بررسي رفتار ستون‌هاي فولادي با مقاطع عرضي مختلف تحت بار انفجار

Investigation of Steel Column Behavior with Different Cross Section under Blast Loading

حملات تروريستي در مجاورت ساختمان‌ها و اماكن حياتي، منجر به فروپاشي جزئي يا كلي آنها و از بين رفتن سرمايه‌هاي يك كشور مي‌شود. انفجار از جمله‌ پديده‌هاي پيچيده و با نرخ كرنش بالا مي‌باشد كه در رفتار المان‌هاي سازه‌اي و مصالح به شدت تاثير‌گذار است. در اين مقاله، رفتار ستون‌هاي فولادي با مقاطع عرضي مختلف، تحت بار انفجار مورد بررسي قرار گرفته است. جهت دستيابي به مقطع با عملكرد مطلوب‌تر، از آناليز ديناميكي غيرخطي توسط نرم‌افزار اجزاء محدود LSDYNA استفاده شده است. جهت صحت‌سنجي، نتايج حاصل از شبيه‌سازي عددي با نتايج يك مدل آزمايشگاهي مقايسه شده است. سپس ستون‌ با مقطع عرضي بال پهن، صليبي و قوطي ، تحت شرايط مرزي دو انتها گيردار و دو انتها مفصل در معرض دو زاويه گسترش موج انفجار صفر و 45 درجه، شبيه‌سازي و پاسخ آن‌ها با يكديگر مقايسه شده است. بررسي‌ها نشان مي‌دهد كه ستون با مقطع عرضي بال پهن در زاويه انفجار صفر درجه نسبت به مقاطع عرضي ديگر حول محور قوي، جابجايي كمتر و عملكرد مطلوب‌تري دارد. همچنين در زاويه انفجار 45 درجه مقاطع عرضي داراي دو محور قوي، جابجايي كمتر و عملكرد مطلوب‌تري را نسبت به مقطع بال‌پهن با يك محور قوي نشان مي‌دهند.

Terrorist attacks and explosions in the vicinity of buildings and vital areas are happening in different countries repetitively. Most of these incidents lead to global and local failure in the main elements of the buildings and in some cases due to intensity of explosions can occur entire structure collapses. Columns are the key bearing elements in the building, and between all columns, the exterior of them are more vulnerable to terrorist attacks. Usually blast resistant design of structure is carried out by simplifying the models and considering a single column with nonlinear behavior under blast loading. Explosion is a complex phenomenon with high strain rate, which affects strongly on behavior and material property of structural elements. Operation of experimental test on structures under blast load is very expensive, difficult and dangerous. Hence, simulation of experimental models using nonlinear finite element software is very useful. In this paper, to achieve better performance of columns under blast loading, the response of steel columns with different crosssections has been investigated. In addition, effects of blast wave incidence angle, blast distance and different boundary conditions are considered. For this purpose, wide flange steel column of experimental test has been simulated under axial force and blast load using LSDYNA software. Numerical model is simulated using shell elements and its result has been validated with the full scale blast experimental data. In the finite element analysis the effects of high strain rate and material nonlinearity are considered. The columns with different cross sections of wide flange, crossIPE and box sections are simulated under two angles of blast waves extensive, zero and 45 degree. Also, two support conditions of fixedfixed end and pinnedpinned end have been considered. The results show in the both boundary conditions for blast with zero angle, the dynamic response of column with wide flange section subject to blast load has been less than the other cross sections. Also, the box section has better performance than crossIPE. In 45 degree blast angle and fixed end boundary conditions, the displacement time history of box column is less than two other sections and it shows better performance respect to other sections. But, under pined end boundary conditions, crossIPE section has better and stronger behavior respect to wide flange and box sections. In addition, the displacement of wide flange section (section with nonidentical strong axes) in 45 degree blast angle has more than zero degree. However, in the columns of box and crossIPE section under the same explosion situation in 45 degree blast angle, the dynamic response is less than zero degree, because they have two identical strong axes. Then for corner columns of buildings that direction of blast wave propagation may be 45 degree the best section (based on minimum deflection criteria) is column section with two strong axes such as box and crossIPE, however for peripheral middle column of building that bending moment of explosion may be accrued about strong axis, the wide flange section with only one strong axis is better. Various distances of explosion from column cause different nonlinear behavior, therefore investigation of optimum column cross section under blast loading depends to distance of explosion from the column. Then displacement criteria may be not enough and use of additional criteria such as residual load bearing capacity can be appropriate.