Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 25 The expressions therefore simplify to: b,Rd y,Ed yy b,y,Rd Ed M k M N N   1.0 (from Expression 6.61) and b,Rd y,Ed zy b,z,Rd Ed M k M N N   1.0 (from Expression 6.62). Values of k yy and k zy may be obtained from EN 1993-1-1, either Annex A or Annex B. Annex A generally provides higher design strength for the rafters and columns in portal frames than Annex B. The choice of Annex may be defined in some countries by their National Annexes. The worked example within this publication adopts Annex B values. The buckling resistances will normally be based on the system length of the rafter and column. Some national regulatory authorities may allow the use of a reduced system length and a buckling length factor. The buckling length factor is 1.0 or smaller, and reflects the increased buckling resistance of members with a degree of end fixity. The buckling length is the product of the length and the buckling length factor, and will be less than the system length. This approach will result in an enhanced buckling resistance. Clause 6.3.5 Lateral torsional buckling of members with plastic hinges. This clause provides guidance for the members in frames that have been analysed plastically. The clause requires restraint to hinge locations and verification of stable lengths between such restraints and other lateral restraints. Both topics are addressed in more detail in Section 6.4. 6.2.1 Influence of moment gradient A uniform bending moment is the most onerous loading system when calculating the lateral torsional buckling resistance of a member. A non-uniform moment is less onerous. Annexes A and B in EN 1993-1-1 allow for the effect of the moment gradient, via coefficients C mi,0 and C mLT etc. These C factors influence the k yy and k zy factors in Expressions 6.61 and 6.62, used when verifying the member. Although it is conservative to take C factors as 1.0, this is not recommended.