Dogbone Eccentrically Braced Frames

where a is the ratio of the absolute value of the smaller-to-larger bending moments at the ttvo ends of the link.

If the applied axial force exceeds 15 per cent of the plastic axial capacity, reduced expressions for the moment and shear plastic capacities are provided in EC 8 to account for the corresponding reductions in their values.

EC8 also provides limits on the rotation '8p' in accordance with the expected rotation capacity. This is given as 0.08 radians for short links and 0.02 radians for long links, whilst the limit for intermediate links can be determined by linear interpolation. The code also gives a number of rules for the provision of stiffeners in short, long and intermediate link zones.

6.6.3 Other frame members

Other members not containing seismic links, such as the columns and diagonals, should be capacity designed. These members should be verified considering the most unfavourable combination of axial force and bending moment with due account for shear forces, such that:

where the actions are similar to those previously defined for concentrically braced frames. However, in this case W is the minimum of the following: (i) min of W. = 1-5VpUnkj/VEdj among all short links, and (ii) min of W. = 1.5MpUnkj/MEdj among all intermediate and long links where VEdi and MEdj are the design values of the shear force and bending moment in link 'i' in the seismic design situation, whilst Vp link. and Mp ink. are the shear and bending plastic design capacities, respectively, of link i. It should also be checked that the individual values of W. do not differ from the minimum value by more than 25 per cent in order to ensure reasonable distribution of ductility.

If the structure is designed to dissipate energy in the links, the connections of the links or of the elements containing the links should also be capacity designed ith due account of the overstrength of the aterial and the links, as before. Semi-rigid and/or partial-strength connections are permitted with some conditions similar to those described previously for MRFs.

Specific guidance is given for link stiffeners in EN 1998-1. Full-depth stiffeners are required on both sides of the link web at the diagonal brace ends of the link as indicated in Figure 6.16. These stiffeners should have a combined width not less than b,—2t and a thickness not less than 0.75t or f w w

10 mm whichever is larger, where bf and tw are the link flange width and link eb thickness, respectively.

Inter ediate eb stiffeners in shear links should be provided at intervals not exceeding (30t -d/5) for a link rotation angle of 0.08 radians, or (52t - d/5) for link rotation angles of 0.02 radians or less, with linear interpolation used

A.Y. Elghazouli and J.M. Castro

Link length (e)

Link length (e)

Seismic Isolation Structure
Figure 6.16 Full-depth web stiffeners in link zones of eccentrically braced frames

in bettveen, where d is the section depth. Links of length greater than 2.6 M ,. JV ,. , and less than 5 M ,. JV , should be provided with intermediate p,link p,link p,link p,link 1

web stiffeners placed at a distance of 1.5 times bf for each end of the link. Both requirements apply for links of length bettveen 1.6 and 2.6 MpljnJVpljnk, and no intermediate web stiffeners are required in links of lengths greater than 5Mp Unk/V. Unk. Intermediate link web stiffeners are required to be full depth. For links that are less than 600 mm in depth, stiffeners are required on only one side of the link eb. Lateral supports are also required at both the top and botto link flanges at the end of the link. nd lateral supports of links should have design strength of 6 per cent of the expected nominal strength of the link flange.

Design of link-to-column connections should be based upon cyclic test results that demonstrate inelastic rotation capability 20 per cent greater than that calculated at the design storey drift. On the other hand, beam-to-column connections away from links are permitted to be designed as pinned in the plane of the eb.

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