بررسي رفتار لرزه اي سازه هاي بتني با آلياژ حافظه دار شكلي تحت اثر زلزله هاي حوزه نزديك

Study on the seismic behavior of SMA –RC frames under near-field earthquakes

در ساختمانهاي با سيستم سازه اي قاب خمشي، عمده شكل پذيري در نواحي انتهايي تيرها، با تشكيل مفاصل پلاستيك به هنگام تغييرشكل هاي بزرگ در حين زلزله تامين مي گردد. باتوجه به ماندگاري تغييرشكل هاي پسماند سازه پس از وقوع زلزله در سازه هاي رايج و عدم امكان بهره برداري از آنها، استفاده از رفتار هوشمند فلزها و آلياژهاي حافظه دار شكلي به جاي ميلگرد هاي طولي در تيرهاي بتني، مورد توجه محققان قرار گرفته است. از مهمترين ويژگي هاي بارز آلياژهاي حافظه دار شكلي قابليت حفظ شكل اوليه در كرنش هاي كمتر از 8%، خاصيت فوق ارتجاعي و كاهش تغييرشكل هاي پسماند مي باشد. از جمله اين مصالح هوشمند مي توان به آلياژهاي حافظه دار شكلي (SMA) اشاره نمود. در اين تحقيق، قاب هاي بتني سه، پنج، هفت و نه طبقه با استفاده از نرم افزار Seismostruct مدلسازي و تحت 6 شتابنگاشت حاصل از زلزله هاي حوزه نزديك قرار گرفته اند و پارامتر هاي تغيير مكان جانبي نسبي طبقات ، تغيير مكان جانبي نسبي پسماند طبقات، تغيير مكان جانبي پسماند بام ها و برش پايه مورد بررسي قرار گرفته است. نتايج تحقيق نشان مي دهند، استفاده از آلياژهاي حافظه دار شكلي در دوانتهاي تيرهاي قاب به جاي ميلگردهاي فولادي رايج، باعث افزايش تغييرمكان جانبي سازه هنگام زلزله مي گردد: اما تغيير مكان جانبي نسبي پسماند سازه، تغييرمكان پسماند بام و برش پايه كاهش يافته كه منجر به افزايش سرويس دهي و هزينه هاي بازسازي پس از وقوع زلزله هاي حوزه نزديك مي گردد

A significant number of concrete structures have been suffered extensive damages during past earthquakes. Since the Northridge (1994) and Kobe (1995) earthquakes, numerous analytical and experimental researches have been undertaken to employ new methods for design and retrofit the seismic resisting concrete structures. Stiffness, strength and ductility are the main parameters in seismic performance of any structure. In general, stiffness and strength are the factors that control structural and non-structural damages; and ductility is a structural characteristic that provides the structure to withstand the inelastic deformations and controls the structural members’ failure. Ductility is the key parameter for earthquake energy dissipation rather than the other effective parameters. It depends on the formation of plastic hinges at the beam ends in concrete structures during an earthquake. Formation of plastic hinges at the beam ends arising from the large displacements causes an increase in the ductility and energy dissipation in moment resisting frames. Although, formation of plastic hinges leads to energy dissipation, but large inelastic deformation results in an increase in the residual displacement of structures. In common reinforced concrete structures, post-earthquake residual strains and displacements play an important role. Therefore, the serviceability of structures may be disrupted after an earthquake and in few cases they need to be re-built. Using Shape Memory Alloy (SMA) materials with the ability of super-elasticity in large strains at beam plastic hinges instead of reinforcing bars reduce residual displacements and deformations. High fatigue and corrosion resistance, ability to regain the original shape after a heat treatment, and high energy dissipation capacity are the advantages of using these material. Also there is no need to replace SMA members after an earthquake. Using shape memory alloy materials in the critical zones of structures such as plastic hinge zones decreases post-earthquake residual displacement, provides serviceability, and prevents the need for destruct or retrofit the structures. One of the most important features of the shape memory alloy materials is the ability to regain their original shape in strains less than 8%. In this paper five concrete moment resisting frames with 3, 5, 7 and 9 stories are modeled and subjected to near-field earthquakes. The amount of damages in the structures that are subjected to near-field ground motions due to the presence of the long-period pulse at the beginning of the record, is more extensive than far-field earthquakes. The non-linear time history analyses have been performed by “SeismoStruct” finite element software. Relative lateral displacement of stories (drift angle), residual relative lateral displacement of stories, lateral displacement of roofs and base shears are investigated. Results showed that using shape memory alloy (SMA) materials instead of steel reinforcing bars at beam plastic hinges reduces the residual displacement of the structure and relative repair cost after earthquake. The relative lateral displacement of stories is increased in the RC frames with SMA. Also residual roof displacements and base shear of SMA RC frames are decreased. In general, the SMA RC frames that are subjected to near-field earthquakes showed desirable performance. It can be deduced that using shape memory materials (SMA) instead of steel reinforcing bars at the beam plastic hinges reduce structural damages.