What is the difference between directional and non-directional wind loads in ASCE 7?

The Directional Procedure (ASCE 7-22 Chapter 27) applies wind from specific directions using true external pressure coefficients (Cp). The Envelope, or non-directional, Procedure (Chapter 28) uses pseudo-pressure coefficients (GCpf) that bound the worst case across all directions. Both rest on the same velocity pressure equation; they differ in the coefficient used and how wind direction is handled.

What is the wind directionality factor Kd and what value does it use?

Kd is the wind directionality factor in the velocity pressure equation qz = 0.00256 Kz Kzt Kd Ke V squared. For buildings it equals 0.85 for both MWFRS and components and cladding. It accounts for the low probability that peak wind speed and the most critical wind direction occur at the same time. Kd applies to both the directional and envelope procedures.

Is the directionality factor Kd the same as choosing a directional procedure?

No. They are separate concepts. Choosing the procedure (directional versus envelope) decides which coefficients and ASCE 7 chapter you use. The directionality factor Kd = 0.85 is a single multiplier inside the velocity pressure equation that reduces pressure for the unlikely worst-case alignment of wind speed and direction, and it applies to both procedures regardless of which you select.

When should I use the directional procedure versus the envelope procedure?

The Directional Procedure (Chapter 27) works for buildings of all heights and uses true wall Cp values from Figure 27.3-1. The Envelope Procedure (Chapter 28) is a non-directional simplification intended for low-rise buildings, using GCpf pseudo-pressures from Figure 28.3-1 that envelope every wind angle. Choose based on building height and the level of directional detail your design requires.

What are the wall pressure coefficients for the directional procedure?

In the Directional Procedure, true external wall pressure coefficients (Cp) come from ASCE 7-22 Figure 27.3-1: windward wall +0.8, leeward wall ranging from minus 0.5 to minus 0.2 depending on building proportions, and sidewalls minus 0.7. The rigid-building gust-effect factor G is 0.85. The Envelope Procedure instead uses combined GCpf pseudo-pressure coefficients rather than separate Cp values.

ASCE 7-22 · MWFRS PROCEDURES

Directional vs Non-Directional Wind Loads

Two ways to apply wind to a building's main force-resisting system: the Directional Procedure aims wind from set directions, while the Envelope Procedure wraps the worst case across all of them.

Ch 27DIRECTIONAL · ALL HEIGHTS

Ch 28ENVELOPE · LOW-RISE

0.85K_d · BOTH METHODS

0.85G · RIGID GUST FACTOR

VISUAL · HOW WIND IS APPLIED

One building, two mental models

Directional reads each wind angle separately with true pressure coefficients. Envelope collapses every angle into one bounding pseudo-pressure pattern.

Directional · Cp

Wind from a chosen direction; true wall C_p from Fig 27.3-1 (windward +0.8, leeward −0.5 to −0.2, sidewall −0.7).

FIG 27.3-1

Envelope · GC_pf

Pseudo-pressure coefficients (Fig 28.3-1) bound the worst case across directions — a non-directional simplification for low-rise.

FIG 28.3-1

SIDE BY SIDE

Directional vs Envelope procedure

Same velocity pressure equation underneath; the difference is the coefficient and how each handles wind direction.

Aspect	Directional Procedure	Envelope Procedure
ASCE 7-22 chapter	Chapter 27	Chapter 28
Coefficient	C_p (true external pressure)	GC_pf (pseudo-pressure)
Coefficient figure	Fig 27.3-1	Fig 28.3-1
Building type	All heights	Low-rise buildings
How direction is handled	Wind applied from specific directions	Worst case enveloped across directions
Wall C_p (directional)	Windward +0.8 · Leeward −0.5 to −0.2 · Sidewall −0.7	—

COMMON MIX-UP

Procedure is not the directionality factor

The choice of procedure (directional vs envelope) decides which coefficients and chapter you use. The directionality factor K_d = 0.85 is a separate term that accounts for the low probability the peak wind speed and the most critical direction occur together — and it applies to both procedures.

The procedure

Directional (Ch 27, C_p) or Envelope (Ch 28, GC_pf) — sets your coefficients and how direction is treated.

METHOD CHOICE

The factor K_d = 0.85

A single multiplier in q_h = 0.00256 K_z K_zt K_d K_e V². Applies to both procedures.

DIRECTIONALITY

KEEP GOING

Pick the right method

Tie the procedure to your design method, the K_d factor, and the MWFRS-vs-C&C decision.

Wind Load Calculators Design Methods Guide

Directional vs Non-Directional Wind Loads

One building, two mental models

Directional · Cp

Envelope · GC_pf

Directional vs Envelope procedure

Procedure is not the directionality factor

The procedure

The factor K_d = 0.85

Pick the right method

Design Methods (ASCE 7)

Wind Directionality K_d

C&C vs MWFRS

Directional vs Non-Directional Wind Loads

One building, two mental models

Directional · Cp

Envelope · GCpf

Directional vs Envelope procedure

Procedure is not the directionality factor

The procedure

The factor Kd = 0.85

Pick the right method

Design Methods (ASCE 7)

Wind Directionality Kd

C&C vs MWFRS

Envelope · GC_pf

The factor K_d = 0.85

Wind Directionality K_d