Materials

Clause 7.2.1(1) The first requirement in clause 7.2 refers to the strength classification of structural concrete:

• The concrete should correspond to class C25/30 as a minimum; C25/30 is a standard value for structural applications in buildings, in particular for slabs.

• The concrete should correspond to class C40/50 as a maximum; the reason behind this restriction is the reduction of the crushing strain ecu with increased concrete strength.

In developments of clauses for ductility of beam sections with a slab, scu2 = 2.5 x 10"3 has been considered as the crushing strain for reinforced concrete under seismic cyclic conditions; this may be taken to correspond to ecu2 = 3.5 x 10"3 under static loading conditions. There is nothing preventing the use of a concrete class above C40/50, as long as the value ecu2 ~ 3-5 x 1 Cr3 is demonstrated under static conditions. Clauses 1.2.2(I), The other requirements in clause 7.2 refer to structural steel and the rebars, with reference 7.2.2(2), to prescriptions for ductility similar to those of Sections 5 (concrete buildings) and 6 (steel

7.2.2(4), buildings). The simpler option in design is to use ductile materials; however, adverse

7.2.3(1) situations are considered: clause 7.2.2(4) covers the situation in which non-ductile welded meshes are used in dissipative zones of composite structures. The rule is to duplicate the non-ductile rebars in dissipative zones by ductile ones of the same cross-sectional area and to rely on these last rebars to evaluate a safe-side plastic capacity. However, both the non-ductile and the ductile rebars present in the case of such a duplication should be used in the evaluation of the upper bound resistance of the cross-section, used as reference for the capacity design of other elements.

The problem covered by this complicated statement is that a reliable negative plastic moment in the beam-to-column connection zone in moment frames can only be based on reinforcement with guaranteed ductility, while the beam plastic moment considered in the capacity design of the column must include all possible contributions of the reinforcements, non-ductile welded mesh included. When duplication of non-ductile reinforcement by means of ductile reinforcement is realized, the capacity design of columns produces an overdesign of these columns.

In practice, the most economic solution can be obtained either by:

• using ductile welded mesh or

• avoiding the continuity of non-ductile reinforcements in dissipative zones; this can be done by using standard ductile rebars there, and by placing the overlap between ductile and non-ductile reinforcement away from the dissipative zone.

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