The paper deals with the analysis of piles of variable cross-sectional properties and, head angular restraint, in non linear soils under both lateral and axial loads.
The analytical procedure follows the stiffness matrix method applied to large deflections and allows contemporarily for both soil and structural nonlinearities.
Complete and detailed solutions are presented and analyzed in view of using AISC interaction formula to take into account beam-column effect on piles.
When designing pile-supported structures engineers have to cope with one of the most interesting problems in civil structural engineering: interaction between soil and structure.
The unquestionable difficulties of the problem arise from the fact that the typical nonlinearities introduced by the soil cannot be correctly evaluated when coupled with linear structural analysis.
The most usual approach to this problem is either to solve the individual pile after determination of the value of the angular elastic restraint to the structure or to simulate the pile with a non laterally restrained beam cantilevered to its lower end, being its length dependent on the pile and soil properties. It is possible to extend the above methods to the case of non linear soils and to solve by a trial-and-error procedure the coupling between pile and structure. Extensive studies are available on this subject and among which stand the basic works of Focht and McClelland (ref. 1, 1955) Reese and Matlock (reC2, 1956), Matlock and Reese (ref. 3, 1961) Davisson and Gill {ref. 4, 1963).
However, besides that introduced by the soil another type of nonlinearity exists in such structures: it arises when in addition to the lateral loads, very high axial loads act on the pile s in such a way that the bean-column effect lay be non longer negligible. This lay happen for instance, in the offshore structures supported by high strength steel piles. In this case, an eventual large displacement at mud line emphasizes the effect and the interdependence of the soil and structural nonlinearities.
When dealing with steel franked structures it is possible to disregard during the analysis the effect of structural nonlinearities because the secondary stresses are accounted for by limiting the maximum permissible primary stresses on the basis of coded regulations. In the case of the AISC code these maximum allowable stresses are dependent through the well known interaction formulae from the effective slenderness ratio and the coefficient, Cm. However for the specific case of piles partially or completely embedded into the soil it is not straightforward to decide on the slenderness ratios and Cm coefficients to evaluate maximum allowable combined stresses. This difficulty is due mainly to the soil nonlinearity because the pile effective slenderness ratio is not constant but increases with the pile lateral displacement.
