AISC LRFD 1.pdf

Comm. A2.] TYPES OF CONSTRUCTION 165this way because the rotation is measured over a gage length that incorporates thecontributions of both the connecting elements and the members being connected.Connection Strength. Referring to Figure C-A2.1, it is presumed that the nominalconnection strength M n can be determined on the basis of an ultimate limit statemodel of the connection or from test data. Further, many PR connections do notexhibit a plateau in their moment-rotation relationship, even at large rotations. Indetermining their strength based on tests, it is necessary to assume a rotation atwhich to define the nominal strength. For this purpose, the connection strength canbe defined at a rotation of approximately n =0.02 radians (Hsieh and Deierlein,1991, and Leon, Hoffman, and Staeger, 1996).An important aspect of the nominal strength of a connection, M n , is its relationshipto the strength of the connected beam M p,beam . A connection is full strength if M n >M p,beam , otherwise the connection is partial strength.A partial strength PR connection must be designed with sufficient ductility to permitthe connection components to deform and to avoid any brittle failure modes.It is also useful to define a lower limit for the strength, below which the connectioncan be treated as simple. Connections that transmit less than 0.2M p,beam at a rotationof 0.02 radians can be considered to have no flexural strength for design. It shouldbe recognized, however, that the aggregate strength of many weak partial strengthconnections (e.g. those with a capacity less than 0.2M p,beam ) can be significant whencompared to that of a few strong connections (FEMA, 1997).Connection ductility. Connection ductility is a key parameter when the deformationsare concentrated in the connection elements, as is the typical case in partialstrength PR connections. The ductility required will depend on the flexibility of theconnections and the particular application. For example, the ductility requirementfor a braced frame in a non-seismic area will generally be less than for an unbracedframe in a high seismic area. Referring again to Figure C-A2.1, the rotation capacity, u , can be defined as the value of the connection rotation at the point whereeither (a) the resisting strength of the connection has dropped to 0.8M n , i.e. to 80percent of the nominal connection strength, or (b) the connection has deformedbeyond a reasonable limit, e.g. 0.03 radians. This second criterion for determining u is intended to apply to connections where there is no loss in strength until verylarge rotations occur. For example, tests of double-angle web connections showthat some details will deform in a ductile manner beyond the point where the beamFig. C-A2.1. Typical moment-rotation response of a partially-restrained connection.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION