بررسي آثار مفيد و مضر اندركنش خاك و سازه با استناد به طيف خرابي

Beneficial and detrimental effects of soilstructure interaction according to damage spectrum

پاسخ سيستم سازه با وجود اندركنش خاك و سازه تابعي از عوامل مانند ابعاد سازه، خصوصيات ديناميكي سازه، پروفيل خاك و خصوصيات زلزله اعمالي است. پاسخ سيستم‌هاي اندركنشي تحت بارگذاري لرزه‌اي عموما به صورت غيرخطي مي‌باشد. اگر چه رفتار غيرخطي خاك و وجود بركنش، انرژي زيادي را در سطح مشترك بين خاك و سازه مستهلك مي‌سازد و اثري مفيد براي در نظر گرفتن اندركنش خاك و سازه به شمار مي‌آيد، اما از طرفي ممكن است باعث تغيير شكل‌هاي ماندگار بيش از اندازه در سازه گردد و لذا اين اثر مخرب نيز بايد به صورت همزمان مدنظر قرار داده شود. در اين مقاله به منظور بررسي اثرات مفيد و مخرب اندركنش خاك و سازه در حوزه رفتار غيرخطي خاك و سازه، مقايسه طيف خرابي سيستم با پاي انعطاف پذير و سيستم با پاي ثابت بر اساس شاخص خرابي پارك و انگ براي سيستم‌هاي يك درجه آزادي ارائه مي‌گردد. اين سيستم‌هاي يك درجه آزادي داراي رفتار غيرخطي هستند كه بر تير با بستر غير ارتجاعي وينكلر قرار دارند. طيف‌هاي خرابي براي چنين سيستم‌هايي با در نظر گرفتن سه زلزله ثبت شده بر خاك نرم و براي نسبت‌هاي مختلف لاغري در سازه، فركانس بي‌بعد و شكل‌پذيري بدست مي‌آيند. نتايج اين مطالعه نشان مي دهد كه اگرچه در بسياري از سازه‌ها اندركنش خاك و سازه اثري مفيد در كاهش نياز لرزه‌اي اعمالي به سازه دارد، اما در برخي از حالات نسبت سختي سازه به خاك، اندركنش خاك و سازه خسارت لرزه‌اي اعمالي را در سازه‌هاي با پريود كم و روي خاك نرم افزايش مي‌دهد.

In spite of extensive studies since 1970 on Soil Structure Interaction (SSI), there is still controversy regarding the seismic performance of the structures rested on soft soil. SSI is known as a phenomenon influencing beneficial and sometimes detrimental effects on the response of structures. The response of soil–structure system depends basically on the size of the structure, its dynamic properties, and the soil profile as well as the applied excitation. Under strong shaking the response of soil–structure system is nonlinear and a combination of different types of nonlinearity as well as sliding, uplift and soil yielding will occur. Although foundation uplift in conjunction with soil yielding may dissipate energy during earthquakes, they may also lead to excessive permanent deformations in the structures. In such case, soil structure interaction (SSI) is not only beneficial, but also detrimental to the seismic response of the structures. In order to investigate the beneficial and detrimental effects of SSI on the nonlinear response of building, damage spectra on the basis of Park and Ang damage Index for the SDOF models are provided by considering and neglecting the SSI effects. The bilinear SDOF models are supported by Beam on Nonlinear Winkler Foundation. Two nondimensional parameters are used to control the modeling. (I) nondimensional frequency a0 which is a statement of the structuretosoil stiffness ratio (II) the aspect ratio of the structure h/r. The systems are subjected to three earthquake ground motions recorded on soft soil. For each period, first the yield strength demand of the structure is calculated by iteration in order to reach the specified target ductility in the fixedbase model within 1% of accuracy when subjected to the ground motions. The dissipated hysteretic energy in the structure is also calculated accordingly. Then, the ductility and the hysteretic energy demands are calculated for the soil–structure systems under different values of a0 and h/r subjected to the same ground motion providing the same yield strength for the structure. Consequently, the damage index is calculated for the structure in the fixedbase state as well as for the structure when located on soil. The result showed that for most of structures with long period SSI decrease the damage index. How ever It is observed that in some cases of structure to soil stiffness, SSI increases the damage index before a threshold period which is closely related to the predominant period of the ground motion. It means that the conventional fixedbase model underestimates the damages sustained by buildings having periods less than this threshold period. In particular, the SSI substantially increases the damage index of shortperiod buildings located on soft soils. It is also observed that increasing the aspect ratio of the structure increases this effect. However, the trend is reversed after the threshold period. it is observed that the increase in the slenderness ratio and ductility ratiob of structures leads to increase in the maximum damage.