Composite connections in dissipative zones

The following design objectives guided clause 7.5.4 of EN 1998-1 for composite connections:

• the integrity of the concrete slab should be maintained during the seismic event

• yielding takes place in the steel section and/or in the rebars of the slab.

The requirements and design calculations for the connections of steel parts to steel parts are identical to those for steel structures. In particular, connections can be of the partial- or full-strength type, according to clause 7.5.2(3). Depending on the chosen option, clause 7.5.2(4) or 7.5.2(5) applies as the rule for the capacity design of adjacent structural components.

The most specific design rules on composite connections in Eurocode 8 refer to conditions to be fulfilled in order to take advantage of the composite resistance of the panel zone in beam-to-column connections of moment-resisting frames. This is achieved by defining the dimensions of the walls of a panel zone 'box' which is filled by concrete. This composite box possesses higher resistance and stiffness than the reference steel panel zone, due to effective

Table 7.1. Limits of wall slenderness in steel members of composite buildings, for different sections, design details and behaviour factor q (note: e = (fy(MPa)/235)"2)

Ductility class of structure

Flange outstand limits c/tf

H or I section in steel or partially or fully encased

Partially or fully encased H or I section plus hoops or straight links

Web width to thickness limit c/t

Partially or fully encased H or I section Partially or fully encased H or I section plus web connected

DCM DCH

Reference value of behaviour factor (q):

20e I4e 9e

30e 2le I3.5e

42e 38e 33e

42e 42e 33e diagonal concrete compression struts activated when the box starts deforming in shear. The confinement of concrete delays its cracking and crushing, so that the contribution of concrete to strength is significant. This typical composite aspect of design can obviate the necessity of costly 'doubler' steel plates welded in the panel zone, as sketched in Fig. 6.10. Clauses 7.5.4(7) and 7.5.4(8) consider, respectively, the cases of fully encased panel zones and of partially encased zones.

Different types of beams framing into different types of columns are envisaged:

• Steel beams, composite fully encased beams and composite partially encased beams.

• Composite partially encased columns, composite fully encased columns (clause 7.5.4(10)) and reinforced-concrete columns (clause 7.5.4(9)). In the last option, the vertical stiffeners of the steel beam placed in the plane of the outside face of the column and contributing significantly to the shear resistance of the panel zone are called 'face-bearing plates'; they are subject to the detailed rules of clause 7.5.4(9).

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