ارزيابي گيرداري جزيي موجود در اتصالات ساده‌ي فولادي با نبشي تحت بارگذاري ثقلي

Evaluation of Partial Rigidity of Simple Steel Frames with Angle Connections under Gravity Loads

اتصالات متداول در قاب‌هاي ساده، كاملاً مفصلي عمل نمي‌كنند و مقداري از لنگر تير را به ستون منتقل مي‌كنند؛ بنابراين فرض رفتار مفصلي ايده‌آل براي قاب‌هاي ساده گرچه محافظه‌كارانه به‌نظر مي‌رسد، ولي ممكن است در برخي حالات به نتايج دور از واقعيت منجر شود. هدف از اين نوشتار، بررسي و تحليل رفتار واقعي اتصالات به كمك نبشي‌هاي جان و نشيمن است كه از انواع اتصالات كاربردي در قاب‌هاي ساده است. براي اين منظور، بعد از مرور برخي مفاهيم مرتبط با اتصالات نيمه‌صلب، رفتار غيرخطي قاب‌هاي ساده‌ي فولادي 3، 5، و 8 طبقه مورد ارزيابي قرار گرفته است. طبق نتايج حاصل، درجات گيرداري اتصالات مذكور كمتر از 10? است. در نظرگرفتن رفتار واقعي اتصالات در مقايسه با رفتار مفصلي ايده‌آل باعث بهبود وضعيت تيرها بين 4 الي 18 درصد مي‌شود، در حالي كه وضعيت ستون‌ها وخيم‌تر مي‌شود. افزايش مقادير طراحي ستون‌ها، در ستون‌هاي كناري، مخصوصاً در طبقات فوقاني، قابل توجه است.

Generally, steel frames are classified as rigid, semi-rigid and flexible frames, according to their connection stiffness. But, in the analysis of steel frame buildings, it is customary to represent joint behavior by an idealized model, either as a rigid or as a pinned joint. However, typical connections in actual structures do not behave in either a perfectly rigid or a perfectly pinned manner. Previous investigations have shown that most connections in braced simple steel frames using angles do not behave in a perfectly pinned manner and exhibit some rotational stiffness. Consequently, an idealized pinned assumption for these types of connections can lead to unrealistic predictions of the response and strength of steel structures. The objective of the current study is to evaluate the effects of this partial rigidity of the beam-to-column connection on the actual behavior of steel frame buildings. Particular attention is devoted to double web angle connections and top and seat angle connections, which are customary types used in most practical cases. In this paper, some basic concepts related to the beam-to-column connection, such as fixity percentage and end fixity factor, are illustrated in detail and then calculated for customary steel frames. For this purpose, particular classes of braced steel buildings, those composed of three, five and eight stories with different span length, are modeled as 2-D frames, and nonlinear static analysis is performed. According to the results, connection fixity quantities are less than 10 percent. Also, some changes in element forces occurred due to the actual semi-rigid behavior of flexible connections. Generally, considering the nonlinear behavior of connections in braced simple steel frames, the designing forces of beams decrease (up to 18 percent in our models), and the designing forces of columns increase. These increases in the designing forces of outer columns, especially in upper stories, are noticeable.