Moment Resisting Frame

q > 4

lass 1

Steel Concentric Braced Frames

investigated by several researchers (e.g. Lay and Galambos, 1967; Kato and Akiyama, 1982; Kato, 1989).

In subsequent sections, the behaviour of the three ain configurations of steel frame structure, namely moment resisting, concentrically and eccentrically braced frames, is discussed. Whereas moment-frames exhibit relatively ductile behaviour under earthquake loading, their lo lateral stiffness may, in some situations, result in high storey drifts, thus leading to unacceptable damage to non-structural components and possible stability problems. On the other hand, concentrically braced frames may provide relatively higher stiffness, but can often suffer fro reduced ductility once the compression braces buckle. Eccentrically braced frames have the potential of providing adequate ductility as ell as stiffness, provided that the shear or bending links are carefully designed and detailed to ithstand the substantial inelastic demands that are imposed on these dissipative zones.

6.4 Moment resisting frames 6.4.1 Frame characteristics

Moment resisting frames (MRFs) are designed such that plastic hinges occur predominantly in beams rather than in columns (weak beam/strong column design) as shown in Figure 6.2. This provides favourable performance, compared to strong beam/weak column behaviour through which significant deformation and second order effects may arise in addition to the likelihood of premature storey collapse mechanisms. The only exception to this requirement is at the base of the ground floor columns, where plastic hinges may form.

Due to the spread of plasticity through flexural plastic hinges, MRFs usually possess high ductility as reflected in the high reference q assigned in

Figure 6.2 Weak beam/strong column and weak column/strong beam behaviour in moment-resisting frames

Weak Column Strong Beam

Figure 6.2 Weak beam/strong column and weak column/strong beam behaviour in moment-resisting frames

EC8. Nevertheless, due to their inherent low stiffness, lateral deformation effects need careful consideration.

6.4.2 Capacity design requirements

In EC8, the 'weak beam/strong column' concept is typically required, with plastic hinges allowed at the base of the frame, at the top floor of multistorey frames and for single-storey frames. The most recent version of EC 8 also allows dissipative zones to be located in the connections provided adequate behaviour can be demonstrated. Rules for moment-resisting frames are described mainly in Section 6.6 of EN 1998-1.

To obtain ductile plastic hinges in the beams, checks are made that the full plastic moment resistance and rotation is not reduced by coexisting compression and shear forces. To satisfy this for each critical section, the applied moment (MEd) should not exceed the design plastic moment capacity (MplRd) (i.e. MEJMplRd < 1.0), the applied axial force (NEd) should not exceed 15 per cent of the plastic axial capacity (NplRd) (i.e. NEJNpl Rd < 0.15). Also, the shear force (VEd) due to the application of the plastic moments with opposite signs at the extremities of the beam should not exceed 50 per cent of the design plastic shear resistance (VplRd) of the section (i.e. VEJVplM < 0.5, in which V, = V, „ + V,,.), where V, „ and V, „ are the shear forces due to

the gravity and moment components on the beam, respectively.

According to Section 6.6.3 of EN 1998-1, columns should be verified for the most unfavourable combination of bending moments MEd and axial forces N„,, based on:

MEd=MEdG+1.1govWMEdE NEd = NEd,G+1.1To„WNEd,E

where Q is the minimum overstrength in the connected beams (Q. =MplM / MEd ). The parameters MEdG and MEd are the bending moments in the seismic design situation due to the gravity loads and lateral earthquake forces, respectively, as shown in Figure 6.3 (Elghazouli, 2007); the same subscripts also apply for axial and shear actions. dditionally, the ost unfavourable

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