a long duration, and the earthquake record is similar to those of far field earthquakes. The fault normal component of the Rinaldi ground motion velocity history is shown in Fig. 1(a). Recorded during the 1994 Northridge earthquake, the Rinaldi station was approximately 7.1 km from the fault. As can be seen in the figure, the Rinaldi ground motion showed a clear and distinguished forward directivity pulse. Figure 1(b) presents the velocity history of the El Centro ground motion recorded from the 1940 Imperial Valley earthquake. The El Centro station was approximately 8.3 km away from the fault but the site was located in the rear of fault rupture direction. A high-amplitude, short duration pulse was not seen in the fault normal component and the ground motion resembles a typical far field ground motion. Velocity (cm/s) Time (sec) (a) Velocity (cm/s) Time (sec) (b) Figure 1. Velocity histories of (a) Rinaldi ground motion and (b) El Centro ground motion Test Specimens To test and analyze the performance of reinforced concrete bridge columns under a near fault ground motion, two one-third scale columns underwent shake table tests at the University of Nevada, Reno, Large Scale Structures Laboratory. The two specimens were designated as NF-1 and NF-2, where NF stands for near fault. Both columns were designed to behave as cantilever members and are representative of typical single column bridge piers. The design of NF-1 was based on the 2004 Caltrans Seismic Design Criteria (SDC) version 1.3 (Caltrans

3 2004), but did not incorporate any of the current near fault guidelines Caltrans provides. This made NF-1 nearly identical to a column, labeled 9F1, tested in the University of Nevada, Reno Large Scale Structures Laboratory during a previous research project (Laplace et al. 2004). Although the design of 9F1 was based on an earlier 1992 version of the Caltrans code, column dimensions and reinforcement were the same in both NF-1 and 9F1. The difference between the two specimens is that NF-1 was subjected to a representative impulsive near fault ground motion and 9F1 was subjected to backwards directivity ground motion, which resembles a typical far field ground motion. The goal was to compare the effects of the two ground motions on similar columns. The design of NF-2 was based on AASHTO s 2002 Standard Specifications for Highway Bridges (AASHTO 2002). Although the Caltrans SDC and AASHTO specifications differ in design philosophy, efforts were made to ensure that NF-1 and NF-2 were both designed to the same criteria. Soil type, seismicity of the site, and dimensions used in the design of NF-1 were also utilized in the design of NF-2. The purpose of testing specimen NF-2 was to further enhance the understanding of near fault ground motion effects on reinforced concrete columns and to analyze how a typical column designed to AASHTO standards stands up to a near fault ground motion. Table 1 presents information for the three columns examined in this study (including 9F1 tested in a prior study). Table 1. Test Model Details Specimen NF-1 NF-2 9F1 Length (mm) Diameter (mm) Longitudina l Steel Ratio (%) Transverse Steel Ratio (%) Ground Motion Rinaldi Rinaldi El Centro Design Criteria Caltrans SDC 2004 AASHTO 2002 Caltrans Code 1992 Experimental Studies NF-1 and NF-2 were scaled down to a size that is compatible with the shake table. A scale factor of was used for both specimens. Design of the full scale prototype was completed first, followed by scaling and design checks of the scaled model. The footing of each specimen was securely attached to the shake table, preventing overturning moments. A mass rig system was connected to the head of the specimen via one rigid link. Threaded rods going through the four PVC pipes in the head held the link and the specimen together. To provide the specimen with the axial load it was designed for, a steel spreader beam was bolted to the top of the column head. In addition, two hydraulic jacks connected to an accumulator were placed on top of the spreader beam. An photo of the shake table test setup is shown in Fig 2. Extensive instrumentations were used to monitor the internal strains, curvatures, displacements, accelerations, and forces for each model.

