Wind Loads (ASCE 7-16 and ASCE 7-22) - MWFRS Directional Procedure

Both Wind Loads (ASCE 7-16) and Wind Loads (ASCE 7-22) include MWFRS (Directional Procedure) per ASCE 7 Chapter 27. This mode calculates direction-dependent wind pressures on walls and roofs using building geometry, wind direction, and external pressure coefficients from Fig. 27.3-1.

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Background

The Directional Procedure evaluates wind pressures for a single wind direction at a time. You choose whether wind acts normal to the long building axis or normal to the short axis, and the calculator determines windward, leeward, sidewall, and roof pressures from interpolated $C_p$ values in Fig. 27.3-1. This differs from the MWFRS (Envelope Procedure) (Chapter 28), which envelopes multiple load cases without selecting a primary wind direction. Use the Directional Procedure when you need explicit direction-specific MWFRS pressures tied to $L/B$, $h/L$, and roof angle.

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Key properties

Under Key Properties, set Type of Calculation to MWFRS (Directional Procedure). Project defaults for Basic Wind Speed, Exposure Category, and Risk Category work the same as other wind load modes. Override them at the top of the sheet if needed.

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Wind Load Parameters

When MWFRS (Directional Procedure) is selected, Gust Effect Factor ($G$) appears in the Wind Load Parameters section below Key Properties. It defaults to 0.85 for rigid buildings (natural frequency greater than 1 Hz) per Cl. 26.11.1.

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Building properties

Define roof geometry, dimensions, and enclosure classification as usual. For the Directional Procedure, Roof Type supports Flat, Gable, Hip, and Monoslope (the Envelope Procedure is limited to flat and gable roofs). Enclosure Type supports Enclosed, Partially Enclosed, Partially Open, and Open. Select Open for structures with no walls — carports, canopies, agricultural sheds, and covered pavilions — where wind flows freely over and under the roof. For Open enclosures the Directional Procedure computes design wind pressure on the free roof per Cl. 27.3.2 (Figs. 27.3-4, 27.3-5, 27.3-7) using the free-roof net pressure coefficients ($C_N$), and enforces the Cl. 27.1.5 minimum design wind force of 16 psf times the projected frontal area. Open enclosure support is currently limited to monoslope and gable free-roof shapes. Directional-specific inputs and computed values include:

• Wind Direction: choose N-S Direction (Normal to Ridge) or E-W Direction (Parallel to Ridge). This sets which building dimension is treated as $L$ (parallel to wind) and $B$ (normal to wind).
• Horizontal Dimension Parallel to Wind Direction ($L$) and Horizontal Dimension Normal to Wind Direction ($B$): derived from Width (Perpendicular to Ridge) and Length (Parallel to Ridge) based on the selected wind direction.
• L/B Ratio (for Directional Procedure): used for the leeward wall $C_p$.
• h/L Ratio (for Directional Procedure): mean roof height divided by $L$; used for roof $C_p$ interpolation.
• Roof Angle in Wind Direction ($\theta_{dir}$): equals the roof slope for most cases, or 0° when wind is parallel to the ridge (E-W on a gable roof).

A diagram under Wind Direction in the summary shows windward and leeward surfaces for the selected direction. Optional Include Roof Overhangs? = Yes adds Roof Overhang External Pressure Coefficient and Roof Overhangs Bottom Surface Pressure for the directional procedure (Cl. 27.3.3).

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Intermediate calculations

The calculator follows the ASCE 7 Chapter 27 workflow:

1. Velocity Pressure ($q_h$) at mean roof height, including exposure, topographic, and ground elevation factors where applicable.
2. Wind Directionality Factor ($K_d$) per Table 26.6-1 (included in ASCE 7-22 directional pressure equations; computed on ASCE 7-16 sheets but not multiplied into the directional summary pressures).
3. Internal Pressure Coefficient ($GC_{pi}$) from enclosure type, optionally reduced by the Internal Pressure Reduction Factor ($R_i$) for large-volume partially enclosed buildings (see Internal pressure reduction (Ri)).
4. Roof External Pressure Coefficients table: $C_p$ for windward wall, leeward wall, sidewall, and roof surfaces from Fig. 27.3-1, with linear interpolation for $L/B$, $h/L$, and $\theta_{dir}$ where permitted by the figure notes. For flat/low-slope roofs with $h/L \geq 1$, the “Roof (0 ft to h/2) Min” coefficient can be reduced with tributary area per Fig. 27.3-1, footnote b (see Cp roof reduction factor (RF_b)).

For sloped roofs ($\theta_{dir} \geq 10°$), the roof table reports separate windward minimum/maximum and leeward coefficients. For low-slope roofs ($\theta_{dir} < 10°$), roof zones are split by distance from the windward edge (0 to $h/2$, $h/2$ to $h$, $h$ to $2h$, and beyond $2h$), with separate minimum and maximum $C_p$ values where Fig. 27.3-1 requires both.

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Internal pressure reduction (Ri)

For Enclosure Type = Partially Enclosed, a Large-Volume, Unpartitioned Building? input appears. Selecting Yes exposes:

• Unpartitioned Internal Volume ($V_i$)
• Total Area of Envelope Openings ($A_{og}$), summing wall and roof openings

The Internal Pressure Reduction Factor is computed per Eq. 26.13-1: $R_i = \min\left(1.0,\ 0.5 \left(1 + \frac{1}{\sqrt{1 + V_i / (22{,}800\, A_{og})}}\right)\right)$ $R_i$ multiplies $GC_{pi}$ and feeds every net design pressure table on the sheet (MWFRS Envelope, MWFRS Directional, and Components and Cladding). It defaults to 1.0 (no reduction) unless the input is set to Yes.

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Cp roof reduction factor (RF_b)

For the MWFRS Directional Procedure on flat/low-slope roofs ($\theta_{dir} < 10°$) with $h/L \geq 1.0$, a Tributary Area for Cp Roof Reduction input appears. The reduction factor $RF_b$ is linearly interpolated from that area per Fig. 27.3-1, footnote b: $RF_b$ multiplies the $-1.3$ roof pressure coefficient in the Roof (0 ft to h/2) Min zone only. It defaults to 1.0 (no reduction) unless the input is engaged.

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Results

The summary includes:

• Wind Pressures: design pressures for each MWFRS surface, reported as Wind Pressure (Positive Internal) and Wind Pressure (Negative Internal) to cover both internal pressure cases.
• Wind Pressures for Windward Wall: height-varying pressures on the windward wall when velocity pressure varies with height (including topographic effects).
• Note: Minimum design wind pressures (Directional): ASCE 7 requires minimum ultimate pressures of 16 psf on windward walls and 8 psf on leeward walls and roofs (Cl. 27.1.5). Compare calculated values against these minimums in your design.

Design pressure form:

• ASCE 7-22: $p = q_h K_d (G C_p \mp GC_{pi})$
• ASCE 7-16: $p = q_h (G C_p \mp GC_{pi})$ (the template applies $K_d = 0.85$ in other modes but omits it from the directional pressure equations shown in the summary)

Both editions report Wind Pressure (Positive Internal) and Wind Pressure (Negative Internal) columns to cover both internal pressure cases. When Include Parapets? is enabled, see Wind Load Parapets (ASCE 7-16 and ASCE 7-22) for parapet pressure outputs.

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Related articles

• Wind Loads (ASCE 7-16) - MWFRS Overview (Envelope Procedure)
• Wind Loads (ASCE 7-22) - Components and Cladding Overview
• Wind Load Parapets (ASCE 7-16 and ASCE 7-22)