Structural Dynamics and Earthquake Engineering

Transcription

1 Structural Dynamics and Earthquake Engineering Course 12 Seismic design of steel structures Course notes are available for download at Design concepts Low-dissipative structural behaviour Dissipative structural behaviour Design concept High dissipative structural behaviour Medium - dissipative structural behaviour Low dissipative structural behaviour Range of the reference values of the behaviour factor q only limited by structural type q < 4.0, also limited by structural type q = Structural ductility class H (high) M (medium) L (low) 1

2 Structural types: behaviour factors q (tab 6.3) Structural types: behaviour factors q (tab 6.3) 2

3 Ductility of steel structures Steel - ductile material ductile steel structures??? Ductile steel structure: Ductile material (steel) Ductile cross-section Ductile elements Appropriate connections Structural ductility Material ductility Material and cross-section ductility f u /f y >1.2 elongation at rupture > 20% elongation at the end of the yield plateau > 1.5% Cross-section ductility Mpl M el elements in tensions: cross-section ductility = material ductility elements in compression: local buckling reduced strength and ductility compression: due to axial forces or due to bending Eurocode 3: four cross-section classes M Clasa 3 Clasa 4 Clasa 2 Clasa 1 Ductility class Behaviour factor q Cross-section class DCH Acc. tab DCM Acc. tab or 2 DCL 1,0 q 1,5 1, 2 or 3 q = 1.0 1, 2, 3 or 4 3

4 Element ductility Buckling reduces both strength and ductility Compression elements: flexural buckling Elements subjected to bending: lateral-torsional buckling Buckling should be prevented for dissipative elements by limiting element slenderness stockier elements lateral restraints Connections Complex behaviour and design: validation through tests Dissipative connections: plastic deformations in connections 4

5 Complex behaviour and design: validation through tests Dissipative connections: plastic deformations in connections Connections Connections Complex behaviour and design: validation through tests Non-dissipative connections: overstrength with respect to the connected dissipative elements 5

6 Connections Non-dissipative connections: designed with an overstrength with respect to the connected dissipative elements R 1,1 R d ov fy FORCE R d non dissipative connections expected strength of dissipative member 1.1 ovr fy ovr fy R fy nominal strength of dissipative member DISPLACEMENT Structural ductility Strength hierarchy in order to promote a global plastic mechanism maximum possible number of plastic zones uniform distribution of ductility demands in the structure 6

7 Moment-resisting frames Horizontal forces are mainly resisted by members acting in an essentially flexural manner Dissipative zones located in plastic hinges in the beams (or the beam-column joints) 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 N Ed / N pl,rd < 0.3 Dissipative zones in beams: Moment-resisting frames M M N N Ed pl,rd Ed pl,rd 10, 0,15 V V Ed pl,rd 0,5 V Ed =V Ed,G + V Ed,M V ed,g - shear force due to gravity loading V Ed,M = (M pl,rd,a +M pl,rd,b ) / L Lateral supports at dissipative zones M pl,rd,a M pl,rd,b M pl,rd,a M pl,rd,b V Ed,G L V Ed,G V Ed,M V Ed,M V Ed L L V Ed 7

8 Moment-resisting frames Columns: 1,1 T M i M /M pl,rd,i ov Ed,i M V V Ed pl,rd 0,5 Moment-resisting frames Dissipative connections experimental proven rotation capacity connection flexibility accounted for in analysis Non-dissipative connections: overstrength over connected elements reduce beam strength increase connection strength Rotation capacity of beam-column connections: 0.04 rad for DCH 0.03 rad for DCM 8

9 Frames with concentric bracings Horizontal forces are mainly resisted by members subjected to axial forces Dissipative zones should be mainly located in the tensile diagonals Type of bracings : 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 K bracings, in which the intersection of the diagonals lies on a column may not be used Frames with concentric bracings Braces shall be placed in such a way that the structure exhibits similar stiffness and strength in opposite senses 9

10 Analysis: Frames with concentric bracings under gravity load conditions, only beams and columns shall be considered to resist such loads in frames with diagonal bracings, only the tension diagonals shall be taken into account in frames with V bracings, both the tension and compression diagonals shall be taken into account Brace design: slenderness limitation X braces slenderness limitation V braces 1,3 2,0 2,0 strength: N pl,rd N Ed Frames with concentric bracings Design of beams and columns N N i Beams in V-braced frames: pl,rd,i / N all non-seismic actions without considering the intermediate support given by the diagonals the unbalanced vertical seismic action effect applied to the beam by the braces after buckling of the compression diagonal E d,i N pl,rd 0.3N pl,rd 10

11 Frames with eccentric bracings Horizontal forces are mainly resisted by axially loaded members, But the eccentricity of the beam-brace connections is such that energy can be dissipated in seismic links by means of either cyclic bending or cyclic shear Seismic links: Frames with eccentric bracings short links (plastic deformations in shear) - e<1.6m pl,link /V pl,link long links (plastic deformations in bending) - e>3.0m pl,link /V pl,link intermediate links (plastic deformations in shear + bending) 11

12 Detailing: stiffeners lateral supports Frames with eccentric bracings Frames with eccentric bracings Elements not containing seismic links (columns, braces, beams): Short links: V i 1,5V pl,link,i / V Ed,i Intermediate and long links: M i 1,5M pl,link,i / M Ed,i 12

13 Buckling restrained braced frames Horizontal forces are mainly resisted by members subjected to axial forces Dissipative zones: buckling restrained braces (BRBs) Buckling restrained braced frames BRBs are composed of a steel core encased in a steel tube filled with mortar, which prevents buckling of the steel core. Stable hysteretic response 13

14 Buckling restrained braced frames Design of braces: Check for axial force strength A f y NEd NRd M 0 Experimental tests to prove a corresponding behaviour of the system. Deformația specifică, % Forța axială, kn Buckling restrained braced frames Beams and columns: non-dissipative elements. Forces in the seismic design situation correspond to attainment of corrected strength in compression and are determined using the following formulas: 14