Local ductility condition

(1)P For the required overall ductility of the structure to be achieved, the potential regions for plastic hinge formation, to be defined later for each type of building element, shall possess high plastic rotational capacities.

(2) Paragraph (1)P is deemed to be satisfied if the following conditions are met:

a) a sufficient curvature ductility is provided in all critical regions of primary seismic elements, including column ends (depending on the potential for plastic hinge formation in columns) (see (3) of this subclause);

b) local buckling of compressed steel within potential plastic hinge regions of primary seismic elements is prevented. Relevant application rules are given in 5.4.3 and 5.5.3;

c) appropriate concrete and steel qualities are adopted to ensure local ductility as follows:

- the steel used in critical regions of primary seismic elements should have high uniform plastic elongation (see 5.3.2(1)P, 5.4.1.1(3)P, 5.5.1.1(3)P);

- the tensile strength to yield strength ratio of the steel used in critical regions of primary seismic elements should be significantly higher than unity. Reinforcing steel conforming to the requirements of 5.3.2(1)P, 5.4.1.1(3)P or 5.5.1.1(3)P, as appropriate, may be deemed to satisfy this requirement;

- the concrete used in primary seismic elements should possess adequate compressive strength and a fracture strain which exceeds the strain at the maximum compressive strength by an adequate margin. Concrete conforming to the requirements of 5.4.1.1(1)P or 5.5.1.1(1)P, as appropriate, may be deemed to satisfy these requirements.

(3) Unless more precise data are available and except when (4) of this subclause applies, (2)a) of this subclause is deemed to be satisfied if the curvature ductility factor | of these regions (defined as the ratio of the post-ultimate strength curvature at 85% of the moment of resistance, to the curvature at yield, provided that the limiting strains of concrete and steel scu and ssu,k are not exceeded) is at least equal to the following values:

where qo is the corresponding basic value of the behaviour factor from Table 5.1 and T1 is the fundamental period of the building, both taken within the vertical plane in which bending takes place, and TC is the period at the upper limit of the constant acceleration region of the spectrum, according to 3.2.2.2(2)P.

NOTE Expressions (5.4) and (5.5) are based on the relationship between | and the displacement ductility factor, |s: | = 2|s -1, which is normally a conservative approximation for concrete members, and on the following relationship between |s and q: |s=q if T1>TC, |s=1+(q-1)TC/T1 if T1<TC (see also B5 in Informative Annex B). The value of qo is used instead of that of q, because q will be lower than qo in irregular buildings, recognising that a higher lateral resistance is needed to protect them. However, the local ductility demands may actually be higher than those corresponding to the value of q, so a reduction in the curvature ductility capacity is not warranted.

(4) In critical regions of primary seismic elements with longitudinal reinforcement of steel class B in EN 1992-1-1:2004, Table C.1, the curvature ductility factor | should be at least equal to 1,5 times the value given by expression (5.4) or (5.5), whichever applies.

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