Dissipative versus lowdissipative structures

Steel buildings can be designed to be 'energy-dissipative' to a larger or smaller extent. As Clauses 6.1.2(1), also explained in Section 2.2.2.1 of this guide, this term refers to the ability of some 6.1.2(2), intentionally selected parts of the structure to safely undergo cyclic plastic deformations, and 6.1.2(4), applies to buildings designed such that the selected zones - and only those - are indeed 6.1.2(5), activated plastically. Then, the global behaviour of the building under seismic loads is 6.1.2(6)

characterized by a load-displacement curve with a significant 'yield plateau' - curve b in the base shear F-displacement d diagram of Fig. 6.1 - rather than by a 'brittle' behaviour - curve a in Fig. 6.1. This ductility aspect of behaviour, which is related to providing a deformation capacity and not only strength, is less prevalent in the design of structures for non-seismic actions.

In what follows, the term 'brittle', intentionally enclosed within quotation marks, has the meaning 'with little or no deformation capacity after reaching the maximum strength'. The meaning is not restricted to the classical meaning of a brittle phenomenon in steel structures (inability to prevent a crack from propagating), which is just one possibility of being 'brittle'.

As already detailed in Section 2.2.2.1 of this guide, structures designed to concept b, i.e. for ductile behaviour, are given a design premium in the sense that they are allowed to be less stiff and less resistant than those designed to concept a. This is achieved by a reduction in the elastic earthquake lateral forces by a factor q (> 1) which is higher for dissipative structures (up to q = 6.5 in steel moment frames). The value of q is different for different structural types, depending on their ability to dissipate energy. Only one structural type is mentioned in Section 6 as unable to be dissipative: frames with K bracings, as explained in Section 6.8.

Regarding connections, these can be of either the full-strength or the partial-strength type, and certain conditions need to be met for both options.

There is a price to pay to be allowed to consider reduced design seismic forces in a building of reduced strength: the structural elements and their connections have to comply with all the Eurocode 8 requirements.

Designing for a reduced earthquake action should in principle generate a more economic structure. This seemingly straightforward conclusion may be wrong in some cases, because a design does not have to comply with the seismic requirements only but with all design requirements, such as the limitation of floor vertical deflection under gravity loading, interstorey drift limits and resistance to wind. As the design checks corresponding to the ultimate limit state (ULS) resistance under the design earthquake are not necessarily the most controlling ones, the complete design process may generate a structure with more strength than strictly needed for the resistance to the design earthquake. Then, some specific Eurocode 8 requirements for ductility, such as the 'overstrength design' of connections relatively to the strength of connected members, or the 'overstrength design' of columns relatively to beams, apply to structural components stronger than strictly necessary for the resistance to earthquakes, leading to a general over-design of the structure. In that case, the option of designing a dissipative structure may not be economic. This is often the case:

• in low seismicity areas

• when a structural system is used that lends itself more for resistance than for ductility, e.g. systems using thin-walled profiles or using partial strength connections

• for flexible structures, in which the serviceability limit states (SLSs) are the controlling ones.

Designing a dissipative structure is certainly uneconomic if the wind resultant force is greater than the earthquake resultant force obtained with the minimum code value of the q factor (q = 1.5). Whether this is the case can be approximately assessed as follows at the pre-project stage. The maximum earthquake horizontal resultant force is estimated as Fb = mSd(T)X (cf. equation (4.5) and equation (D4.5) in Section 4.5.2.3). Without knowledge

Concept a Concept b
Fig. 6.1. Definition of design concepts a and b

Ductile link i ('fuse') Other links Computed design action effect Ed: E6I Ed/ (here Ed/ = Edj)

Design resistance required fid: fldi>Edi y(RdJ/Edi) Ed;

Ductile link i ('fuse') Other links Computed design action effect Ed: E6I Ed/ (here Ed/ = Edj)

Design resistance required fid: fldi>Edi y(RdJ/Edi) Ed;

Fig. 6.2. Principle of capacity design of the structure period and of the soil, a safe-side approach uses the maximum spectral ordinate 2.5agSm/q (clause 3.2.2.5) for the most adverse conditions, S = 1.5, q = 1.5 and A = 1, so that Fb = 2.5agm is an estimate of the earthquake horizontal resultant force Fh to be compared with the wind resultant force Fw. If Fh < Fw, designing for dissipative structural behaviour is uneconomic.

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