The equivalent single-wheel load. A critical analysis

The design of the gigantic B29 bombers, outfitted with complex landing apparatus, was the occasion for working up new calculation procedures having as their aim to substitute the multi-wheel undercarriage strut, with which these aircraft were outfitted, with fictitious undercarriage struts outfitted with a single wheel yet able to create in the pavement the same stress or strain states. Their advantage consisted in permitting analysts to continue to use the classical theories (Westergaard, Burmister, Peltier, Hogg, etc.) based in fact on the load transmitted by a single wheel only. The equivalent single-wheel load concept (ESWL) took on during succeeding years ever greater importance, to the point of being inserted in many countries’ codes governing pavement design and of being used to define the limit conditions on runway use by modern civil jet aircraft. However, the development of ever more refined calculation methods and the extraordinary power of the latest-generation electronic computers enable us to surpass the limits of the classical theories and to forsake the artificial and not very credible determination of the ESWL. To require for cement-concrete pavements, as the FAA suggests, that the ESWL be supposed to create the same stress state as generated by the real undercarriage strut leads to underestimating the vertical displacements by over two times. But to hypothesize the equality of the strain states induced by the real and fictitious undercarriage struts overestimates the actual stress state. In the study great prominence was given to the influence of the materials constituting pavements, to the intensity of the loads transmitted by the Boeing 747-200-300-400 (Jumbo) landing gear and, most especially, to the dimensions of the plate fields subjected to the ESWL; this last is not much dealt with in the international scientific literature.