Under what conditions we should use load combinations mentioned under Subject heading in UBC -97? As per my research we should use them only for " Element supporting Discontinuous System"-UBC 97-1630.8.2 and for "Collector Elements"-UBC 97 1633.2.6. So, there is no need to use these combinations for all structures and its components (Exceptions are mentioned above). Is my understanding correct? Please reply with referrence