رفتار چرخه‌اي ستون‌هاي مركب با بست افقي و منحني‌هاي رفتاري آن‌ها

Cyclic Behavior and Backbone Curves for Batten Columns

در اين مقاله، رفتار ستون‌هاي مركب با بست افقي تحت اثر بارهاي محوري ثابت و جانبي چرخه‌اي مورد بررسي قرار گرفته و منحني‌هاي رفتاري آن‌ها نيز استخراج و ارايه شده‌اند. منحني‌هاي رفتاري اعضاي سازه‌اي، بيانگر رابطه نيرو-تغييرمكان اعضا تحت بارهاي چرخه‌اي در محل تشكيل مفاصل پلاستيك است. اين منحني‌ها داراي كاربردهاي گسترده در تحليل‌هاي استاتيكي غيرخطي سازه‌ها به منظور انجام طراحي لرزه‌اي بر اساس عملكرد و يا انجام مطالعات ارزيابي و بهسازي لرزه‌اي است. در حال حاضر منحني‌هاي رفتاري براي ستون‌هاي مركب با بست افقي در دستورالعمل‌هاي ارزيابي لرزه‌اي سازه‌ها، موجود نيست. با توجه به تفاوت‌هاي موجود در رفتار لرزه‌اي، حالات شكست و شكل‌پذيري ميان ستون‌هاي مركب و توپر، استفاده از منحني‌هاي رفتاري ستون‌هاي توپر براي ستون‌هاي مركب صحيح نيست. در پژوهش حاضر، منحني‌هاي چرخه‌اي تحت بارهاي جانبي براي ستون‌هاي مركب با شرايط مختلف هندسي، با استفاده از تحليل‌هاي اجزا محدود غيرخطي، به دست آمده و با استفاده از آن‌ها، منحني‌هاي رفتاري اين ستون‌ها تهيه شده و بررسي شده است. در پايان بر اساس نتايج به دست آمده، يك منحني رفتاري پيشنهادي براي ستون‌هاي مركب با بست افقي براي استفاده در موارد كاربردي ارايه شده است.

In this paper, the cyclic behavior and available ductility of batten columns subjected to constant axial and cyclic lateral load (seismic condition) and their failure mode are evaluated numerically using nonlinear finite element analysis. The column specimens were steel I-shape sections and were analyzed as an equivalent cantilever column. Batten columns are compression members composed of two or more similar longitudinal components (chords) that are connected at points along their length with batten plates as transverse connectors. These connectors ensure that the column behaves as one integral unit to achieve maximum axial capacity. In the past decades, many research activities were conducted on the buckling problem of batten columns. When a batten column is subjected to lateral load or bending moment about its hollow axis (axis perpendicular to battens) in addition to axial compression, the additional internal actions will be imposed to its members (chords and battens). In this case, it is expected that the batten column will have different behavior and failure modes. If the lateral load or displacement is due to seismic actions, more complexities will exist in the column behavior due to nonlinearities and its post-failure response. Few researches were reported about the behavior of batten columns in seismic conditions and their ductility. In this research, the backbone curves for batten columns have been also developed based on their cyclic response. The component backbone curve represents the nonlinear behavior of component in plastic hinge locations and was used in the nonlinear pushover analysis. The backbone curve for some structural components has been found in many standard and guidelines of seismic evaluation like as FEMA356. Using the backbone curve, the available ductility of column considering its post failure response under cyclic lateral loads, could be evaluated. The backbone curve for batten columns does not exist in any guideline or research reports. Because of differences between behavior and failure modes of batten and solid web columns under seismic action, it was expected that their backbone curves had been substantially different. In this research, cyclic response of batten columns with different geometries have been investigated subjected to cyclic lateral and 3 level of constant axial load. Using cyclic curves, the backbone curves of considered batten columns have been developed. The results show that the available ductility of batten columns is considerably low compared with solid web columns. The failure mode of batten columns is local buckling of bottom chords (in flanges and web) in combination with overall buckling of these chords symmetrically. It is also shown that the backbone curves of batten columns are different from solid web columns. The backbone curves of batten columns are semi-ductile (Type 2) based on FEMA356 classification and don’t have any residual strength. Finally, a conservative backbone curve has been proposed for engineering applications.