بررسي اثر ميانقابي ديوارها بر روي رفتار لرزه اي اتصالات داراي نشيمن

On the Influence of Infill Panels on the Seat Angle Connections

اتصال ساده تير با نبشي نشيمن انعطاف‌پذير يكي از اتصالاتي است كه در ساختمانهاي فولادي مورد استفاده قرار مي‏گيرد كه در آن، تير بر روي يك نبشي نشيمن قرار مي‌گيرد. در اين نوع اتصال علاوه بر نبشي نشيمن كه وظيفه انتقال برش از تير به ستون را دارد، يك نبشي در بالاي تير نيز به كار مي‌رود كه وظيفه آن تنها تامين تكيه‌گاه جانبي براي بال فشاري تير است. هرچند كه در تحليل هاي مرسوم كه در آنها از اثر ميانقاب در سازه صرفنظر مي‌شود نيروي زلزله به اتصالات وارد نمي شود ولي بررسي رفتار لرزه اي اين اتصالات در دهانه‌هاي داراي ديوار حاكي از اعمال نيرويي بيش از حد مورد انتظار به نبشي‌هاي اتصال است كه مي تواند موجب ايجاد شكست در جوشهاي اتصال اين نبشي‌ها يا تغييرشكلهاي بزرگ و ماندگار در آنها ‏شود. در اين مقاله سعي مي‌شود ابتدا نيروهاي ناشي از كنش ميانقابي ديوار معرفي و سپس اثر اين نيروها بر روي اتصالات مفصلي به خصوص از نوع نبشي نشيمن بيان و روش طراحي مناسبي براي آنها ارايه شود، ضمن اين كه مطالبي در اين زمينه براي نبشي موجود در اتصالات خورجيني نيز بيان شده است.

Infills are normally used in buildings for architectural purposes. It has been shown that they have considerable influences on stiffness and strengths of the structures. However, their effects are just considered as 20% reduction in the building natural period of vibration and other global and local structural effects are normally ignored in analysis and design phases. Experience of previous earthquake shows that infills have significant influences specially on connections of the surrounding frames. Some failure modes of connection on during previous earthquakes are shown in this paper. Harmful influences of infills on connections are investigated in this paper, specially for simple connections with seat angles as well as for saddle connections. Simple connec-tions having flexible seat angle is very common in steel structures. In this kind of connections, the beam is placed on an angle which transfers the shear of the beam to the column. Another angle is applied on top of the beam as well to prepare lateral support for compression flange of the beam. This angle is normally designed only for 10% of the shear force that applies on the bottom angle or even not designed. Behavior of simple connections, with seat angles, in bays having infill walls shows that the infill reaction force practically applied to the connection, is greater than the assumed one of the designing phase. The local force of the infill to the frame may cause large and residual displacement or even failure in the connection weldings. In this regard, saddle connections, in which the beam is connected to the column by two angles at the top and the bottom, is similar to seat angle connections and may be affected by infill panels. Furthermore, the saddle connections may have a considerable twisting moment as well. In this paper, the influence of infill walls on the loads of such connections are investigated and some suggestions are proposed for their safety in earthquakes. The obtained results and formulas of this paper can be applied for designing of new buildings with simple or other types of connections, and seismic rehabilitation of existing buildings for considering infill panel effects. In this regard, some formulas of the literature for estimating the applied load of the infill to the frame are discussed. Then an example is solved to show inadequacy of connections, which are designed based on the codes, in which influences of infills are ignored. Subsequently, a formula is presented to estimate the applied loads on the connections.