[en] MODSAP is a modified and expanded version of the program SAP-IV originally developed at the University of California, Berkeley. MODSAP contains additional finite elements, time integration operators, and other options for user convenience. In addition, MODSAP permits the economical analysis of structures with localized nonlinearities. The report is divided into three parts. The first part presents the construction of the program, an overview of the analysis capabilities, the finite element library, and the numerical techniques used. References are included for further details. The second part of the report discusses several sample problems. The last part of the report is a series of appendices including the user's manual and detailed discussions of the theory for many of MODSAP's features