The basis for the crane vertical impact forces in Section 4.9.3 of ASCE 7-16 are not provided in the standard itself or the commentary. The 25% increase in wheel loads has been in the standard since at least the 1972 edition, when it was ANSI A58.1. The 10% increase in wheel loads was added in the 1995 edition. The 1995 edition was also the first edition where crane live loads were organized in their own subsection and the tabular layout of the vertical impact forces was included.
The crane vertical impact forces were likely derived from crane industry standards. Several such standards are referenced in the ASCE 7-16 commentary, Section C4.9.
The crane vertical impact force provisions, and the corresponding commentary text, have been revised in the recently published 2022 edition of ASCE 7. While the changes do not provide clarification of the basis for the forces, the forces are now associated with the crane service class which considers the overall use of the crane. The 25% increase applies to heavy and severe classes (service classes D, E, and F), while the 10% increase applies to infrequent, light, and moderate service classes (service classes A, B, and C).