Flat slabs as primary seismic elements in second-generation Eurocode 8

Michael N. Fardis

doi:10.1007/s10518-022-01352-9

Flat slabs as primary seismic elements in second-generation Eurocode 8

2022-03-05

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Michael N. Fardis *

DOI：10.1007/s10518-022-01352-9

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Over eleven hundred tested slab-column joints are used to address four issues in seismic design or assessment: (a) the slab stiffness for the analysis; (b) the capacity-design column moments on a slab; (c) the slab's cyclic punching shear resistance; and (d) its cyclic rotation capacity and ductility. Equivalent beams between adjacent columns, with the reinforcement ratios of the support strip and non-overlapping width extending on each side into the slab beyond the column's transverse dimension by up to 25% of the clear span, have on average the same secant-to-yield stiffness as the flat slab. Applied to them, the 50%-of-uncracked-stiffness approximation on average overestimates the secant-to-yield stiffness of the slab by the same ratio as for other types of RC members. The moment from the column for punching shear verifications should be taken as at least 75% of the hogging moment resistance of the slab's full tributary width at edge joints, or 80% of the sum of the hogging and sagging ones at interior joints, or 1.25-times the sum of sagging and hogging moment resistances of the proposed equivalent beams (or their hogging moment resistance at edge joints). The punching shear resistance model in (draft) second-generation Eurocode 2 agrees well on average with cyclic tests, if the linear approximation of biaxial eccentricity at corner or edge joints is abandoned. A variant of the Annex I approach in Eurocode 2 is proposed for the cyclic punching resistance: it is very simple and agrees well with tests. If protected from shear failure, flat-slabs have ductility and cyclic deformation capacity consistent with the predictions of Eurocode 8 for ductile prismatic members and large enough for design as primary seismic elements with a mean chord-rotation ductility factor of about 3. This value translates to an upper limit of 2 for the behaviour factor of buildings having flat-slab-frames as primary seismic elements, supplemented with walls or stiff beam-column frames to limit second-order effects and interstorey drifts. Such systems should not be used in Ductility Class 3 (or High).

Keyword:

Capacity design

Eurocode 8

Flat slabs

Punching shear

Reinforced concrete buildings

Seismic design

Flat slabs as primary seismic elements in second-generation Eurocode 8

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