Behaviour factors of structural types similar to steel

Clauses 7.3.1, The values of q factors for composite structures are the same as in steel structures with 7.3.2 the same structural system, namely moment-resisting frames,84 or frames with concentric or eccentric bracings, as given in Table 6.2 in EN 1998-1. There are, however, some limitations for these types of composite structures, which deserve an explanation. These limitations are:

• Braces of composite concentrically braced frames cannot be composite (clause 7.3.1(l)b). Clause 7.3.1(1) Assessing the real behaviour of a concentrically braced frame in a simple way is not easy, as explained in Section 6.10. Using composite brace members in which the concrete is effective under compression only, would make the design even more complex. In a simplified analysis, the resistance of the diagonals under compression is neglected (see Section 6.10), and only the diagonals which are in tension are assumed to resist the lateral earthquake loads. However, in the capacity design of the entire system the diagonals which experience compression under seismic loading need to be considered. As explained in Section 6.10, if the buckling capacity of a diagonal under compression at a certain storey is more than 50% of the tensile capacity N u Rd of the other diagonal at that same level, the columns will undergo larger axial forces than considered in the simplified analysis. This justifies the lower bound limit on A in clause 6.7.3(1) of EN 1998-1. Introducing a composite brace would increase the difficulty to design a slender diagonal member complying with A >1.3.

• Clauses 7.3.1(l)(c) and 7.9.3(1) require that pure steel sections are used to realize Clauses 7.3.1(1), seismic links in composite eccentrically braced frames (they can possibly be composite 7.9.3(1), with slabs). The links may not include encased steel sections. Clause 7.9.1(1) requires 7.9.1(1) them to be designed such that the dissipative action occurs essentially through yielding in shear of the links.

Capacity design requires a correct evaluation of the plastic resistance of dissipative zones. In long links, in which plastic bending is developed, plastic rotations are larger than in moment frames undergoing the same global displacement. The experimental background which would allow reference to the composite resistance of composite long links is missing. An initial composite resistance is certainly developed, probably followed by a concrete deterioration which leads to a link with the steel section capacity alone so that two values of link resistances should probably be considered:

the composite resistance, which would serve in the capacity design of braces, beams and columns

- the steel resistance, which would serve to evaluate the global resistance of the structure at ULS.

However, there are uncertainties for both of these values. Disconnection of the slab in order to refer to a pure steel section in bending raises similar questions, because it is not obvious that a local disconnection allows a reference to steel resistance only. Indeed, experiments in the context of moment frames have shown that disconnections of a very local character have a limited effect: the composite resistance is still there. The conclusion is that links working in bending in beam elements with a slab raise serious questions and cannot be designed in a reliable way.

In beams, only short links, made of an unencased steel section possibly with a slab, working in shear correspond to a well-controllable situation, because:

- the plastic resistance in shear of the steel section can be reliably computed the slab contribution in the shear resistance of the links is negligible.

Vertical steel links also correspond to a controllable situation.

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