Author: Reiss, M;Sokal, J
First published: N/A
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Reiss, M;Sokal, J
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.
A proposed plastic design method, depending on tension chord yield, for two-layer parallel-chord space trusses is discussed. The method is compared with the conventional
elastic method and the strip method originally used in reinforced concrete slab design. The disadvantages of these last methods are shown in a comparison of complete
analyses of three designs. The essential point made is that when tension chords begin to yield a set of membrane forces is developed: these forces are compressive in the region of the yielding tension members and increase in magnitude as the yielded tension zone increases. The proposed design method allows for this change of force distribution so that no compression member instability occurs, thereby furnishing a favourable load-deflexion characteristic for the structure. Reference is made to experimental results of tension member yield in planar trusses in order to justify the use of yielding tension members; it is noted that further work is necessary.
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 öx, öy, öz 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.