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What is the difference between the "Directional Procedure" and the "Envelope Procedure" as referred to in ASCE 710 for wind design? Posted on 20140630 Topics: ASCE 710, Wind Provisions Return to Blog Index
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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, Cp, is used in this method, that is distinct and separate from the gust effect factor, G. The Cp is different for the windward face, the leeward face, the side faces and the roof. The Cp coefficients are based on past wind tunnel testing of prototype buildings. The “Directional Procedure” is found in Part 1 of ASCE 710 Chapter 27 (Main WindForce Resisting System or MWFRS) and Part 3 of ASCE 710 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 710, is found in Part 2 of ASCE 710 Chapter 27 (MWFRS) and Part 4 of ASCE 710 Chapter 30 (C&C).
The “Directional Procedure” is a function of the wind direction with different Cp 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 backcalculated 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, Cpf is lumped together with the gust effect factor,
G, and GCpf 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 710 Chapter 28 (MWFRS) and Part 1 of ASCE 710 Chapter 30 (C&C). A simplified "Envelope Procedure" is found in Part 2 of ASCE 710 Chapter 28 (MWFRS) and Part 2 of ASCE 710 Chapter 30 (C&C). The simplified "Envelope Procedure" was called Method 1 Simplified Procedure in ASCE 705.
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