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Accidental Eccentricities

User_795178 2017-07-17 16:27:08

can you please explain the three different options for the accidental eccentricities shown in the image?



TonyC 2017-07-17 17:48:08

The meaning of these three options is as follows:

For symmetrical structures
The effect of accidental eccentricity is evaluated for the only verticalyl resistant elements (columns, walls, shear walls, etc) by amplifying the stresses on each element by means of the factor (δ) in relation (4.12) of UNI EN 1998-1:2005.
This criterion can be used for buildings with a symmetrical distribution of lateral rigidity and masses in the plan view.

For generic structures with Torque
The effect of accidental eccentricity is evaluated by introducing a torque effect on the nodes, the result of which will be equal to the moment induced by the seismic plane by the distance due to the displacement of the center of mass (see image With Torque).
This more general criterion can be used for concrete and/or masonry buildings as they have vertical structures with high torsional rigidity. It is, however, less suitable for use in steel or wood-framed structures as inserting a torque couple on the structure nodes to simulate the effects of a center of mass displacement of +5% of the transversal distance in direction of the seismic force, may introduce torsional stresses in those structural members that are not capable of being contasted by the section inertia.

For generic structures with forces
The effects of accidental eccentricity is evaluated by introducing a system of forces which are statically null, directed in the direction of the earthquake, which generate with respect to the center of gravity, a total torque equal to the moment produced by the seismic Plane for moving the center of mass due to accidental eccentricity (see image With Forces).
This criterion can be used for steel or wood-framed buildings (or even reinforced concrete if very flexible) as mentioned earlier, they reduce or completely eliminate the stressing forces on the structural elements.