Connection rotation, 0

FIGURE 23.5 Four-parameter power model of semirigid connection.

Three prediction moment-rotation equations were incorporated with Kishi and Chen's database: the polynomial model shown in Equation 23.2, a modified exponential model [17], and the three-parameter power model shown in Equation 23.3. The validity of either the three-parameter power model or the modified experimental model for practical use was examined by Kishi et al. [18], who concluded that the three-parameter power model adequately describes the experimental test data for modeling the connection moment-rotation behavior for practical use.

23.3.6 Connection Classification

In practice, steel framed structures are to be designed in accordance with one of the three types of steel framing constructions: continuous construction, simple construction, and semirigid construction. Therefore, designers must be knowledgeable as to when the connections can be assumed to be rigid, semirigid, or flexible. The purpose of connection classification is to select a suitable basis on which to conduct the frame analysis and design. Based on the stiffness and strength of a connection, the connection can be classified as follows:

• Stiffness criterion. A connection is categorized as rigid, semirigid, and flexible based on the ratio of its rotational stiffness (R) to the stiffness of the beam (EIIL) it connects, RL/EI, where L and EI are the length and bending rigidity of the beam, respectively.

• Strength criterion. The most important aspect of the strength of a connection is its relationship to the strength of the connected beam. Based on comparison of the moment resistance (Mn) of a connection to the plastic moment resistance (Mp) of the beam it connects, the connection can be classified as a full strength, partial strength, or flexible connection.

Table 23.2 summarizes three classifications of connections presented by Bjorhovde et al. [19], Eurocode 3 [4], and LRFD [2] based on the foregoing criteria. The initial stiffness of the connection (R;) was adopted in classifications by Bjorhovde et al. and Eurocode 3, while the secant stiffness of the connection (Rs) was used in the LRFD classification. As shown in Table 23.2, the secant stiffness, Rs, is defined on the basis of either the moment, Ms, or the rotation, 0s, which would occur under the applied loads. The LRFD classification suggests that two distinct values of secant stiffness should be adopted to characterize the connection behavior under the two limit states of serviceability and ultimate load. The moment resistance of the connection, Mn, can be determined on the basis of an ultimate limit-state model of the connection or from tests. When a semirigid connection does not exhibit a plateau in its moment-rotation relationship, the moment resistance of the connection is defined at the rotation dn = 0.02 radians [2,12,13,20].

In addition to the strength and stiffness of the connection, a third important characteristic that should be considered in connection classification is the ductility of the connection. Connection ductility

Connection rotation, 0

TABLE 23.2 Summary of Connection Classification Systems

Bjorhovde classification [19]

Eurocode 3 classification [4]

AISC LRFD classification [3]

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