Issue 10

The latest edition of the structural steelwork code, BS 5950-1: 20001, requires secondary forces, generated by geometrical changes to a structure under load, to be considered during the analysis and design of ‘sway sensitive’ frames. This paper discusses the fundamental steps required in developing a computerised method for the second order plane frame analyses of steel structures, at any loaded stage up to and including collapse. Design engineers with some knowledge of a personal computer programming language in addition to an understanding of matrix structural analysis, who require a better and more in-depth command of second order effects, can comfortably adopt and develop the procedure to form a useful working tool. The load deflection characteristics including plastic hinge order and position, in addition to collapse loads, collectively obtained from this method, compare very favourably with those determined both experimentally and theoretically from previously published papers. John Graham, PhD, BSc, CEng, FIStructE Engineering Design, Nottinghamshire County Council

The replacement of the suspended structure of the Lions' Gate Suspension Bridge in 2000-2002 was the culmination of a long study of the bridge that revealed information about suspension bridges not generally known before, notably in the areas of bridge dynamics, aerodynamics (including criteria), traffic loading, load factors, cable stretch, seismic effects, and bridge temperature compared to ambient. The need to rehabilitate the bridge while keeping traffic flowing resulted in some innovative designs, including replacing the concrete deck of the viaduct with a wider steel orthotropic deck, rotation of some foundations, and replacement of the entire suspended structure. Constraints on the reconstruction options provided some fascinating challenges. These were overcome by going back to first principles, and by some creative solutions – exercised with great care. Peter G. Buckland, LLD, CEng, FCSCE, MICE Buckland & Taylor Ltd, North Vancouver, BC, Canada