Criterion for the formation of a global plastic mechanism

Clauses 6.7.3(5), The design requirement for diagonals in clause 6.7.3(5) simply expresses the need for a 6.7.4(1) section able to take the computed action effect NFd: Npl Rd > /VEd. To form a globally dissipative bracing structure, which means achieving a global plastic mechanism in which yielding affects a significant number of diagonals, two conditions must be fulfilled:

(1) The beams and columns have to be capacity designed to the real strength of the diagonals. This prevents pre-emptive yielding in non-dissipative elements, such as columns.

(2) The design criterion should give to each diagonal a significant probability of yielding under seismic loading.

To fulfil condition 1, beams and columns are capacity designed according to the real yield strength of the diagonals, for bending with axial force NEd given by equation (D6.1) However, the ratios Qi = Npi Rd ¡/Nm i are normally different in each diagonal, as NUcl; or /Vpl Rd i are different. In fact, due to the heightwise distribution of the seismic shear in the structure, NEi decrease from the bottom to the top of the frame. So, if all the diagonals have the same section, yielding will take place only in the diagonals of the first storey, possibly reaching the second storey due to strain hardening in the ground storey diagonals. In this case, a 'soft-storey' type of mechanism will form (Fig. 6.14). Clause 6.7.3(8) In order to force the formation of a global mechanism, a second condition is imposed, which aims to fit better the distribution of diagonal strengths ATIjRd to the distribution of computed action effect NEi. The ratios = AT (> Rdti/NEdwhich define the section overstrength of diagonals with respect to their minimal required strength (A^ ¡) may not vary widely over the height of the structure. Q considered in equation (D6.1) being the minimum J?„ the values of all other Qts should be in the range Q to 1.25i7. In practice, this condition forces the use of different sections of diagonals over the height of the structure.

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