Unfavourable factors for local ductility due to the composite character of structures

The use of composite steel-concrete frames can have some adverse affects on local ductility; these are in addition to the phenomena described in Section 6.4 for steel structures:

• Concrete crushing in compression. Concrete failure in compression is not ductile. Many aspects of Section 7 of Eurocode 8 aim at defining conditions to avoid such failure by keeping stresses and strains in concrete below their values at failure:

- The limit values ofxld in steel beams with a slab defined in clauses 7.6.2(7), 7.6.2(8) and in Table 7.4, which limits the position of the neutral axis in a composite T beam of moment frames in order to keep the maximum strains econcrete in the slab below an acceptable value (Fig.7.1).

- The design conditions of the rebars of the slab presented in Annex C of EN 1998-1, which are defined with the twofold objectives of:

(a) taking advantage of the concrete strength in order to maximize the potential of resistance of sections made of a steel H beam composite with a concrete slab, by defining steel reinforcement of the slab which works as 'ties' to equilibrate concrete compression 'struts'

Clauses 7.6.2(1), 7.6.2(7), 7.6.2(8), 7.6.4(2), 7.6.5(1)

(b) mitigating slab cracking around columns in moment-resisting frames, by creating a concrete-confining effect. Clauses • Concrete disintegration under alternate shear. Concrete failure under alternate shear is

7.5.4(7), not ductile, because alternate cracks may rapidly cause complete disintegration of

7.6.4(7), concrete, reducing concrete strength to practically nil. This explains why the resistance

7.6.5(2) V Rd of the panel zone of composite columns, which can include the contribution of concrete compression struts inside the 'box' formed by the column flanges and the transverse stiffeners, must be capacity designed to the beam plastic strength in bending by complying with equation (7.3) in EN 1998-1:

The analogous condition for steel structures is equation (D6.6) in Chapter 6: ^wp, Ed - Kv, Rd- ^ is clear that increased safety is required against panel 'failure' in the case of composite columns than in the case of steel columns, precisely in order to keep the shear stresses in the panel zone below the yield stress of steel and below the ultimate stress of concrete.

The dramatic decrease in the shear strength of sections subjected to plastic bending and in which concrete fails under alternate shear also justifies that, in dissipative zones of columns made of an encased steel profile, no confidence is given to the shear resistance of the composite section. Clauses 7.6.4(7) and 7.6.5(2) express this, by requiring that the shear resistance of the steel section alone should be considered.

Clauses 7.6.2, • Negative impact of the rise of the neutral axis in beams composite with a concrete slab.

7.7.1 (4) In composite beams made of a steel H section with a concrete slab connected by shear connectors to the steel section, the neutral axis is raised to the upper part of the section (currently the steel flange), which involves increased strains eSiC0mp0Site in the bottom flange of the steel section, in comparison with the strains es sted developed at equal rotation in a symmetrical steel section (Fig. 7.1). These higher strains cause faster strength degradation due to buckling, and accordingly reduce the ductility of the section. This effect is considered by values of the limits of wall slenderness c/t of webs which are more restrictive for webs fully in compression (in beams with a slab) than for webs in bending (in symmetrical steel sections). The limit values of the wall slenderness c/f for flanges remain unchanged.

9 Negative impact of the increase in effective resistance of composite girders. The composite action significantly increases the effective strength of steel girders, particularly in sections where curvature of the girder places the top flange into compression. It is of primary importance that capacity design of non-dissipative structural elements is based on the values of the composite strength, which is the case in EN 1998-1. Design ignoring the effective composite strength of girders can result in structural problems. For instance, in moment-resisting steel frames, the development of a global 'weak column-strong beam' mechanism or a local 'weak panel zone' condition, or the reduction of the effectiveness of reduced beam section design. It has been evaluated that the distress in moment connections in the Northridge (1994) earthquake was partly explained by the higher stress and strain demand in the bottom flange of composite beams.

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