Closed-form solutions for laterally loaded free and fixed-head piles in elastic-plastic media have been developed and implemented into a spreadsheet program called GASLFP. Underpinned by a generic limiting force profile (LFP), the solutions offer an expeditious and sufficiently accurate method for predicting response of lateral piles. They also allow parameters to be deduced using measured pile response, as is evident in the study to date on -70 test (elastic) piles. The solutions also well capture the impact of structure nonlinearity of pile body by employing reduced pile bending stiffness (EpIp). The law of the reduction in EpIp, however, needs to be verified. In this paper, the solutions are utilised to examine the response of four test-piles exhibiting structural nonlinearity. For each nonlinear concrete pile, the shear modulus of soil and the three parameters (of Ng, ao and n) for constructing the LFP were deduced first using elastic pile assumption; and subsequently the variations of flexural rigidity with bending moment, the cracking moment Mcr and the ultimate moment Mult were back-figured against measured pile deflection. Thie study provides parameters for modelling nonlinear piles in sand and clay, and justification using pertinent expressions.