Introduction and scope

Field experience and analytical and experimental research have demonstrated the overall Clause 2.2.2(6)

beneficial effect of masonry infills attached to the structural frame on the seismic performance of buildings, especially when the building structure has little engineered earthquake resistance.

If they are effectively confined by the surrounding frame, infill panels reduce, through their in-plane shear stiffness, storey drift demands, increase, through their in-plane shear strength, the storey lateral force resistance and contribute, through their hysteresis, to the global energy dissipation capacity.58 In buildings designed for earthquake resistance, non-structural masonry infills normally constitute a second line of defence and a source of significant overstrength. EN 1998-1 adopts this attitude, and does not encourage the designer to reduce the earthquake resistance of the structure to account for the beneficial effects of masonry infills.

If the contribution of masonry infills to the lateral strength and stiffness of the building is large relative to that of the structure itself, the infills may override the seismic design of the structure and invalidate both the efforts of the designer and the intention of Eurocode 8 to control the inelastic response by spreading the inelastic deformation demands throughout the structure and the building. For instance, loss of integrity of ground storey infills will produce a soft storey there, and may trigger collapse of the structural frame itself. Concentration of inelastic deformation demands in a small part of the building is much more likely if the infills are not uniformly distributed in plan or - more importantly - in elevation. This situation may also have serious adverse effects on seismic performance and safety. Last but not least, the infills may have adverse local effects on the structural frame, possibly causing pre-emptive brittle failures. It is against such local or global adverse effects that EN 1998-1 strives to provide safeguards, in the form of guidance to the designer or even mandatory rules.

Clauses The rules of EN 1998-1 for buildings with masonry infills are mandatory when the, structure itself is designed for relatively low lateral force stiffness and strength but for high, ductility and deformation capacity. This is the case for unbraced moment frame systems (in (4) concrete, also of frame-equivalent dual systems) designed for DCH, i.e. for high ductility and a high value of the q factor. Structural systems of lower ductility class (DCL or DCM) are considered as designed for lateral strength which is sufficient to overshadow that of infill walls. Steel or composite frames with concentric or eccentric bracings and concrete wall (or wall-equivalent dual) systems are also considered as stiff enough not to be affected by the presence of masonry infills. For these two categories of structural systems, the safeguards specified by Eurocode 8 against the negative effects of infill walls are not mandatory; however, the designer is advised to consider them as guidance for good practice. Clause (5) If structural connection is provided between the masonry and the surrounding frame members (through shear connectors, or other ties, belts or posts), then the structure should be considered and designed as a confined masonry building, rather than as a concrete, steel or composite frame with masonry infills.

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