Author: Reiss, M;Sokal, J
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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. L.C. Schmidt
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 Williams in introducing his paper quoted from an unpublished illustrated report on the first submerged tube tunnel under the River Thames at Rotherhithe which it was proposed to build in 1811 but was later abandoned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.