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Dr. E. C. Hambly (M) : I wish to challenge the authors' statement that grillage analysis is inappropriate for decks with less than four cells. I will substantiate this challenge by demonstrating that the shear-flexible grillage can be used to reproduce the structural behaviour of a box-girder deck with the accuracy of a more sophisticated technique. I advocate grillage analysis of cellular decks because-it is an automatic method which does not require a high level of concentration in the design office over a long period, it is relatively inexpensive for analysis of multiple load cases, it can accommodate variations of geometry in plan and section, and it can be used with accuracy. There are shortcomings but they are no worse than those associated with grillage analysis of beam and slab decks. I should add that my enthusiasm for grillage analysis has developed considerably since I changed to the C&CA program which has very simple and clear data praparation and output.
The author's experience on several codes-of-practice committees has led him to the conclusion that one of the greatest fields of uncertainty lies in the specifying of effective lengths of continuous columns. This is coincidental with the difficulty of calculating elastic critical load factors and dealing with frame instability. Comprehensive design charts are given for effective lengths of columns with any local
degree of end restraint, both for sway and no sway conditions. To design for the limit-state of collapse it is essential to discover new 'desk' methods of dealing with overall frame instability when effective lengths based on local restraints are not accurate enough. A new technique of 'stiffness distribution', akin to moment distribution, will be acceptable in a drawing office, both for steel and reinforced concrete frames, at the same time providing rapid estimates of the other limit-state of permissible sidesway. Important modifications for strain hardening and composite panel stiffening are included, indicating the urgent need for more full-scale tests on composite action with cladding. The emphasis throughout has been on simplicity of theory and of the many worked examples.
A numerical procedure to determine the load-deformation response, up to the point of instability, of restrained steel beam-columns under biaxial bending is presented in this paper. The rotational restraints and the end loads need not be symmetric. The member is prismatic, originally straight and untwisted. The influence of residual strains as well as the warping strains that result from the twisting of the cross-section of column are included in the analysis. The equilibrium equations are written with respect to an arbitrary system of co-ordinate axes. This way the shift in the shear centre, and the shift and rotation of principal axes of the elastic core, as the yielding of the cross-section progresses is taken care of automatically.
S. Vinnakota and Y. Aoshima