Columns 1
(1)P In primary seismic columns the design values of shear forces shall be determined in accordance with the capacity design rule, on the basis of the equilibrium of the column under end moments Mi;d (with /'= 1,2 denoting the end sections of the column), corresponding to plastic hinge formation for positive and negative directions of seismic loading. The plastic hinges should be taken to form at the ends of the beams connected to the joints into which the column end frames, or (if they form there first) in the columns (see Figure 5.2).
(2) End moments M,d in (1)P of this subclause may be determined from the following expression:
where
YRd is the factor accounting for overstrength due to steel strain hardening and confinement of the concrete of the compression zone of the section, taken as being equal to 1,1;
MRc,i is the design value of the column moment of resistance at end ' in the sense of the seismic bending moment under the considered sense of the seismic action;
(3) The values of Mrc,, and EMRc should correspond to the column axial force(s) in the seismic design situation for the considered sense of the seismic action.
- Figure 5.2: Capacity design shear force in columns
5.4.2.4 Special provisions for ductile walls
(1)P Uncertainties in the analysis and post-elastic dynamic effects shall be taken into account, at least through an appropriate simplified method. If a more precise method is not available, the rules in the following clauses for the design envelopes for bending moments, as well as the magnification factors for shear forces, may be used.
(2) Redistribution of seismic action effects between primary seismic walls of up to 30% is allowed, provided that the total resistance demand is not reduced. Shear forces should be redistributed along with the bending moments, so that the in the individual walls the ratio of bending moments to shear forces is not appreciably affected. In walls subjected to large fluctuations of axial force, as e.g. in coupled walls, moments and shears should be redistributed from the wall(s) which are under low compression or under net tension, to those which are under high axial compression.
(3) In coupled walls redistribution of seismic action effects between coupling beams of different storeys of up to 20% is allowed, provided that the seismic axial force at the base of each individual wall (the resultant of the shear forces in the coupling beams) is not affected.
(4)P Uncertainties regarding the moment distribution along the height of slender primary seismic walls (with height to length ratio hwllw greater than 2,0) shall be covered.
(5) The requirement specified in (4)P of this subclause may be satisfied by applying, irrespective of the type of analysis used, the following simplified procedure.
The design bending moment diagram along the height of the wall should be given by an envelope of the bending moment diagram from the analysis, vertically displaced (tension shift). The envelope may be assumed linear, if the structure does not exhibit significant discontinuities of mass, stiffness or resistance over its height (see Figure 5.3). The tension shift should be consistent with the strut inclination taken in the ULS verification for shear, with a possible fan-type pattern of struts near the base, and with the floors acting as ties.
Key a moment diagram from analysis b design envelope al tension shift
Figure 5.3: Design envelope for bending moments in slender walls (left: wall systems; right: dual systems).
(6)P The possible increase in shear forces after yielding at the base of a primary seismic wall, shall be taken into account.
(7) The requirement specified in (6)P of this subclause may be satisfied if the design shear forces are taken as being 50% higher than the shear forces obtained from the analysis.
(8) In dual systems containing slender walls the design envelope of shear forces in accordance with Figure 5.4 should be used, to account for uncertainties in higher mode effects.
|
Key |
Post a comment