(1)P Steel buildings shall be assigned to one of the following structural types according to the behaviour of their primary resisting structure under seismic actions (see Figures 6.1 to 6.8).
a) Moment resisting frames, are those in which the horizontal forces are mainly resisted by members acting in an essentially flexural manner.
b) Frames with concentric bracings, are those in which the horizontal forces are mainly resisted by members subjected to axial forces.
c) Frames with eccentric bracings, are those in which the horizontal forces are mainly resisted by axially loaded members, but where the eccentricity of the layout is such that energy can be dissipated in seismic links by means of either cyclic bending or cyclic shear.
d) Inverted pendulum structures, are defined in 5.1.2, and are structures in which dissipative zones are located at the bases of columns.
e) Structures with concrete cores or concrete walls, are those in which horizontal forces are mainly resisted by these cores or walls.
f) Moment resisting frames combined with concentric bracings.
g) Moment resisting frames combined with infills.
(2) In moment resisting frames, the dissipative zones should be mainly located in plastic hinges in the beams or the beam-column joints so that energy is dissipated by means of cyclic bending. The dissipative zones may also be located in columns:
− at the base of the frame;
− at the top of the columns in the upper storey of multi-storey buildings;
− at the top and bottom of columns in single storey buildings in which NEd in columns conform to the inequality: NEd / Npl,Rd < 0,3.
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(3) In frames with concentric bracings, the dissipative zones should be mainly located in the tensile diagonals.
The bracings may belong to one of the following categories:
− active tension diagonal bracings, in which the horizontal forces can be resisted by the tension diagonals only, neglecting the compression diagonals;
− V bracings, in which the horizontal forces can be resisted by taking into account both tension and compression diagonals. The intersection point of these diagonals lies on a horizontal member which shall be continuous.
K bracings, in which the intersection of the diagonals lies on a column (see Figure 6.9) may not be used.
(4) For frames with eccentric bracings configurations should be used that ensure that all links will be active, as shown in Figure 6.4.
(5) Inverted pendulum structures may be considered as moment resisting frames provided that the earthquake resistant structures possess more than one column in each resisting plane and that the following inequality of the limitation of axial force: NEd< 0,3 Npl, Rd is satisfied in each column.
a) b) c)
Figure 6.1: Moment resisting frames (dissipative zones in beams and at bottom of columns). Default values for αu/α1 (see 6.3.2(3) and Table 6.2).
Figure 6.2: Frames with concentric diagonal bracings (dissipative zones in tension diagonals only).
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Figure 6.3: Frames with concentric V-bracings (dissipative zones in tension and compression diagonals).
Figure 6.4: Frames with eccentric bracings (dissipative zones in bending or shear links). Default values for αu/α1 (see 6.3.2(3) and Table 6.2).
a) b)
Figure 6.5: Inverted pendulum: a) dissipative zones at the column base; b) dissipative zones in columns (NEd/Npl,Rd < 0,3). Default values for αu/α1 (see 6.3.2(3)
and Table 6.2).
Figure 6.6: Structures with concrete cores or concrete walls.
Figure 6.7: Moment resisting frame combined with concentric bracing (dissipative zones in moment frame and in tension diagonals). Default value for αu/α1 (see
6.3.2(3) and Table 6.2).
Figure 6.8: Moment resisting frame combined with infills.
Figure 6.9: Frame with K bracings (not allowed).
6.3.2 Behaviour factors
(1) The behaviour factor q, introduced in 3.2.2.5, accounts for the energy dissipation capacity of the structure. For regular structural systems, the behaviour factor q should be taken with upper limits to the reference values which are given in Table 6.2, provided that the rules in 6.5 to 6.11 are met.
Table 6.2: Upper limit of reference values of behaviour factors for systems regular in elevation
Ductility Class STRUCTURAL TYPE
DCM DCH
a) Moment resisting frames 4 5αu/α1
b) Frame with concentric bracings Diagonal bracings
V-bracings
4 2
4 2,5 c) Frame with eccentric bracings 4 5αu/α1
d) Inverted pendulum 2 2αu/α1
e) Structures with concrete cores or concrete walls See section 5 f) Moment resisting frame with concentric bracing 4 4αu/α1
g) Moment resisting frames with infills
Unconnected concrete or masonry infills, in
contact with the frame 2 2
Connected reinforced concrete infills See section 7 Infills isolated from moment frame (see
moment frames) 4 5αu/α1
(2) If the building is non-regular in elevation (see 4.2.3.3) the upper limit values of q listed in Table 6.2 should be reduced by 20 % (see 4.2.3.1(7) and Table 4.1).
(3) For buildings that are regular in plan, if calculations to evaluate αu/α1, are not performed, the approximate default values of the ratio αu/α1 presented in Figures 6.1 to 6.8 may be used. The parameters 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 plastic 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.
(4) 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 Figures 6.1 to 6.8.
(5) Values of αu/α1 higher than those specified in (3) and (4) of this subclause are allowed, provided that they are confirmed by calculation of αu/α1 with a nonlinear static (pushover) global analysis.
(6) The maximum value of αu/α1 that may be used in a design is equal to 1,6, even if the analysis mentioned in (5) of this subclause indicates higher potential values.