The awareness of favourable and unfavourable factors for energy dissipation, which are Clause 6.2(2) listed in Section 6.4 below, allows the design of reliable dissipative zones. It remains to ensure that energy dissipation takes place in dissipative zones and not in 'brittle' ones. This is achieved through capacity design.
In the capacity design concept, all 'brittle' structural elements or components are protected against failure, by providing them with a strength greater than that corresponding to the development of the maximum feasible strength in the plastic regions.
The following features characterize the procedure:
• potential plastic regions within the structure are clearly defined and designed to have dependable strengths
• potentially 'brittle' regions or those components not suited for stable energy dissipation are protected, by ensuring that their strength exceeds the demands originating from the plastic regions.
To highlight the concept of capacity design, the chain shown in Fig. 6.2 is often considered.68 As the strength of a chain is the strength of its weakest link, one ductile link may be used to achieve ductility for the entire chain. The nominal tensile strength of the ductile link is subject to uncertainties of material strength and strain hardening effects at high strains. The other links are presumed to be 'brittle', but their failure can be prevented if their strength is in excess of the real strength of the ductile weak link at the level of ductility envisaged.
As detailed below, sound application of the capacity design principle thus requires knowledge of the material properties, in particular of the yield stress, both of the plastic zones and of the neighbouring ones.
Capacity design develops a hierarchy of strength. To be effective, the real strength of the 'brittle' parts and of the ductile parts must be under control. For the 'brittle' parts, the standard nominal yield stress / is a lower bound which guarantees that those parts have at least the design strength needed to remain elastic. For ductile zones, however, the yield strength of the material must be limited by an upper bound value, in addition to the standard minimum value fy corresponding to the grade of steel. This raises practical problems, because the definition of an upper bound yield stress is not a usual practice for steel products and also because of a lack of statistical data on that subject. Eurocode 8 tackles this problem considering different-possible circumstances.
Case (a) in clause 6.2(3) refers to the standard situation, in which the real yield stress of Clause 6.2(3) the material of dissipative zones is not known at the design stage. Then, the upper bound yield stress is estimated as 70v/y; 70V is a coefficient based on statistics of yield stresses characterizing steel products, which may vary from one steel plant to another. For European rolled sections, the estimate is 7ov = 1.25, but the designer may choose a different value, larger or smaller. Then, the upper bound of the actual yield stress of the material provided at the construction stage for dissipative zones should not be greater than/ M = 1 •l7ov/j„ in which 1.1 provides an additional margin.
If the actual yield strengths of the materials to be used in the construction are accessible at the design stage, then a real value of 70V can be used as 70V act =/ ac//y, as stated in case (c) of clause 6.2(3).
Case (b) in clause 6.2(3) refers to a situation in which steel producers would provide the market with a 'seismic' steel grade for which both a nominal (lower bound) value and an upper bound value / max are defined. If the design of all sections is made considering only that one 'seismic' steel grade and if the steel for the non-dissipative parts is specified to belong to a higher grade than the 'seismic' one, then the general hierarchy criterion is automatically fulfilled. This would, for example, be the case if an S235 steel with fy, max= 355 MPa is a 'seismic' grade used for dissipative zones, with S355 steel used for non-dissipative parts or members. In such a case, there is no need for 70V, and its value can be set equal to 1.
Sound application of the capacity design principle requires, in addition to knowledge of the material properties, notably the yield stress, of both the plastic and neighbouring zones, a correct evaluation of the stresses, and strains sustained by the various components of the plastic zones: steel profiles, welds, bolts and plates. In or near connections, the real distribution of stresses and strains are very different from what they are in beams, columns, etc., and can only be defined by sophisticated numerical or experimental studies. There are several reasons for this: plane sections do not remain plane, the presence of stress concentrations, etc. Without such sophisticated studies, safety is based on simple estimates by means of the 'overstrength design factor', equal to 1.1 in the relationship/, max = I.I7mf
Sound application of the capacity design principle also requires adequate materials for the plastic zones, where 'adequate' refers to the required properties: elongation,/,//, toughness and weldability. It also requires good design of the plastic zones, avoiding in particular a localization of strains. This requirement is explained in Section 6.7.
6.4. Design for local energy dissipation in the elements and their connections
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