Uniformity symmetry and redundancy

Uniformity, symmetry and redundancy are related characteristics which are normally correlated to structural simplicity

The advantage of structural uniformity in the seismic design context is that it allows the inertial forces created in the distributed masses of the building to be transmitted via short and direct paths, avoiding longer or indirect paths.

Structural uniformity of the building should be sought both in plan and in elevation. To achieve plan uniformity (and symmetry), it may be useful to subdivide the entire building into more uniform structural blocks through the use of seismic joints. These blocks will behave as dynamically independent units, but it should be checked that pounding of individual units is prevented, by providing appropriate width to these joints (as indicated in clause 4.4.2.7 of EN 1998-1).

Furthermore, the in-plan uniformity of a building structure should, in most cases, be in line with the more or less uniform distribution of floor masses that occurs in buildings. This close relationship between the distribution of structural elements and masses will thus tend to eliminate large eccentricities.

The symmetrical or quasi-symmetrical distribution of the structural elements in plan is also a very positive feature for the seismic response of buildings because it decouples the vibration modes of the building in two independent horizontal directions, and thus its response to the seismic excitation is much simpler and less prone to torsional effects.

On the other hand, uniformity of the building structure in elevation tends to eliminate the occurrence of large variations in the ratio between demand and resistance among the different vertical structural elements and thus avoids the appearance of sensitive zones where concentrations of stress or large ductility demands might prematurely cause collapse.

Finally, the use of evenly distributed structural elements increases redundancy and allows a more favourable redistribution of action effects and widespread energy dissipation across the entire structure.

Clauses

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