Elevation Regularity Eurocode

Clauses There is plenty of evidence from damage observation after earthquakes that regular buildings

4.2.3.1(1), tend to behave much better than irregular ones. However, a precise definition of what is a

4.2.3.1(2), regular structure in the context of the seismic response of buildings has eluded many

4.2.3.1(3) attempts to achieve it. There are so many variables and structural characteristics that may (or should) be considered in such a definition that the classification of a building as 'regular' is, in the end, mostly intuitive. EN 1998-1 recognizes this difficulty, and does not attempt to establish very strict rules for the distinction between regular and non-regular buildings. Rather, it provides a relatively loose set of characteristics that a building structure should possess to be classified as regular. This classification serves the purpose of, essentially, establishing some distinctions regarding concerns relating to the more or less simplified structural model and the method of analysis to be used as well as in concerns relating to the value of the behaviour factor.

With this approach, EN 1998-1 does not forbid the design and construction of non-regular structures but, rather, attempts to encourage the choice of regular structures both by making it easier to design them and also by making them more economic (as a consequence of using in such cases higher values of the behaviour factor).

As in most other modern seismic design codes, the concept of building regularity in EN 1998-1 is presented with a separation between regularity in plan and regularity in elevation. Moreover, regularity in elevation is considered separately in the two orthogonal directions in which the horizontal components of the seismic action are applied, meaning that the structural system may be characterized as regular in one of these two horizontal directions but not in the other. However, a building assumes a single characterization for regularity in plan, independent of direction.

In order to reduce stresses due to the constraint of volumetric changes (thermal, or due to concrete shrinkage), buildings which are long in plan often have their structure divided, by means of vertical expansion joints, into parts that can be considered as separate above the level of the foundation. The same practice is recommended in buildings with a plan shape consisting of several (close to) rectangular parts (L-, C-, H-, I- or X-shaped plans), for reasons of clarity and predictability of their seismic response (see points 2 and 3 in Section 4.3.2.1). The parts into which the structure is divided through such joints are considered as 'dynamically independent'. Structural regularity is defined and checked at the level of each individual dynamically independent part of the building structure, regardless of whether these parts are analysed separately or together in a single model (which might be the case if they share a common foundation, or if the designer considers a single model as convenient for comparing the relative displacements of adjacent units to the width of the joint between them).

Unlike US codes (e.g. see Building Seismic Safety Council39 and Structural Engineers Association of California40), which set quantitative - albeit arbitrary - criteria for regularity:

• in plan, on the basis of the planwise variation of floor displacements as computed from the analysis

• in elevation, based on the variation of mass, stiffness and strength from storey to storey.

Eurocode 8 introduces qualitative criteria, which can be checked easily at the preliminary design stage by inspection or through simple calculations, without doing an analysis. This makes sense, as the main purpose of the regularity classification is to determine what type of linear analysis maybe used for the design: in three dimensions (3D), using a spatial model, or in two dimensions (2D), using two separate planar models, depending on the regularity in plan; and static, using equivalent lateral forces, or response spectrum analysis, depending on the regularity in elevation. So, it does not make sense to first do an analysis to find out what type of analysis is allowed to be used at the end. Moreover, the regularity in plan and in elevation affects the value of the behaviour factor q that determines the design spectrum used in linear analysis.

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Responses

  • Elvia
    What does regularity in plan and elvation in structure?
    2 years ago
  • autumn
    What is irregularity and regularity in plane and elevation mean?
    1 year ago

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