پديد آورندگان :
وهداني، رضا دانشگاه سمنان - دانشكده مهندسي عمران - گروه مهندسي زلزله , انصاري، مختار دانشگاه بزرگمهر قائنات - دانشكده مهندسي - گروه عمران , انصاري، مسعود دانشگاه سمنان - دانشكده مهندسي عمران
كليدواژه :
مولفه ي قائم زمين لرزه , نيروي محوري ستون , نسبت نياز به ظرفيت برشي , حوزه ي نزديك گسل
چكيده فارسي :
با توجه به خسارت هاي سازه اي به وجود آمده در زمين لرزه اي نزديك گسل اخير كه به مولفه ي قائم زمين لرزه نسبت داده مي شود و بروز آسيب هاي متعدد در ستون هاي بتني كه باعث تخريب پيش رونده نيز مي شود، علاقه مندي براي بررسي اثر مولفه ي قائم در پاسخ هاي لرزه اي افزايش يافته است.در اين پژوهش، قاب هاي بتني ميان مرتبه طراحي شده با ضوابط لرزه اي، تحت تحريك همزمان مولفه قائم و افقي زلزله و تحريك مولفه افقي زلزله به تنهايي قرار گرفت. براي تحليل تاريخچه زماني از دو گروه شتاب نگاشت هاي نزديك و دور از گسل، استفاده گرديد. پاسخ هاي سازه اي نيروي محوري كششي، فشاري و نوسان نيروي محوري ستون ها، نسبت نياز به ظرفيت برشي ستون ها و لنگر بيشينه ي وسط دهانه ي تير، در دو حالت با و بدون تحريك مولفه ي قائم، مقايسه شد و اثر حضور مولفه ي قائم، براي شتاب نگاشت هاي دور و نزديك گسل، براي دهانه ي كناري و مياني، قاب هاي بتني به صورت مجزا به دست آمد .نتايج نشان مي دهد كه نيروي كششي و بروز كشش در ستون هاي كناري بحراني تر از ستون هاي مياني، خواهد بود. حضور مولفه ي قائم باعث كاهش نيروي فشاري كمينه و حركت به سمت كشش مي شود كه اين مقدار در بحراني ترين حالت، به صورت ميانگين بين شتاب نگاشت هاي نزديك گسل به 84% كاهش ميرسد. حضور مولفه ي قائم باعث افزايش نياز به ظرفيت برشي ميشود و در بيشترين حالت به 31% افزايش نسبت نياز به ظرفيت برشي منجر ميشود.
چكيده لاتين :
According to the observations after the recent near-field earthquakes, structural damages are mostly attributed to the vertical component of the ground motion, i.e. concentration of the damages in column members leading to progressive structural collapse. This is why investigation of ground motion’s vertical component effect has been widely regarded in recent studies. In seismic design, this component is considered less than other components of earthquake. However, in near fault earthquakes, large vertical acceleration components cause extensive damages compared to the ones with horizontal acceleration. Failure and damage in concrete columns is among the examples of the negative effects of vertical component. Vertical component of earthquake is considered in the design of specific members on the recommendation of seismic codes such as EC-8 and FEMA 356. The design is intended to use the scaled horizontal component, where this can result in incorrect answers due to lack of stimulation because of the specific characteristics of vertical component of earthquake and structural properties in the vertical direction. Also, the vertical component of earthquake is less studied in seismic risk analyses. In this study, the effects of vertical earthquake excitations on medium-rise concrete moment frames are investigated in two separate stages including near field and far field records.
In this research, various structural models, representative of real structures and designed in accordance to seismic codes and under actual gravitational loads have been subjected, simultaneously, to horizontal and vertical components of near- and far-field ground motion records at two stages. Nonlinear time history and progressive dynamic analyses have been performed in this regard. Furthermore, the effect of elevation or reduction of initial gravitational forces as well as columns’ initial axial forces have been investigated by applying differing gravitational loading coefficients. Structural response parameters including tensional and compressional axial loads of the columns as fluctuating forces, columns’ uplift forces at various plan positions and under various gravitational coefficients, the interactive axial-flexural forces of the columns at different gravitational coefficients, shear demand-to-capacity of columns, axial deformation of the columns in presence and absence of vertical component of the earthquake, have been comparatively investigated and the effect of vertical ground motion component has been assessed, separately, for far- and near-field acceleration records and for external and internal columns placed at different stories.
The obtained results reveal that tensional uplift forces are more critical in external columns than the internal ones. This is mainly true for lower stories, while at the upper stories the tensional forces experienced by internal columns are seen to be more critical. The existence of vertical component of earthquake leads the minimum compression forces to increase and change toward tension range. The amount of this reduction has been witnessed to reach to 84% in the more extreme case. It was also seen that for smaller gravitational coefficients, tensional axial forces are more frequently observed. The presence of earthquake’s vertical component has been shown to amplify the columns’ shear demand by the values reaching to 31% at the most extreme cases.
