Influence Of PGV And PGD On Structural Nonlinear Seismic Response Of Steel Buildings

Destruction and collapse of structures and buildings under earthquake are the main cause for casualties and property damage. Therefore, it has been a research hotspot to reveal characteristics of ground motions for the purpose improving seismic design for buildings in these years. By this means, structural dynamic response is greatly decreased under the force of earthquake. However, ground motion is a complicated process and various factors influence the process. Earthquake disaster in decades indicates that amplitude, frequency spectrum and duration are considered as the representative characteristic parameters. Among these three parameters spectral characteristic are frequently applied as the engineering characteristics for ground motions. Nevertheless, once the inelastic behavior occurs to the buildings, frequency spectrum can not generally reflect the engineering characteristics of ground motions. Peak characteristics generates prodigious impacts on structural nonlinear dynamic response and the specific parameters describing the peak values of ground motions are gradually formed including peak acceleration, peak velocity and peak displacement. Studies on the influence of peak characteristics are one of the favourite topics nowadays, especially on the influence on the structural elastic-plastic response of the peak velocity and peak displacement.In this dissertation, based on a specific acceleration response spectrum,4 sets of artificial ground motions are synthesized with identical peak velocity and peak displacement. These ground motions are introduced as the input motions to the steel buildings models with the total floors of 6, 16,21 and 52. Structural inelastic seismic response analyses are carried out for the selected four buildings. Structural distortion, internal force and energy dissipation parameters are computed and compared with different set of ground motions. Comparative analysis suggests the influence laws of the peak velocity and peak displacement. The main analysis procedures and the conclusions are listed as follows,1. Characteristic analysis for the ground motionsBased on a specific acceleration response spectrum curve,4 sets of ground motion time histories are artificially synthesized with 30 samples in each set. The peak accelerations of the 1st and 2nd sets are identical with value of 1 m/s2 and their peak velocities are similarly the same with the value of 0.20 m/s. Their peak displacements differ from each other, with the values at 0.2 dm for the 1st set and 0.4 dm for the 2nd set. The peak accelerations and peak displacements are consistent with the peak velocity of 0.15 m/s for the 3rd set and 0.30 m/s for the 4th set.The ground motions are separately scaled to various levels to render certain different degree of inelastic seismic response. After seismic response analysis, the influence of peak displacement and peak velocity are easily compared and concluded. Positive enveloping and reverse enveloping are figured out and compared for 120 time histories. Enveloping curves indicate that the main durations for 4 sets basically coincide. With enveloping analysis, probability of the influence from durations is eliminated and the influence laws of peak characteristics get more reliable.2. Calibration and updating of numerical modelsFour buildings with 6 stories,16 stories,21 stories and 52 stories are selected with natural periods respectively at 0.687s,2.137s,2.405s and 5.882s. They are representative examples as lower high-rise, middle high-rise, high-rise and super high-rise structures. The periods are widely distributed and the buildings have been instrumented for strong motions. Several moderate earthquakes were recorded with high signal-noise ratio in the time histories. Based on the recordings, the natural characteristics are recognized using system identification method to calibrate the results from modal analysis of the numerical models. When the natural periods identified from actual recordings and modal analysis are approximately equal, the models are used for dynamic seismic response analysis with the input ground motions from the recordings on the basement. The accelerations and displacements between the recordings and the numerical analyses are compared and calibrated. If they match well, the models are satisfactory. If they greatly deviate, the models are required to be updated until they finally matched. Twice corrections of natural characteristics and two response parameters guarantee the models coincide with actual buildings and they are adaptable for further analytical investigation on nonlinear seismic response.3. Variable coefficient of the structural responseAfter double check of natural vibration characteristics and structural responses, four load cases with peak accelerations of 100gal (gal denotes cm/s2),200gal,400gal and 800gal are applied to four selected models and elastic-plastic seismic response analyses are carried out. Three parameters including deformation, internal force and energy dissipation are computed for its distribution and mean. Among the deformation, they are story drift ratio, inter-story drift ratio, beam plastic rotation and column plastic rotation. Column-end shear force stand for the internal force and ductility is used as the parameter for energy dissipation capacity. Elastic-plastic states have large differences under the input of four specific levels because the earthquake performances for the buildings are distinctive. As indicated, the variable coefficients are close in elastic states and they are enlarged with the increase of input motions when the responses are nonlinear, while the variable coefficients for the internal forces change less.4. Influence on structural deformation of peak characteristicsComparative analyses of deformation for the models indicate the peak chacteristics have influences on the structural deformation. The influences are various for each building. The influence degrees go up with increase of input motions. The influence extents of peak displacements on the 6-story,16-story and 21-story are minor and a bit of them are negative. The influences on the story drift ratio, inter-story drift ratio, beam and column plastic rotation reach the maximum as 45.9% for the 52-story building. However, the influences of peak velocities on the deformation go relatively larger than these of peak displacements and the maximum is 83.4%, much bigger than 45.9%.5. Influence on structural internal force of peak characteristicsStress states in beams are complicated and influenced by several factors. It is difficult to find out the influence law of peak characteristics in beams. According to the layout of columns, the axial forces remain equal and they are not changed by external load cases. Therefore, only columns are applied for internal force analysis. Column-end shear force analysis for the selected buildings indicates that peak displacements and peak velocities bring in the variation of shear force and they increase with the amplitude of the input motions. Smaller increases owing to the changes of peak displacements occur and the shear force decreases for the 21-story model. Maximum increase is 10.0% on the 52-story building. The increase ranges are relatively bigger of the column-end shear force brought about by peak displacements and the maximum increase on the 52-story model is 13.9%, larger than that of peak velocities.6. Influence on structural energy dissipation parameters of peak characteristicsBuildings absorb and dissipate earthquake energy by its ductility to cut down the destructive effect from earthquakes. Structural ductility can be considered as a valid parameter to symbolize the energy-dissipating capacity. The influence on the structural ductility of peak characteristics is corresponding to the influence on the energy-dissipating capacity. With the development of structural nonlinear response, the ductility coefficient gradually strengthens. Analyses make it clear that the impact of peak characteristics on structural ductility cannot be neglected and the larger the input strong motions are, the stronger the influences of peak characteristics are. The influence of peak velocities on the structural ductility for the selected four buildings is larger than that of peak displacements. Its maximum is 60.7% on the 52-story model and the maximum of the enlargement for peak displacements is 21.3% on the 52-story model as well.7. Influence trend analysis for peak characteristicsThe elastic-plastic seismic response analyses for the buildings selected indicate that the changes in peak velocities and peak displacements affect the deformation, the internal force and the energy dissipation parameters. Particularly, when the models develop its nonlinear state, the influence extent keeps positive correlation with the input acceleration amplitude. System identification and modal analysis jointly manifest the periods for the selected buildings and they are increased from 0.687s to 5.882s with the increase of building stories.After analyzing on the structural deformation, internal force and energy dissipation parameters, it is inferred that the influence appears a tendency to increase with the amplitude of input ground motions. Analyses also indicate the influence on the structural response of peak velocity is predominant and reach its maimum on the 52-story buildings. The influence of peak displacement is weaker and becomes evident when the structural natural period reaches 5.882s.Aforementioned analyses illustrate peak velocities have large influences on nonlinear responses of various buildings. The influence of peak displacements is relatively smaller to building with moderate natural period. The influence will become gradually remarkable when the buildings have large natural period. Therefore, when ground motions are selected for structural seismic design, it is not appropriate to merely consider spectral characteristics, ignoring the influence of peak characteristics.