ABSTRACT
Fundamental decisions taken at the initial stages of planning a building structure usually play a crucial role in determining how successfully the finished building achieves its performance objectives in an earthquake. This chapter describes how EC8 sets out to guide these decisions, with respect to siting considerations, foundation design and choice of superstructure. 4.2 Fundamental principles
4.2.1 Introduction
In EC8, the fundamental requirements for seismic performance are set out in Section 2. There are two main requirements. The first is to meet a ‘no collapse’ performance level, which requires that the structure retains its full vertical load bearing capacity after an earthquake with a recommended return period of 475 years; longer return periods are given for special structures, for example casualty hospitals or high risk petrochemical installations. After the earthquake, there should also be sufficient residual lateral strength and stiffness to protect life even during strong aftershocks. The second main requirement is to meet a ‘damage limitation’ performance level, which requires that the cost of damage and associated limitations of use should not be disproportionately high, in comparison with the total cost of the structure, after an earthquake with a recommended return period (for normal structures) of 95 years. Note that Section 2 of EC8 (and hence these basic requirements) applies to all types of structure, not just buildings.EC8’s rules for meeting the ‘no collapse’ performance level in buildings are given in Section 4 of Part 1 with respect to analysis procedures and in Sections 5 to 9 of Part 1 with respect to material specific procedures to ensure sufficient strength and ductility in the structure. The rules for meeting the ‘damage limitation’ performance level in buildings are given in Section 4 of Part 1; they consist of simple restrictions on deflections to limit structural
and non-structural damage, and some additional rules for protecting non-structural elements.EC8 Part 1 Section 4.2.1 sets out some aspects of seismic design specifically for buildings, which should be considered at conceptual design stage, and which will assist in meeting the ‘no collapse’ and ‘damage limitation’ requirements. It is not mandatory that they should be satisfied, and indeed since they are qualitative in nature, it would be hard to enforce them, but they are sound principles that deserve study. Related, but quantified, rules generally appear elsewhere in EC8; for example, the structural regularity rules in Section 4.2.3 supplement the uniformity and symmetry principles given in Section 4.2.1. Six guiding principles are given EC8 Part 1 Section 4.2.1 as follows, and these are now discussed in turn. • Structural simplicity. • Uniformity, symmetry and redundancy. • Bi-directional resistance and stiffness. • Torsional resistance and stiffness. • Adequacy of diaphragms at each storey level. • Adequate foundations. 4.2.2 Structural simplicity
This entails the provision of a clear and direct load path for transmission of seismic forces from the top of a building to its foundations. The load path must be clearly identified by the building’s structural designer, who must ensure that all parts of the load path have adequate strength, stiffness and ductility.Direct load paths will help to reduce uncertainty in assessing both strength and ductility, and also dynamic response. Complex load paths, for example involving transfer structures, tend to give rise to stress concentrations and make the assessment of strength, ductility and dynamic response more difficult. Satisfactory structures may still be possible with complex load paths but they are harder to achieve. 4.2.3 Uniformity, symmetry and redundancy
Numerous studies of earthquake damage have found that buildings with a uniform and symmetrical distribution of mass, strength and stiffness in plan and elevation generally perform much better than buildings lacking these characteristics.Uniformity in plan improves dynamic performance by suppressing torsional response, as discussed further in Section 4.2.5 below. Irregular or asymmetrical plan shapes such as L or T configurations may be improved by dividing the building with joints to achieve compact, rectangular shapes (Figure 4.1), but this introduces a number of design issues that must be
solved; these are avoiding ‘buffeting’ (impact) across the joint, and detailing the finishes, cladding and services that cross the joint to accommodate the associated seismic movements.Uniformity of strength and stiffness in elevation helps avoid the formation of weak or soft storeys. Non-uniformity in elevation does not always lead to poor performance, however; for example, seismically isolated buildings are highly non-uniform in elevation but are found to perform very well in earthquakes.Redundancy implies that more than one loadpath is available to transmit seismic loads, so that if a particular loadpath becomes degraded in strength or stiffness during an earthquake, another is available to provide a backup. Redundancy should therefore increase reliability, since the joint probability of two parallel systems both having lower than expected capacity (or greater than expected demand) should be less than is the case for one system separately. Redundant systems, however, are inherently less ‘simple’ than determinate ones, which usually makes their assessment more complex. 4.2.4 Bi-directional resistance and stiffness
Unlike the situation that often applies to wind loads on buildings, seismic loads are generally similar along both principal horizontal axes of a building. Therefore, similar resistance in both directions is advisable. Systems such as cross-wall construction found in some hotel buildings, where there are many partition walls along the short direction but fewer in the long direction,
work well for wind loading, which is greatest in the short direction, but tend to be unsatisfactory for seismic loads. 4.2.5 Torsional resistance and stiffness
Pure torsional excitation in an earthquake may arise in a site across which there is significantly varying soils, but significant torsional excitations on buildings are unusual. However, coupled lateral-torsional excitation, arising from an eccentricity between centres of mass and stiffness, is common and is found to increase damage in earthquakes. Such response may be inadequately represented by a linear dynamic analysis, because yielding caused by lateral-torsional response can reduce the stiffness on one side of a building structure and further increase the eccentricity between mass and stiffness centres.Minimising the eccentricity of mass and stiffness is one important goal during scheme design, and achieving symmetry and uniformity should help to satisfy it. However, some eccentricity is likely to remain, and may be significant due to a number of effects that may be difficult for the structural designer to control; they may arise from uneven mass distributions, uneven stiffness contributions from non-structural elements or non-uniform stiffness degradation of structural members during a severe earthquake. Therefore, achieving good torsional strength and stiffness is an important goal. Stiff and resistant elements on the outside the building, for example in the form of a perimeter frame, will help to achieve this, while internal elements, such as a central core, contribute much less. Quantified rules are provided later in Section 4 of EC8 Part 1, as discussed in Section 4.5 of this chapter. 4.2.6 Adequacy of diaphragms at each storey level
Floor diaphragms perform several vital functions. They distribute seismic inertia loads at each floor level back to the main vertical seismic resisting elements, such as walls or frames. They act as a horizontal tie, preventing excessive relative deformations between the vertical elements, and so helping to distribute seismic loads between them. In masonry buildings, they act to restrain the walls laterally. At transfer levels, for example between a podium and a tower structure, they may also serve to transfer global seismic forces from one set of elements to another.Floor diaphragms that have very elongated plan shapes, or large openings, are likely to be inefficient in distributing seismic loads to the vertical elements. Precast concrete floors need to have adequate bearing to prevent the loss of bearing and subsequent floor collapse observed in a number of earthquakes. In masonry buildings, it is especially important to ensure a good connection between floors and the masonry walls they bear onto in order to provide lateral stability for the walls.
