Sideways Collapse of Deteriorating Structural Systems under Seismic Excitations Phase II Shake Table Collapse Test

Transcription

1 Sideways Collapse of Deteriorating Structural Systems under Seismic Excitations Phase II Shake Table Collapse Test Melissa E. Norlund SUNY University at Buffalo REU Faculty Advisors Professor Helmut Krawinkler, Stanford University Professor Andrew Whittaker, SUNY University at Buffalo Professor Andrei Reinhorn, SUNY University at Buffalo PhD Mentor Dimitrios G. Lignos, Stanford University paper 12 1 Seattle, Washington August 8-12, 2007

2 Abstract An experiment, entitled NEES Collapse, was conducted to validate an analytical model which predicts the behavior of a four story frame under seismic excitations. The frame (scale 1:8), designed with reduced beam sections (RBS), was made of aluminum with steel plate, dog bone, connectors. The frame was connected to a mass simulator using four horizontal links. Strain gauges were placed on all dog bone steel plates, 80 clip gauges, and on the horizontal links. In total, 234 channels were used to record data for further analysis. A series of four tests were planned using shake table #1 at the Structural Engineering and Earthquake Simulation Laboratory (SEESL) at the University at Buffalo. A fifth test was conducted to fully collapse the structure since the originally planned level of intensity that would lead the structure to collapse was proved to be slightly lower than the real collapse level. The five tests preformed were: service level, design level, Maximum Considered Earthquake level, collapse level (1.9*Canoga Park record), and final collapse level (2.2*Canoga Park record). After collection and analysis of the data recorded, it was found that the analytical model was fairly accurate for the collapse prediction of the building. Introduction The first objective of this experiment was to safely collapse a four story frame. The second objective was to validate a computer analysis created to predict the behavior of the frame and the IDA algorithm to reach collapse. The experiment was designed at Stanford University, CA and conducted at the SEESL facility at the University at Buffalo, NY. The third objective was to validate the computer program Bispec. Theory The analytical work done to predict the behavior of a structure under seismic loading is based on the equation of motion (Eq.1) (Tongue, 2005). The four story frame used in the experiment can be thought of as a single degree of freedom (SDOF) system allowing for a simplification of the analysis. An example of a SDOF system is shown in figure 1. mu + cu + ku = p (t) (1) Where m-mass u-velocity c-damping k-stiffness p eff (t)-effective support excitation loading eff paper 12 2 Seattle, Washington August 8-12, 2007

3 Figure 1: SDOF Lollipop model The SDOF often known as the lollipop model shown in figure 1 assumes that the mass on the end of a massless rod will behave like a simple harmonic oscillator (SHO) with a certain damping. When a force, from a seismic record, is applied to the bottom of the massless rod the particle will feel only the components of ground motion with periods near the natural period of this SHO (USGS, 2007). A record can then be found of the particle motion. From this record the maximum displacement can be determined. Taking the second derivative of the displacement record with respect to time will produce an acceleration record. The program Bispec uses this theory and allows a user to input ground motion and will output a displacement versus time graph. Comparing the analytical model produced by Bispec to the experimental model will show the accuracy of Bispec. Ground Motion Two different frames were tested to validate the effectiveness of analytical models that model deterioration explicitly. The first frame was subjected to the ground motion from the Northridge 1994 Canoga Park record while the second frame was subjected to different ground motions, including the 1985 Chilean record, to investigate the effect of different loading histories to collapse capacity of a building. For final collapse level of Frame #2 the Canoga Park record was also used and it turned out that the collapse level of the building for the certain ground motion was very consistent. Canoga Park record is illustrated in figure 2. Due to brevity only the first test (ground motion Canoga Park) will be discussed. paper 12 3 Seattle, Washington August 8-12, 2007

4 Figure 2: Northridge 1994 Canoga Park Record, Ground Motion The Northridge 1994 Canoga Park record was chosen based on the comparison of its spectral acceleration and the spectral acceleration of the design spectrum at the range of periods close to the period of the prototype structure. As seen in figure 3, the design spectrum matches the Canoga Park acceleration spectrum at the period of interest (1.32s). Therefore, there is a direct comparison with the design specification. Northridge 1994 Canoga Park: Acceleration Spectrum, ζ=5% Canoga Park Record Design Spectrum Sa/g Period T (sec) Figure 3: Ground motion, Canoga Park paper 12 4 Seattle, Washington August 8-12, 2007

5 Experimental Setup The four story two bay frame used in NEES Collapse is a special moment resisting frame (SMRF) with fully restrained reduced beam section (RBS) moment connections designed based on FEMA-350 (Lignos, 2008) specifications. The basic dimensions of the tested structure are shown in figure 4. Figure 4: 4-story 2-bay frame The frame was connected to a mass simulator with four horizontal links using the flag pole technique, as shown in figure 5. The flag pole technique allows for the gravity associated with each floor to be considered to the scaled model since gravity is mainly triggering collapse apart from deterioration of actual components of the building. The plates of the mass simulator were constructed using hinges so each plate could move freely with the frame. Figure 5: Experimental setup paper 12 5 Seattle, Washington August 8-12, 2007

