Clauses 7.4.2, The mechanical properties of structural elements and structures made of two structural 7.5.3 materials depend on the properties of each material and on the interconnection between them. In particular, in the case of steel-concrete composite elements, the following should be considered:

• Concrete offers resistance to compression, but not (reliably) to tension. This implies that both stiffness and resistance at yield of composite sections depend on the sign of the stresses in the concrete. As a result, two values are defined for each mechanical property of sections in structural elements in which there is a reversal of the sign of the bending moment and/or normal force. This contributes to the apparent complexity of design for composite structures.

• The evaluation of the global resistance of a structure should be based on the nominal strengths of the materials, which are lower than the mean values. Capacity design of zones which are to remain elastic should, in contrast, normally be based on estimates of the real resistances of dissipative zones, which are average or upper bound values; this results, in some cases, in two values for the resistance of sections, contributing to the apparent complexity of seismic design of composite structures.

• The periods of a structure depend on its stiffnesses. Considering a lower bound value of the modulus E of concrete causes underestimation of the stiffness, an overestimation of the periods and of seismic displacement demands and underestimation of the force action effects (base shear, etc.).

• In the seismic context, there are types of mechanical stress, such as alternate shear, which induce fast degradation of the concrete; this situation results in the necessity of considering non-composite resistances of some sections.

8 The value of the maximum resistance to compression of a given concrete varies widely, depending on the confinement achieved in the most stressed zones;/cd is the design value of strength considered in resistance calculations, but the design checks may implicitly include the favourable influence of confinement at the ULS.

• In sections working in a composite way, there is generally shear transfer at the concrete-to-steel interface; in dynamic cyclic conditions, requirements for the effectiveness of this transfer may be higher than under static loading, because, for instance, there may not be strong confidence in high bond resistance. This will imply that in certain cases more shear connectors are required in seismic design than in static design.

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