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

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

Clauses 7.6.1 (4), • The positive effect of concrete encasement around steel profiles. Concrete encased in a 7.6.4(8), profile, or between the flanges of a profile, prevents inward local buckling of steel walls

7.6.4(9), and reduces strength degradation due to buckling. For this reason, some limits of wall

7.6.4( 10), slenderness for composite sections are higher than those of pure steel sections. The limit

7.6.5(3), values for web slenderness of H sections are higher by one class than those for steel

7.6.5(4), sections, as indicated in Eurocode 4,82 to which the basic clause 7.1.1(1) refers, provided

7.6.5(6) that the steel web is connected to the concrete in the way specified in clause 5.5.3(2)

of Eurocode 4. For partially or fully encased H sections, the limit values of flange slenderness given in Table 7.3 are the same as those of sections of class 1,2 and 3 in Table 5.2 of Eurocode 4,82 which in turn are the same as in Table 5.2 of Eurocode 383 for steel

Fig. 7.1. Strains in steel and composite sections sections. Fully encased beams are at present outside the scope of Eurocode 4; only partially encased beams are considered. Because of the lack of data, it was not possible in Eurocode 8 to make a distinction between partially and fully encased beam sections. In addition to the standard situations envisaged in Eurocode 4, Eurocode 8 introduces the possibility of increasing the limits of flange slenderness by mitigating the buckling of flanges of encased H profiles. The limits can be increased by up to 50% in certain cases by means of specific measures:

- additional stirrups for fully encased profiles (see clauses 7.6.4(9) and 7.6.4(10)) additional straight bars welded to the inside of the flanges for partially encased profiles (see clauses 7.6.5(4) and 7.6.5(6) and Fig. 7.8 in EN 1998-1). These details can improve the design at relatively low cost, since the additional rebars or stirrups need be present only in the critical regions of columns or in the length of dissipative zones of beams, which are of the order of the beam depth. The positive effect of steel plates and sections encased in a concrete wall. Consideration of steel inside concrete in this way may seem like a repetition of the previous case of the positive effect of concrete encasement around steel profiles. There is a difference, though, in the reference structural element, which is a concrete wall. By transforming a concrete wall into a composite one, the designer can significantly improve the ductility and strength of the wall and solve typical design problems such as providing a higher shear resistance within concrete dimensions limited by architectural considerations. Sections 7.10 and 7.11 of EN 1998-1, which are presented in Section 7.15 below, provide design guidance for reinforced-concrete walls composite with steel plates and sections. Increased damping, in comparison with steel structures, due to cracking and to friction at steel-concrete interfaces. Although commonly recognized, this positive effect on energy dissipation is not explicit in the design process because it is considered that, at the ultimate limit state (ULS), the energy dissipation by damping due to structural elements is secondary with respect to the energy dissipated in plastic mechanisms.

Clauses 7.10, 7.11

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