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We have read the letter that appeared in The Structural Engineer, Vol. 5 0 , No. 1, January 1972, p. 28, with some surprise and not a little interest. Since this Company originated the idea and production of permanent woodwool formwork units in 1959, something like 2 x 106 yd2 of Spanform formers have been installed in the UK and overseas, and this criticism, to our knowledge, is the first of its kind.
The buckling load of a latticed structure is determined by considering the equilibrium of the joints in the deformed state, taking into account the effect of axial loads and deformation. The displacements U, V, W and the rotations ox, oy, oz are considered as unknowns at each joint. A stiffness matrix is developed based on ‘second order theory’. The loads causing buckling are those for which the determinant of the coefficients of the equilibrium equations at the joints vanish. As this results in a nonlinear eigen value problem, it is solved by iteration. A general computer program is developed to find the load causing instability of latticed structure of any geometry and for any support conditions.
Mr. Williams in introducing his paper quoted from an unpublished illustrated report on the first submerged tube tunnel under the River Thames at Rotherhithe which it was proposed to build in 1811 but was later abandoned because of differences among its promoters.