ارزيابي شاخص خرابي قاب‌هاي خمشي فولادي با در نظر گرفتن اثرات اندركنش خاك-سازه تحت زلزله‌هاي متوالي

Evaluation Damage Index of Steel Moment frames by Considering the Effects of Soil-Structure Interaction under Seismic Sequence

هدف از اين مقاله ارزيابي شاخص خرابي قاب‌هاي خمشي فولادي، با در نظر گرفتن اثرات اندركنش خاك-سازه تحت زلزله‌هاي متوالي مي‌باشد. سازه‌هاي انتخابي داراي نسبت هاي لاغري (H/B) متفاوتي مي‌باشند، تحت اثر زلزله هاي متوالي با محتواي فركانسي متفاوت و با در نظر‌گرفتن اثرات خاك‌هاي مختلف با استفاده از روش زيرسازه مورد تحليل و بررسي قرار‌گرفته‌اند. در نهايت به منظور بررسي تاثير توالي لرزه‌اي، جابه‌جايي طبقات و شاخص خرابي طبقات با حالت زلزله منفرد مقايسه شده است. نتايج نشان دهنده‌ي آن‌است كه اثر‌گذاري زلزله‌هاي متوالي با در نظرگرفتن اندركنش خاك- سازه وابسته به شرايط محتواي فركانسي زلزله‌هاست. نياز‌هاي جابه‌جايي سازه‌ها در زلزله‌هاي متوالي نسبت به زلزله تكي با توجه به محتواي فركانسي ممكن است افزايش يابد، به‌گونه اي كه اين مقادير تا %20 نسبت به زلزله تكي افزايش داشته و به طور كلي نياز‌هاي جابه‌جايي سازه با نرم‌تر شدن خاك تا حدود %10 افزايش مي‌يابد. با افزايش نياز به جابجايي در اثر نرم شدن خاك، افزايش ميزان شاخص خرابي در سطوح مختلف خرابي اتفاق افتاده است؛ در زلزله تكي شاخص تا %15و در توالي لرزه‌اي %10 تفاوت ديده مي‌شود. با افزايش سطح آسيب‌پذيري سازه، ميزان خرابي سازه از سطح خرابي محدود تا سطح خرابي كامل تا حدود 5/1 برابر افزايش مي‌يابد.

One of the main targets in designing structures is achieving adequate level of inter-story drift and ductility for controlling damage level. When a structure is subjected to sequential earthquakes, damage control becomes more critical. This seismic sequence as a result of the occurrence of strong and moderate earthquake ground motion in a short time strongly affects the seismic response of structures. Studies revealed that variation in the seismic responses such as displacement, ductility, absorbed energy etc. cause modifications in structural damage states. A damage index can numerically quantify the structural damage of buildings. Several damage indices have been developed for evaluating the seismic performance of structures. The aim of this paper is to investigate the effect of seismic sequence of earthquake ground motion on the damage indexes variation and the seismic performance of steel moment frame structures considering soil-structure interaction (SSI). For the present work, the drift damage index is selected for measuring the story damage due to the inter-story drift. This index has been calculated by performing the nonlinear time-history analysis on a set of selected group of steel frames subjected to selected seismic excitations and sequential earthquakes. 2. Methodology To this end, four steel moment frames with 8 and 16 stories are considered each of which three and six bays have. Soil-structure-interaction is considered by employing dampers and springs having specified damping and stiffness through cone method. For modeling SSI effects, three types of soil based on Standard 2800 are considered. For each soil-structure model, different responses are measured under different scenarios of sequential earthquakes. 3. Results and Discussion The results of time-history analysis show that the relative displacement of the structure and inter-story drift are increased when the structure is subjected to the seismic sequence. As a result, the drift damage index induced to the structure. The study findings demonstrate as soil becomes softer, the structural demands increases and consequently the damage index rises and SSI amplified the structural damage level. Therefore, frames built on soft soil subjected to sequential earthquakes experienced more damages. 4. Conclusions The obtained responses show that a special attention should be paid for designing of steel structures due to the preventing extensive structural damage in high seismic zone when they are subjected to seismic sequence excitation. Furthermore, soil-structure-interaction is an important parameter that should be considered for such conditions. Therefore, it is recommended that the effects of seismic sequence and soil-structure-interaction should be accounted for better estimation and evaluated accurate responses of steel shear frame.