4 Figure 2. Shake Table Test Setup for Specimen NF-1 Both specimens were tested on a shake table at the University of Nevada Reno Large Scale Structures Laboratory using a real near fault ground motion record. In this manner, the dynamic near fault directivity pulse can be accurately simulated. The Rinaldi ground motion from the 1994 Northridge earthquake was selected for both shake table tests. This ground motion was chosen because the fault normal component displays a clear and definite pulse in the velocity history. In addition, the Rinaldi ground motion has one of the highest peak ground velocity ever recorded. The fault normal component has a peak ground acceleration of g, a peak ground velocity of 1660 mm/s, and a peak ground displacement of 289 mm. The Rinaldi station was located 7.1 km away from the fault. The S228 component of the Rinaldi motion was used and represents the fault normal component of the fault in plan view. The Rinaldi ground motion serves as a satisfactory contrast to the El Centro motion used for the 9F1 specimen test. The time scale of the Rinaldi acceleration history was compressed by a factor of to take into account scaling and the differences in axial and inertial loading. Each specimen was subjected to a series of Rinaldi ground motions in which the acceleration amplitude was scaled by an increasing factor in subsequent runs. The series started at a low amplitude motion, the amplitude of which then progressively increased after each run until failure occurred. Failure is defined as rupture in the longitudinal bars or the point when the shake table parameter limits are met and a higher amplitude motion cannot be produced. Between each run, cracks that were visible on the column were marked and damage was assessed. The reason for subjecting each column to a series of runs was for the purpose of evaluating the column performance at different damage states: from pre-yielding, to yielding, to post-yielding, and then to failure. Both columns were loaded in the north-south (Rinaldi fault normal) direction only. The Rinaldi fault normal component is deemed to be the most critical component. Measured Response Comparisons A total of 11 runs were performed for specimens NF-1 and NF-2 each. The longitudinal bars ruptured at 135% of the original Rinaldi magnitude in both specimens (Fig. 3). Virtually

5 no damage was seen on the upper two-thirds of the column during the entire test sequence. As expected for a cantilever member, extensive damage was localized to the plastic hinge region. The lower south side of the column suffered damage primarily from compression, which was seen through the spalling of concrete and buckling of longitudinal bars. The lower north side of the column experienced damage primarily from tension, which was seen through the extensive amount of flexural cracking that grew wider with each subsequent run (Fig. 4). Figure 5 shows the specimens after the completion of testing. As can be seen, both specimens displayed a significant amount of residual displacement. (a) (b) Figure 3. Rupture of Reinforcement and Core Damage on South Side of Specimens (a) NF-1 and (b) NF-2 (a) (b) Figure 4. Major Flexural Cracking on North Side of Specimens (a) NF-1 and (b) NF-2

6 (a) (b) Figure 5. (a) NF-1 and (b) NF-2 at Completion of Shake Table Testing Force-Displacement Hysteresis Relationships The measured force displacement hysteretic curves for all the runs cumulatively are shown in Fig. 6 for both NF-1 and NF-2. The hysteretic data for both specimens show motion that is biased in one direction and becomes more prevalent after each subsequent run. This onesided bias is attributed to the pulse in the near fault Rinaldi ground motion. The high velocity pulse caused each specimen to swing in a whip-like fashion, generating high specimen displacements in one direction. When most of the earthquake energy is localized into a single, short duration pulse, as with the near fault Rinaldi record, the hysteretic response tends to be biased in one direction. This trend is particularly true when the pulse is asymmetric. The Rinaldi ground motion had a peak velocity pulse amplitude of 1660 mm/s in one direction and a peak velocity pulse amplitude of 721 mm/s in the other direction. This asymmetry in the directivity pulse generated an asymmetric response in both specimens Force (kn) Force (kn) Displacement (mm) Displacement (mm) (a) (b) Figure 6. Accumulated Force Displacement Hysteresis for Specimen (a) NF-1 and (b) NF-2

