مرکز منطقه ای اطلاع رساني علوم و فناوري - بررسي تأثير ارتفاع پايه پل ها بر دقت روش تك مودي آيين نامه آشتو در تحليل لرزه اي پل ها و ارائه پيشنهاد براي اصلاح و توسعه روش

چكيده لاتين :

Based on the AASHTO provisions, the equivalent static and dynamic method of analysis are the two methods used for the seismic analysis of bridges. These methods are selected for the bridge analysis depending on the bridge type and its seismic performance category (SPC). There are four methods of analysis in AASHTO for seismic analysis of the bridges among which the single mode method is used for the analysis of the regular bridges classified as SPC B, C or D with 2 to 6 spans. The piers height effect is not regarded as a criterion in the selection of the above methods of AASHTO. In the single mode method the earthquake load is evaluated by applying a uniform load over the total length of the bridge, and calculating the static displacements of the bridge deck. Based on these displacements, the bridge period is obtained and used in the calculation of the seismic coefficient and seismic design loads. In the present paper, the effects of the bridge piers height on their seismic performance is studied and the results reveal that the procedure mentioned above is not able to accurately estimate the internal forces developed, due to earthquake motions, in the bridges with high piers .To demonstrate this deficiency, 6 sample bridges, with 4 uniform 25m length spans, are selected. The height of piers is the same in each bridge but varies from 5m to 45m in samples. The bridges are analyzed by the single mode method, and the internal forces of the piers are compared with those obtained from the multimode analysis method by including 25 modes in the analysis. The results of the multimode analysis are used as the reference to calculate the errors in the single mode method. The axial and shear forces, as well the bending moments in the piers are compared with the reference solutions. The fundamental natural period of the bridges are also obtained by using the multimode analysis and compared with those obtained from the single mode method. These comparisons indicate that the single mode method is not able to accurately predict the natural period of the bridges, as well the internal forces of the piers. The results of the present study show that the discrepancy of the results from the reference values are mainly due to the distribution and applyingthe equivalent loads, and partially due to the equation used for the evaluation of periods. To reduce these errors, it was proposed in this paper, that the piers need to be included effectively in the analysis procedures. For this purpose, it is found that the following modifications have to be made on single mode method: 1. The uniform loading, which is the first step in the single mode method, should be extended to the piers also. 2. The piers weight need to be properly included in the calculation of period. 3. The effective weight of piers needs to be included in the calculation of equivalent seismic loads. 4. In calculating the internal forces caused by the earthquake motions, the equivalent seismic loads should be calculated associated with all individual piers and also bridge deck. The above modifications are implemented in the single mode method and the 6 selected sample bridges are analyzed again. The results of the fundamental periods and internal forces are obtained and compared with the reference solutions. The comparison of the results shows that the proposed modifications highly improve the accuracy of the single mode method in predicting the seismic behavior of bridges with high piers. The main conclusions of the present work can be summarized as follows: • The accuracy of the single mode method in calculating the fundamental period of bridges with pier-height/span ratio different to 0.5 is not good. • The single mode method is not accurate to predict the internal forces of the piers. This is more visible in the prediction of pier axial forces of the bridges with small pier-height/span ratio. This lack of accuracy is mostly due to the presence of vertical vibration modes in the seismic vibration of such bridges which is not considered in the single mode method. • The accuracy of the single mode method for the prediction of internal forces of the piers is deteriorated by differentiation of pier-height/span ratio from 0.5. Hence, this ratio can be considered as criteria such that for bridge with pier-height/span ratio differs to 0.5, the proposed method can be considered to obtain accurate estimates. • The accuracy achie^d in/the proposed method reveals the necessity of inclusion of piers weight in me analysis an^Mso reveals how the distribution of equivalent seismic load can affect the results. 4 ^