(1)P Concrete buildings shall be classified into one of the following structural types (see 5.1.2) according to their behaviour under horizontal seismic actions:
a) frame system;
b) dual system (frame or wall equivalent);
c) ductile wall system (coupled or uncoupled);
d) system of large lightly reinforced walls;
e) inverted pendulum system;
f) torsionally flexible system.
(2) Except for those classified as torsionally flexible systems, concrete buildings may be classified to one type of structural system in one horizontal direction and to another in the other.
(3)P A wall system shall be classified as a system of large lightly reinforced walls if, in the horizontal direction of interest, it comprises at least two walls with a horizontal dimension of not less than 4,0 m or 2hw/3, whichever is less, which collectively support at least 20% of the total gravity load from above in the seismic design situation, and has a fundamental period T1, for assumed fixity at the base against rotation, less than or equal to 0,5 s. It is sufficient to have only one wall meeting the above conditions in one of the two directions, provided that: (a) the basic value of the behaviour factor, qo, in that direction is divided by a factor of 1,5 over the value given in Table 5.1 and (b) that there are at least two walls meeting the above conditions in the orthogonal direction.
(4)P The first four types of systems (i.e. frame, dual and wall systems of both types) shall possess a minimum torsional rigidity that satisfies expression (4.1b) in both horizontal directions.
(5) For frame or wall systems with vertical elements that are well distributed in plan, the requirement specified in (4)P of this subclause may be considered as being satisfied without analytical verification.
(6) Frame, dual or wall systems without a minimum torsional rigidity in accordance with (4)P of this subclause should be classified as torsionally flexible systems.
(7) If a structural system does not qualify as a system of large lightly reinforced walls according to (3)P above, then all of its walls should be designed and detailed as ductile walls.
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5.2.2.2 Behaviour factors for horizontal seismic actions
(1)P The upper limit value of the behaviour factor q, introduced in 3.2.2.5(3) to account for energy dissipation capacity, shall be derived for each design direction as follows:
5 ,
w 1
o ≥
=q k
q (5.1)
where
qo is the basic value of the behaviour factor, dependent on the type of the structural system and on its regularity in elevation (see (2) of this subclause);
kw is the factor reflecting the prevailing failure mode in structural systems with walls (see (11)P of this subclause).
(2) For buildings that are regular in elevation in accordance with 4.2.3.3, the basic values of qo for the various structural types are given in Table 5.1.
Table 5.1: Basic value of the behaviour factor, qo, for systems regular in elevation
STRUCTURAL TYPE DCM DCH
Frame system, dual system, coupled wall system 3,0αu/α1 4,5αu/α1
Uncoupled wall system 3,0 4,0αu/α1
Torsionally flexible system 2,0 3,0
Inverted pendulum system 1,5 2,0
(3) For buildings which are not regular in elevation, the value of qo should be reduced by 20% (see 4.2.3.1(7) and Table 4.1).
(4) α1 and αu are defined as follows:
α1 is the value by which the horizontal seismic design action is multiplied in order to first reach the flexural resistance in any member in the structure, while all other design actions remain constant;
αu is the value by which the horizontal seismic design action is multiplied, in order to form plastic hinges in a number of sections sufficient for the development of overall structural instability, while all other design actions remain constant. The factor αu may be obtained from a nonlinear static (pushover) global analysis.
(5) When the multiplication factor αu/α1 has not been evaluated through an explicit calculation, for buildings which are regular in plan the following approximate values of αu/α1 may be used.
a) Frames or frame-equivalent dual systems.
− One-storey buildings: αu/α1=1,1;
− multistorey, one-bay frames: αu/α1=1,2;
− multistorey, multi-bay frames or frame-equivalent dual structures: αu/α1=1,3.
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b) Wall- or wall-equivalent dual systems.
− wall systems with only two uncoupled walls per horizontal direction: αu/α1=1,0;
− other uncoupled wall systems: αu/α1=1,1;
− wall-equivalent dual, or coupled wall systems: αu/α1=1,2.
(6) For buildings which are not regular in plan (see 4.2.3.2), the approximate value of αu/α1 that may be used when calculations are not performed for its evaluation are equal to the average of (a) 1,0 and of (b) the value given in (5) of this subclause.
(7) Values of αu/α1 higher than those given in (5) and (6) of this subclause may be used, provided that they are confirmed through a nonlinear static (pushover) global analysis.
(8) The maximum value of αu/α1 that may be used in the design is equal to 1,5, even when the analysis mentioned in (7) of this subclause results in higher values.
(9) The value of qo given for inverted pendulum systems may be increased, if it can be shown that a correspondingly higher energy dissipation is ensured in the critical region of the structure.
(10) If a special and formal Quality System Plan is applied to the design, procurement and construction in addition to normal quality control schemes, increased values of qo may be allowed. The increased values are not allowed to exceed the values given in Table 5.1 by more than 20%.
NOTE The values to be ascribed to qo for use in a country and possibly in particular projects in the country depending on the special Quality System Plan, may be found in its National Annex.
(11)P The factor kw reflecting the prevailing failure mode in structural systems with walls shall be taken as follows:
( )
≤ +
−
=
systems flexible
sionally tor
and equivalent -
wall wall, for 0,5, than less not but , 1 3 / 1
systems dual
equivalent frame
and frame for , 00 , 1
o
w α
k (5.2)
where αo is the prevailing aspect ratio of the walls of the structural system.
(12) If the aspect ratios hwi/lWi of all walls i of a structural system do not significantly differ, the prevailing aspect ratio αo may be determined from the following expression:
∑ ∑
= wi wi
o h / l
α (5.3)
where
hwi is the height of wall i; and
lwi is the length of the section of wall i.
(13) Systems of large lightly reinforced walls cannot rely on energy dissipation in plastic hinges and so should be designed as DCM structures.
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