6 Both the frame and the mass simulator were braced by the bracing system shown in figure 5. The bracing systems are used in order to prevent the out of plane motion of the 2 sub structures. The bracing system is also used as a safety measure. Angles were welded onto the ends to keep the mass simulator from causing major damage during collapse. Steel plate, dog bone connectors were used at each joint on the frame. A strain gauge was placed on all 80 plates in order to record small deformations during elastic testing. The gauges on the dog bones become ineffective once plastic deformation takes place. Because data is needed after the point of plastic deformation eighty clip gauges or home made extensometers were positioned in accordance with the dog bones. The clip gauges were set in notches made in the steel plates and attached using rubber bands so as not to slip during high deformations and in particular near collapse. Strain gauges were placed on every clip gauge in order to record data after the point which the steel plates on the strain gauges had stopped accurately recording. The Krypton system was also used during NEES Collapse to record data. Forty LED s were placed strategically about various plastic hinges to record deformations, and confirm deformations recorded by strain gauges. In total, 234 channels were used to collect data. A typical plastic hinge of the model building is shown in figure 6. The steel plate dog bones, home made extensometers clip gauges, and LED s are all shown in this figure. Testing and Results Figure 6: Plastic Hinge White Noise: Before each test a series of white noise tests were preformed to determine modal characteristics of the structure and to ensure that the frame was moving freely on its hinges. The main problem encountered during the first white noise test was the friction damping from the hinges of the mass simulator. With the amount of damping paper 12 6 Seattle, Washington August 8-12, 2007

7 encountered, the frame and mass simulator were moving in harmony with the table. To correct the problem grease was applied to all hinges and other sites throughout the frame where friction could be a potential problem. Service Level (SL): (0.4*Canoga Park record) at this level it is assumed that the building was designed for a 50% chance of an earthquake in 50 years. This test is an elastic test, meaning there was no permanent deformation. Data was recorded using the strain gauges mounted on the dog bone connectors and they were well correlated with the other instruments. Design Level (DL): (1.0*Canoga Park record) at this level it is assumed that the building was designed for a 10% chance of an earthquake in 50 years. This test is an inelastic test, meaning there is a small amount of permanent deformation. Figure 7, shows the plastic hinge at the base of the structure after the completion of a design level test. Figure 7: Column 1 floor 0 after DL testing Maximum Considered Earthquake Level (MCE): (1.5*Canoga Park record) at this level it is assumed that the building was designed for a 2% chance of an earthquake in 50 years. From figure 8, it is obvious that there was permanent deformation. The steel plate in compression is shown to be buckled. paper 12 7 Seattle, Washington August 8-12, 2007

8 Proceedings of the 2007 Earthquake Engineering Symposium for Young Researchers Figure 8: Column 1 floor 0 after MCE level testing Collapse Level 1: (1.9*Canoga Park record) at this level, the frame was expected to collapse, however, the frame did not reach the state of collapse. As shown in figure 9, the plates experienced great deformation. Another test was conducted to collect data from the frame reaching collapse. Figure 9: Column 1 floor 0 after Collapse Lever (1.9*Canoga Park) Final Collapse: (2.2*Canoga Park record) at this level, the frame reached collapse almost immediately. As seen in figure 10, the plate in compression is completely buckled and the plate in tension, fractured. It was determined after the data analysis that a factor of 2.0*Canoga Park would have been a more appropriate value to achieve collapse. paper 12 8 Seattle, Washington August 8-12, 2007

9 Figure 10: Column 1 floor 0, after Final Collapse testing Bispec: The Canoga Park ground motion was used to test the validity of this program. Once the displacement v time graph, produced by the program was compared to that produced from the experimental data, the accuracy of the program could be determined. Figure 11 shows the comparison. As shown, the model produced by Bispec is fairly accurate up to about eight seconds. Displacement (in) Comparison Between SDOF Model vs Collapse Model Time (sec) Analytical Model Experiment Figure 11: Bispec v. Experiment Results: From the data collected it was found that the analytical model predicted fairly well the collapse capacity of the building for the certain ground motion. A picture of the predicted collapse mechanism alongside the actual collapse mechanism is shown in figure 12. As seen, the predicted collapse mechanism is identical with the actual collapse mechanism. paper 12 9 Seattle, Washington August 8-12, 2007

10 Figure 12: Collapse mechanism Conclusion A four story two bay moment resisting frame was safely collapsed using the testing facility at the University at Buffalo. The computer program and analytical models used to determine the behavior of a structure subjected to seismic loading were validated through the experiment. As shown, collapse indeed can happen in a reasonable level of intensity, hence the collapse level should be considered into the design provisions. Bispec was tested and proved to be valid in the initial stages of collapse. Friction is a main cause of error in the Bispec model. The simplified Bispec model uses only a SDOF system. The experiment has many connections and joints in which friction can act. Greasing the structure was able to reduce the amount of friction, however there was still friction which the Bispec model could not account for, thus the model can not be assumed accurate in later stages of collapse where friction plays a larger role. References Lignos, D. G., Krawinkler, H (2008) Sidesway Collapse of Deteriorating Structural Systems Under Seismic Excitations, PhD dissertation, Stanford University, CA 2008 (to appear) FEMA-350 (2000) Recommended seismic design criteria for new steel moment-frame buildings. Tongue, Benson H. Sheppard, S. T. Dynamics Analysis and Design of Systems in Motion, USA 2005 US Geological Survey (USGS) Definition of Spectral Acceleration (SA) last accessed 09/22/07 paper Seattle, Washington August 8-12, 2007