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This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Steel Designers' Manual - 6th Edition (2003) 448 Beams 16.4 Lateral bracing For design to BS 5950: Part 1, unless the engineer is prepared to supplement the code rules with some degree of working from first principles, only restraints capable of acting as rigid supports are acceptable. Despite the absence of a specific stiffness requirement, adherence to the strength requirement together with an awareness that adequate stiffness is also necessary, avoiding obviously very flexible yet strong arrangements, should lead to satisfactory designs. Doubtful cases will merit examination in a more fundamental way. 7,8 Where properly designed restraint systems are used the limits on lLT for Mb = Mc (or more correctly pb = py) are given in Table 16.7. For beams in plastically-designed structures it is vital that premature failure due to plastic lateral – torsional buckling does not impair the formation of the full plastic collapse mechanism and the attainment of the plastic collapse load. Clause 5.3.3 provides a basic limit on L/ry to ensure satisfactory behaviour; it is not necessarily compatible with the elastic design rules of section 4 of the code since acceptable behaviour can include the provision of adequate rotation capacity at moments slightly below Mp. The expression of clause 5.3.3 of BS 5950: Part 1, L m 38ry £ 2 2 [ fc/ 130 + ( py/ 275) ( x/ 36) ] Table 16.7 Maximum values of l LT for which p b = p y for rolled sections p y (N/mm 2 ) Value of l LT up to which p b = p y 245 37 265 35 275 34 325 32 340 31 365 30 415 28 430 27 450 26 1 2 makes no allowance for either of two potentially beneficial effects: (16.4) (1) moment gradient (2) restraint against lateral deflection provided by secondary structural members attached to one flange as by the purlins on the top flange of a portal frame rafter. The first effect may be included in Equation (16.4) by adding the correction term
This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Steel Designers' Manual - 6th Edition (2003) Bracing action in bridges – U-frame design 449 of Brown, 9 the basis of which is the original work on plastic instability of Horne. 10 This is covered explicitly in clause 5.3.3.A method of allowing for both effects when the beam segment being checked is either elastic or partially plastic is given in Appendix G of BS 5950: Part 1; alternatively the effect of intermittent tension flange restraint alone may be allowed for by replacing L m with an enhanced value L s obtained from clause 5.3.4 of BS 5950: Part 1. In both cases the presence of a change in cross-section, for example, as produced by the type of haunch usually used in portal frame construction, may be allowed for. When the restraint is such that lateral deflection of the beam’s compression flange is prevented at intervals, then Equation (16.4) applies between the points of effective lateral restraint. A discussion of the application of this and other approaches for checking the stability of both rafters and columns in portal frames designed according to the principles of either elastic or plastic theory is given in section 18.7. 16.5 Bracing action in bridges – U-frame design The main longitudinal beams in several forms of bridge construction will, by virtue of the structural arrangement employed, receive a significant measure of restraint against lateral – torsional buckling by a device commonly referred to as U-frame action. Figure 16.8 illustrates the original concept based on the half-through girder form of construction. (See Chapter 4 for a discussion of different bridge types.) In a simply-supported span, the top (compression) flanges of the main girders, although laterally unbraced in the sense that no bracing may be attached directly to them, cannot buckle freely in the manner of Fig. 16.4 since their lower flanges are restrained by the deck. Buckling must therefore involve some distortion of the girder web into the mode given in Fig. 16.8 (assuming that the end frames prevent lateral movement of the top flange). An approximate way of dealing with this is to regard each longitudinal girder as a truss in which the tension chord is fully Fig. 16.8 Buckling of main beams of half-through girder compression boom
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