تأثير اندركنش خاك و سازه در تقاضا و سطوح قابليت اطمينان احتمالاتي ساختمان هاي فولادي نامنظم هندسي در ارتفاع

Soil-structure interaction effects on demand and probabilistic confidence level of geometric vertically irregular steel buildings

در اين مقاله، تاثير اندركنش خاك و سازه در تقاضاي تغيير مكان هاي جانبي نسبي و سطوح قابليت اطمينان احتمالاتي ساختمان هاي فولادي داراي نامنظمي هندسي در ارتفاع مورد بررسي قرار مي گيرد. براي اين منظور، دسته اي از سازه هاي نامنظم هندسي در ارتفاع با درجات گوناگون پس رفتگي در تراز طبقات گوناگون، تحت تاثير مولفه هاي افقي زلزله آناليز مي شوند. اثرات انعطاف پذيري پي سازه با استفاده از روش تير غيرخطي وينكلر مدل مي شود كه در آن، با استفاده از مجموعه اي از فنرهاي غيرخطي، رفتار خاك تحت بارهاي ديناميكي مدل سازي مي شود. تحليل ها با استفاده از روش تحليل ديناميكي افزايشي صورت مي گيرد كه به موجب آن، سطوح قابليت اطمينان سازه از مرحله رفتار خطي تا حد فروريزش سازه مورد بررسي قرار مي گيرد. پس از برآورد سطوح عملكردي گوناگون سازه، درصد قابليت اطمينان سازه براي دامنه شدت ارتعاشات و سطح تقاضاي متناظر با آن بدست مي آيد. نتايج حاصله حاكي از آن است كه ماكزيمم تقاضاي تغييرمكان جانبي نسبي بين طبقات سازه ها، بخاطر اثرات انعطاف پذيري پي افزايش مي يابد. اين نرخ افزايش بسته به موقعيت و ميزان پس رفتگي در تراز طبقات سازه تغيير مي يابد. همچنين، سطح اطمينان قابل قبول براي تامين قابليت بهره برداري بي-وقفه در سازه هاي با پاي گيردار و انعطاف پذير تحت زلزله هاي با سطح خطر كم تا متوسط حاصل مي آيد. با اين حال، در سطوح خطر لرزه اي بالاتر، انعطاف پذيري پي باعث كاهش سطح اطمينان عملكردي سازه مي شود كه به موجب آن، بسته به محل و درصد پس رفتگي در سازه، متوسط كاهش 40 تا 60 درصدي در سطوح قابليت اطمينان سازه مشاهده مي گردد.

In this paper, the effect of soil-structure interaction is investigated on the drift demand and probabilistic seismic confidence level of geometric vertically irregular steel buildings. A series of vertically irregular steel buildings (known as setback buildings) with different setback ratios were designed based on the regulations in the current edition of Iranian seismic design code (Standard 2800). Foundation design of the structures was accomplished with the assumption of sandy soil with shear wave velocity of 200m/s under the footings. The three dimensional model of nonlinear soil-structure system was built in Open System for Earthquake Engineering Simulation (OpenSees). Concentrated plastic hinges at the end of frame elements were used to model the nonlinear behavior of these elements. Soil-foundation system of the structures was modeled with the Beam on Nonlinear Winkler Foundation (BNWF) approach. In this approach, a series of nonlinear springs are used to model the soil behavior under the dynamic excitation. Simplicity and efficiency of this modeling approach make it popular in soil structure interaction problems. The seismic analysis of the structures was performed under the simultaneous action of orthogonal components of real ground motions. Twenty earthquake records were used for this porpose. The selection of earthquake ground motions was accomplished based on appropriate specifications of earthquake components like magnitude, shear wave velocity, the distance from faults, and etc. Incremental dynamic analysis (IDA) was accomplished to estimate the structural performance of the regular and vertically irregular setback buildings from the linear phase of behavior to the nonlinear phase and up to the global instability of the structures. Based on the results, the median IDA curve was evaluated. This curve was used to estimate the structural performance objectives. Four common performance objectives namely Immediate Occupancy (IO), Life Safety (LS), Collapse prevention (CP) and Global Instability (GI) were specified on the median IDA curve of each structure. Following the performance-based earthquake engineering framework, the confidence level of meeting a specific performance level was evaluated at each limit state. Based on the results, curves were generated to specify the confidence level of meeting a specific performance level for the range of earthquake intensities and corresponding maximum inter story drift ratio. The performance based confidence level of flexible base setback buildings was compared to that of the fixed base structures at five seismic hazard levels. The selected hazard levels have the return periods from 25 to 4975 years. It is observed that all the fixed and flexible base buildings have the ability to continue their immediate occupancy with the confidence level of 100% under the earthquakes with low to medium hazard levels (i.e. with the return period of 43 years). However, as the level of seismic hazard increases the difference between the confidence level of flexible base structure and the fixed base ones increases. Depend on the position and ratio of the setback, 40 to 60% reduction is observed in the performance based confidence level of flexible base structures. Meanwhile, soil-structure interaction increases the maximum drift demand in structures. Based on the given results, it is observed that up to 35% increase of maximum drift happens in structures with flexible foundation.