004 012 0125 02

Mn Ms

Mn Ms


Semirigid: Ei/10d<R<Ei/2d


Mn > 0.7Mp Semirigid:

Stiffness criterion


RiL/EI> 25 (unbraced frame) RiL/EI > 8 (braced frame) Semirigid: 2 < RiL/EI< 20 (unbraced frame) 2<RiL/EI<8 (braced frame) Flexible: RiL/EI < 2

Strength criterion Full strength:

Partial strength: 0.25Mp<Mn<j Flexible: Mn < 0.25Mp






Full strength: Mn > Mp Partial strength: 0.2Mp < M.

Mn_6=0.02 " moment resistance of the connection at a rotation of 0.02 radians

M* = M/Mp; 0* = 0/0p is a key parameter when joint deformations are concentrated in the connection elements, as is the typical case for partial strength connections where the strength of the connected beam exceeds that of the connection. The connection ductility can be represented by its rotational capacity, 6u, which is defined in the LRFD classification as the value at which the moment resistance of the connection has dropped to 0.8Mn or the connection has deformed beyond 0.03 radians. That is, a connection is classified as ductile if its rotation capacity is equal or greater than 0.03 radians, otherwise the connection is considered to be nonductile.

The difference between the LRFD and Eurocode 3 classifications is that the latter one acknowledges the fact that the extent of semirigid action is largely associated with the type of structure, such as a braced or unbraced frame. Therefore, in Eurocode 3 the connections are classified separately with regard to braced and unbraced frames. The Eurocode 3 classification considers the load-carrying capacity of frames, which is more rational. For example, the stiffness boundaries between the rigid and semirigid zones are chosen such that the drop in load carrying capacity, due to semirigid behavior, is not more than 5% (in terms of the Euler buckling load). However, the ductility demand is not considered in Eurocode 3.

As the purpose of connection classification is to facilitate frame design for different limit states, it is necessary that a classification system be able to reflect the connection assumptions used in the frame analysis and design at each limit state. To that purpose, the connections should be classified corresponding not only to the ultimate limit state but also to the serviceability limit state, as a connection may respond differently at the two different limit states.

23.4 Structural Analysis of Semirigid Framed Structures

The structural analysis procedures for semirigid frames presented in this section are based on the following assumptions and idealizations:

1. All members are prismatic and straight.

2. Only the moment-rotation behavior of connections is considered (i.e., axial and shear deformations in a connection are ignored).

3. Members display linear-elastic or second-order elastic behavior, while the connections display nonlinear moment-rotation behavior.

4. Connection dimensions are assumed to be negligible compared to the lengths of the beams and columns; that is, the rotational deformation of a connection is assumed to be concentrated at a point at the end of the semirigid member.

5. The effects of eccentricity at joints are neglected.

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