7 Displacement Ductility Capacity The envelopes of the measured hysteresis curves provided a representative forcedisplacement relationship that was then idealized by an elasto-plastic curve to determine the effective yield displacement and the ductility capacity. The measured displacement ductility capacity for NF-1, NF-2, and 9F1 were 11.1, 9.5, and 7.8, respectively. NF-2 had a slightly lower ductility capacity than NF-1 because its longitudinal reinforcement was 10% larger than that of NF-1. The spiral steel ratio in NF2 was also higher. However, the gain in confinement was not sufficient to offset the effect of the higher longitudinal steel ratio in NF-2. Both NF-1 and 9F1 were designed to have the same stiffness and the same displacement ductility capacity. In fact, the Caltrans SDC bases its design procedure for reinforced concrete bridge columns on displacement ductility. From the test results, however, ductility capacities between NF-1 and 9F1 differ by a significant amount. A biased, one-sided response implies that the column will not deteriorate as rapidly as a more symmetric response since deterioration primarily occurs only on one side of the column. Hence, 9F1 reached a lower displacement ductility capacity than NF-1 because the El Centro ground motion caused a somewhat symmetric response and similar deterioration on both sides of the column. This resulted in a higher rate of deterioration and a lower ultimate displacement in 9F1. Residual Displacements During moderate amplitude testing of NF-1 and NF-2 it was noticed that the columns exhibited relatively large residual drifts. Figure 7 shows a plot of residual drift ratio versus the target peak ground acceleration (PGA). Residual drift ratio is defined as residual displacement divided by the length of the column. The data show that residual displacements in NF-1 and NF-2 were considerably higher than those of 9F1. As peak ground acceleration increased, the residual displacements in NF-1 and NF-2 also increased in a nearly exponential manner. By comparison, the residual displacement seen in 9F1 was virtually nonexistent until a PGA of approximately 1.2 g. Residual Drift Ratio NF-1 NF-2 9F PGA (g) Figure 7. Residual Drift versus PGA

8 The higher residual displacements seen in NF-1 and NF-2 are due to the highly asymmetric near fault ground motion pulse. Again, 9F1 and NF-1 were nearly identical columns, yet NF-1 showed residual displacements up to fifty times higher than what was recorded for 9F1. The asymmetric pulse can cause large displacements in one direction. Since near fault ground motions tend to also have higher peak ground accelerations due to their proximity to the fault, the pulse can be strong enough to push a column far past the elastic range and not allow the column to return to its original position. The permanent displacement of the column after each run led to even more asymmetric response in subsequent runs, thus increasing the residual drift. This issue presents several problems for the bridge serviceability after moderate earthquakes. Currently there are no written guidelines for the design of reinforced concrete bridge columns with respect to control of residual displacement in either the AASHTO specifications or the Caltrans SDC. Although failure in the test specimens was defined as rupture in the reinforcement, in reality, a high residual displacement in a bridge column could indicate that the bridge is unsafe. In Japan, reinforced concrete bridge columns with residual drift ratios of more than 1.75% were demolished and rebuilt after the Hyogo-ken Nanbu earthquake (Kawashima and MacRae 1998). Strain Rates and Plastic Hinge Lengths The measured strain rates for NF-1, NF-2, and 9F1 are listed in Table 2. The strain rates presented are the peak instantaneous strain rate measured before yielding in tension or compression occurred. Researchers have hypothesized that columns would experience significantly higher strain rate from a near fault ground motion due to the high velocity pulse (Gibson et al. 2002). This study did not validate this hypothesis, however. Results show that the measured peak strain rate for NF-1 and NF-2 were comparable to the measured values for 9F1. The plastic hinge lengths were calculated using Paulay and Priestley s method (Paulay and Priestley 1992). It is known that the theoretical value tends to be conservative for conventionally reinforced concrete columns. For all three specimens examined for this study, the theoretical plastic hinge length value was computed to be 286 mm, which includes the bar strength increase due to strain rate. Concerns have been raised that current methods for calculating the plastic hinge length may underestimate the actual value in structures subjected to impulsive loading (Hamilton et al. 2001). Table 2 lists the measured experimental plastic hinge lengths for NF-1, NF-2, and 9F1. The results show that all three specimens had plastic hinge lengths longer than those predicted using Paulay and Priestley s equations. Therefore, based on the specimens tested for this study, no modification to the plastic hinge equation seems to be necessary to specifically account for the near fault effect. Table 2. Measured Peak Strain Rates and Plastic Hinge Lengths

