Scope

This chapter covers the design of concrete buildings for earthquake resistance according to Clause 5.1.1 the provisions of Section 5 of EN 1998-1. It summarizes the important points of Section 5 without repeating them, and provides comments and explanations for their application, as well as background information.

The scope of Section 5 in EN 1998-1 covers buildings made of cast-in-place or precast concrete. It is stated clearly in Section 5 that its provisions do not fully cover buildings in which 'flat slab frames' (i.e. frames of columns connected through flat slabs, instead of beams) are used as primary seismic elements. In such frames, strips of the flat slab between columns act and behave like beams in the event of an earthquake. The effective width of such strips increases with the magnitude of the seismic demands, as measured in this case by interstorey drift; nonetheless, it is very uncertain. There is also large uncertainty about the behaviour of these strips under inelastic cyclic loading, and especially of the regions around the columns. Irrespective of this uncertainty, the stiffness and flexural capacity of these strips is relatively low compared with the columns, conducive to a beam mechanism with column plastic hinging only at the base, as in a strong-column-weak-beam design. However, due to the flexibility of the strips of the flat slab that act like beams, such frames may develop large second-order (P-A) effects.

Although not explicitly excluded from the scope of Section 5, the use of prestressing in primary seismic elements is not fully covered in EN 1998-1. In buildings, prestressing could conceivably be used to advantage in long-span primary seismic beams. However, it is mainly at the ends of beams that plastic hinges are expected to form in the event of an earthquake, and Section 5 indeed gives rules for the design and detailing of the end regions of primary seismic beams for ductility and energy dissipation. These rules are limited to reinforced concrete beams, hence the implicit exclusion of the use of prestressing in primary seismic elements.

Concrete buildings designed according to Section 5 for energy dissipation may include flat slabs or prestressed concrete beams, provided that these elements as well as the columns connected to them are considered and designed as secondary seismic elements. As an alternative, concrete buildings with flat slabs or prestressed concrete beams may be designed considering all elements as primary seismic ones, but for almost fully elastic response under the design seismic action, i.e. for Ductility Class Low (DCL) and a value of the behaviour factor q of not more than 1.5. It should be recalled, though, that this alternative is recommended in EN 1998-1 only for low-seismicity regions.

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