Eccentrically braced frames Frame characteristics

In this type of structural system, as shown in Figure 6.13, the bracing members intersect the girder at an eccentricity 'e', and hence transmit forces by shear and bending. The length of the girder defined by e is termed a 'link beam', which may behave predominantly in either shear or bending. While retaining the advantages of CBFs in terms of drift control, eccentrically braced frames (EBFs) also represent an ideal configuration for failure mode control. Another important advantage is that by providing an eccentricity, a higher degree of flexibility in locating doors and windows in the structure is achieved. By careful design of the link beam, significant energy dissipation capacity can be obtained. Moreover, zones of excessive plastic deformations

Eccentrically Braced Frame
Figure 6.13 Possible configurations of eccentrically braced frames

can be shifted away from beam-column connections, thus improving the overall integrity of the frame.

The length of the link zone has a direct influence on the frame stiffness. The relation bettveen eccentricity ratio (e/L) and the lateral stiffness (K) is illustrated in Figure 6.14. As e/L tends to unity, the stiffness of the MRF is obtained, while the zero eccentricity ratio corresponds to the CBF stiffness. There is also a direct relationship between the frame drift angle (8) and the rotational demand in the link (y). Simple analysis of plastic collapse mechanisms of a single link in EBF gives a relationship bettveen frame and link deformations (see Figure 6.15) as:

Eeccentrically Braced Frames
Figure 6.14 Relationship between link length and lateral stiffness of eccentrically braced frames
Eccentrically Braced Frame Link
Figure 6.15 Relationship between link length and ductility demand in eccentrically braced frames

Since the span of the frame is significantly larger than the eccentricity, e, it follows that the ductility demand in the link is considerably higher than that for the frame. It is also evident that shorter links would have higher demand for the same level of frame drift.

As in other codes, eccentrically braced frames are designed in EC8 so that beams are able to dissipate energy by formation of plastic bending or plastic shear mechanisms in the links. Specific rules are given to ensure that yielding in the bending/shear links of the beams will take place prior to yielding or failure in other members, which would therefore be capacity designed. The most recent version of EC 8 incorporates detailed provisions (mainly in Section 6.8 of EN 1998-1) that are largely in accordance with North erican design procedures.

6.6.2 Link beams

Whereas short links suffer from high ductility demands, they yield primarily in shear. Experimental evidence (e.g. Hjelmstad and Popov, 1983; Kasai and Popov, 1986; Engelhardt and Popov, 1989) showed that shear link behaviour in steel is superior to that of flexural plastic hinges. However, other considerations such as architectural requirements may necessitate the use of long links. Assuming no strain-hardening or moment-shear interaction, the theoretical dividing length (ec) between shear and flexural yielding is:

where Mp lmk and Vp lmk are the plastic moment and plastic shear capacities of the cross section, respectively.

Experimental evidence, however, shows that strain-hardening is significant in link behaviour. The ultimate shear and bending strengths may be significantly higher than Vpljnk and Mlmk, with different ratios. Accordingly, in EN 1998-1, for I-section links where equal moments occur at both ends, the links are defined as:

On the other hand, in designs in which only one plastic hinge forms at one end in I-sections:

short links e < es = 0.8 (1 + a) Mplmk/Vplmk (6.12)

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