US: Tel: 1-206-279-3300
EU: Tel: +30 6986 007 252

Structures with irregularities in plan or in elevation present special ductility demands in certain locations contrary to the general demand of uniform ductility distribution in normal buildings.

Criteria for regularity

A. Criteria for regularity in elevation

EC8(§4.2.3.3) considers as regular in elevation the buildings that satisfy all the following conditions:

• All lateral load resisting systems, such as cores, structural walls, or frames, shall run without interruption from their foundations to the top of the building or, if setbacks at different heights are present, to the top of the relevant zone of the building.
• Both the lateral stiffness and the mass of the individual storeys shall remain constant or reduce gradually, without abrupt changes, from the base to the top of a particular building.
• In framed buildings the ratio of the actual storey resistance to the resistance required by the analysis should not vary disproportionately between adjacent storeys.
• for gradual setbacks preserving axial symmetry, the setback at any floor shall be not greater than 20 % of the previous plan dimension in the direction of the setback.
• for a single setback within the lower 15 % of the total height of the main structural system, the setback shall be not greater than 50 % of the previous plan dimension (see Figure 1.c). In this case the structure of the base zone within the vertically projected perimeter of the upper storeys should be designed to resist at least 75% of the horizontal shear forces that would develop in that zone in a similar building without the base enlargement;
• if the setbacks do not preserve symmetry, in each face the sum of the setbacks at all storeys shall be not greater than 30 % of the plan dimension at the ground floor above the foundation or above the top of a rigid basement, and the individual setbacks shall be not greater than 10 % of the previous plan dimension (see Figure 1.d).

The irregularity in elevation is expected to have more severe effects in the design and in the seismic response of the building rather than the irregularity in plan. So:

• the static analysis with (equivalent) horizontal seismic loads is allowed to be applied only in buildings that are regular in elevation, and their basic period satisfies for the two main directions the equations:   Τ≤ 2 sec and Τ≤ 4Τ_C.

In buildings irregular in elevation, the behavior factor q is reduced 20% ο in comparison with buildings that are regular in elevation.

Figure 1: Criteria of regularity in elevation for buildings with setbacks (EC8 §4.2.3.3)

B. Criteria of regularity in plan

EC8(§4.2.3.2) considers as regular in plan the buildings that satisfy all the following conditions:

• With respect to the lateral stiffness and mass distribution, the building structure shall be approximately symmetrical in plan with respect to two orthogonal axes
• The plan configuration shall be compact, i.e., each floor shall be delimited by a polygonal convex line. If in plan set-backs (re-entrant corners or edge recesses) exist,  regularity in plan may still be considered as being satisfied, provided that these setbacks do not affect the floor in-plan stiffness and that, for each set-back, the area between the outline of the floor and a convex polygonal line enveloping the floor does not exceed 5 % of the floor area.
• The in-plan stiffness of the floors shall be sufficiently large in comparison with the lateral stiffness of the vertical structural elements.
• At each level and for each direction of analysis x and y, the structural eccentricity eo and the torsional radius r shall be in accordance with the two conditions below, which are expressed for the direction of analysis y (EC8 § 4.2.3.2(6)):

0.3r_x ≥ e_x and 0.3r_y ≥ e_y

r_x ≥ l_s and r_y ≥ l_s

where the radius of gyration of one floor in the two directions can be calculated from the polar moment of inertia of the floor mass in plan as:

Figure 1.2: Design response spectrum according to EC8 (Spectrum Type 1, soft soils Type C, Elghazouli, 2009).

FREE VERSION OF RCsolver Please sign up to download the free version of RCsolver and use all the capabilities of our software for free! The free version of RCsolver contains advertisements. It is a full version, containing all features. Easily upgrade to the full version without advertisements only for 150 USD! Download RCsolver and join the world of Concrete Design according to Eurocodes 2 and 8 and ACI 318-11 with an impressive and modern software! RCsolver: Learn more Advertise with RCsolver Feel free to contact us at info@rcsolver.com for more information, pricing options and special offers!