In 2016, a photographer recreated historic photos of Chinese cities to highlight the changes the cities have undergone. For example, here is a spot in Chengdu in 1994 and 2016. (I agree, 1994 is not that historic).
In 1999, when I started working on OpenSees (G3 at the time), the framework was much like the 1994 Chengdu photo. A couple buildings dotted the horizon–Frank had the truss element; Elastic, ElasticPP, and Parallel uniaxial materials; and the elastic beam-column element from his 1997 Ph.D. dissertation.
Remo implemented the force-based beam-column element and the fiber section representations (patches, layers, etc.) for his Spring 1999 CE 224 class project. Greg implemented the zero length element in December 1999. That same year, I got my feet wet porting Filip’s
Steel1 FORTRAN subroutines to C++. A snapshot of OpenSees in 2001, when a few more models came in to the picture, is available here.
From there, construction of the OpenSees framework took off. Like the buildings in the 2016 Chengdu photo, several new models have filled in the spaces around the basic line elements and uniaxial materials of 1999 OpenSees.