Effective width of slabs

Tables 7.5.1 and 7.5.II EN 1998-1 for the effective width b,, of the slab in composite beams have been established for moment-resisting frames with rigid connections in which:

• The local plastic mechanism in dissipative zones is such that the integrity of the concrete slab is maintained during the seismic event. In fact, the values provided for be achieve that goal in the first stage of plastic rotations, but, as the rotations increase, some degradation of concrete adjacent to the column may occur. However, the resistance of the elements in bending is kept constant due to other factors: strain hardening of steel, other resistance mechanisms in the slab, strength of the concrete around the column higher than/cd due to confinement, etc.

• Yielding takes place at the bottom part of the steel section and/or in the rebars of the slab. Extensive experimental and numerical work provided the background to the Eurocode 8 values of effective width, and showed that they have to be different for the elastic and the ultimate (plastic) behaviour: the values for computing MRd are 2-3 times higher than those used to compute I. As detailed in Annex C of EN 1998-1, other influences have also been considered, such as the presence of 'transverse' beams (perpendicular to the beam for which an effective width is being defined), the type of façade transverse beam, the design of rebars connected to these façade beams, and the sign of the bending moments at beam ends.

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