Collectors are structural components that play a critical role to transmit inertia forces in the floor diaphragms to the vertical seismic-force resisting system in a building structure. Yet little research has been done on collectors. A three-phase test program was conducted on a half-scale, two-story steel building by using the NHERI@UCSD large high performance outdoor shake table. The main objectives of this project were to investigate the inertial force load path in the floor diaphragms and the seismic behavior of collectors and their connections. Phase 1 tests were performed as a “single-story phase” with only the first story with a composite slab constructed. An innovative experimental technique was developed such that the absolute acceleration history response of any floor in a multi-story prototype building experiencing nonlinear response and higher-mode effects could be simulated by using a re-usable single-story specimen through a transfer function approach. Test results validated this testing technique. Phase 2 tests were conducted after a second story with a bare steel roof deck was added to the test building; the conventional testing method with the scaled historical ground acceleration as the input motion was used. In Phase 3, two buckling-restrained braces were added to the second story to modify the building dynamic characteristics and the collector seismic load path. Earthquake simulation tests were conducted again until the failure of side-lap connections of the roof deck occurred. Test results showed that the current collector design would overestimate the axial forces in the roof collectors because it neglects the effect of flexural rigidity of the collector connections, which would mobilize gravity columns to transfer some inertial forces to the story below. An improved design method for estimating the roof collector axial forces that considers the flexural rigidity of the collector connections was proposed. Test results also showed that the unintended moment demand produced by the connection rigidity would cause the steel connections in composite collectors to experience axial forces higher than that assumed in design. Recommendations including connection design requirements and collector width-to-thickness ratios were also made.