A person shall be regarded as practicing structural engineering within the meaning of this Act who is engaged in the design, analysis, or supervision of the construction, enlargement or alteration of structures, or any part thereof, for others, to be constructed by persons other than himself or herself. Structures within the meaning of this Act are all structures having as essential features foundations, columns, girders, trusses, arches or beams, with or without other parts, and in which safe design and construction require that loads and stresses must be computed and the size and strength of parts determined by mathematical calculations based upon scientific principles and engineering data. Nothing in this Section imposes upon a person licensed under this Act the responsibility for the performance of any acts or practice unless such person specifically contracts to provide it. Nothing in this Section precludes an employee from acting under the direct supervision or responsible charge of a licensed structural engineer.