9 Specimen Peak Strain Rate (microstrain/s) Plastic Hinge Tension Compression Length (mm) NF-1 30,000 22, NF-2 33,400 18, F1 32,900 19, Near fault ground motions effect is often compared to that of blast loading due to impulsive type loading similarities. The pulse of a blast load is typically comprised of an extremely short period and high amplitude. This can generate high strain rates and more localized plastic hinge lengths, resulting in a potentially brittle and early failure. In blast loading, the pulse is typically a few tenths of a second in duration. Near fault directivity pulses usually have pulses with periods measured in whole seconds, however. Although the Rinaldi ground motion shares the characteristic of being an impulsive type load, it did not cause strain rates or plastic hinge lengths that are different from the responses caused by a far field ground motion. In fact, NF-1 and NF-2 both showed predominately flexural deformations and large ductility capacities, which are responses that are not expected from a severe blast load (Munshi 2004). This implies that near fault ground motions have more in common with far field motions than with blast loading. Conclusions study: The following conclusions were made based on the experimental results obtained in this 1. An asymmetrical, high amplitude velocity pulse has the tendency of producing a biased, one-sided response in bridge columns. Asymmetrical impulsive loading causes a whiplike behavior in columns, which generates large displacements in one direction. This displacement is only partially recovered leading to a significant residual displacement. 2. Biased, one-sided column responses lead to somewhat higher ductility capacities than symmetrical responses. The rate of deterioration is slower in biased responses because the deterioration is primarily occurring only on one side of the column. For symmetrical responses, similar deterioration occurs on both sides of the column resulting in a higher rate of degradation and a lower ultimate displacement. 3. The plastic hinge length in columns subjected to near fault ground motions is comparable to those subjected to far field motions. No modifications are necessary to specifically account for the near fault effect. 4. Strain rates induced by near fault ground motions are comparable to strain rates induced by far field ground motions. Pulse durations under a near fault excitation are generally not short enough to produce significantly higher strain rates. References American Association of State Highway and Transportation Officials, LRFD Bridge Design

10 Specifications, Washington, D.C. California Department of Transportation, Seismic Design Criteria, Sacramento, CA. Gibson, N., A. Filiatrault, and S. Ashford, Impulsive Seismic Response of Bridge Column-Cap Beam Joints, American Concrete Institute Structural Journal 99 (6), Hamilton, C.H., G.C. Pardoen, R.P. Kazanjy, and Y.D. Hose, Experimental and Analytical Assessment of Simple Bridge Structures Subjected to Near-Fault Ground Motions, Proceedings, The International Conference of Engineering Mechanics and Computation, Cape Town, South Africa. Kawashima, K., G.A. MacRae, J. Hoshikuma, and K. Nagaya, Residual Displacement Response Spectrum, American Society of Civil Engineers Journal of Structural Engineering 124 (5), Laplace, P.N., D.H. Sanders, M. Saiidi, M.B. Douglas, and S. El-Azazy, Performance of Concrete Bridge Columns Under Shaketable Excitation, American Concrete Institute Structural Journal 102 (3), Munshi, J., Seismic Versus Blast Loading, American Concrete Institute Concrete International 26 (5), Paulay, T., and M.J.N. Priestley, Seismic Design of Reinforced Concrete and Masonry Buildings, John Wiley & Sons, Inc., New York. Phan V., M. Saiidi, and J. Anderson, Near Fault (Near Field). Ground Motion Effects on Reinforced Concrete Bridge Columns, Center of Civil Engineering Earthquake Research, Department of Civil Engineering, University of Nevada, Reno, Nevada, Report No. CCEER Somerville, P.G., Characterizing Near Fault Ground Motion for the Design and Evaluation of Bridges, Third National Conference and Workshop on Bridges and Highways, Portland, Oregon.