Seismic progressive-failure analysis of tall steel structures under beam-removal scenarios

Investigating progressive collapse of tall structures under beam removal scenarios after earthquake is a complex subject because the earthquake damage acts as an initial condition for the subsequent scenario. An investigation is performed here on a 10 story steel moment resisting structure designed to meet the life safety level of performance when different beam removal scenarios after earthquake are considered. To this end, the structure is first subjected to the design earthquake simulated by Tabas earthquake acceleration. The beam removal scenarios are then considered at different locations assuming that both ends connections of the beam to columns are simultaneously detached from the columns;thus the removed beam falls on the underneath floor with an impact. This imposes considerable loads to the structure leading to a progressive collapse in all the scenarios considered. The results also show that the upper stories are much more vulnerable under such scenarios than the lower stories. Hence, more attention shall be paid to the beam-to-column connections of the upper stories during the process of design and construction.

活动平面导板矫治对成人安氏Ⅱ类深覆患者髁突位置的影响

目的比较平面导板打开咬合对成人安氏Ⅱ类深覆患者髁突在关节窝中位置的影响。方法选择有正畸治疗需求的18~32岁安氏Ⅱ类错患者30例。矫治前行锥形束CT(cone beam CT,CBCT)扫描,对原始影像进行3D重建,利用QR-NNT软件测量重建后3D影像的关节前、上、后间隙,运用Pullinger分析法计算髁突在关节窝中的位置。矫治过程中,利用上颌活动式平面导板打开安氏Ⅱ类深覆患者前牙咬合,待前牙咬合打开、后牙建立稳定咬合接触后再次行CBCT,对矫治前后测量值进行统计学分析。结果平面导板打开咬合后,安氏Ⅱ类错患者关节前间隙减小[(2.16±0.13)mm vs(2.03±0.19)mm,P<0.000 1],关节上间隙增大[(2.81±0.32)mm vs(3.30±0.33)mm,P<0.000 1],后间隙增大[(2.68±0.41)mm vs(2.97±0.42)mm,P=0.000 3]。结论正畸治疗上颌活动式平面导板打开安氏Ⅱ类患者前牙咬合后,安氏Ⅱ类患者髁突在关节窝内有前移的趋势。