Beam/Column Plastic Moment Capacity Ratio

Transcription

1 COPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEBER 2001 STEEL FRAE DESIGN UBC-ASD Technical Note This Technical Note describes how the program checks or designs joints. When using UBC97-ASD design code, the structural joints are checked or designed for the following:! Check for the requirement of continuity plate and determination of its area (see Technical Note Design of Continuity Plates Steel Frame Design UBC97-ASD)! Check for the requirement of doubler plate and determination of its thickness (see Technical Note Design of Doubler Plates Steel Frame Design UBC97-ASD)! Check for ratio of beam flexural strength to column flexural strength! Reporting the beam connection shear! Reporting the brace connection force Beam/Column Plastic oment Capacity Ratio In Seismic Zones 3 and 4, for Special oment-resisting Frames, the code requires that the sum of beam flexure strengths at a joint should be less than the sum of column flexure strengths (UBC ). The column flexure strength should reflect the presence of axial force present in the column. To facilitate the review of the strong-column/weak-beam criterion, the program reports a beam/column plastic moment capacity ratio for every joint in the structure. For the major direction of any column (top end) the beam-to-column strength ratio is obtained as: R maj = nb n =1 pcax pbn cos θ + pcbx n (UBC ) Page 1 of 5

2 For the minor direction of any column the beam-to-column strength ratio is obtained as: R min = nb n =1 pcay pbn sin θ + n pcby (UBC ) R maj, min = Plastic moment capacity ratios, in the major and minor directions of the column, respectively pbn θ n pcax,y pcbx,y n b = Plastic moment capacity of n-th beam connecting to column, = Angle between the n-th beam and the column major direction, = ajor and minor plastic moment capacities, reduced for axial force effects, of column above story level. Currently, it is taken equal to pcbx,y if there is a column above the joint. If there is no column above the joint, pcax,y is taken as zero. = ajor and minor plastic moment capacities, reduced for axial force effects, of column below story level, and = Number of beams connecting to the column. The plastic moment capacities of the columns are reduced for axial force effects and are taken as pc = Z c (F yc - f a ), (UBC ) Z c F yc f a = Plastic modulus of column, = Yield stress of column material, and = aximum axial stress in the column. Beam/Column Plastic oment Capacity Ratio Page 2 of 5

3 For the above calculations, the section of the column above is taken to be the same as the section of the column below assuming that the column splice will be located some distance above the story level. Evaluation of Beam Connection Shears For each steel beam in the structure, the program will report the maximum major shears at each end of the beam for the design of the beam shear connections. The beam connection shears reported are the maxima of the factored shears obtained from the loading combinations. For special seismic design, the beam connection shears are not taken less than the following special values for different types of framing. The requirements checked are based on UBC Section 2213 for frames in Seismic Zones 3 and 4 and on UBC Section 2214 for frames in Seismic Zones 1 and 2 (UBC , , 2213, 2214). No special requirement is checked for frames in Seismic Zone 0.! In all Seismic Zones except Zone 0, for Ordinary oment Frames, the beam connection shears reported are the maximum of the specified loading combinations and the following additional loading combinations (UBC , ): 1.0 DL + 1.0LL ± Ω 0 EL (UBC , )! In all Seismic Zones except Zone 0, for Special oment-resisting Frames, the beam connection shears that are reported allow for the development of the full plastic moment capacity of the beam (UBC , ). Thus: V = C L pb + V (UBC , ) DL + LL V = Shear force corresponding to END I and END J of beam C = 0 if beam ends are pinned or for cantilever beam, = 1 if one end of the beam is pinned, Evaluation of Beam Connection Shears Page 3 of 5

4 = 2 if no ends of the beam are pinned, pb = Plastic moment capacity of the beam, ZF y, L = Clear length of the beam, and V DL+LL = Absolute maximum of the calculated factored beam shears at the corresponding beam ends from the dead load and live load combinations only.! In all Seismic Zones except Zone 0, for Eccentrically Braced Frames, the beam connection shears reported are the maximum of the specified loading combinations and the following additional loading combination: 1.0 DL + 1.0LL ± Ω 0 EL Evaluation of Brace Connection Forces For each steel brace in the structure, the program reports the maximum axial force at each end of the brace for the design of the brace-to-beam connections. The brace connection forces reported are the maxima of the factored brace axial forces obtained from the loading combinations. For special seismic design, the brace connection forces are not taken less than the following special values for different types of framing. The requirements checked are based on UBC Section 2213 for frames in Seismic Zones 3 and 4 and on UBC 2214 for frames in Seismic Zones 1 and 2 (UBC , , 2213, 2214). No special requirement is checked for frames in Seismic Zone 0. In all Seismic zones except Zone 0, for Ordinary Braced Frames, the bracing connection force is reported at least as the smaller of the tensile strength of the brace (F y A) and the following special loading combination (UBC , ). 1.0 DL + 1.0LL ± Ω 0 EL (UBC , )! In all Seismic Zones except Zone 0, for Special Concentrically Braced Frames, the bracing connection force is reported at least as the smaller of the tensile strength of the brace (F y A) and the following special loading combination (UBC , ): Evaluation of Brace Connection Forces Page 4 of 5

5 1.0 DL + 1.0LL ± Ω 0 EL (UBC , )! In all Seismic Zones except Zone 0, for Eccentrically Braced Frames, the bracing connection force is reported as at least the brace strength in compression that is computed as 1.7F a A (UBC , ). 1.7F a A is limited not to exceed F y A. Evaluation of Brace Connection Forces Page 5 of 5