What is the difference between the envelope procedure and directional procedure, and which approach is used in the WFCM?

The WFCM uses the envelope procedure, which is a simplified method where GCpf values are developed to provide maximum structural actions from boundary-level wind tunnel tests of low-rise buildings meeting certain limitations. The directional procedure uses GCp values that are based on general aerodynamic theory and is more generally applicable to all buildings.