Short links versus long links

Clauses 6.8.2(3), Seismic links are designed for the computed seismic action effect in shear or in bending of 6.8.2(4), the link, by complying to

6.8.2(9) which Vp_link and Mp Mnk are, respectively, the plastic shear and bending resistance of the link. In H sections,

K, link = (fy/^K(d - if) Mp> link =fybtl(d - Q (D6.10)

Equation (6.17) in EN 1998-1 allows computation of Vp link taking into account the interaction of shear with axial force, while equation (6.18) in EN 1998-1 allows computation of Mp link taking into account the interaction of bending with axial force.

The local plastic mechanism in seismic links depends on the topology of the structure. The topology of a structure can be such that bending moment diagrams in the link are symmetric (Fig. 6.16) or unsymmetric (Fig. 6.15).

The plastic mechanism in seismic links also depends on their length, e. Short links yield essentially in shear; the energy dissipated in the plastic mechanism is Wv = Vp iink(9pe.

Long links yield essentially in bending; in a long link subjected to the symmetrical action effect M as in Fig. 6.16, the energy dissipated in the plastic mechanism is WM = 2Mp link6>p.

The limit between long and short links corresponds to the situation in which yielding may take place either in shear or in bending:

WM = Wv => 2Mp,link0p = Kp,link0p e e = 2Mp_link/Fp,!ink (D6.ll)

However, for values of e around the limit of equation (D6.ll), significant bending moments and shear forces exist simultaneously, and their interaction has to be considered. In EN 1998-1, the value of e for considering only plastic mechanism only in shear is e<es = 1.6MpJiJVpAink (D6.12a)

The value of e for considering only a plastic bending mechanism is e>eL = 3MPitmk/Vp,link (D6.12b)

Fig. 6.1S. Conversion of concentric V bracing to eccentric bracing with a vertical link
Eccentric Bracing

Moment M and shear 1/ in link

Fig. 6.16. Conversion of concentric V bracing to eccentric bracing with a horizontal link

Moment M and shear 1/ in link

Fig. 6.16. Conversion of concentric V bracing to eccentric bracing with a horizontal link

Between these two values, es and eL, links are termed 'intermediate', and the interaction between shear and bending has to be considered by making use of equations (6.17) to (6.20) in EN 1998-1.

If the topology of the structure is such that the bending moment diagram is not symmetric, only one plastic hinge will form if the link is long, so that WM = Mp link#p. Then, the limit length between long and short links corresponds to e=MpJink/VpiUnk

This is, for instance, the case for the vertical shear links in Fig. 6.15. Values of es and eL are defined in equations (6.24) to (6.26) in EN 1998-1 to cover this situation. These expressions are made continuous with those for symmetric situations by means of a factor a which characterizes the shape of the diagram of bending moments. In the example of Fig. 6.15, a ■

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