The “Directional Procedure” is the traditional wind design procedure that can be used for structures of any height. When we say it is the “traditional wind design procedure,” we mean that it has been around since the 1972 edition of the standard that is now ASCE 7, which at that time was ANSI Standard A58.1, ANSI standing for the American National Standards Institute. With the “Directional Procedure,” the wind direction matters; you get different wind pressure diagrams for different wind directions. An external pressure coefficient, C_p, is used in this method, that is distinct and separate from the gust effect factor, G. The C_p is different for the windward face, the leeward face, the side faces and the roof. The C_p coefficients are based on past wind tunnel testing of prototype buildings. The “Directional Procedure” is found in Part 1 of ASCE 7-10 Chapter 27 (Main Wind-Force Resisting System or MWFRS) and Part 3 of ASCE 7-10 Chapter 30 (Components and Cladding or C&C). A simplified “Directional Procedure” applicable to buildings having mean roof height not exceeding 160 ft and introduced for the first time in ASCE 7-10, is found in Part 2 of ASCE 7-10 Chapter 27 (MWFRS) and Part 4 of ASCE 7-10 Chapter 30 (C&C).

The “Directional Procedure” is a function of the wind direction with different C_p values for
the windward face, leeward face, side faces, and roof

The “Envelope Procedure” is significantly different. One can say that the wind direction does not matter. It is a wind design procedure in which the effect of wind direction has been eliminated by testing models in a wind tunnel under all conceivable wind directions and by providing pseudo external pressure coefficients. The pseudo pressures are back-calculated from observed values of key structural actions (uplift, horizontal shear, bending moment, etc.) and the pseudo external pressure coefficients are chosen to produce key structural actions that envelope their maximum values among all possible wind directions. The pseudo external pressure coefficient, C_pf is lumped together with the gust effect factor, G, and GC_pf is given for a number of areas into which the exterior surface of a building is divided. The “Envelope Procedure” can be quite onerous because it requires wind to be applied to each of the four corners of a structure in turn, and in two orthogonal directions at each corner. It becomes less burdensome if a building is singly symmetrical and hardly burdensome at all for doubly symmetrical buildings. The “Envelope Procedure” is found in Part 1 of ASCE 7-10 Chapter 28 (MWFRS) and Part 1 of ASCE 7-10 Chapter 30 (C&C). A simplified “Envelope Procedure” is found in Part 2 of ASCE 7-10 Chapter 28 (MWFRS) and Part 2 of ASCE 7-10 Chapter 30 (C&C). The simplified “Envelope Procedure” was called Method 1 – Simplified Procedure in ASCE 7-05.

The “Envelope Procedure” is not a function of wind direction and uses an envelope approach that is
inclusive of maximum values among all possible wind directions